Somatic growth contributes to population variation in marine fishes.

Understanding population fluctuations is a major goal of population ecology. In unpredictable marine environments, population variation is thought to be caused primarily by varying survival rates through a critical early life-history stage. However, there is increasing evidence that somatic growth variation is common and causes population fluctuations. We examine the relative effects of empirically validated variability in somatic growth and recruitment on two response metrics across eight different life-history archetypes of marine fish. We evaluate how much variation is propagated into mature biomass (MB), a proxy for population resilience, and population production, a measure of population rebuilding capacity. Production is defined as the biomass produced by the stock above what is needed to sustain the population at a constant level. We used empirical estimates of reproductive success and somatic growth rate, coupled with a population model, to evaluate the relative role of both types of variation in population fluctuations. The effects of this variation on population production and MB were examined across three variation scenarios, in which somatic growth only, reproduction only or both processes varied temporally. We also examined three levels of age truncation to explore whether modified population age structure altered these dynamics. The contribution of somatic growth to biomass variability exceeded that of recruitment for some species (2/8), while in others (5/8 species), recruitment variation was more influential. When population production was examined, somatic growth variation contributed more to population variation for three species. The relative importance of the two processes was not clearly correlated with key life-history traits (i.e., growth and mortality rates), but instead was determined by time-series characteristics of growth and recruitment variation. Increasing age truncation slightly increased the relative effect of recruitment variation on MB variation for three species. These results suggest somatic growth variation can be as important as early life-history survival in driving population fluctuations in some marine fish species. This analysis provides a counterexample to the commonly held assumption of many marine population dynamics models: That population variability is induced primarily through variation in reproductive success.

Fishing amplifies forage fish population collapses.

Forage fish support the largest fisheries in the world but also play key roles in marine food webs by transferring energy from plankton to upper trophic-level predators, such as large fish, seabirds, and marine mammals. Fishing can, thereby, have far reaching consequences on marine food webs unless safeguards are in place to avoid depleting forage fish to dangerously low levels, where dependent predators are most vulnerable. However, disentangling the contributions of fishing vs. natural processes on population dynamics has been difficult because of the sensitivity of these stocks to environmental conditions. Here, we overcome this difficulty by collating population time series for forage fish populations that account for nearly two-thirds of global catch of forage fish to identify the fingerprint of fisheries on their population dynamics. Forage fish population collapses shared a set of common and unique characteristics: high fishing pressure for several years before collapse, a sharp drop in natural population productivity, and a lagged response to reduce fishing pressure. Lagged response to natural productivity declines can sharply amplify the magnitude of naturally occurring population fluctuations. Finally, we show that the magnitude and frequency of collapses are greater than expected from natural productivity characteristics and therefore, likely attributed to fishing. The durations of collapses, however, were not different from those expected based on natural productivity shifts. A risk-based management scheme that reduces fishing when populations become scarce would protect forage fish and their predators from collapse with little effect on long-term average catches.