Goulburn River Trout Fishery

Goulburn River Trout Fishery: Monitoring and Assessment Paul Brown

March 2008

Fisheries Victoria Research Report Series No. 20

Goulburn River trout assessment

© The State of Victoria, Department of Primary Industries, 2008.

Authorised by the Victorian Government, 1 Spring Street, Melbourne

This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.

Printed by DPI Snobs Creek, Victoria

Preferred way to cite this publication: Brown, P. (2008) Goulburn River Trout Fishery: Monitoring and Assessment. Fisheries Victoria Research Report Series No. 20. ISSN 1448‐7373 ISBN

978‐1‐74199‐986‐0

Author Contact Details: Paul Brown Fisheries Research Branch Private Bag 20, Alexandra Victoria 3714


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Published by the Department of Primary Industries. Copies are available from the website: www.dpi.vic.gov.au/fishing General disclaimer This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.

Executive Summary This work is part of a suite of Goulburn River Trout fishery research projects designed to gather information to assist management of one of Victoria’s most popular recreational trout fisheries.

>350 mm in the sample is used as a performance measure. Population estimates were made for each site sampled, each year, using a mark‐ recapture approach, with fish tagged at each survey.

Regular calls from the community to enhance the Goulburn River trout fishery by stocking, and address uncertainties associated with the impact of fishing on trout numbers and size composition, have lead to a further need to assess the fishery more quantitatively. This monitoring and assessment project has had evolving aims in order to address these emerging issues. However, the primary purpose of this report is to assess the size and species structure of the trout population; determine factors that may limit the sustainability of the trout population; and recommend options for enhancing the fishery in the Goulburn River.

Trout movement, recapture rates, and mortality were determined from tag‐recovery data. Recording the location of recaptures reported by recreational fishers allowed us to monitor the movement of trout tagged during fish surveys the previous winter. The number of tagged brown and rainbow trout recaptured and reported each fishing season was determined. Tagged fish recaptures, during the season following the winter in which they were tagged, were analysed to estimate what proportion of wild trout present at the start of each season are caught by anglers each season.

The Goulburn River and its tributaries upstream from Seymour are predominantly trout fisheries, with brown trout more common and reaching a larger size than rainbow trout. During spring to autumn the Goulburn carries irrigation water releases from Lake Eildon to the distribution system commencing at Goulburn Weir. Considerable fluctuations in flow and level occur.

Summary of Methods Winter surveys to assess the size‐structure of the trout population and collect information to estimate population size, have been undertaken annually since 1997 at a range of sites between the Eildon fish trap and Molesworth. This report concentrates on analysis of samples collected during winter 2000–2004, with some comparison with previous data. Since the year 2000 sampling effort has concentrated on three sites that were consistently fishable and represented popular fishing access points: the breakaway‐bridge pool, the Rubicon River confluence pool and the gauge pool downstream of the canoe launch. Survey‐design was shifted in 2001 from a simple estimate of relative‐abundance, to a population‐estimate based on a mark‐recapture survey at each site. Surveys were at night, using an electro‐fishing boat. Size‐structure of the trout population was monitored and the proportion of brown trout

To gather information on potential tag reporting rate, we ran a simulation‐experiment during the 2002/3003 trout season. To determine the incidence and rate of tag‐ shedding, we conducted trials both in the river, and in the more controlled situation of laboratory tanks. An estimate of total mortality rate for catchable‐ sized brown trout was also made from the rate‐ of‐return of tagged fish recaptures. To investigate dispersal and ‘longevity’ of stocked trout, within the recreational fishery; for to the 2003 fishing season, approximately 500 tagged rainbow and brown trout were released in the Goulburn River. Subsequent reports, by anglers, of the capture of these tagged fish were logged and collated.

Summary of Results Size structure of brown trout population is relatively stable from year‐to‐year, although there are indications of a stronger than average year‐class originating in 1999, perhaps due to advantageous growth and survival in the moderate water temperatures of 2001. The proportion of the brown trout sample >350 mm is used as an indicator of quality size. Linear regression shows that the increasing trend for quality size is statistically significant, and the percentage of the stock at quality size has


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increased at an average rate of 2.9% per year since 1997.

equates to an annual survival rate of 4% year‐1 averaged over all size and age‐classes.

Size structure of rainbow trout population fluctuates more than for brown trout. Only two years, 1997 and 2001, stand out with rainbows present in significant numbers.

During 1997–2003 fishing seasons, anglers recaptured and reported locations for 49 brown trout and 14 rainbow trout after periods of up to 355 days at liberty. Of these trout, 42% were recaptured after net‐movements of 0–500 m, 68% had moved up to 0–5 km, and 17% had moved over 10 km from their release locations. The longest movement recorded was a brown trout that was tagged at Eildon and recaptured 148 km downstream at Kirwinʹs Bridge, Goulburn Weir.

Only data from 2001 to 2004 are used for population estimates. Although there is quite high variability among and between sites, the overlapping confidence limits on the estimates suggests no statistically significant difference in size of trout populations between years or between sites. Fishing mortality rate, estimated from tag‐ recaptures, is influenced by tag reporting rate and tag shedding rate. Using a simulated tag experiment described above during the 2002/2003 season we estimated a 16% tag reporting‐rate (ie. 16% of anglers catching a tagged trout may report it). Brown and rainbow trout show different tag‐shedding characteristics, with rainbow trout retaining tags better than browns. Tag loss is significant over the duration of the fishing season. In the tank trial only approximately 57% of the brown trout and approximately 84% of the rainbow trout retained their tags over this period. Tag‐loss in trout in the Goulburn River was roughly consistent with the tag loss within the tank‐trial for similar periods of liberty. During 1997–2004 , 2454 trout were tagged between May and September. Most of the data is for brown trout (n=2193). Of these, 54 were captured and reported by anglers in the fishing season following their tag and release. For brown trout the annual reported recapture rate varied from 0.3% in 1997 to 6.5 % in 2004 and was 2.7 % on average over the whole period. For rainbow trout recapture rate was more variable ranging from 0% in 1997, 1998 and 2002 to 10% in 1999 and 2004. The proportion of pre‐season abundance that is captured by anglers (exploitation rate) is calculated using the recapture data, reporting rate, and tag‐retention rates. There is strong inter‐year variability between 1997 and 2003 however, on average 31% and 34% of wild brown and rainbow trout (>200 mm TL at the start of the season) were captured by recreational anglers in a season. Total mortality rate of wild brown trout during the 1997–2003 was estimated from the rate of recaptures, weighted by the tag‐shedding rate over. The estimated monthly mortality rate


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Dispersal and longevity of stocked, trout in the 2003 fishing season was revealed by the 46 rainbow trout (19%) and eight (7%) brown trout, during the 2003 season that were recaptured and reported. Most of the rainbow trout recaptures were reported within 10‐weeks of stocking. The stocked trout dispersed both upstream and downstream however, downstream movements tended to be larger on average. Hence, the net dispersal of all stocked fish for either species was downstream. Upstream movement and no‐ movement, were also recorded and the average distance moved in either direction was 3 to 8 km. The largest movement recorded for individual stocked trout was a journey of 22 km for a brown trout stocked in the Goulburn near the Eildon pondage gates and recaptured up the Rubicon River. Similarly, a rainbow trout stocked in the Goulburn near the Breakaway Bridge, swam 20 km before recapture near the Eildon pondage gates.

