Trends in size-at-age, sex ratios, and age-at-return in hatchery and wild Chinook salmon across BC

Annual change in mean age, age composition, mean size, size-at-age and female composition were estimated within each Chonook salmon population using linear regressions and age-specific patterns across populations and geographical regions. The clearest trends observed were in mean size and size-at-age. Both showed declines across most populations with statistical significance. Declines varied in magnitude across populations and age classes. Mean age showed variable, yet statistically significant declines across most stocks, and appeared to be driven by reductions in the relative proportion of the oldest (ocean-4) and increases in the youngest age classes (ocean-2). Low-enhancement stocks (stocks with mean pHOS < 10%) showed mixed trends in mean age and age composition. The proportion of females showed variable declines across populations, of which only a small proportion were significant. There was no consistent pattern for the few wild and low-enhancement populations. Our study shows that the size and age of BC Chinook salmon have declined since the 1970s. Declines such as these may be caused by fishing, predation, competition and hatchery enhancement, however in this review we did not explore quantitative models that incorporate these factors. Simple analyses show some evidence for the effect of hatchery enhancement on declining mean size and size-at-age, with some populations showing increases in size for the youngest age class but declines for the oldest.