Productivity-Susceptibility Analysis Completed for Hawaii Longline Fishery

Decisions on catch limits and other fishery regulations involve some level of risk to the economic, social, and ecological systems within which the fisheries operate. Managers should fully assess those risks with the help of scientists, fishers, conservationists, and other stakeholders. A useful framework for assessing ecological risks of fishing has been developed by scientists in the Australian Fisheries Management Authority. This approach, endorsed by fishery experts in the U.S, takes into account uncertainties in the information and models used to establish and evaluate management policy and the complex nature of ecosystems. One of the components of the risk assessment is a semi-quantitative step called the "Productivity-Susceptibility Analysis", or PSA, which evaluates the vulnerability of target species and bycatch species to fishing on the basis of their productivity attributes (e.g., age at maturity, fecundity) and susceptibility attributes (e.g., availability to capture, post-release survival). Vulnerability can be viewed as the extent to which the productivity of the stock is diminished by targeted and indirect fishing pressure and is expected to differ between stocks based on their life history characteristics and susceptibility to the fishery.

PIFSC Fishery Biologists Donald R. Kobayashi and Keith A. Bigelow used the PSA method to evaluate the vulnerability of fish stocks to overfishing by the Hawaii longline fishery. They generated PSA plots for both the tuna sector (using deep-set gear) and swordfish sector (using shallow-set gear) of the fishery, altogether examining a complex of 33 fish species listed in the Fishery Management Plan (FMP) governing the fishery. Their results are presented in the graphic below.

When a significant difference in vulnerability exists between stocks in the exploited complex, some stocks will likely decline over time while others remain constant or increase in abundance. If these differences in vulnerability are not identified, unmonitored stocks could become overfished and lead to ecosystem instability. Therefore, all stocks listed in an FMP, including targeted stocks (such as tuna or swordfish) and others in the ecosystem regarded as incidental bycatch stocks (such as shark), need to be evaluated for vulnerability to the fishery. Then managers can apply or adjust management measures to ensure that no stock, including the most vulnerable stock, is overfished.

PSA is a practical and comprehensive tool for evaluating the vulnerability of stocks based on their life history characteristics and susceptibility to the fishery. It allows evaluation of all stocks within a consistent, yet flexible, analytical framework. It is applicable to well-studied stocks for which data quality is high, as well as little known stocks for which data quality is low.

Kobayashi and Bigelow are members of the NMFS Vulnerability Evaluation Working Group (VEWG) producing technical guidance for use by agency stock assessment scientists for evaluating vulnerability of a fish stock to a specified fishery. The VEWG intends to provide several case studies to demonstrate how the PSA tool is used across a diverse array of stocks, habitats, and fisheries. The final guidance document will be published by the end of 2008. The Hawaii longline fishery case study by Kobayashi and Bigelow will be part of the guidance document. The NMFS guidelines will enable the agency to comply with new annual catch limit and accountability requirements as described in the Magnuson-Stevens Reauthorization Act of 2006. The objective of these requirements is to end overfishing in Federal fisheries.

Group/Species#PSA Plots by Fishery Sector
Tunas
Albacore 1
Bigeye tuna 2
Bullet tuna 3
Kawakawa 11
Northern bluefin tuna 17
Skipjack tuna 21
Wahoo 28
Yellowfin tuna 29
Billfishes
Black marlin 3
Blue marlin 8
Striped marlin 15
Sailfish 20
Shortbill spearfish 23
Broadbill swordfish 24
Sharks
Blue shark 6
Bigeye thresher shark 7
Longfin mako shark 13
Salmon shark 14
Pelagic thresher shark 19
Shortfin mako shark 22
Oceanic whitetip shark 30
Silky shark 31
Common thresher shark 32
Others
Pacific pomfret 5
Dolphin fish (mahimahi) 9
Brilliant pomfret 10
Spotted moonfish (opah) 12
Oilfish 16
Roudi escolar 18
Flathead pomfret 25
Dagger pomfret 26
Sickle pomfret 27
Escolar 33

PSA plots for fish stocks exploited in the tuna and swordfish sectors of the Hawaii longline fishery.  Numbers in the plot symbols identify the 33 fish stocks and assemblages in the complex. The farther a stock’s score is from the plot origin, the more vulnerable the stock is to overfishing.

For more information contact: Donald Kobayashi