Summary of Conclusions •

There has been no detectable change in the size of the brown trout population based on winter mark‐recapture estimates at three fixed sites between 2001–04. The structure of the brown trout population is a relatively stable with three and four age‐classes represented in two distinct length modes each winter. Since 1997 there has been a significant increase in the proportion of large trout (>350 mm LCF) in the winter population. The lack of significant rainbow trout spawning stock present during winter indicates the occasional summer‐abundance as anything other than escaped fish from commercial aquaculture enterprises.

•

The sporadic presence of abundant rainbow trout enhances angling. However, they present an impediment to significant planned, strategic, stock enhancement. Their unpredictable presence in addition to any

significant stock enhancement is likely to result in overstocking. If so, consequences for the sustainability of the wild brown trout fishery are unknown. •

Fishing exploitation rate in most years is still below that which would be expected to limit abundance. However, capture probability strongly varies from year‐to‐year and in some years a high enough proportion of catchable‐sized fish are caught to potentially limit the abundance of the population.

•

The recapture data from an experimental batch of stocked rainbow and brown trout suggests that rainbows of catchable size stocked at the start of the fishing season don’t generally disperse far before they are caught within about ten weeks. Most brown trout stocked at catchable size, remain in residence and uncaptured throughout the season.


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Table of Contents Executive Summary............................................................................................. iii Summary of Methods..............................................................................................................................................iii Summary of Results ................................................................................................................................................iii Summary of Conclusions ....................................................................................................................................... iv

Introduction ............................................................................................................ 1 Project Design ........................................................................................................ 3 Objectives....................................................................................................................................................................3

Methods ................................................................................................................... 4 Study area....................................................................................................................................................................4 Fish surveys ................................................................................................................................................................4 Trout movement, recapture rates, and mortality..................................................................................................5 Experimental Stock‐enhancement ..........................................................................................................................6

Results...................................................................................................................... 7 Size structure of brown trout population..............................................................................................................7 Size structure of rainbow trout population ..........................................................................................................8 Population Estimates.................................................................................................................................................9 Fishing Mortality .....................................................................................................................................................11 Total mortality rate of wild brown trout .............................................................................................................12 Movement of wild trout .........................................................................................................................................12 Dispersal and longevity of stocked‐catchable sized trout: 2004 season ........................................................13

Discussion and Implications for Management ............................................. 14 Trout Population Structure ....................................................................................................................................14 Trout Population Sustainability ...........................................................................................................................15 Trout Movement, Dispersal and Persistance......................................................................................................16

Conclusions .......................................................................................................... 17 Acknowledgements............................................................................................. 18 References ............................................................................................................. 19


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List of Tables Table 1. Population estimates for catchable (>200 mm LCF) brown trout with approximate confidence limits from mark‐recapture experiments at three sites on the Goulburn River, during winter 2001, 2002, 2003 and 2004. L95% and U95% are the lower and upper 95% confidence intervals on the population estimate. Estimates for Rubicon Junction site in 2001 made using modified Schnabel estimator (Ricker, 1975) all other estimates made using the Adjusted Petersen estimator (Ricker, 1975)................................................................................................................................................................... 10 Table 2. Wild tagged trout (>200 mm, LCF) recaptured and reported by anglers during the fishing‐season following the winter they were tagged. The annual pre‐season abundance‐captured by anglers is weighted by a 16% reporting rate determined experimentally, and by a 57% and 84% tag retention rates estimated for brown and rainbow trout respectively........................................................................ 12

List of Figures Figure 1. Length‐frequency distribution of brown trout sampled by electrofishing during winter (June– September), from the Goulburn River from 1997–2000................................................................................ 7 Figure 2. Length‐frequency distribution of brown trout sampled by electrofishing during winter (June– September), from the Goulburn River from 2001–2004................................................................................ 7 Figure 3. Size structure of winter electrofishing samples of brown trout from the Goulburn River since 1997, showing the proportion in each winter smaller and larger than 350 mm (text in bars)(ie. the size based catch‐limits in current fishing regulation). Numbers in parenthesis = n. ....................................... 8 Figure 4. Length‐frequency distribution of rainbow trout sampled by electrofishing during winter (June– September), from the Goulburn River from 1997–2004................................................................................ 8 Figure 5. Mark‐recapture population estimates for catchable brown trout at three sites (see legend) standardised to fish per kilometre. Bars indicate 95% confidence intervals on the estimate. Missing values indicate no sample................................................................................................................................. 9 Figure 6. Cumulative tag loss rates in a combined tank trial of two species of trout. Brown trout (diamonds, n=56) and rainbow trout (squares, n=54) were double‐tagged using Hallprint (PDX40mm) dart‐tags and held in a single tank during August 2003–August 2004. Both tags (tag1, tag2) were inserted in dorsal musculature adjacent to dorsal fin. Proportion of sample that shed tags at each position noted on right‐hand side of figure. ................................................................................... 11 Figure 7. Regression of natural log of number of angler recaptures on time‐at‐liberty (T, months) (diamonds and dotted line), and the same data weighted to account for tag‐loss rates estimated from tank experiments triangles and solid line) for tagged brown trout >200mm TL in the Goulburn River. ............................................................................................................................................................................ 12 Figure 8. Cumulative reported catch of rainbow trout stocked at catchable size in the Goulburn prior to the 2003 season (triangles). Heavy line marks total number stocked. Dotted line shows the same data assuming only 20% of the tags were reported. ............................................................................................ 13


