P u b l i s h i n g
Marine Freshwater Research Volume 52, 2001 © CSIRO 2001
A journal for the publication of original contributions in physical oceanography, marine chemistry, marine and estuarine biology and limnology
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Mar. Freshwater Res., 2001, 52, 1271–5
Relationship between settlement of southern rock lobster pueruli, Jasus edwardsii, and recruitment to the fishery in Tasmania, Australia C. Gardner, S. D. Frusher, R. B. KennedyA and A. Cawthorn Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, PO Box 252-49, Tas. 7001, Australia. email:
[email protected] A Present address: New Zealand Ministry of Fisheries, PO Box 1020, Wellington, New Zealand
Abstract. Puerulus catches on artificial collectors were measured monthly at four sites around Tasmania from 1991 to April 2000, with the aim of predicting future changes in recruitment to the fishery. Support for the potential of catch-rate prediction in Tasmania was provided at the two sites that have overlap of several years between indices of puerulus settlement and indices of the abundance of recruits to the fishery. At Bicheno, on the northeast coast, correlations between annual puerulus index and commercial catch rates were highly significant, with a lag of 5 years (P < 0.01). Similar interannual trends in puerulus index and estimates from a stock-assessment model of the biomass of recruits to the fishery provided additional support for a link with puerulus index. A 5-fold interannual variation in puerulus index detected at Bicheno, with a peak in 1995, was preceded by 3 years of relatively low puerulus catch. The peak in puerulus index appears to lead to an increase in the abundance of sublegal males in research sampling 3 years later. Correlation between annual measures of puerulus index and catch rate also appeared significant at King Island (P = 0.06) although data at this site had less contrast.
Introduction Prediction of the magnitude of future recruitment to the fishery is a desirable tool for management and business alike, as it reduces risk in decision making. Current management of the Tasmanian rock-lobster resource is heavily influenced by outputs of the stock assessment model described by Punt and Kennedy (1997) with minor modifications (Frusher and Gardner 1999). This model produces estimates of historical recruitments, driven largely by commercial catch and effort data. Estimates of recruitment are selected randomly for simulation of probable future patterns in the fishery under various management scenarios. The broad range of recruitment strengths observed historically results in a spread of simulation results under any future management scenario. These projections provide a probability of whether future targets will be reached, but they cannot provide a true prediction, so considerable uncertainty remains in relation to the outcome of each fishing year. The ability to predict future recruitment to the fishery would overcome this problem and provide improved accuracy in model projections. Research on the development of predictive capacity through monitoring of puerulus catch has been underway in Tasmania since 1989 although the original, more conservative, aim was to develop the ability to detect periods of prolonged low or declining puerulus settlement © CSIRO 2001
(Kennedy et al. 1991, 1994). In addition to this aim of predicting trends in future harvests, the data collected through puerulus monitoring were expected to provide insights into the biology of early settlement stages. Impetus for the research came from success in similar work with Panulirus cygnus in Western Australia (Phillips 1986) and progress toward the prediction of recruitment of Jasus edwardsii in New Zealand (Booth and Bowring 1988; Breen and Booth 1989). Although indices of puerulus catch derived from collectors have demonstrated value in some fisheries, a range of factors can decouple trends in the catch of puerulus on collectors from the magnitude of future recruitment to the fishery. These factors include high variability in growth between individuals, which is compounded when growth is slower, the absence of a relationship between catches on collectors and settlement on adjacent natural habitat, and localised variation in settlement that conceals regional trends in settlement. For example, in Florida, the magnitude of the catch of P. argus pueruli on collectors is not necessarily correlated with abundance of puerulus settling into neighbouring habitat (Butler and Herrnkind 1992). Likewise, puerulus catch indices on collectors did not predict juvenile abundance, a pattern interpreted as evidence that density-dependent mortality mechanisms were important, notably habitat availability (Herrnkind and Butler 1994; Butler et al. 1995). 10.1071/MF01032
