Body Size Patterns and Density-Dependent Habitat Selection in Age-0 Herring from Northern Puget Sound

Identifying key variables that underlie juvenile body size variation has important implications for individual growth trajectories, life history schedules, predation risk, and spawner size structure. For Pacific herring, debate remains over whether juvenile body size is governed by density-dependent or -independent processes and little work has evaluated whether the relative importance of either process shifts over the course of early ontogeny. We examined body length variation in young-of-the-year Pacific herring from Skagit Bay (northern Puget Sound, Washington) sampled over a nine year period (2001-2009) and tested for temperature and abundance relationships soon after an ontogenetic habitat shift in to open, pelagic waters (midsummer) and at the end of the growing season (fall). Midsummer body size was positively related to temperatures experienced during the egg / yolk sac and larval stages and unrelated to abundance. Fall body size, however, was negatively correlated with abundance and uncorrelated with both midsummer body size and temperature, indicating shifting control from density-independence to density-dependence over the course of the growing season. We went on to evaluate whether density-dependent growth may be associated with density-dependent shifts in habitat use, consistent with the “basin model” wherein spatial overlap between herring and their prey resources declines during high recruitment years. We show that herring distributions do expand as a function of abundance, and spatial overlap with prey-rich waters declines with increased recruitment. Evidence of density-dependent growth in marine fish population is often attributed to exploitative competition, but our results suggest that such patterns may partly be mediated by density-dependent distribution expansions in to prey-poor waters.