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Introduction The Goulburn River, from Eildon pondage downstream to Alexandra, is a popular recreational trout fishery. Since the 1980s it has been managed as a wild trout fishery without stock‐enhancement. Recent research reports (Brown 1998; 2000; 2007; Brown and Gason 2007) and management strategies (FV 2002) have pointed out that its popularity, and proximity to the large population of Melbourne, suggests that careful monitoring and assessment are required to maintain, or enhance, the productivity of this quality trout‐stream. To effectively manage a fishery requires information on a suite of factors such as fish habitat, levels of exploitation, population‐ dynamics and behaviour of the fish. In the Goulburn River little of this was known until a program of research began in 1997. In the summer, the Goulburn River flows strongly with cool water released from Eildon Dam. Flows in the Goulburn River downstream of Eildon reservoir are managed to supply irrigation to agriculture further downstream; stock and domestic supply along the valley; and some hydro power‐generation (at Eildon). The demand from the irrigation industry is such that there are strong seasonal fluctuations in stream flow (Ladson and Finlayson 2002). Superimposed upon these, flows in the reach downstream from Eildon to Alexandra also fluctuates (albeit less markedly), according to the demand for elec‐ tricity by the power grid. During winter Goulburn River flows downstream of Eildon are greatly reduced. The assumption in the Goulburn Eildon Region Fisheries Management Plan, was that low flows reduced the available wetted habitat and were likely to be a limiting factor for the abundance of the trout stock. However, in a study of the relationship between flows and habitat it was shown that typical summer irrigation flows reduced the available habitat for adult brown trout, by increasing the area with sub‐optimal high water velocities. The same study suggested that a lack of suitable fry habitat may be limiting trout abundance (Brown 2003). For the Mid‐Goulburn River system, winter stock assessments have been made using electrofishing and a fish‐trap, since prior to the first regulated trout‐fishing season in 1997. These annual

surveys (Brown 1998; Brown 2000) were initially qualitative, and designed to detect any trends towards a change in the size‐structure of the stock that may be correlated to the new management regime. Tagging and releasing the trout sampled in these winter surveys has been ongoing since 1997. Tagging was done initially to estimate the fishing exploitation rate, and also more recently to assist with the mark‐recapture method of population estimation. The present study includes experimental estimates of reporting, and tag‐retention rate to enable more accurate interpretation of the tag‐recapture data. The present study also follows on from two studies of trout behaviour. These radio‐tracking studies showed that sudden changes in flows had little effect on the behaviour of wild brown trout (Douglas 2003), and that stocked brown and rainbow trout behaved similarly to their wild counterparts (Brown 2007). Within the small groups of radio‐tagged trout, both stocked brown and stocked rainbow trout had lower survival rates than resident brown trout. However, this was mainly due to a higher capture‐rate of stocked fish than resident‐fish by anglers. In both studies, all fish showed restricted dispersal from their release site. A quantitative analysis of the harvest and economic impact of the recreational fishery was completed parallel to the present study (2002–03 and 2003–04 seasons) using a randomised questionnaire survey of anglers. That ʺcreel surveyʺ provides a useful contrast to the analyses of the fish population presented here (Brown and Gason 2007). The creel survey showed that the summer fishery is often dominated by rainbow trout, and that over 60% of all trout caught are released. Recent calls from the community to enhance the fishery by stocking and to address uncertainties associated with the impact of fishing pressure on trout numbers and size composition (FV 2002) have lead to a further need to assess the fishery more quantitatively. The present project, in combination with those mentioned above, has had evolving aims to address these emerging issues. However, the primary purpose of the present report is to assess the size and species structure of the trout population; determine factors that may limit the sustain‐ability of the


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trout population; and recommend options for enhancing the fishery in the Goulburn River.


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Project Design The study had the following objectives.

Objectives 1.

To describe the size and structure of the mid‐Goulburn River trout population

2.

To determine factors that may be limiting this population

3.

To assess the sustainability of the fishery

4.

Recommend options for enhancing the fishery


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Methods Study area The Goulburn River and its tributaries upstream from Seymour are predominantly ‘trout waters’ (Weigall 2003), with brown trout more common and reaching a larger size than rainbow trout (Brown 1998). Fifteen other species of fish have also been recorded since 1997 (DPI, unpublished data). During spring to autumn the Goulburn River carries irrigation water releases from Lake Eildon to the distribution system commencing at Goulburn Weir. Considerable fluctuations in flow and level occur. Winter and spring flows are stored in Lake Eildon and passing‐flow targets in the Goulburn downstream are 130–250 ML/d depending on recent storage inflows. At the start of the irrigation season, the flows increase and may flow at 6,000–10,000 ML/d over the summer period, depending on demand and supply level in the storage. The Goulburn River alternates between sinuous gravel and cobble‐bedded reaches based on relatively short pool‐run‐riffle sequences; and long, deep, straight sections with silty margins. Even the ‘pools’ tend to have cobble or gravel beds along the thalweg (or current‐line) due to the high‐velocity of summer flows. Large woody debris is common especially within the longer straight pools. Willow thickets often provide thick cover and shade along the margins. Access for the trout fisher is good along most of the reach upstream of Alexandra. Multiple‐ vehicle car‐parks exist at 14 points between Alexandra and Eildon and three commercial caravan parks also have river‐frontage.

Fish surveys Winter surveys to assess the size‐structure of the trout population and collect information to estimate population size have been undertaken annually since 1997 at a range of sites between the Eildon fish trap and Molesworth. Early information from winter 1997–19 was reported largely as a comparison of size‐frequencies from year‐to‐year (Brown 1998; Brown 2000). This report will concentrate on analysis of samples collected during winter 2000–04, with some comparison with previous data. Since the year 2000, sampling effort has concentrated on three sites. These sites were consistently fishable and


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represented popular fishing access points. These were the breakaway‐bridge pool, the Rubicon River confluence pool and the gauge pool downstream of the canoe launch. Due to the need for more quantitative information, survey‐design was shifted in 2001 from a simple estimate of relative‐abundance, to a population‐estimate based on a mark‐recapture survey at each site. Surveys are conducted after‐dark, using an electro‐fishing boat equipped with lights. During 2003 and 2004, only two‐out‐of‐three sites were fished. A third site remained un‐fishable during each winter due to sustained highly turbid conditions that limited electrofishing efficiency.

