Sensitivity Analysis Completed for New Stock Assessment of Bigeye Tuna in Western and Central Pacific

Probability distributions of the ratio B/BMSY for bigeye tuna in the WCPO under various scenarios. BLACK = base case (i.e., data and assumptions as used in the current OFP stock assessment); GREEN = assumes equatorial purse seine catch of bigeye has been underestimated by as much as 50%; DARK BLUE = assumes bigeye catch in Philippine and Indonesian fisheries has been overestimated; LIGHT BLUE = assumes the stock-recruitment curve for bigeye is less steep than assumed in the base case; RED = assumes catchability of bigeye in the Japanese longline fishery is greater than assumed in the base case.

FBSAD scientists Pierre Kleiber and Keith Bigelow worked with staff of the Oceanic Fisheries Program (OFP), Secretariat of the Pacific Community, on a stock assessment of bigeye tuna in the central and western Pacific Ocean (WCPO). The assessment was prepared by the OFP for the Western and Central Pacific Fisheries Commission (WCPFC) and reported online at http://www.wcpfc.int/sc4/pdf/SC4-SA-WP1-rev.1%20[BET%20Assessment].pdf.

As part of the assessment, the group studied the sensitivity of the assessment's conclusions and management implications to uncertainty in the input data and key assumptions. The assessment was repeated under several alternative scenarios. Under each scenario, sensitivity was measured by comparing probability distributions of B/BMSY — the ratio of current stock biomass (B) to the stock biomass producing Maximum Sustainable Yield (BMSY) — in the base case (i.e., current assessment) and the alternative scenario.

The analysis showed that the probability distribution of B/BMSY was little affected by certain kinds of errors in bigeye catch estimation — either overestimating the catch in Indonesia and the Philippines, or underestimating the catch by purse seine vessels. Other scenarios result in probability distributions of B/BMSY significantly different from the base case, indicating a higher probability that the stock biomass is overfished. For example, such results occur when assuming the stock-recruitment curve is less steep near its origin than assumed in the base case or assuming that catchability of bigeye in the Japanese longline fishery is greater than assumed in the base case. In all of the scenarios described here, a similar sensitivity analysis based on the ratio of current fishing mortality to the mortality generating the Maximum Sustainable Yield (F/FMSY) indicates that overfishing is occurring.

For more information contact: Pierre Kleiber