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Hart, A. M., Thomson, A. W., and Murphy, D. 2011. Environmental influences on stock abundance and fishing power in the silver-lipped pearl oyster fishery. – ICES Journal of Marine Science, 68: . Stock variability, fishing power, and the contributing environmental factors were examined for the Australian silver-lipped pearl oyster, Pinctada maxima. The approach was to compare the longer-term time-series of stock abundance derived from catch per unit effort against the shorter-term 0+ spat-settlement index to seek agreement on causal factors. Indices of stock abundance were established with generalized linear models that simultaneously examined the influence of technological and environmental factors, resulting in a predictive model with a 3-year forecast. The advent of global positioning systems caused a 30% increase in fishing power. A negative relationship between abundance and rainfall, and a positive relationship between abundance and temperature, was detected for both spat settlement and fishery abundance. Northerly winds (negative northings) from December to February significantly enhanced settlement, but easterly winds (negative eastings) in the main fishing month of May influenced fishing power positively. After standardizing for the effects of fishing power, a 150% increase in stock abundance of the exploited component of P. maxima stocks was detected between 2004 and 2009. A major contributor to this increase was an exceptionally high settlement of spat in 2005, associated with a rare combination of environmental conditions. Once this year class grows beyond the target size classes, abundance is predicted to decrease to the normal levels.

Blue swimmer crab (Portunus pelagicus) fisheries in Western Australia have generally been considered robust to recruitment overfishing, as the minimum legal size for retention of these crabs in both the commercial and recreational crab fisheries are set well above the size at sexual maturity allowing crabs to spawn at least once before entering the fishery. However, the Cockburn Sound crab stock suffered a recruitment collapse, with three key factors: (a) the fishery is near the edge of this species distribution and hence vulnerable to environmental fluctuations; (b) a number of consecutive years of poor environmental conditions resulted in poor recruitments; and (c) high fishing pressure continued on these low recruitments. This study indicates that water temperatures at the start of the spawning season positively influence the strong stock-recruitment relationship for P. pelagicus in Cockburn Sound. Apparently, warm water temperatures at the onset of spawning result in the larger females producing additional broods of eggs, and therefore a far greater number of larvae over the short spawning season. This relationship produces catch predictions for this fishery a year ahead and provides information for the development of biological reference points for management.

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Size at maturity in the western rock lobster (Panulirus cygnus) fishery in Australia increases from north to south and female lobsters carry either one or two batches of eggs per season depending on their carapace length. There has been a steady decrease in the size at first maturity of western rock lobster over the last 30 years, and there are now significantly more small females, many below the legal minimum size, contributing to the brood stock than any time since the fishery has been researched. Biological data were used to show the current contribution to egg production in different management regions of the fishery and historical length-frequency data were used to show how egg production has changed in those regions overtime. Reasons for the change in size at maturity in this fishery are unclear, but the outcome has been that egg production is now more evenly distributed across management zones. Egg production was high at Abrolhos Islands in the 1990s owing to strong year classes of breeding animals entering the fishery over those years. In the period since 2000, egg production has fallen sharply at the islands, but has increased in the coastal population compared with the 1980s and 1990s.

Law courts occasionally have had to deal with evidence on the measurement of rock lobsters that have been cooked, or sometimes both cooked and frozen. A point of contention in legal argument involving breaches of legal minimum size regulation, has been whether processing the lobsters could have affected measurement of the original carapace length. We show that Panulirus cygnus shrink slightly with cooking (p < 0:01) and with freezing (p < 0:01), but that this shrinkage is minimal. The overall shrinkage in carapace length from the combination of cooking and freezing is 0.11 mm (s.e. D 0.0076). These results are compared with a similar study on the effects of cooking on the carapace length of Jasus edwardsii. In light of results from this investigation, a tolerance of 0.3 mm is considered appropriate before prosecuting shers with cooked, or cooked and frozen sub-legal sized western rock lobsters. RÉSUMÉLes cours de tribunaux ont eu de temps en temps à traiter des mesures des langoustes ayant été cuits ou quelquefois à la fois cuits et congelés. Un point de litige parmi les arguments juridiques concernant les infractions au règlement sur la taille légale minimale a été que les procédés de préparation des langoustes pourraient affecter la longueur originale de la carapace. Nous avons montré que Panulirus cygnus rétrécit légèrement à la cuisson (p < 0,01) et à la congélation (p < 0,01) mais que ce rétrécissement est minime. Le rétrécissement de longueur de la carapace par combinaison de la cuisson et de la congélation est de 0,11 mm (s.e. 0,0076). Ces résultats ont été comparés à ceux d'une étude comparable sur les effets de la cuisson sur la longueur de la carapace de Jasus edwardsii. A la lumière des résultats de cette investigation, une tolérance de 0,3 mm doit être considérée comme appropriée avant de poursuivre les pêcheurs qui ont des langoustes cuits, ou cuits et congelés de taille minimale.Western rock lobsters, Panulirus cygnus George, 1962 are subject to a number of management regulations, one of which is a legal minimum size of 77 mm carapace length (CL) between 15 November and 31 January during the migratory phase of the shery and a 76 mm CL between 1 February and 30 June during the nonmigratory phase. Transgressors of this regulation are generally charged in court or given warnings or infringement notices.Most cases resulting from breaches of the legal minimum size regulation involve the seizure of whole uncooked rock lobsters. Occasionally, however, law courts have had to deal with seized sub-legal sized rock lobsters that have been frozen, boiled, or boiled and frozen. A point of contention in legal argument has been whether freezing or cooking could have distorted the carapace in such a way as to have led to legal-sized animals becoming undersize as a result of these processes.Ibbott et al. (2001) have recently shown cooking to have no statistically detectable effect on carapace length of southern rock lobster, Jasus edwardsii (Hutton, 1875). This note covers a similar investigation into the effects of...

Mail surveys have been used over the last 17 years to estimate the recreational catch of western rock lobster Panulirus cygnus in an Australian fishery. Because nonresponse bias has been recognized as having a likely effect on these survey results, different inducements have been used over the years to increase returns. If these inducements have succeeded in reducing the effects of nonresponse bias, a catch model built from the resulting catch estimates would need to include inducement as a factor. At the end of the 2002-2003 season, 5,000 recreational license holders were sent identical surveys, along with one of the six different inducements that had been tried in previous seasons. The results showed that none of the inducements was likely to have affected total catch estimates in previous seasons. The age of the licensee and the license type held were identified as possible strata for future mail surveys to help reduce nonresponse bias.