Size‐structure Trout captured in the fish surveys were each weighed (g) and measured (mm)(length to caudal fork, LCF) under anaesthesia. Fish were returned alive to the water on completion of sampling. Length‐frequency data were pooled from all sites each year and grouped in 10‐mm length classes. Length‐frequency charts were constructed for comparison between years, for each of brown and rainbow trout. The proportion of brown trout >350 mm LCF in the sample is used as a performance measure. Linear regression analysis was used to detect whether the increasing trend was significant at the p=0.05 level.

Population estimates Population estimates were made for each site sampled each year using a mark‐recapture approach. During sampling, all trout >200 mm LCF were tagged with a single dart‐tag inserted in the dorsal musculature at the base of the dorsal fin. After recovery from anaesthesia in oxygenated water, all trout were released at the capture‐location. The principle behind population‐estimates is that a known number of fish are sampled, marked and returned to mix randomly with the unmarked fish population. When a second sample is then drawn from the population, the fraction of marked fish in the second sample is directly proportional to the ratio of the original sample‐size to the population‐size. Two important assumptions that should be satisfied for this method are: •

no significant movement into or out‐of the population and no mortality or recruitment between marking and recapturing, and

•

marked fish should mix thoroughly with the population before recapture survey.

The first assumption is probably not completely adhered to, as the pools we sampled are open at either end. It is impractical to use stop‐nets to close the large pools on the Goulburn River. In‐ practice movement between pools during this low‐water period is probably negligible. Radio‐ tracking studies of trout in the Goulburn River show that individual trout show a high degree of site‐fidelity, and are likely to remain at a given location for weeks‐at‐a‐time (Douglas 2003). Importantly, any departure from this assumption is likely to be similar each year, so that temporal comparisons are still valid. At worst, this series of population estimates can be considered as an index of relative abundance that is closer to the true population abundance than simply using the catches standardised by effort. The second assumption was dealt with by allowing sufficient time to pass between mark and recapture for the fish to re‐distribute themselves. In 2001 anglers re‐captured the second sample for us. As insufficient tagged fish were re‐caught, a sample was re‐captured using electro‐fishing several weeks after the initial release. In 2002 and subsequently, the time between release of marked fish and recapture surveys was reduced to 1–4 days to minimise any departures from assumptions about the method. Two methods of population estimation were used depending upon the total number of recaptures achieved after a single recapture, or multiple‐recapture census. In most cases, adjusted Petersen estimates (Ricker 1975) were used at sites as a single recapture census resulted in at least 4 recaptures. Modified Schnabel estimates (Ricker 1975) were made at sites where a multiple recapture census was required to make a total of at least 4 recaptures.

Trout movement, recapture rates, and mortality Movement Recording the location of recaptures reported by recreational fishers allowed us to monitor the movement of trout tagged during fish surveys the previous winter. Posters advertising the presence of tagged trout and instructions on how to record and report their capture, were maintained at all major river access‐points and in local fishing‐tackle stores.

Anglers usually reported tags via the telephone number printed on the tag. A short telephone interview usually yielded information on date and location of recapture. Sometimes length at recapture was also recorded along with whether the tagged fish was returned or retained. Data were recorded in the Snobs Creek angling database (SCAD) and anglers reporting recaptures were sent a letter detailing the initial capture information along with the information recorded at recapture and time at liberty (days) for ‘their’ fish.

Recapture rate The total number of tagged brown and rainbow trout recaptured and reported each season was determined for each year during 1997–2004. Tagged fish recaptures during the season following the winter in which they were tagged were analysed to estimate what proportion of wild trout present at the start of each season, are caught by anglers each season. Estimates of reporting‐rate and tag retention rate need to be taken into account in the estimation of annual estimates of angling‐exploitation (E) such that

Ri Ei =

a

(Ti × l )

where R is the number of trout caught and reported by anglers during the season (i); a is the estimated reporting rate for re‐captured tags; T is the number of fish tagged at the start of the season; and l is the estimate of annual retention rate for tags. Tag Reporting Experiment To gather information on potential reporting‐rate for tags in the Goulburn trout fishery, we ran a simulation‐experiment during the 2002–03 trout season. One hundred ‘tickets’ were printed containing a simulated tag‐number, the phrase ‘This numbered card represents the simulated “capture” of a tagged fish. Please report this to Fisheries Victoria at Snobs Creek 03 57742208’ and the Departmental logo. Tickets were approximately 14 x 4 cm and printed on white paper. Throughout the fishing season tickets were randomly placed in a prominent position, under the windscreen wipers of cars (driver’s side) parked in fishing‐access points between


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Eildon and Alexandra. A few were also similarly distributed along the nearby Rubicon River. As these ‘simulated’ captures were reported, they were recorded with the same procedure as normal tag recaptures. The proportion of distributed tickets that were reported was used as an estimate of tag reporting rate. To determine the incidence and rate of tag‐ shedding amongst the trout routinely tagged as part of our winter surveys, we conducted trials both in the river, and in the more controlled situation of laboratory tanks. Tag‐shedding experiment: River trial Trout tagged in the Goulburn River during winter 2003, as part of our winter population surveys, were mainly double‐tagged. Two numbered dart tags (Hallprint, PDX 40 mm) were implanted instead of the normal, single tag. Each tag was positioned, as normal, in the dorsal‐ flank musculature adjacent to the dorsal fin. The implanted tags were approximately 10 mm apart. Tag‐shedding experiment: Tank trial On 25 August 2003, 56 brown trout (weight range 115–251g) and 54 rainbow trout (weight range 111–327g) were obtained from a commercial hatchery and tagged, each with two numbered dart tags (Hallprint, PDX 40 mm) as above. These fish were held in a 2000 litre, flow‐through system at the Snobs Creek aquaculture research facility. Fish were fed a maintenance diet (pellets). The tank was fitted with screens, and inspected every 1–3 days to collect lost tags. Lost tags were collected and the date of loss recorded as the inspection date. On 25 August 2004, the surviving fish were anaesthetised, weighed and measured; tags were counted and the condition of the remaining tag and implant‐site noted. Factors such as skin‐necrosis, fungal infection at the tag site were noted, along with the presence of a tag‐scar (in the absence of a tag).

Total Mortality An estimate of total mortality rate for catchable‐ sized brown trout was also made from the angler recaptures of tagged fish. Total mortality rate includes both natural and fishing mortality. Recaptures by anglers were collated from SCAD as frequency of recaptures per 30‐day time‐ intervals at liberty. The instantaneous rate of mortality was estimated for brown trout as the slope of the relationship between the natural log of number of recaptures and time at liberty (Gillanders et al. 2001). Gillanders et al. (2001), regressed the number of recaptures per month against time (months) at liberty and considered


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only recaptures made within the first 35 months at liberty. The present analysis considers the number of recaptures per 30‐day period for 7 periods (total 210 days), approximately the duration of a fishing season. The estimate obtained is of monthly mortality and is converted to an annual estimate by dividing by 30 days and multiplying by 365 days. The recaptures from 2003–04 are excluded to eliminate the effects of stocked‐fish that year.