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The importance of density-dependent mortality of early benthic stages in Tasmania was unknown at the start of this study, although evidence from most other research was that the level of puerulus settlement was the principal determinant of recruitment (Phillips et al. 1994). Support for the monitoring project also came from early work by (Lewis 1977), which had indicated that catches of J. edwardsii pueruli on collectors represented patterns on other substrata. We present results here from Tasmania, which indicate that commercial catch rates of J. edwardsii can be predicted from puerulus indices. Materials and methods Indices of puerulus settlement Detail on the design and layout of Tasmanian puerulus monitoring sites is given elsewhere (Kennedy et al. 1994; Gardner et al. 1998). Briefly, puerulus collectors were similar in design to the crevice collectors described and illustrated by Booth and Tarring (1986). These collectors are deployed on sand at 3 to 9 m depth in sheltered locations adjacent to reef at five sites in Tasmania (Fig. 1). Each of these sites contains several individual collectors ranging from N = 8 at King Island and Flinders Island to N = 24 at Bicheno. The number of collectors deployed at each site influences the power to detect interannual change in the magnitude of catches of pueruli. This change was assessed most recently by Gardner et al. (1998) with the aim of providing an 80% chance of detecting a significant difference (P < 0.05) when interannual change in the magnitude of puerulus catch is at least 30%.
Fig. 1. Location of Jasus edwardsii puerulus settlement monitoring sites (), and regions for which commercial catch and effort data were drawn to compare against puerulus settlement data from King Island and Bicheno (shaded). Size-structure data were obtained by research fishing in the shaded region along eastern Flinders Island.
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Data presented here focus on results from Bicheno and King Island, as these are the longest-running sites and are also in regions of higher growth rates. This faster growth results in higher number of years of comparison between puerulus indices and measures of recruitment to legal size. Collectors have been checked monthly since late 1990 at Bicheno and King Island, although several months have been skipped at King Island. Low-catch-rate months (March–June) have not been monitored at Bicheno since 1998, although this change is too recent to affect data compared against recruitment, which is for all months. Data presented here are from the commencement of monitoring up to December 1999 and are grouped by calendar year. Monthly services involve placing mesh bags around the collectors before they are hauled to the surface for cleaning and collection of pueruli (Booth and Tarring 1986). Collectors occasionally became damaged during the course of this project and were replaced. Preliminary work by Kennedy et al. (1994) demonstrated that conditioning of collectors affected catch rate, although no effect was detectable after 120 days. This observation was similar to those by Booth (1979). Because conditioning appears to affect catch rates, damaged collectors were replaced with collectors from conditioning sites in the same area. Although collectors are set on sand, juveniles were occasionally encountered, and these appear to have migrated from neighbouring reef. These animals were excluded from analyses presented here. As noted by Booth and Tarring (1986), evidence of immigration implies that undetected emigration may also occur. We are unable to quantify it and have proceeded on the assumption that the net effect is constant from year to year. Indices of recruitment to the fishery Information on recruitment to the fishery was obtained from three sources. The first is simply the interannual change in catch rate reported by commercial fishery participants through compulsory logbooks. Fishers report catch as numbers and weight and report effort as the number of traps or pots hauled, on the spatial scale of quarterdegree blocks (30 × 30 minute; Fig. 1). In order to compare puerulus indices with commercial catch rate data on a broader regional scale, we grouped quarter degree blocks. This grouping was intended to combine data from blocks with similar interannual patterns in catch rates, as these patterns indicate similar processes leading to recruitment to the fishery (such as similar growth rates, settlement patterns, and postsettlement processes). The grouping of blocks was by correlation between the original block that contained the collector site and neighbouring blocks along the coast. These correlations used annual catch-rate data from 1990–1999, and blocks were grouped when Pearson product-moment correlations ( r -values) were above an arbitrary value of r > 0.80 (Fig. 1). Although the commercial season runs from November through August, catch rate data were restricted to the months of December through April in an attempt to minimise the influence of factors other than recruitment on interannual trends in catch rates. Male rock lobsters moult in spring, and the proportion moulting in November tends to vary from year to year (Gardner 1999). Catchability of recently moulted males is exceptionally high, so the proportion of the male population moulting in November can influence the catch rates in this month. Catch data for November were therefore excluded. Data for the fishing season from May through August were also excluded, as catch rates typically drop during this period and become more volatile because of smaller sample sizes. Catch rate measured as kilograms per pot lift is especially responsive to changes in recruitment to the fishery where the residual recruited biomass is low and the fishery is based mainly on recruits. This appears to be the case for most areas around Tasmania, including coastal areas near both the King Island and Bicheno puerulus monitoring sites (Gardner 1999).
