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Stock Assessment to Improve Management for Sharks

SharkPIs: Dr. Elizabeth A. Babcock, University of Miami, and Dr. Ellen Pikitch, Institute for Ocean Conservation Science.

Completed 2008

Overfishing is one of the greatest threats to marine ecosystems around the world. To stop overfishing and rebuild depleted species, it is essential to create effective, scientifically-based management plans for each exploited fish species. Each plan must include an understanding of the species’ “population dynamics” – that is, the changes in the fish’s population size, average age and physical size over time, and the biological and environmental factors that influence these characteristics.

Unfortunately, inadequate population dynamics data exists for many fish species, including sharks. In the absence of such data, scientists have had to devise ways to reliably calculate fish population estimates despite the knowledge gaps. Institute for Ocean Conservation Science researchers have been at the forefront of developing and applying advanced mathematical modeling methods that make the most of limited data sets.

Institute scientists are globally recognized experts in “Bayesian methods,” an assessment approach that incorporates data from diverse sources to create more reliable stock assessments. Recent Institute projects have used Bayesian methods to calculate the population sizes of large coastal sharks and open-ocean sharks, as well as of white marlin, bluefin tuna, and swordfish.

Bayesian analysis enables scientists to combine multiple hypotheses about fish species’ abundance, biology, life history, and population dynamics, along with any hard data that may be available regarding a fish’s total catch and its abundance trends. This multi-sourced analysis of fish stocks is useful for the establishment of comprehensive fisheries management strategies.

The Institute has made its modeling techniques available to global scientists by developing the “Bayesian Surplus Production” model software. That software is available through the International Commission for the Conservation of Atlantic Tunas’ (ICCAT) catalog of methods, and is now used by the National Marine Fisheries Service to determine the population status of blacktip and sandbar sharks. ICCAT, an intergovernmental organization involved in stock assessment and management of tuna and other fish species, also uses the Institute’s software to assess the status of shortfin mako and blue sharks.

Institute modeling methods have informed important shark management decisions at the national and global level. These include U.S. quotas on the number of pounds of coastal shark that can be caught annually, known as the “Total Allowable Catch”; as well as ICCAT’s international ban on shark finning, the wasteful practice in which commercial fishermen slice off a shark's fin for later sale and throw the shark’s carcass overboard. The fins fetch a high price in mostly-Asian food markets.

More Information:   Ellen Pikitch, PhD  |  Demian Chapman, PhD  |  Elizabeth Babcokc, PhD

Blue Shark
Blue Shark.
Photo Credit: Neil Hammerschlag http://www.neil4sharks.org

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