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Population parameters, migration and exploitation of the cod (Gadus morhua L.) in Masfjorden, western Norway

1992; Elsevier BV; Volume: 15; Issue: 3 Linguagem: Inglês

10.1016/0165-7836(92)90019-p

1872-6763

Anne Gro Vea Salvanes, Øyvind Ulltang,

Marine Bivalve and Aquaculture Studies

This study is a component of a comprehensive ecosystem analysis in connection with a cod enhancement experiment in Masfjorden, western Norway. The main objective was to estimate stock size by age group, recruitment, growth, catches and mortalities both prior to and for the first year after the first large-scale release of juvenile cod, and to study migration. The quantitative estimates are essential for a future evaluation of whether the cod population in the fjord increases after large-scale releases of juveniles. A method to estimate population parameters based on a short time series of data has been developed. Age compositions for the years 1986–1989 were estimated using samples from experimental net groups. Recruitment was estimated from tagging experiments with small-small release of 0-group cod in the autumns of 1985 and 1986, and large-scale release in 1988. Combining absolute and relative abundance estimates for the 1985 and 1986 year-classes, the average catchability coefficient, q, for the net groups was found. Assuming constant q, the numbers of each age group of cod present in the fjord were estimated. The corresponding biomass estimates for each sampling year and age group were obtained by multiplying numbers present by average weight. Total commercial catches were estimated using log books containing daily catch statistics from the commercial fishery, including length measurements of individual cod. These length distributions were converted to age distributions using age-length keys from the experimental fishery, and catches by age groups were estimated. Total recreational catches were estimated using additional data from the tagging experiments. By pooling the catches from commercial, recreational and experimental fisheries, estimates of total catches by age group were found. The total instantaneous mortality, Z, was estimated by age group and year from the data on catch per unit of effort, assuming a constant catchability coefficient q. Subsequently, the corresponding instantaneous fishing mortalities were obtained from the catch equation utilizing the estimated Z values, obtained annual mean numbers and total catch by age group.