Experimental Stock‐enhancement Immediately prior to the fishing season in 2003 catchable‐sized rainbow trout (n=241) and brown trout (n=250) were released spread across six sites in the Goulburn River (Breakaway Bridge Pool, McMartins Lane, Goulburn Valley Highway Bridge, Thornton beach, Walnut’s Reserve and downstream of the pondage). Each trout was tagged using numbered dart‐tags. Subsequent reports, by anglers, of the capture of both tagged groups were logged and collated to investigate dispersal and ‘longevity’ within the recreational fishery.

Results Size structure of brown trout population

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Examination of the length structure in the brown trout population samples (Figure 1 and Figure 2) show that a stable pattern of size‐structure continued until winter 2001.

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Figure 1. Length‐frequency distribution of brown trout sampled by electrofishing during winter (June–September), from the Goulburn River from 1997–2000.

Figure 2. Length‐frequency distribution of brown trout sampled by electrofishing during winter (June–September), from the Goulburn River from 2001–2004.


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The proportion of the brown trout sample >350 mm is used as an indicator of quality size. Linear regression shows that the increasing trend for quality size is statistically significant (p=0.01, R2adj=62%) and the percentage of the stock at quality size has increased at an average rate of 2.9% per year since 1997 (Figure 3).

10

frequency

The increased relative abundance of the 1 and 2 year‐olds in 2001 suggested that recruitment of these young trout was strong. This gave rise to a strong size‐class in 2002 that were 3 year‐olds from the 1999 year‐class. In 2003, the size structure re‐stabilised to a typical bi‐modal pattern.

that of 2001; however, the size structures of these two samples are quite different.

frequency

The winter 2002 sample showed a dominance of large fish (>300 mm LCF). Previous age determination data has shown that the first identifiable length‐class of brown trout 250200 mm) trout, including 15 tag‐ recaptures (11 brown trout and four rainbow trout). Recaptures for each site varied from 3 to 10 trout. No recaptures were observed that had strayed from other sites.

1500 1000 500 0 2001

2002

2003

2004

3500 Rubicon Junction 3000

fish/km

2500 2000 1500 1000 500

During winter 2002, four sites were fished in total during July and August. The limited data collected from site at Killingworth road was not included in the analysis as this site was found to be difficult to fish effectively. A total of 391 brown (n=340) and rainbow trout (n=51) were captured by electrofishing. During July and August at three sites, 184 trout were initially sampled by boat electrofishing and tagged with dart tags and released. During follow‐up surveys, within 4 days of these initial captures, an additional 144 were sampled including 31 recaptures. Again, recaptures varied from three to 16 for each site. During 2003 three sites were surveyed in August and September. High flows and turbidity after local rain precluded the population estimate at the third site (Savages). However, mark‐ recapture estimates were made at sites at the Breakaway Bridge, and Rubicon River confluence. Brown trout marked at these sites numbered 39 and 34 respectively. Subsequent surveys, two days after the initial marking resulted in capture of 26 and 23 brown trout of which 13 and two were recaptures, respectively. Population estimates for each site were calculated and are presented in Figure 5.

2000

0 2001

2002

3500

2003

2004

Breakaway Bridge

3000

fish/km

2500 2000 1500 1000 500 0 2001

2002

2003

2004

Figure 5. Mark‐recapture population estimates for catchable brown trout at three sites (see legend) standardised to fish per kilometre. Bars indicate 95% confidence intervals on the estimate. Missing values indicate no sample.


9

During 2004 three sites were again surveyed in August and September. High flows and turbidity after local rain again precluded the population estimate at the third site (Rubicon Junction pool). However, mark‐recapture estimates were made at the Breakaway Bridge, and Savages. Brown trout marked at these sites numbered 104 and 33 respectively. Subsequent surveys, two days after the initial marking resulted in capture of 50 and 53 brown trout of which 23 and 10 were recaptures respectively. Population estimates for brown trout >200mm LCF at each site were calculated and are presented in Figure 5.

For catchable sized brown trout, estimates of population size and their 95% confidence intervals were made for the individual sites. Using the known site dimensions these were weighted up to a per‐kilometre basis for comparison among sites and years (Table 1). Although there is quite high variability among and between sites, the overlapping confidence limits on the estimates suggests no statistically significant difference in size of trout populations between years or between sites.

Table 1. Population estimates for catchable (>200 mm LCF) brown trout with approximate confidence limits from mark‐recapture experiments at three sites on the Goulburn River, during winter 2001, 2002, 2003 and 2004. L95% and U95% are the lower and upper 95% confidence intervals on the population estimate. Estimates for Rubicon Junction site in 2001 made using modified Schnabel estimator (Ricker, 1975) all other estimates made using the Adjusted Petersen estimator (Ricker, 1975) Estimate Site

2001 Savages

Site length (m)

L95% (n)

U95% (n)

Population Estimate (n/km)

L95% (n/km)

U95% (n/km)

800

369

127

1843

462

158

2303

Rubicon junction

400

363

189

764

908

472

1911

Breakaway Bridge

200

179

88

447

896

438

2234

800

647

396

1115

809

495

1394

2002 Savages

Rubicon junction

400

77

31

191

191

78

478

Breakaway Bridge

200

115

65

223

577

327

1113

2003

Savages

800

Rubicon junction

400

28

18

47

70

44

117

Breakaway Bridge

200

38

26

57

190

131

287

2004

Savages

800

134

76

258

168

95

323

Rubicon junction

Breakaway Bridge

200

174

101

326

870

505

1630


10

Site Estimate (n)

Some insight into the angling mortality rate can be gained from the rate of angling recapture of tagged fish. However, the rate at which anglers report tags, and the rate at which tags are lost from fish both need to be accounted for in estimating angling mortality.

Tag Reporting Using a simulated tag experiment described above during the 2002–03 season we estimated a 16% tag reporting‐rate (i.e. 16% of anglers catching a tagged trout may report it). Therefore this is used as the annual tag reporting‐rate in further analyses of angling recaptures to estimate exploitation rates.

