UW Aquatic & Fishery Sciences Quantitative Seminar

Ray Hilborn

School of Aquatic and Fishery Sciences, University of Washington

The impact of fishing forage fish on dependent predators: has it been greatly overestimated?

Abstract

Cury et al showed that the reproductive success of many seabirds depends upon the abundance of the fish the fish species that are their major prey at the time of breeding. Pikitch et al 2012 and Smith et al. 2011 both used ecosystem models to suggest that fishing forage fish will reduce the abundance of the forage fish and thus the abundance of higher trophic level dependent predators.

In this talk I will explore several issues that I believe will have caused these papers to overestimate the impact of fishing forage fish on their dependent predators. First is that many forage fish undergo very large, (sometimes two orders of magnitude) fluctuations even in the absence of fishing, and thus the impact of fishing on abundance must be compared to natural fluctuations. Second even for forage species that do not go through large long-term cycles, recruitment in forage fish is largely (if not totally) unrelated to spawning stock size so the size of the cohorts of forage species is not impacted by fishing. Third, most avian and fish predators of forage fish take small individuals younger than the ages at which the fishery operates, so if recruitment is independent of spawning stock size, and the predators take fish before the fishery reduces the cohort, then fishing the forage fish has no impact on the food available to the predators.

I will give some examples of the recruitment dynamics and size selectivity to illustrate that much more work is needed before we can understand the general impact of fishing forage fish on dependent predators.

Cury, P. M., Boyd, I. L., Bonhommeau, S., Anker-Nilssen, T., Crawford, R. J., Furness, R. W., Mills, J. A., Murphy, E. J., Österblom, H., and Paleczny, M. 2011. Global seabird response to forage fish depletion—one-third for the birds. Science. 334: 1703-1706.

Pikitch, E., Boersma, P. D., Boyd, I., Conover, D., Cury, P., Essington, T., Heppell, S., Houde, E., Mangel, M., and Pauly, D. 2012. Little fish, big impact: managing a crucial link in ocean food webs. Lenfest Ocean Program, Washington, DC. 108.

Smith, A. D., Brown, C. J., Bulman, C. M., Fulton, E. A., Johnson, P., Kaplan, I. C., Lozano-Montes, H., Mackinson, S., Marzloff, M., and Shannon, L. J. 2011. Impacts of fishing low–trophic level species on marine ecosystems. Science. 333: 1147-1150.