CCAMLR

If we could determine the relationship between catch rates in a trap survey and absolute population size, then we could estimate the standing stock of snow crabs before the fishing season from a survey conducted with traps. One way to calibrate the catch rates is to examine available historical Leslie analyses of commercial catch data obtained over several years and quantify the relationship between catch rate at the beginning of the season and the corresponding population estimate obtained by the Leslie analysis. Data from several different regions can be made comparable by expressing catches and efforts on an areal basis. Results obtained from data from Newfoundland and Nova Scotia suggest that this approach may be feasible.

Over 6000 male snow crabs were tagged during a 6-year period in Conception Bay, Newfoundland, in order to estimate the increase in size at the time of molting. Ninety-two animals were recaptured which had usable information on growth increments. Based on the amount of growth, we hypothesized that 20 of these had molted once while the remainder molted twice. Two lines of evidence support this interpretation. First, animals in the group presumed to have molted twice were at liberty on average twice as long as those presumed to have molted once. Second, a regression line fitted to data on single-molters predicted the size after two molts in close agreement with a regression line fitted to data on double-molters. A nonlinear regression model was developed to estimate the parameters of the relationship between post- and pre-molt sizes using the combined data set for single and double molters. The method was also generalized to account for a quadratic relationship between post- and pre-molt size. For crabs in the size range 80-110 mm carapace width, the predicted size after molting in mm is equal to 7.398 + 1.038 x pre-molt size. A similar study conducted in Bonavista Bay, Newfoundland, yielded growth information for 18 animals. The molt increments appear similar to those observed from Conception Bay.

Catch rate and catch composition of male snow crabs were compared for four sampling methods. These methods were: (1) large-meshed commercial traps, (2) small-meshed traps, (3) bottom trawl deployed during the day. and (4) bottom trawl deployed at night. Catches were characterized in terms of crab body sizes, shell conditions, and claw allometry. We concluded that: 1) mean and modal size of crabs captured in large-meshed traps was larger than those captured in small-meshed traps which, in turn, were larger than those caught in the trawl; the size of crabs caught in the trawl at night was larger than those caught during the day; 2) large-clawed crabs predominated in the catches from traps whereas small-clawed animals predominated in the trawl catches; 3) soft-shell crabs were more common in trawl than in trap catches whereas old-shell crabs were more common in trap than in trawl catches; 4) mean size of the crabs caught increased with depth for all sampling methods, but especially so for traps; and 5) catch per unit effort for both large- and small-clawed crabs increased with depth for all sampling methods.

Change-in-ratio and index-removal estimators are presented in a general form suitable for fisheries studies. We also show how to combine the two approaches in a single estimator. It is necessary to sample the population before and after the fishery, and to determine the total harvest and its composition, in order to use these methods. We use the methods to estimate the population of legal-size snow crabs (Chionoecetes opillio) 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 pre-recruits but this requires the assumption of equal catch ability of all animals, 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 populations of large crustaceans.

The reproductive performance of commercially important crustaceans may be considerably affected by symbiotic nemertean egg predators and parasitic castrators. Because these parasites commonly affect only females or feminize males, they may be protected by management practices that protect females. To manage a parasitized stock, we suggest that strategies should vary depending on the recruitment dynamics of both host and parasite. For a certain spatial scale, recruitment to a population may be "open" or "closed" depending on the behavior of planktonic larvae, the duration of these planktonic stages, and the flush rate of the environment of the adult host. Both hosts and parasites may have open or closed larval recruitment. We developed mathematical models to investigate the impact of protection of females on a hypothetical fishery for different combinations of host and parasite recruitment dynamics. The models suggest that the common practice of releasing females is not advantageous when a fishery is affected by a parasite. Retaining females in the catch is preferable in most cases. Treating or culling infected females may be advisable when host recruitment is closed.

A method is developed for calculating asymptotic confidence intervals for trawl surveys using the swept area method, using likelihood ratios from Aitchison's delta distribution. Simulation tests of the method show that unbiassed estimates of the density and biomass can be obtained and that the estimated confidence intervals have close to the nominal coverage probability. Performance deteriorates in cases where few of the hauls contain fish, and the coefficient of variation is high. The lower confidence bound appears to be more reliable than the upper.