Margaret C. Siple

Fishing amplifies forage fish population collapses

Significance Forage fish provide substantial benefits to both humans and ocean food webs, but these benefits may be in conflict unless there are effective policies governing human activities, such as fishing. Collapses of forage fish induce widespread ecological effects on dependent predators, but attributing collapses to fishing has been difficult because of natural fluctuations of these stocks. We implicate fishing in forage fish stock collapses by showing that high fishing rates are maintained when stock productivity is in rapid decline. As a consequence, the magnitude and frequency but not duration of stock collapses are far greater than expected from natural fluctuations. Risk-based management policies would provide substantial ecological benefits with little effect on fishery catches. 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.

Population diversity in Pacific herring of the Puget Sound, USA

Demographic, functional, or habitat diversity can confer stability on populations via portfolio effects (PEs) that integrate across multiple ecological responses and buffer against environmental impacts. The prevalence of these PEs in aquatic organisms is as yet unknown, and can be difficult to quantify; however, understanding mechanisms that stabilize populations in the face of environmental change is a key concern in ecology. Here, we examine PEs in Pacific herring (Clupea pallasii) in Puget Sound (USA) using a 40-year time series of biomass data for 19 distinct spawning population units collected using two survey types. Multivariate auto-regressive state-space models show independent dynamics among spawning subpopulations, suggesting that variation in herring production is partially driven by local effects at spawning grounds or during the earliest life history stages. This independence at the subpopulation level confers a stabilizing effect on the overall Puget Sound spawning stock, with herring being as much as three times more stable in the face of environmental perturbation than a single population unit of the same size. Herring populations within Puget Sound are highly asynchronous but share a common negative growth rate and may be influenced by the Pacific Decadal Oscillation. The biocomplexity in the herring stock shown here demonstrates that preserving spatial and demographic diversity can increase the stability of this herring population and its availability as a resource for consumers.

Contributions of adult mortality to declines of Puget Sound Pacific herring

Contributions of adult mortality to declines of Puget Sound Pacific herring Margaret C. Siple,* Andrew O. Shelton, Tessa B. Francis, Dayv Lowry, Adam P. Lindquist, and Timothy E. Essington School of Aquatic and Fishery Sciences, University of Washington, P.O. Box 355020, Seattle, WA 98105, USA Northwest Fisheries Science Center, NOAA, 2725 Montlake Blvd E, Seattle, WA 98112, USA Puget Sound Institute, University of Washington Tacoma, 326 E D St, Tacoma, WA 98421, USA Washington Department of Fish and Wildlife, Marine Fish Science Unit, Olympia, WA, USA *Corresponding author: tel: þ1 (206) 661-8403; fax: +1 (206) 685-7471; e-mail: siplem@uw.edu

A heuristic model of socially learned migration behaviour exhibits distinctive spatial and reproductive dynamics

&NA; We explore a “Go With the Older Fish” (GWOF) mechanism of learned migration behaviour for exploited fish populations, where recruits learn a viable migration path by randomly joining a school of older fish. We develop a non‐age‐structured biomass model of spatially independent spawning sites with local density dependence, based on Pacific herring (Clupea pallasii). We compare a diffusion (DIFF) strategy, where recruits adopt spawning sites near their natal site without regard to older fish, with GWOF, where recruits adopt the same spawning sites, but in proportion to the abundance of adults using those sites. In both models, older individuals return to their previous spawning site. The GWOF model leads to higher spatial variance in biomass. As total mortality increases, the DIFF strategy results in an approximately proportional decrease in biomass among spawning sites, whereas the GWOF strategy results in abandonment of less productive sites and maintenance of high biomass at more productive sites. A DIFF strategy leads to dynamics comparable to non‐spatially structured populations. While the aggregate response of the GWOF strategy is distorted, non‐stationary and slow to equilibrate, with a production curve that is distinctly flattened and relatively unproductive. These results indicate that fishing will disproportionately affect populations with GWOF behaviour.

Reply to Szuwalski and Hilborn: Forage fish require an ecosystem approach

In response to our recent paper (1), Szuwalski and Hilborn (2) make several points about the timing of recruitment failures, the effect of fishing on productivity, and our choice of using biomass, not recruitment, as the indicator for collapses. We address these points here to show that not only do they not affect our conclusions, but that we are largely in agreement regarding the biological processes and the implications for fisheries and conservation.