Bird Study Cormorant Phalacrocorax carbo ...

This article was downloaded by: [201.242.171.154] On: 22 March 2014, At: 07:40 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Bird Study Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tbis20

Cormorant Phalacrocorax carbo occupancy and impact at stillwater game fisheries in England and Wales D.A. Callaghan , J.S. Kirby , M.C. Bell & C.J. Spray Published online: 29 Mar 2010.

To cite this article: D.A. Callaghan , J.S. Kirby , M.C. Bell & C.J. Spray (1998) Cormorant Phalacrocorax carbo occupancy and impact at stillwater game fisheries in England and Wales, Bird Study, 45:1, 1-17, DOI: 10.1080/00063659809461074 To link to this article: http://dx.doi.org/10.1080/00063659809461074

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

Bird Study (1998) 45, 1–17

Cormorant Phalacrocorax carbo occupancy and impact at stillwater game fisheries in England and Wales

Downloaded by [201.242.171.154] at 07:40 22 March 2014

DES A. CALLAGHAN,1 JEFF S. KIRBY, *1 MIKE C. BELL1 and CHRIS J. SPRAY2 1The Wildfowl & Wetlands Trust, Slimbridge, Gloucestershire, GL2 7BT, UK and 2Association of Stillwater Game Fisheries Managers, c/o Northumbrian Water Ltd, Abbey Road, Pity Me, Durham DH1 5FJ, UK This study provides an assessment of Cormorant occupancy and impact at stillwater game fisheries in England and Wales, during 1988/89 to 1992/93. A total of 167 waterbodies operated as ‘put-and-take’ trout fisheries was included within a questionnaire survey, and this provided the bulk of the data used in the analyses. The results show that Cormorants are widespread at inland stillwater game fisheries in England and Wales throughout the year, with higher densities present during October–March. Peak counts by fishery managers at 45 of the fisheries increased by about 20% per year, regardless of season. Site use by Cormorants is investigated and the results imply that large, low altitude sites that are close to rivers in southern England are more likely to be used by these birds. The survey revealed that Cormorants are widely perceived by fishery managers to be responsible for significant economic losses through consumption and/or injury of stock fish. While this may be justified locally, we found no overall relationship between Cormorant density and anglers’ catches of Rainbow Trout, the principal stock fish. Cormorant status and distribution The bulk of the British Cormorant population is comprised of the North Atlantic subspecies Phalacrocorax carbo carbo. Counts on estuaries and standing waters suggest that, during summer, Britain supports about 16% of the estimated world breeding population of this subspecies,1 and about 15% during winter.2 Some 5–10% of the British wintering population may comprise immigrants of the Continental race P. c. sinensis2 and individuals of this subspecies have recently been observed breeding at three inland colonies in Britain: Abberton Reservoir (Essex), Besthorpe Gravel Pits (Nottinghamshire) and Deeping St James (Lincolnshire) (G.R. Ekins & R.M. Sellers, pers. comm. 1994). Cormorants breed around much of the coast of Britain and the size of this population was estimated at 6400 pairs in 1969–70 and 7200 pairs in 1985–87, indicating an increase of about *Correspondence author.

© 1998 British Trust for Ornithology

3% per year, which has subsequently continued in most areas.3–5 Additionally, since 1981, when nine pairs nested at Abberton Reservoir, at least six inland colonies of tree-nesting birds have become established in Britain, supporting a total of about 617 pairs in 1992,4 indicating an increase of about 47% per year at inland breeding sites. Soon after breeding, Cormorants disperse widely around the coasts of Britain, with some visiting continental Europe.6 Since about 1960, however, substantial numbers have increasingly used inland sites in Britain during the winter months, particularly in southeast England.2 The total British winter population was estimated to have reached 19 000 in 1990/91,2 increasing by about 6% per year during 1987/88 to 1991/92.7 Cormorants and fisheries There is growing concern amongst fishery managers that Cormorants are responsible for significant economic losses at fisheries in


2

D.A. Callaghan et al.