Relationship between settlement and recruitment, J. edwardsii
The second source of data on recruitment to the fishery is change in catch rates of male rock lobsters in legal sizes in research sampling conducted at Eastern Flinders Island, ~200 km north of the puerulus monitoring site at Bicheno. Data on change in size structure at Flinders Island in relation to puerulus index at Bicheno are presented here, despite the spatial separation, as puerulus settlement patterns are influenced by large-scale oceanic patterns (Booth and Stewart 1992). Consequently, settlement trends detected at Bicheno may also have occurred at Flinders Island. Although a puerulus monitoring site has been established at Flinders Island, it has been running for insufficient duration to permit comparison between puerulus index and size structure of rock lobsters in research fishing. On the basis of comparison of growth rates of legal-sized animals (105 mm and 110 mm carapace length for females and males respectively), we would expect pueruli to recruit to legal size at Flinders Island after fewer years than at Bicheno (Punt et al. 1997). Size-structure data presented here are for males only, as catch rates of females in this region are erratic because of reproductive behaviour interactions with catch rate (Frusher, unpublished). All research sampling was conducted in November and December with cane pots. In total, 532 pot lifts were undertaken in 1995, 390 in 1996, and 564 in 1998. The third source of recruitment data to the fishery is estimates derived from the Tasmanian rock lobster stock-assessment model (Punt and Kennedy 1997; Punt et al. 1997). This model incorporates both commercial catch and effort data and size-structure data from research sampling, so it is not completely independent from the previous two measures of recruitment. Nonetheless, it provides the ability to estimate the relative magnitude of legal-sized biomass that recruited in the current year, as opposed to total legal-sized biomass, which is measured indirectly by commercial catch and effort data. Outputs from this model are divided over 8 regions around the Tasmanian coast. Data used here are for the two sexes combined and are drawn from a single area off the East Coast around the Bicheno region (between latitudes 41°S and 42°S, Fig. 1; ‘Area 3’ in Punt and Kennedy 1997). To examine relationships between puerulus indices and abundance indices, we tested correlations using Pearson product-moment r-values.
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Fig. 2. Correlation between interannual change in commercial catch rates of Jasus edwardsii for eastern Tasmania and puerulus index from Bicheno. Puerulus index has been shifted forward 5 years to simulate growth from settlement to legal size. Arrows indicate the extent of the current overlap, which is 5 years (r = 0.974; P < 0.01).
Results Puerulus index at Bicheno showed a fourfold range in magnitude from 1991 to 1999 with a large degree of interannual variation that provided a clear signal for comparison with trends in indices of recruitment to the fishery. Transposition of puerulus index data on commercial catch rate data resulted in nonsignificant correlation (P > 0.3) for all lags except 5 years, which was highly significant (r = 0.97, P < 0.01; Fig. 2). Only 4 years of overlapping data were available for comparison between puerulus index and model estimated recruitment with a 5-year lag, which is one less than for catch-rate data, because of the more extensive data requirements for producing a hindcast model fit. Interannual trends in puerulus index and model estimated recruitment were similar with a 5-year lag although the correlation was not significant (r = 0.90, P = 0.097; Fig. 3). We note that correlation analysis with only four points is unlikely to be significant except where trends are extremely closely matched; nevertheless, the similar though nonsignificant trends shown here support the significant relation shown between puerulus index and catch rate.
