The effect of fishing on size composition and sex ratio of offshore lobster stocks
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The lobster fishery in the offshore waters of southern New England is in the early stages of development. The assumption that canyon fishing grounds closest to shore are the most heavily fished was supported by a lower catch per day (CPE), a smaller mean size, and a higher total mortality than exists in more distant canyons. At Hudson and Veatch Canyons, less than 250 km offslzore, the average CPE was 552 kg; Oceanographer and Lydonia Canyons extend 300 km offshore and had a CPE of 670 kg; and at Corsair Canyon, which is 400 km from shore, CPE was 741 kg. Lobsters were smallest at the canyons wllicll were closest to shore and had the lowest CPE. The size frequency was higher at the canyons of intermediate distance and the CPE was higher. The largest lobsters were from Corsair Canyon, which is the furthest from shore, and had the highest CPE. The modal carapace length of lobsters at Veatch Canyon was over 12 em in 1956 and is now less than 9 cm. In three successive years of Data provided by JAMES THOMAMSa,i ne Department of Sea and Shore Fisheries. sampling (1965-67), the contribution of small lobsters has increased and in 1967 more than 90 percent of the legal-sized lobsters were from the 8 to 10 cm length class. The progressive change at Veatch Canyon and magnitude of size differences among canyons are greater than expected from the influence of temperature or depth of fishing. Observed changes in sex ratio support the basic assumptions made in THOMA(S19' 55) model and follow the expected changes at different levels of mortality. The proportion of females to males increases gradually from 8 cm to 13 cm and then declines. Because females molt less frequently than males, their numbers at a given size decline more rapidly. The model assumes that females molt only every two years; whereas, males molt annually and have a lower intermolt mortality. The change in sex ratio was used to estimate mortality in the offshore fishing grounds. The more distant canyons had a lower mortality, higher catch per unit of effort, and larger size composition. The size composition and sex ratio of coastal lobster fisheries indicated that exploitation was more intense than in offshore fisheries. The increased proportion of young lobsters in the offshore catches followed expected changes from exploitation, but also suggested an increase in the population birth rate. This possibility was supported by the observation that in ten years the percentage of berried females at smaller sizes of maturity had increased substantially. However, additional data are needed to determine whether the improved recruitment has resulted from other factors. Additional data and analyses are needed to test the hypothesis that fishing has caused the changes in size composition, sex ratio, and catch rates. If the assumptions hold and the level of fishing continues or increases, the more distant canyons are expected to experience changes in lobster size, sex ratio, and catch rates-comparable to those at the canyons closest to shore. The size at maturity, frequency of molting, and rate of growth used in the model are not entirely consistent with observations in the offshore fishery. However, the basic concepts seem applicable and the necessary adjustments and corrections can be incorporated into the model when sufficient data are available.
SeriesFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 15 no 3