An experiment was conducted to study trap selectivity for male snow crab (Chionoecetes opilio) in St. Mary's Bay, Newfoundland. The finishing effort of large-mesh commercial traps and small-mesh control traps was not equal because two commercial traps were fished for every control trap. We show how the SELECT (Share Each Length's Catch Total) method extends to this situation and, more generally, to selectivity trials in which catches are sampled in unequal proportion. Application to the crab data showed that a good fit was provided by the logistic selection curve under the assumption of unequal fishing efficiencies of commercial and control traps. The carapace widths at the retention probabilities of 0.25, 0.50, and 0.75 were estimated to be 86.4, 94.6, and 102.9 mm, respectively, and the retention probability at the minimum legal size of 95 mm was estimated to be 0.51. The commercial traps had almost twice the fishing efficiency of the control traps and consequently caught more legal-sized crabs per trap.
1993. Change-in-ratio and index-removal methods for population assessment and their application to snow crab (Cbaionoecetes opilio). Can. J. Fish. Aquat. Sci. 50: 1467-1496 Change-in-ratio and index-removal estimators are presented in a general form suitable for fisheries studies of closed populations. We also show how to combine the two approaches in a single estimator. In order to use these methods, it is necessary to sample the population before and after the fishery and to determine the total harvest and its composition. We used the methods to estimate the population of legal-size snow crab (Claionoecetes opiliu) in St. Mary's Bay, Newfoundland, before and after the fishery and to estimate the catchability coefficient and exploitation rate. It is also possible to estimate the abundance of prerecruits but this requires the assumption of equal catchability of ail anirnals, a condition that may not be met. These methods have been largely neglected by fishery scientists; however, they seem to be ideally suited for studies of many temperate popultttions of large sedentary crustaceans, particularly those subjected to fisheries of short duration.Des estimateurs des changements dans les proportions et le prdevernent sont present& sous une forme g6nerale qui se prGte aux etudes d'halieutique de populations fermees. Nous montrons aussi comment combiner les deux apprsches en un estimateur unique. Afin d'utiliser ces methodes, i 0 est necessaire d'echantillonner la population avant et apres la peche, et de determiner la recolte totale ainsi que sa composition. Nous avons appliqu6 ces rnethodes 2 I'estimation de la population d a m crabe des neiges de taille legale (Chionoecetes opilio) dans la baie St. Mary's, Terre-Neuve, avant et aprPs la g@che, et nous les avons applicguees 2 I'estimation d u coefficient du potentiel de capture ainsi que du taux d'exploitation. II est egalement possible d'estimer ['abondance des jeunes qui ne sont pas encore recrutes, mais pour cela, i B faut adopter I'hypothGse selon laquelle tous les sujets sont egasernent sujets A Gtre captures, ce qui peut se reveler inexact. Ces mkthodes ont kt6 largemen! negligees par les spt5cialistes des pGcl~es; toutefois, elles semblent convenir parfaitement aux 6tudes de bon nombre de populations en none ternperee de gros cru~taces sedentaires, particuli6rernent ceux qui font l'sbjet d'une p@che de courte durke.
Monthly length-frequency data of green tiger prawns from April 1985 to August 1990 were analyzed to estimate the von Bertalanffy growth curves and related parameters . The mean asymptotic carapace lengths (L ce ) and growth constants (K) over 5 seasons were 36 .6 mm and 1 .6 yr -1 for males and 51 .2 mm and 1 .7 yr-1 for females, respectively. Annual total mortality rates (Z) from 1985/86 to 1989/90 ranged from 2 .7 to 6 .5 yr -1 for males and from 2 .8 to 6 .8 yr1 for females . The mean exploitation rates calculated using the mean natural mortality rates (M) of 2 .5 yr1 for males and 2 .4 yr1 for females showed that the male green tiger prawns in Kuwait waters were under exploited, whereas the catch of females was very close to the sustainable yield . Chi-square tests indicated that the sex ratio was close to one for adults (CL > 26 mm), but significantly favoured males for recruits (CL < 26 mm) . A major recruitment was detected from June/July in all seasons and a minor recruitment in some seasons in August/September . The relative yield-per-recruit (Y' /R) reached the optimum at the length-at-first capture (L a ) of 25 mm CL for males, and increased as L, increased for females . The observed mean L, values were 23 .4 mm for both males and females. The Y'/R under the current exploitation level is close to the maximum sustainable yield for females, but lower for males . More fishing effort could be exerted to exploit males, but doing so may not be biologically or commercially possible with current trawling practices .
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