Fig. 3. Correlation between interannual change in estimated recruitment of Jasus edwardsii to eastern Tasmania (between 41°S and 42°S) and puerulus index from Bicheno. Puerulus index has been shifted forward 5 years to simulate growth from settlement to legal size. Arrows indicate the extent of the current overlap, which is 4 years (r = 0.904; P = 0.097).
A prominent feature of the puerulus-index data from Bicheno is the peak catch in 1995, where mean annual puerulus catch per collector reached 127. This peak was preceded by 3 years of relatively low settlement, so we hypothesised that, if puerulus index indicated future recruitment to the fishery, this cohort should be detected in length- tructure of rock lobsters collected in research fishing surveys (Fig. 4). The abundance of male rock lobsters of ~105 mm carapace length increased markedly between samples taken in 1996 and 1998, but as illustrated in Fig. 1, the location of this research sampling was about 200 km north of the Bicheno puerulus-monitoring site. Consequently, viewed in isolation, this apparent change in
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overlap than at Bicheno. Note that the relationship between puerulus index and commercial catch rates at King Island is also consistent with a 5-year lag, as at Bicheno, and this option cannot be discounted. A spike in puerulus index was recorded at King Island in 1999, although insufficient time has elapsed to compare this peak against any index of recruitment. This spike may provide future opportunity for assessing the appropriate lag at this site. Discussion Fig. 4. Interannual change in size-structure data of male Jasus edwardsii from eastern Flinders Island. Samples were collected by research fishing with escape gaps on the traps closed.
size structure in response to a peak in puerulus index 3 years earlier is not conclusive evidence of a link between puerulus index and later recruitment. Nonetheless, the striking change in abundance of undersized males appears to support the hypothesis that puerulus index can be used to predict catch rate along eastern Tasmania. A 5-year lag between puerulus settlement and recruitment to the fishery is consistent with the limited information available on growth of sublegal J. edwardsii from eastern Tasmania and elsewhere (Booth and Kittaka 1994). We used the apparently strong relationship between puerulus index at Bicheno and catch rate along eastern Tasmania with a 5-year lag as a benchmark for comparisons between puerulus index and catch rate from the King Island region. Growth rate of legal-sized lobsters is faster at King Island than around the Bicheno region (Punt et al. 1997), so we applied a shorter lag period of 4 years at King Island. Correlation between puerulus index and catch rate at King Island was significant ( r = 0.86, P = 0.06; Fig. 5), although less interannual variation was apparent in both sets of data over the period of
Fig. 5. Correlation between interannual change in commercial catch rates of rock lobsters Jasus edwardsii, from the King Island region and puerulus index from King Island. Puerulus index has been shifted forward 4 years to simulate growth from settlement to legal size (r = 0.862; P = 0.060).
The high correlation between puerulus catches at Bicheno and indices of the abundance of juveniles and recruits indicates that prediction of catch rate is viable in eastern Tasmania. Puerulus index at Bicheno is closely correlated with commercial catch rate, and further support is indicated by interannual changes in size structure and correlation between puerulus index and model predicted recruitment to the fishery. The relationship between puerulus index and catch rate at King Island indicates the potential of puerulus monitoring for prediction at this site (P = 0.06), although we remain cautious of results from this site, as there was little interannual signal and some sample periods were missed. Similar correlations between puerulus indices and the abundance of later stages have been reported elsewhere, so this link is not unique (Breen and Booth 1989; Phillips et al. 1994; Booth et al . 2000). Nonetheless, the results are informative, as it was unclear whether a relationship would be demonstrated in Tasmania, because of the potential of density-dependent growth, density-dependent survival, variation in predation, and variation in migration to mask any relationship. Gardner and Vanputten (1998) evaluated the cost effectiveness of puerulus monitoring in Tasmania and concluded that benefits would strongly outweigh costs provided a predictive ability was achieved within 20 years of establishment of the program; the development of this capacity now appears probable. Critical aspects of linking puerulus indices to future catch rate are the estimated interval between settlement and recruitment and the extent of any difference between sexes. Direct measures of juvenile growth have been used to establish appropriate lag intervals in New Zealand (Annala and Bycroft 1985; Breen and Booth 1989; Booth et al . 2000). In Western Australia, information on growth rate of juvenile rock lobsters is also available and provides a guide to the interval between settlement and recruitment (Phillips et al. 1992; Caputi et al. 1995). In Tasmania, information on growth of juveniles is limited, so the selection of lag time applied here was iterative. Our use of a 5-year lag between puerulus index and recruitment at Bicheno resulted in a correlation with catch rate (P < 0.01) that would be highly unlikely to result from random variation in either index. This 5-year lag is consistent with our limited understanding of growth of sublegal animals of both sexes from eastern Tasmania (Gardner and Vanputten 1998).
