Fishing amplifies forage fish population collapses

Abstract

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.marine conservation | population collapse | fisheries | ecosystem-based management

Stop-loss order for forage fish fisheries

“…It should lay to rest any question that fishing impacts population fluctuations, even in circumstances where environmental factors are clearly exerting a major influence on stock dynamics, and is an important culprit in collapses. The methods developed by Essington et al (4) are applicable to any population, and should prove useful for detecting the fingerprint of fishing in other situations that have heretofore been seen as an irresolvable challenge. Finally, minimum biomass thresholds for nonforage fish species set at appropriate levels may have substantial ecological benefits and relatively low costs, and deserve much wider evaluation and application.…”

“…These findings underscore the importance of (i) furthering understanding of the determinants of forage fish collapses, and (ii) of devising exploitation strategies that will maintain their crucial ecological and economic roles. Essington et al (4) significantly advance understanding on both counts, applying novel methods to uncover fishing's fingerprint in regulating population fluctuations and demonstrating the effectiveness of a simple remedy to prevent collapses.…”

Climate, Anchovy, and Sardine

“…In particular, the recovery of stocks from low to high abundance may be slowed or prevented by even modest mortality caused by directed fishing, bycatch, or other factors, including marine mammals, seabirds, jellyfish, and disease , Essington et al 2015, Lindegren et al 2013, MacCall et al 2016, Ward et al 2001. Similarly, the rate of decline of a stock caused principally by changes in climate may be accelerated by mortality of the types just listed (Essington et al 2015, Lindegren et al 2013, Pinsky & Byler 2015. Finally, the duration of a period of low abundance may be prolonged by enhanced mortality (Essington et al 2015).…”

Tradeoffs between fisheries harvest and the resilience of coral reefs

“…A more complex consideration is maintaining the ecosystem function of the target species. A well-known example is the importance of forage fish and krill to the diet of large fish, sea birds, and marine mammals (3)(4)(5), which generates a tradeoff between human and nonhuman (natural) consumption of the stock. Measuring such direct trophic tradeoffs is challenging enough (6), but some ecosystem functions act through complex ecological interactions that might involve multiple trophic levels and result in nonlinear impacts on the ecosystem (7).…”