Britain and elsewhere in Europe, especially in the light of human population increases and greater usage of inland freshwaters.2 Complaints about Cormorants from fishery managers are now common in Britain, especially at stillwater and river game fisheries.30 Such complaints mainly concern damage to fish populations owing to Cormorant predation and/or injury to fish which have been grasped by the birds but not consumed. There is little doubt that Cormorants often feed on stocked trout,8 but such impact can only become a concern when it is economically damaging to a fishery. In most cases, complaints are based on subjective information and there is an urgent need for quantitative evidence to assess the true scale of Cormorant predation at inland fisheries. Study objectives The Association of Stillwater Game Fishery Managers (ASGFM) is a major fisheries body concerned with the management of stillwater game fisheries in England and Wales, and embraces representatives from both the private and commercial sectors, including all water companies. This paper presents the results of a questionnaire survey of the stillwater game fisheries included within the membership of this organisation, which was conducted in order to investigate: (i) occupancy of these sites by Cormorants; (ii) perceived Cormorant impact and preventative measures taken to reduce it; (iii) the characteristics of fisheries used by Cormorants; and (iv) any relationship between Cormorant presence/absence and angler catch-rates. Such information is useful in the attempt to assess the nature and geographic scale of any Cormorant damage and is fundamental to the development of management plans that may be designed to reduce any severe damage attributable to this species. METHODS Questionnaire survey In January 1994, detailed questionnaires were sent to the members of ASGFM who manage stillwater game fisheries in England and Wales, targeting a total of 167 waterbodies (Fig. 1)

© 1998 British Trust for Ornithology, Bird Study,

45, 1–17

amongst the 90 members. It was estimated that this represents about 25% of all stillwater trout lakes in England and Wales, as listed in ref. 9. The questionnaire requested details on: a Location, surface area and mean depth of the waterbodies. b Fish stocking regimes and catch rates (including the number, species and mean weight of fish stocked per three-month period, and the number of fish caught per year). c Peak numbers of Cormorants present at each waterbody per three-month period and their activities (loafing, roosting, unsuccessful breeding, successful breeding and feeding). d Perceived impact of Cormorants (whether the fishery manager believes the birds are consuming and/or injuring an economically significant amount of fish, with supporting data if available). e Control measures (what action has been taken, or is planned, to reduce any perceived impact). Figures were requested covering the five years 1988/89 to 1992/93 inclusive. Data quantity and quality Fully completed questionnaires were provided by 24 members (27%), while 33 members (37%) supplied partially completed questionnaires. The remainder (33 members) contributed only very basic information, either by letter or verbally, which included details on (i) the number and size of lakes managed as putand-take lakes within the fishery; (ii) whether Cormorants regularly visit any of these lakes; and (iii) whether the fishery manager believed that Cormorants were causing a significant amount of damage to game fish stocks at any of the lakes. Thus, all of the ASGFM members approached supplied at least these basic details. We considered that the comprehensive data were likely to have been supplied from waterbodies where Cormorants are present and perceived to be a problem. This is confirmed by the fact that Cormorants have occurred regularly at 94% of these sites in comparison with just 48% of all waterbodies included in the survey. Such bias in the data received should be borne in mind in any interpretation of the analyses presented. A comparison was made of the peak Cormorant counts provided on the question-


Cormorants and game fisheries

3

Figure 1. Location of the 167 stillwater game fisheries in England and Wales included in the questionnaire survey.

naires with those recorded for the Wetland Bird Survey (WeBS) at a subsample of sites. WeBS represents the UK’s long-term monitoring scheme for non-breeding waterbirds. Counts are undertaken by experienced volunteers, mostly on a predetermined date in each month and mainly between September and March.10,11 Counts of Cormorants from both WeBS and the survey questionnaire were available for 24 waterbodies (Fig. 2). Pearson correlation analysis indicated that there was a very high level of agreement between the counts obtained from the two sources (r = 0.920, P < 0.001), whilst major axis regression12 indicated the relationship: M = 11.71 + 1.11W, where M is the peak Cormorant counts from fishery managers

and W is the corresponding WeBS counts. This indicates that the fishery managers’ counts obtained from the questionnaires were, overall, about 11 ± 6% (95% CL) higher than the WeBS counts. Thus, it can be concluded that the questionnaire results provided a broadly accurate reflection of the peak seasonal Cormorant numbers at the included sites. Statistical analysis An analysis of covariance was used to investigate the relationships between Cormorant densities and sites, seasons and years, and any interactions between them, using the SAS GLM procedure.13 Comprehensive Cormorant counts


45, 1–17

4


1000

WeBS counts

100


10

1 1

10

100

1000

Fishery manager counts

Figure 2. Relationship between peak Cormorant counts obtained from fishery managers and participants in the Wetland Bird Survey (WeBS). Peak counts at 24 sites from October–December and January–March during 1988/89 to 1992/93 are represented. Note log scales.