Tag Loss Rate In a 12‐month tank‐trial starting August 2003 we double‐tagged 56 brown and 54 rainbow trout and observed the rate of tag loss. Brown trout (n=95) and rainbow trout (n=15) tagged during winter surveys in the Goulburn were also double tagged that year in a parallel experiment. Three main things were learned from the tank‐ trial. Brown and rainbow trout show different tag‐shedding characteristics, with rainbow trout retaining tags better than browns. Tag‐loss is negligible over short time‐scales (0–100 days) and certainly over periods used to estimate the populations using mark‐recapture ( 100 %) and are not included in calculating the mean.


11

Table 2. Wild tagged trout (>200 mm, LCF) recaptured and reported by anglers during the fishing‐ season following the winter they were tagged. The annual pre‐season abundance‐captured by anglers is weighted by a 16% reporting rate determined experimentally, and by a 57% and 84% tag retention rates estimated for brown and rainbow trout respectively. Fishing Brown trout Season

Rainbow trout

Tagged Recaptured & Recapture + pre‐ reported reporting season during season rate % (T) (R )

% pre‐season Tagged abundance‐ pre‐ captured by season anglers (T)

Recaptured & Recapture + % pre‐season reported reporting abundance‐ during season rate % captured by (R) anglers

(E)

(E)

1997

286

1

0.3

4

25

0

0.0

0

1998

107

2

1.9

20

4

0

0.0

0

1999

232

6

2.6

28

10

1

10.0

74

2000

215

14

6.5

71

11

1

9.1

68

2001

570

7

1.2

13

103

3

2.9

22

2002

542

9

1.7

18

79

0

0.0

0

2003

108

6

5.6

61

20

2

10.0

74

2004

133

2

1.5

16

9

0

0.0

0

During the 1997–2003 fishing seasons the average instantaneous total mortality rate (Z) of wild brown trout tagged and released during winter surveys was 0.30 month‐1, or 3.89 year‐1 (Figure 7). This equates to a mean annualised survival rate of only 2% year‐1. This is slightly negatively biased by tag shedding throughout the year. When the tag recaptures are weighted by the estimated rate of tag‐loss the annual survival rate increases to 4% year‐1. Tag recaptures are not weighted by the estimated reporting rate as reporting rate is a constant and has no effect on rate of recaptures. We have no estimate of initial tag mortality for wild fish, however this was low for those fish in the tank trial.

Movement of wild trout During 1997–2003 fishing seasons, anglers recaptured and reported locations for 49 brown trout and 14 rainbow trout after periods of between 4 days and 355 days at liberty. Of these trout, 42% were recaptured after net‐movements of 0–500 m, 68% had moved up to 0–5 km, and 17% had moved over 10 km from their release locations. The longest movement recorded was


12

from two rainbows and a brown trout that travelled approximately 20 km downstream, and a single brown trout that was tagged at Eildon and recaptured 148 km downstream at Kirwinʹs Bridge, Goulburn Weir. 3.0

Ln(weighted recaptures) = -0.26x + 2.79

2.5 Ln(recaptures)

Total mortality rate of wild brown trout

Ln(recaptures) = -0.30x + 2.87

2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

10

time (30 day blocks days )

Figure 7. Regression of natural log of number of angler recaptures on time‐at‐liberty (T, months) (diamonds and dotted line), and the same data weighted to account for tag‐loss rates estimated from tank experiments triangles and solid line) for tagged brown trout >200mm TL in the Goulburn River.

A total of 46 rainbow trout (19%) and eight (7%) of the brown trout, that were stocked as catchable trout before the 2003 season, were recaptured and reported. The final reported tag‐ recapture rate was around 11%. Most of the rainbow trout recaptures were reported quickly (Figure 8). Over 95% were reported within 10‐ weeks of stocking. Our previously estimated reporting rate of 16% was too low this time, producing an overestimate of fish‐returned. A better fit to the data is gained with the assumption of at least 20% tags caught being reported. These stocked trout sometimes dispersed upstream or downstream however, downstream movements tended to be larger on average. Hence, the net dispersal of all stocked fish in either species was downstream. There was a net total movement of 23 km downstream for the 46 rainbow, and 31 km for the eight brown trout respectively. However, upstream movement and no‐movement, were also recorded and the average distance moved in either direction was 3–8 km. The largest movement recorded for individual stocked trout was a mainly‐ downstream journey of 22 km for a brown trout stocked in the Goulburn 800 m downstream from the Eildon pondage and recaptured up the Rubicon River. Similarly, there was an upstream swim of 20 km for a rainbow trout stocked in the Goulburn near the Breakaway bridge, and recaptured near the Eildon pondage gates.

Tagged rainbow trout recapture

Dispersal and longevity of stocked‐catchable sized trout: 2004 season

300 250 200 150 100 50 0 0

5

10

15

20

25

Weeks after stocking

Figure 8. Cumulative reported catch of rainbow trout stocked at catchable size in the Goulburn prior to the 2003 season (triangles). Heavy line marks total number stocked. Dotted line shows the same data assuming only 20% of the tags were reported.


13

Discussion and Implications for Management Trout Population Structure The brown trout population structure seems relatively stable in the Goulburn River. Since 1997, annual surveys have shown two main size‐ classes with length modes of approximately 150– 220 mm (age 1+ year) and approximately 270– 380 mm (a mixture of age 2+ and 3+ years). Brown trout >400 mm are likely to be age 4 years and over. Variation in individual growth rate, timing of surveys and annual variations in environmental‐conditions, are likely to account for the spread of size‐at‐age. In contrast to the Goulburn River, many other streams in Victoria exhibit highly variable streamflows, and also more variable trout abundance (Brown 2000). The Goulburn River is a ʹtailraceʹ fishery downstream of Lake Eildon, and the yearly regularity in pattern and magnitude of streamflows in the fishery is probably a key factor in its population‐stability. Recent research modelling the effects of flow variability on trout habitat in the Goulburn River has suggested that the quantity of suitable fry‐ habitat may be limiting the population (Brown 2003). Most of the natural variability in stream trout populations is conferred by variable egg production and juvenile mortality (Elliott 1985; 1989). If fry habitat is acting as a ʹbottleneckʹ, then potentially variable egg‐production or alevin survival may not be reflected in the population dynamics. If only a fixed number of fry survive to recruit to the fishery this may also stabilise the dynamics of the adults. The current estimates of population density, based on mark‐and‐recapture experiments, largely confirm this population‐stability. Sampling variability is high and therefore estimates come with large confidence intervals. However, the hypothesis that there is no difference in catchable brown trout density between sites and between years during 2002–04 cannot be rejected. Previous estimates of trout density in the Goulburn River 1979–91, made using the piscicide rotenone, are similar (540–927 brown trout/km) (Baxter et al. 1992) lending further confidence to our estimates. In smaller Victorian streams, most population estimates of


