Larry B. Crowder

Stephen B. Toth Professor of Mar ine Biology, Duke University

Shifting Gears: Impacts of Bycatch from Populations to Ecosystems

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http://moray.ml.duke.edu/faculty/crowder/

Abstract

Bycatch of non-target species is one of the most critical problems in fisheries. Wasteful bycatch of finfish accounts for nearly a quarter of global landings. Threatened and endangered species like sea turtles, marine mammals, seabirds, and sharks are also killed in fishing gear. By far, shrimp trawling has the highest bycatch—it is particularly devastating to sea turtles. Turtle excluder devices (TEDs) can reduce sea turtle bycatch significantly and TED use is now spreading globally. Gill nets can be deadly to air breathers like turtles and marine mammals. Technological fixes, like acoustic pingers, can reduce bycatch for some small cetaceans. But the long-term effectiveness of pingers is still in question. Shifting from setting on dolphins to setting on logs has reduced marine mammal bycatch in the tuna purse seines, but led to increases in bycatch of many other species, including sea turtles. Longlines generally take less bycatch than high seas driftnets, but remain problematic, particularly for seabirds, sea turtles, and sharks. Setting technologies can reduce seabird bycatch, but currently there are no solutions to bycatch of sea turtles and sharks. Although gear fixes can reduce bycatch, problems remain with implementation and enforcement. A recent analysis of the number of dead sea turtles stranding along the Texas coast failed to detect a TED effect. Instead, strandings remain high due to lack of compliance to the TED regulations. Similarly, proven technologies to reduce seabird mortality on longlines have not been implemented in the North Pacific, where takes of Black-footed albatrosses are still excessive.

Population models have been used to evaluate the population-level implications of bycatch, but food web and ecosystem effects of fishing have only recently been addressed. Food web models based on ECOPATH/ECOSIM allow us to examine the potential direct and indirect effects of alternative fishing gears on marine food webs. Although preliminary, current models for various North Pacific ecosystems show that the ultimate impact of fishing on protected species depends upon both direct and indirect effects of food web alterations.

Bio

Dr. Crowder's research centers on predation and food web interactions, mechanisms underlying recruitment variation in fishes, and on population and food web modeling in conservation biology. He has studied food web processes in estuaries and lakes, and has used observational, experimental and modeling approaches to understand these interactions in an effort to improve fisheries management. He co-directed the South Atlantic Bight Recruitment Experiment (SABRE) and continues to conduct research on the life histories of estuarine-dependent fishes. He continues to conduct model and statistical analyses to assist in endangered species management for both aquatic (sea turtles, seabirds) and terrestrial species (red-cockaded woodpeckers). Recently he has begun developing more extensive programs in marine conservation, including research on bycatch in longline and other fisheries, spatial analysis of marine megafauna, nutrients and low oxygen, marine invasive species and integrated ecosystem management.

Readings

1. Crowder, L.B. and S.A. Murawski. 1998. Fisheries bycatch: Implications for management. Fisheries 23(6):8-17.

2. Heppell, S.S., H. Caswell, and L.B. Crowder, 2000. Life histories and elasticity patterns: Perturbation analysis for species with minimal demographic data. Ecology 81(3):654-665.

3. Crowder, L. 2000. Leatherback's survival will depend on an international effort. Nature 405:881.