by fisheries managers were available for 45 waterbodies, located throughout England (Fig. 3). These counts were converted into densities (birds/ha) owing to the considerable size variation amongst waterbodies (x-= 3.73 ha; range = 0.1–1200 ha). Cormorant density (log-transformed) was fitted as the dependent variable with site and season as main effects and year as the covariate. Regression analysis of Cormorant densities (logtransformed) on year was subsequently used in order to quantify the overall annual change in Cormorant densities at the 45 waterbodies over the 1988/89 to 1992/93 period. The characteristics of fisheries favoured by Cormorants were examined in relation to both presence/absence of Cormorants, and their numbers during autumn/winter (October– March) and spring/summer (April–September). This necessitated the quantification of nine site variables for the 167 waterbodies included in the questionnaire surveys. The variables selected were: water area, mean water depth,


45, 1–17

fishery age (i.e. the number of years since game fish stocking began), altitude, distance to the nearest coast, distance to the nearest river more than 10 m in width, distance to the nearest lake outside the fishery of more than 10 ha in size, and National Grid easting and northing. Some of these details had been collected in the questionnaires, whereas others were estimated from 1:50 000 Ordnance Survey maps. Prior to analysis, these site variables were transformed using the Box-Cox procedure12 which finds the optimum transformation to statistical normality. Investigation into the presence and absence of Cormorants at the 167 waterbodies proceeded with one-way analysis of variance to compare the characteristics of sites with and without Cormorants. Stepwise discriminant analysis, using the SAS STEPDISC procedure, was used to identify variables contributing to the best function distinguishing between sites with and without Cormorants. Two variables, fishery age and mean water depth, were



5

Figure 3. Location of 45 waterbodies for which comprehensive counts of Cormorants were provided via the questionnaire survey.

omitted owing to large numbers of missing values. Discriminant analysis, using the SAS DISCRIM procedure, was applied using the variables identified as important in the previous analysis, and these were standardized (i.e. the variables were transformed so that the mean equalled zero and the standard deviation equalled one) for easier interpretation of the discriminant function. Five-year mean peak counts (log-transformed) were used to represent Cormorant numbers for both autumn/winter (October–March) and spring/summer (April–September). Sufficient count data were available for 45 sites, and these figures were analysed in relation to the nine site variables described above, using multiple regression analysis (SAS REG procedure). Water depth, of which there was one missing value, was omitted after it was found not to contribute

significantly to any model in which it was included. There were four missing values for fishery age, but since this factor contributed significantly to models for both spring/ summer and autumn/winter numbers, it was retained in the analyses, thus reducing the number of sites included to 41. Mallow’s model selection criterion Cp was used to identify the optimum model among the set of all possible models nested within that containing all site variables.14 Comprehensive data concerning the stocking and catches of Rainbow Trout Oncorhynchus mykiss and the numbers of Cormorants were provided for 22 waterbodies where the birds were perceived by the fishery managers to be consuming significant quantities of this fish at their fisheries. These data comprised the number of angler visits and fish caught per


45, 1–17


6


year, and seasonal figures (i.e. January–March, April–June, July–September and October– December) detailing the number of fish stocked, their average stocking weights and the peak number of Cormorants present during 1988/89 to 1992/93. Unfortunately, the quantity of such data received for Brown Trout, the only other salmonid commonly stocked at the fisheries included in the present survey, was insufficient to allow analysis for this species. Nor was it provided in sufficient quantity for Rainbow Trout in waterbodies where Cormorants were absent. In order to identify any differences in the number of Rainbow Trout stocked and/or their stocking weight between seasons and/or years, these variables were analysed using two-way analysis of variance. One-way ANOVA was used to investigate any differences in the total number of angler visits and the number of Rainbow Trout caught during each year. The SAS GLM procedure was used to undertake these analyses. The data were then analysed to investigate any relationship between Cormorant numbers and Rainbow Trout catches. Each of the variables, except the mean weight of each fish stocked, was converted to densities owing to the large variation in the size of the sites. It was necessary to summarize data provided on a seasonal basis (i.e. data concerning the number and weight of Rainbow Trout stocked, and the peak number of Cormorants) to annual figures so that these variables could be directly related to two other variables (i.e. the number of angler visits and the number of fish caught per year). Annual figures for Cormorant densities were estimated by calculating a mean figure from the

peak seasonal densities during seasons when Rainbow Trout were stocked (e.g. if Rainbow Trout were stocked from April–September in a particular year, the mean of the peak Cormorant densities for April–June and July–September was used). Multiple regression analysis was used to examine the relationship between the number of Rainbow Trout caught per year at the 22 sites as a whole and the number of fish stocked, the number of angler visits, the average weight of fish stocked and the density of Cormorants. RESULTS Cormorant abundance, distribution and activities Counts of Cormorants for the period 1988/89 to 1992/93 were received for 45 waterbodies (located within 35 fisheries). Since these data were single peak counts for each three-month period, estimates of Cormorant abundance will be inflated. This ought to be borne in mind in subsequent analyses. Cormorants were recorded during October–December and January–March at all sites where count data were available (n = 45), at 30 (67%) sites during April–June and 34 (76%) during July–September. Thus, Cormorants were recorded at fewer sites during the periods April–June and July– September, than during October–December and January–March (χ23 = 31.828, P < 0.001). Examination of counts from these sites (Table 1) indicates that numbers are highest in October–December and January–March, and lowest in April–June. Numbers have increased

Table 1. Sums and trends of peak counts of Cormorants at 45 stillwater game fisheries in England and Wales during 1988/89 to 1992/93.

