Collaborative Blue Shark Stock Assessment Completed

Annual catch of North Pacific blue shark in number of fish by driftnet (red) and longline (purple) fisheries. Driftnet fishing ended on the high seas in 1993 but continues within the EEZ of Japan.

The Center recently published a NOAA Technical Memorandum describing a stock assessment of blue shark (Prionace glauca) in the North Pacific Ocean co-authored by Pierre Kleiber of the Center's Fishery Biology and Stock Assessment Division and colleagues from Japan, Canada, and the University of Hawaii Joint Institute for Marine and Atmospheric Research.

The analysis used statistics on fishing effort and the number and size of blue sharks caught by Japanese, Taiwanese, and Hawaii-based longline fleets as well as data from Japanese, Korean and Taiwanese driftnet fleets (mostly now defunct). The total North Pacific blue shark catch increased dramatically during the 1980's and declined sharply in the early 1990's after the use of high-seas driftnets was banned.

Kleiber and colleagues encountered difficulties in assembling reliable data on blue shark catch, in large part because fishermen rarely retain this species for the market (although some foreign vessels remove and sell the fins) and consequently underreport the catch of blue sharks in their logbooks. Therefore, the catch data were "filtered" to identify the more reliable logbook records. On average, estimates of catch derived from the logbook data were of similar magnitude as estimates North Pacific blue shark catches derived independently from shark fin market data.

The team of scientists analyzed the data using two models: a simple surplus production model and a more complex, integrated model incorporating age structure and spatial structure. The two approaches were largely in agreement with each other. As the accompanying chart shows, the models predicted an increase in blue shark biomass (red lines) during the 1970s, a decline during the 1980s when driftnet fleets were active in the North Pacific, then a recovery to (or above) biomass levels early in the time series.

Estimated trajectories of blue shark stock biomass for the base case. Red lines show biomass history of the exploited stock and green lines show projected biomass history in the absence of fishing. In each scenario, solid lines denote total stock biomass and dotted lines denote spawning stock biomass.

The "base case" integrated model analysis indicated a low probability (around 30%) that at the end of the time series stock biomass was less than the biomass level supporting maximum sustainable yield (BMSY) and an even lower probability that fishing mortality was greater than the associated level of fishing mortality (FMSY). These results indicate that that the stock was not overfished at the time, nor was overfishing occurring. Sensitivity analyses showed considerable variability between the base case and a few alternative scenarios. Some alternatives gave more "pessimistic" results than the base case (i.e., trending closer toward conditions of overfishing or an overfished state) and some were more "optimistic". Despite the degree of uncertainty, all scenarios showed an overall increasing trend in the model prediction of the historical blue shark stock biomass had there been no fishing at all (green lines in the chart). This indicates that other forces, in addition to fishing, have been affecting the blue shark population.

As the catch chart above shows, toward the end of the time series there was an increase in blue shark catch by longline fleets. During this period there was also an increase in total longline fishing effort and this trend may have continued. Accordingly, it would be prudent to assume that the population abundance of blue shark is at least close to its MSY level (BMSY) and that fishing mortality is approaching, or above, its MSY level (FMSY).