Relationship between settlement and recruitment, J. edwardsii
Management of the Tasmanian rock lobster fishery was changed from output controls to quota management in 1998 with the aim of rebuilding the legal-sized biomass. We expect this change to reduce volatility of annual catch rates, as recruits form the bulk of the harvest currently. Although the rebuilding of the legal-sized biomass may obscure the relationship between puerulus index and catch rate to some extent, the prediction of future recruitment events will be of value in the setting of annual catch quotas. Puerulus monitoring sites at King Island and Bicheno appear to be of value for predicting future catch rates for regions contributing over a quarter of the catch from the state. Expansion of this coverage may be possible in the future from the three other monitoring sites. Acknowledgments We thank Sam Ibbott, Jac Gibson, Craig MacKinnon, Brett Hislop, Rod Pearn, Dave Tarbath, John Mauric and the numerous other people who assisted in the collection of puerulus-index data. Size-structure data could not have been collected without the generous help of Michael White, Arnold White, Tony Harper, Chris Rhodes, and crews. This project was initiated with FRDC funding in 1989. References Annala, J. H., and Bycroft, B. L. (1985). Growth rate of juvenile rock lobsters (Jasus edwardsii ) at Stewart Island, New Zealand. New Zealand Journal of Marine and Freshwater Research 19, 445–55. Booth, J. D. (1979). Settlement of the rock lobster, Jasus edwardsii (Decapoda: Palinuridae), at Castlepoint, New Zealand. New Zealand Journal of Marine and Freshwater Research 13, 395–406. Booth, J. D., and Bowring, L. D. (1988). Decreased abundance of the puerulus stage of the rock lobster, Jasus edwardsii, at Kaikoura, New Zealand. New Zealand Journal of Marine and Freshwater Research 22, 613–16. Booth, J. D., and Kittaka, J. (1994). Growout of juvenile spiny lobster. In 'Spiny Lobster Management'. (Eds B. F. Phillips, J. S. Cobb and J. Kittaka.) pp. 424–45. (Blackwell Scientific Publications: London.) Booth, J. D., and Stewart, R. A. (1992). Distribution of phyllosoma larvae of the red rock lobster Jasus edwardsii off the east coast of New Zealand in relation to the oceanography. Bureau of Rural Resources Proceedings 15, 138–48. Booth, J. D., and Tarring, S. C. (1986). Settlement of the red rock lobster , Jasus edwardsii , near Gisborne, New Zealand. New Zealand Journal of Marine and Freshwater Research 20, 291–7. Booth, J. D., Bradford, E., and Renwick, J. (2000). Jasus edwardsii puerulus settlement levels examined in relation to the ocean environment and to subsequent juvenile and recruit abundance. New Zealand Fisheries Assessment Report. pp. 47. (National Institute of Water and Atmospheric Research.) Breen, P. A., and Booth, J. D. (1989). Puerulus and juvenile abundance in the rock lobster Jasus edwardsii at Stewart Island, New Zealand. New Zealand Journal of Marine and Freshwater Research 23, 519–23.