1988/89 1989/90 1990/91 1991/92 1992/93 Change (% per year, 95% CL) aIncludes

Oct–Dec

Jan–Mar

1 117 1 218 1 599 1 952 2 395

1 239 1 156 2 069 1 945 2 211

22 (20–24)

18 (10–41)

Apr–Jun

Jul–Sep

289 430 565a 541b 625c

634 1 001 959a 1 262b 1 663c

19 (14–25)

24 (19–29)

five breeding pairs at one site. bIncludes 36 breeding pairs at two sites. cIncludes 71 breeding pairs at

two sites.


45, 1–17

by about 20% per year overall (Table 1), with no significant difference between seasons (see confidence limits in Table 1). Successful breeding pairs (i.e. those producing fledged young) have been recorded at two sites, both of these colonies having become established since 1991. At a further three sites breeding has been attempted but was unsuccessful (i.e. nests have been built but no young have been reared to fledging). Mean density of Cormorants estimated for the 45 waterbodies during 1988/89 to 1992/93 also varied seasonally, with highest densities overall in October to March and lowest in April to September (Fig. 4). Analysis of covariance revealed that there was considerable variation in densities between sites (F44,804 = 1.92, P < 0.001), that densities have, overall, increased significantly over the years (F1,804 = 26.7, P < 0.001), and that there were significant differences in the rate of change in densities between sites (F44,804 = 1.91, P < 0.001) (see also Fig. 5). Regression analysis of Cormorant densities suggested the following relationship between years: ln Y = –1.5 + 0.104 ln C (R2 = 1.5%, P < 0.001), where Y is year and

7

C is change in density. This indicated that Cormorant densities were increasing overall at the 45 waterbodies by about 11% per year over the included years. This overall increase and the seasonal variation is apparent in Fig. 5. During 1988/89 to 1992/93, Cormorants were regularly present at 80 (48%) of the 167 waterbodies sampled during this survey. The geographical distribution of these sites, and those without Cormorants is shown in Fig. 6. From this it is apparent that fisheries where Cormorants are present are widespread throughout England and Wales. With reference to the comprehensive Cormorant counts received for 45 waterbodies, it is apparent that most peak Cormorant counts were fewer than 25 birds both during autumn/winter and spring/summer (Fig. 7). Classification of the activities of Cormorants at 65 waterbodies indicated that feeding and loafing were the primary activities, recorded at 98% and 85% of sites, respectively. Some of these waterbodies also supported overnight Cormorant roosts (31%), while unsuccessful breeding was recorded at three sites and successful breeding at two sites. Thus,

0.5 0.45

Cormorant density (birds/ha)

45

0.35

40

44

45

45 45

44

0.4 43

0.3

42 41

43

0.25 0.2 0.15

29

29 26

30

27

22

0.1

43

34

30

0.05 Jul–Sep

Apr–Jun

Jan–Mar

Jul–Sep

1993 Oct–Dec

Apr–Jun

Jan–Mar

1992 Oct–Dec

Jul–Sep

Apr–Jun

Jan–Mar

1991 Oct–Dec

Jul–Sep

Apr–Jun

Jan–Mar

Jul–Sep

1990 Oct–Dec

Apr–Jun

1989 Jan–Mar

Oct–Dec 88



Figure 4. Seasonal variation in mean peak Cormorant densities from 1988/89 to 1992/93. Sample sizes are noted above se bars.


45, 1–17

8

D.A. Callaghan et al. 20 18

Number of sites

16 14 12 10 8 6


4 2 0 –3.5 –3 –2.5 –2 –1.5 –1 –0.5 0 0.5

1 1.5 2

2.5 3

3.5

4

Change in density (Cormorants/ha) Figure 5. Change in Cormorant density between 1988/89 and 1992/93. Data from 45 sites are represented. ❐, Autumn/winter; ■, spring/summer.

Figure 6. Presence and absence of Cormorants during 1988/89 to 1992/93 at 167 stillwater game fisheries in England and Wales. ●, Present; ❍, absent.


45, 1–17


9

40 35

Frequency

30 25 20 15 10 5


0