14

brown trout vary between 100 and 700 fish per km of stream (Brown 2000). As a marked exception to the normal stable‐ pattern, there was unusually high relative abundance of brown trout aged 2+ in winter 2002. Results of this winter survey predicted that larger fish (>300 mm) should have been relatively abundant in the catch for the subsequent 2002 fishing season. The results from a creel survey (Brown and Gason 2007) supported this with brown trout catch in 2002 being significantly higher than in the following year (2003). The cause of this good year‐class is uncertain. Fish straying from those stocked into Eildon pondage were identifiable by fin‐clip, and made little contribution to samples in most years. There were none recorded in the 2002 sample. The increased abundance may have resulted from exceptional natural recruitment to the fishery from the 1999 and/or 2000 year‐classes. Strong growth may have conferred a survival advantage on the fish aged 1+ year in 2001, resulting in increased abundance of fish aged 2+ years in the fishery for 2002. Conditions for growth were exceptional during 2001 due to river temperature remaining between 10oC and 20oC all year (Brown 2004). Fast‐growing fish are vulnerable for less time to size‐limited predators such birds and other fish and can therefore be expected to have lower natural mortality rates (Dannewitz and Petersson 2001; Hambright 1991; Kirby et al. 1996). It should be noted that population estimates for winter 2001 were made with intervals between mark‐and‐recapture of months, whereas population estimates for winter during 2002– 2004 were made with intervals between mark‐ and‐recapture of days. Almost all mark‐recapture methods rely on a set of assumptions about the population. Primarily, that there is negligible movement of trout into or out of the experimental site plus negligible mortality or recruitment between times of mark and recapture. These assumptions are more likely to have been adhered to over the shorter sampling intervals that were used during 2002–2004. Thus it is not surprising that the winter 2002–2004 estimates have tighter, more precise confidence intervals.

There were anecdotal reports of ‘mass‐escapes’ of rainbow trout from unknown aquaculture sources along the Goulburn River in late spring and summer 2003. The abundance and physical condition (eg. eroded fins) of rainbow trout in the creel catch supports the anglers reports (Brown and Gason 2007). However, such summer‐ abundance of rainbow trout was not detected by winter surveys either before, or after the season anglers were catching them. As our sampling methods were adequate to detect rainbow trout, if present, this suggests that the rainbows were short lived; arriving after our sampling in winter 2003 and gone by the following winter.

Trout Population Sustainability Data from recapture and reporting of tags, by anglers provides the first estimates of the probability of capture for individual trout in this fishery. Our estimates vary considerably between years (4–71% for brown trout and 0–74% for rainbows in Table 2) but overall averages suggest that approximately 30% of brown and rainbow trout, that are >200 mm before the fishing season, are caught during the subsequent fishing season. The proportion of the pre‐season population that is harvested (as opposed to caught‐and‐released) by anglers is the exploitation rate. Data from the Goulburn River creel survey 2002–2003 suggested that 38% of all trout caught, were harvested (Brown and Gason 2007). Therefore the average recapture‐probability equates to an exploitation‐rate suggesting around 11% of the pre‐season abundance of catchable trout are harvested in an ʹaverageʹ season. In Minnesota trout streams it was found that angling limited abundance only when harvest was greater than 40–50% of pre‐season abundance (Thorn 2000). When harvest is lower, higher natural mortality accounts for a similar overall reduction in abundance. If this is the case for the Goulburn River, then the exploitation rate is probably high enough in some years (e.g. when recapture probability exceeds 70%) to enable angling‐ exploitation to limit abundance. Assuming the ‘catchability’ of trout does not change dramatically from year‐to‐year, then exploitation rates are probably set by the level of effort in the fishery and the proportion of captured‐trout that are released by anglers. Release rates on the Goulburn are partially due to voluntary “catch‐ and‐release” ethics amongst the anglers, and partially due to the fish size‐distribution and anglersʹ preferences to release small fish and regulations prohibiting the retention of more than two fish >350 mm TL. If the recent shift

continues towards an increasing proportion of larger fish in the winter population, it is possible that anglers may choose, or may be required, to release less of their catch. Trends in effort, and social trends such as “catch‐and‐release” fishing, may change over time and should be monitored to ensure the future sustainability of this fishery. The low annual survival rate of brown trout tagged as part of our winter surveys may be a concern from a methodological point‐of‐view. Recaptures of tagged trout by anglers suggest that few survive the year subsequent to their sampling and tagging. Whereas the size and age‐ structure of the population suggests higher‐ numbers must be surviving into most age‐ classes. The discrepancy may be partly due to the age and size bias in the recapture data. The estimate of the annual survival rate of angler‐ recaptured fish includes fewer fish aged 1+ year and aged 2+ years than in the general population. Most fish that are tagged are relatively near the end of their natural life span. It is also possible that tagging is adversely affecting an individual trout’s survival chances. If so, the estimates of survival, probability of capture and exploitation rate may be underestimated. Emigration of tagged fish is unlikely to be a methodological issue, as most fish show restricted movement (Brown 2007), and in any event the recreational fishery extend far beyond the immediate study‐ area so tags could also be reported from fish that had emigrated. Tag retention trials suggest that tag loss and tag‐ related injury are significant to the individual fish concerned, and more so with brown trout than rainbows. The rates of tag retention observed in the present tank‐study are similar to those published in a study of lake‐dwelling arctic char with a similar type of Floy tag (Rikardsen et al. 2002). Although lower than those observed in rainbow trout, with Floy anchor tags, in an Ozark stream (Walsh and Winkelman 2004). While our low tag retention rate might reduce the usefulness of tagging as a method for some application, the experimental estimation of the tag‐retention rate enabled us to correct for tag‐ loss bias and use the angling recapture data to estimate capture‐probability. Furthermore, the use of tagging in mark‐recapture estimates over a few days remains relevant; as does the use of tagging to determine movement and dispersal patterns. Electrofishing of salmonids is known to impose injury and growth penalties on the individuals sampled (Dalbey et al. 1996; Schill and Beland


15

1995). Given the observed, restricted movement and home‐range behaviour of trout in the Goulburn (Douglas 2003); repeatedly electrofishing during mark‐recapture estimates also has the potential to reduce populations at fixed sites although this is not supported by any observed decline in abundance estimates.