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Butler, M. J., and Herrnkind, W. F. (1992). Are artificial "Whitham" surface collectors adequate indicators of Caribbean spiny lobster, Panulirus argus , recruitment ? Proceedings of the Gulf and Caribbean Fisheries Institute 42, 135–6. Butler, M. J., Hunt, J. H., Herrnkind, W. F., Childress, M. J., Bertelsen, R., Sharp, W., Matthews, T., Field, J. M., and Marshall, H. G. (1995). Cascading disturbances in Florida Bay, USA: cyanobacteria blooms, sponge mortality, and implications for juvenile spiny lobsters Panulirus argus. Marine Ecology Progress Series 129, 119–25. Caputi, N., Brown, R. S., and Chubb, C. F. (1995). Regional prediction of the western rock lobster, Panulirus cygnus, commercial catch in Western Australia. Crustaceana 68, 245–56. Frusher, S. D., and Gardner, C. (1999). Tasmanian rock lobster fishery 1998. Tasmanian Aquaculture and Fisheries Institute Stock Assessment Report. 43 pp. Gardner, C. (1999). Tasmanian Rock Lobster Fishery 1998/99. Tasmanian Aquaculture and Fisheries Institute Stock Assessment Report. 58 pp. Gardner, C., and Vanputten, I. (1998). An evaluation of the costs and benefits of the puerulus monitoring program and proposed changes. Marine Resources Division, Tasmanian Department of Primary Industry and Fisheries Internal Report No. 48. 42 pp. Gardner, C., Cawthorn, A., Gibson, I., Frusher, S., Kennedy, R., and Pearn, R. (1998). Review of the southern rock lobster Jasus edwardsii puerulus monitoring program: 1991–1997. Marine Resources Division, Tasmanian Department of Primary Industry and Fisheries Technical Report No. 52. 51 pp. Herrnkind, W. F., and Butler, M. J. I. (1994). Settlement of spiny lobster, Panulirus argus (Latreille, 1804), in Florida: pattern without predictability? Crustaceana 67, 46–64. Kennedy, R. B., Wallner, B., and Phillips, B. F. (1991). Preliminary investigations of puerulus settlement of the rock lobster Jasus novaehollandiae in southern Australia. Revista Investigaciones Marinas 12, 76–82. Kennedy, R. B., Pearn, R. M., Tarbath, D. B., and Terry, P. (1994). Fishery status report: assessment of spatial and temporal variation in puerulus settlement of the southern rock lobster Jasus edwardsii. Marine Resources Division, Tasmanian Department of Primary Industry and Fisheries Internal Report No. 5. 65 pp. Lewis, R. K. (1977). Rock lobster puerulus settlement in the south east. South Australian Fishing Industry Council Magazine 13, 9–11. Phillips, B. F. (1986). Prediction of commercial catches of the western rock lobster Panulirus cygnus. Canadian Journal of Fisheries and Aquatic Sciences 43, 2126–30. Phillips, B. F., Palmer, M. J., Cruz, R., and Trendall, J. T. (1992). Estimating growth of the spiny lobsters Panulirus cygnus, P. argus and P. ornatus. Australian Journal of Marine and Freshwater Research 43, 1177–88. Phillips, B. F., Cruz, R., Brown, R. S., and Caputi, N. (1994). Predicting the catch of spiny lobster fisheries. In 'Spiny Lobster Management'. (Eds B. F. Phillips, J. S. Cobb and J. Kittaka.) pp. 285–301. (Blackwell Scientific Publications: London.) Punt, A. E., and Kennedy, R. B. (1997). Population modelling of Tasmanian rock lobster, Jasus edwardsii, resources. Marine and Freshwater Research 48, 967–80. Punt, A. E., Kennedy, R. B., and Frusher, S. D. (1997). Estimating the size–transition matrix for Tasmanian rock lobster, Jasus edwardsii. Marine and Freshwater Research 48, 981–92.
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