Prey tell: what quillback rockfish early life history traits reveal about their survival in encounters with juvenile coho salmon

Predation is a major source of mortality in the early life stages of fishes and a driving force in shaping fish populations. Theoretical, modeling, and laboratory studies have generated hypotheses that larval fish size, age, growth rate, and development rate affect their susceptibility to predation. Empirical data on predator selection in the wild are challenging to obtain, and most selective mortality studies must repeatedly sample populations of survivors to indirectly examine survivorship. While valuable on a population scale, these approaches can obscure selection by particular predators. In May 2018, along the coast of Washington, USA, we simultaneously collected juvenile quillback rockfish Sebastes maliger from both the environment and the stomachs of juvenile coho salmon Oncorhynchus kisutch. We used otolith microstructure analysis to examine whether juvenile coho salmon were age-, size-, and/or growth-selective predators of juvenile quillback rockfish. Our results indicate that juvenile rockfish consumed by salmon were significantly smaller, slower growing at capture, and younger than surviving (unconsumed) juvenile rockfish, providing direct evidence that juvenile coho salmon are selective predators on juvenile quillback rockfish. These differences in early life history traits between consumed and surviving rockfish are related to timing of parturition and the environmental conditions larval rockfish experienced, suggesting that maternal effects may substantially influence survival at this stage. Our results demonstrate that variability in timing of parturition and sea surface temperature leads to tradeoffs in early life history traits between growth in the larval stage and survival when encountering predators in the pelagic juvenile stage.

Revisiting daily age determination in juvenile anchoveta Engraulis ringens

Recent studies using otolith microstructure analysis have suggested that the duration of the juvenile stage in anchoveta (Engraulis ringens) is shorter than previously suspected, which suggests that the ages being entered into the traditional age-based stock assessment are incorrect. However, the interpretation of young pelagic fish otoliths remains problematic. To clarify the age interpretation of larval and juvenile E. ringens, newly hatched larvae were reared in a quasi-natural environment for periods of up to 103 days. The sagittal otoliths were subsequently examined and measured by international otolith experts in a double-blind workshop study. The young anchovy could be aged both accurately and precisely using otolith microstructure, after measures were taken to correct for unresolvable increments formed immediately after hatch. The presence of a fast-growth transition zone characterised by either considerable splitting or subdaily increments or both was confirmed. This study confirms the hypothesis of rapid growth and young age through the juvenile stage for anchoveta, suggesting that a critical appraisal of the annual age determinations used as the basis for anchoveta stock assessment is warranted. The otolith interpretation principles outlined in this study may apply broadly across many small pelagic fish species.

Dynamics of growth-based survival mechanisms in Japanese anchovy (Engraulis japonicus) larvae

Three growth-based survival mechanisms were tested for multiple cohorts of Japanese anchovy (Engraulis japonicus) larvae in Sagami Bay. Through otolith microstructure analysis, growth trajectories and histories of samples of the survivors were compared with those of the original populations to examine size- and growth-selective mortality to test the “bigger is better” and “growth-selective predation” mechanisms, respectively. The effects of growth rates on the timing of metamorphosis were examined to test the “stage duration” mechanism. The bigger is better and growth-selective predation mechanisms were detected to be effective in two and six of eight seasonal cohorts, respectively. Results contrary to the bigger is better and growth-selective predation mechanisms were obtained from three and two of eight cohorts, respectively. The stage duration mechanism was evaluated to be effective for both of two cohorts that were testable. Overall, none of the three mechanisms were universally appropriate for all of the cohorts. The relative contributions of the three mechanisms were dynamic, although the growth-selective predation mechanism was identified to be the major one in anchovy larvae in the study site.