Trout Movement, Dispersal and Persistance Tag returns from anglers suggest that some Goulburn River trout can be repeatedly found near a given location (within 500 m), while some are capable of substantial movements of 5–25 km over time‐scales of several spring and summer months. Tag returns from electrofishing surveys during the winter suggest that many trout can be repeatedly found within the same pool over time‐scales of up to 2–3 winter months. Both findings largely agree with results of a study using radio‐telemetry to track wild brown trout movements in the Goulburn River during October 2002–June 2003 (Douglas 2003) and September 2003–June 2004 (Brown 2007). In both previous studies, fish stayed within a small home‐range for months, and occasionally shifted home range by approximately 5 kilometres. Stocking catchable‐sized rainbow trout from Eildon to the Breakaway Bridge before the start of the 2003 season seemed to supplement the fishery for up to 10 weeks (until mid‐November). By this time most had probably been caught. Few of the stocked catchable‐sized brown trout were


16

reported as recaptured by anglers. The lack of tag returns that were reported from downstream of Alexandra, despite a net movement of stocked trout downstream, probably indicates the low fishing effort in this reach. This experimental trial also supports the assertion that the reporting rate of tags is relatively low (around 20%). Elsewhere, catchable‐sized rainbow trout, have shown poor persistence within tailwater fisheries; however, this was due largely to ‘natural’ mortality and emigration rather than susceptibility to angling (Bettinger and Bettoli 2002). Rainbow trout stocked into Victorian lake fisheries show similar levels of persistence and low survival rate (Douglas and Hall 2004). Our data on restricted dispersal agrees with that for rainbow trout in USA tailwaters, (Gido et al. 2000; Simpkins et al. 2000) and for wild, resident brown trout in the Goulburn River (Douglas 2003) and a Victorian stream (Jackson 1980). Simply put; in the Goulburn River, rainbow trout of catchable size stocked at the start of the fishing season do not generally disperse far before they are caught within about ten weeks, while most brown trout stocked at a catchable size remain in residence and uncaptured throughout the season.

Conclusions •

There has been no detectable change in the size of the brown trout population based on winter mark‐recapture estimates at three fixed sites between 2001 and 2004. Annual variation in the estimates can be accounted for by sampling variability. However this variability is large and therefore the power of this method to detect change is low.

•

The structure of the brown trout population is a relatively stable with 3–4 yearly age‐ classes represented in two distinct length modes each winter. Since 1997 there has been a significant increase in the proportion of large trout (>350 mm LCF) in the winter population. One strong year‐class could be identified (2000 year‐class) and this may be the result of optimal stream‐temperatures producing exceptional growth and survival in 2001.

•

season. The sporadic presence of abundant, ʹescapedʹ rainbow trout undoubtedly represents a bonanza for the angler. However, they also present an impediment to significant planned, strategic, stock enhancement. Their unpredictable presence in addition to any significant stock enhancement is likely to result in overstocking. If so, consequences for the sustainability of the wild brown trout fishery are unknown.

Winter sampling did not corroborate reports and observations of abundant rainbow trout during the summer seasons for 2002–03 and 2003–2004. The lack of significant rainbow trout spawning stock present during winter makes it difficult to explain the summer‐ abundance as anything other than escaped fish from commercial aquaculture enterprises. The lack of persistence and/or survival of these rainbows through until winter is noted, and supported by the estimated large recreational catch of rainbow trout during the creel survey 2002– 04, and the similar response of the tagged catchable rainbow trout released in the 2003

•

Only approximately 20% of anglers, who catch a tagged fish, report it. Furthermore the annual tag retention rate for the type of dart tags used is low (57% for brown trout and 82% for rainbow trout).

•

Fishing exploitation rate in most years is still below that which would be expected to limit abundance. However, capture probability strongly varies from year‐to‐year and in some years a high enough proportion of catchable‐sized fish are caught (some more than once) to potentially limit the abundance of the population.

•

The recapture data from an experimental batch of stocked rainbow and brown trout suggests that rainbows of catchable size stocked at the start of the fishing season do not generally disperse far before they are caught within about ten weeks. Most brown trout stocked at a catchable size remain in residence and uncaptured throughout the season.


17

Acknowledgements This work would not have been possible without the assistance of most, if not many, of the staff of the Freshwater Fisheries Science section at Snobs Creek, and the cooperation of landowners Sid Savage, Richard Reed and Craig Gloury. I thank them all. I particularly acknowledge the assistance of (alphabetically)


18

Annie Giles, Peter Grant, Karl Pomorin, Andrew Pickworth, Daniel Stoessel, Russell Strongman and others. Thanks also to John Douglas, Wayne Fulton, Anne Gason and Terry Walker, Fisheries Research Branch, for insightful comments on the draft.

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predator mouth width and prey body depth. Transactions of the American Fisheries Society 120, 500‐508. Jackson PD (1980) Movement and home range of brown trout, Salmo trutta Linnaeus, in the Aberfeldy River, Victoria. Aust. J. Mar. Freshwater Res. 31, 837‐845. Kirby JS, Holmes JS, Sellers RM (1996) Cormorants Phalacrocorax carbo as fish predators: an appraisal of their conservation and management in Great Britain. Biological Conservation 75, 191‐199. Ladson A, Finlayson B (2002) Rhetoric and reality in the allocation of water to the environment: A case study of the Goulburn River, Victoria, Australia. River Research and Application 18, 555– 568. Rikardsen AH, Woodgate M, Thompson DA (2002) A comparison of Floy and soft VIalpha tags on hatchery Arctic charr, with emphasis on tag retention, growth and survival. Environmental Biology of Fishes 64, 269‐273. Schill DJ, Beland KF (1995) Electrofishing injury studies ‐ a call for population perspective. Fisheries 20, 28‐29. Simpkins DG, Hubert WA, Wesche TA (2000) Effects of a Spring Flushing Flow on the Distribution of Radio‐Tagged Juvenile Rainbow Trout in a Wyoming Tailwater. North American Journal of Fisheries Management 20, 546–551. Thorn WC (2000) Management for large brown trout in southeast Minnesota streams. Minnesota Department of Natural Resources, Draft Staff Report No., Minnesota. Walsh M, Winkelman D (2004) Anchor and visible implant elastomer tag retention by hatchery rainbow trout stocked into an Ozark stream. North American Journal of Fisheries Management. 24, 1435‐1439.


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