Genetic variation and local differences in Pacific cod Gadus macrocephalus around Japan

The population structure of the Pacific cod Gadus macrocephalus was examined using 15 microsatellite loci and mitochondrial DNA (ND2 region). In total, 274 individuals were sampled from 16 locations around Japan to estimate the level of genetic differentiation and effective population size (Ne). Pairwise FST, analysis of molecular variance and Bayesian clustering analysis suggested the presence of two genetically distinct groups in waters around Japan, with a higher Ne value in the eastern group than in the western group. A possible factor that restricts gene flow between groups may be related to the water temperature differences in the south‐western part of the Sea of Japan, where the Tsushima Warm Current flows around the area inhabited by the western group, which may limit migration between the west and east

Annual Fecundity of Tautog in Long Island Sound: Size Effects and Long-Term Changes in a Harvested Population

Abstract.—Few studies estimate the impact of individual size on annual reproductive output, which is an important consideration where size-selective harvest may truncate size distributions and sharply reduce populationwide reproductive potential. We conducted a 2-year study of reproduction in field-collected and captive tautog Tautoga onitis from Long Island Sound to investigate the influence of individual size on components that constitute annual fecundity: batch fecundity, spawning frequency, and season duration. Estimates of spawning frequency in field-caught females relied on time-varying features of postovulatory follicles that we validated in experiments conducted on captive spawners.Mature females collected in the wild demonstrated midseason peaks in spawning frequency and batch size. Both spawning frequency and batch fecundity increased significantly with size. As a result, annual fecundity increased sharply with size: large (500-mm) females produced 24–86 times as many eggs as did small (250-mm) females. Average (400-mm) females spawned 10–16 million eggs over a season, or about 10,000 eggs/g of whole body mass. We estimated temporal changes in populationwide egg production with data from a 22-year trawl survey in Long Island Sound. Over this period, an index of abundance declined by a factor of six and size distributions shifted to smaller fish. Despite the shift in size distribution, estimated annual egg production declined no more than the index of abundance because the sex ratio of the population has become female biased. Estimates of tautog annual fecundity were higher than those reported previously in the southern portion of the species’ range, reflecting genetic differentiation or phenotypic responses to environmental effects. Given the relatively large reproductive output of large females, their abundance is likely to influence the rate of population recovery in Long Island Sound