Indices of larval king mackerel abundance in the Gulf of Mexico for use in population assessments

The stock status of the Gulf of Mexico migratory group of king mackerel (Scomberomorus cavalla) is currently evaluated by using age-based sequential virtual population analysis (VPA). We examined king mackerel larval occurrence and abundance indices from annual ichthyoplankton surveys, developed an age-adjusted abundance index, and questioned whether larval indices of abundance are useful for calibrating the king mackerel VPA. Gulf of Mexico king mackerel larval occurrence and abundance increased over a fourteen-year time series from 1982 to 1995 and were highly correlated with spawning stock size. Correlations between stock size and larval occurrence, and between stock size and larval abundance, were 0.82 and 0.84, respectively. The correlation between larval occurrence and stock size for the years 1986-95 increased to 0.91, owing to the addition in 1986 of a fall survey with added coverage during peak spawning. Daily instantaneous mortality rate (Z) was estimated by regression of larval catch curves. Although a large amount of interannual variability in mortality rates was noted, no statistical differences were detected among years. The instantaneous daily mortality rate estimated by pooling all years, Z = 0.53, was used to develop an age-adjusted index for king mackerel in order to eliminate the influence of variable larval age composition among years. This adjusted index did not improve correlations between stock size and larval abundance (r=0.78). For now, indices of larval occurrence and unadjusted larval abundance from ichthyoplankton collections reflect trends in spawning stock size and provide useful variables for calibrating the king mackerel VPA.