Indicators of the spatial distribution of blue shark (Prionace glauca) in the North Pacific
The Western and Central Pacific Fisheries Commission at its 14th Annual Session (paragraph 378, WCPFC14 Summary Report) requested the Scientific Committee to provide advice as to whether blue shark in the North Pacific should be designated as a “northern stock”, defined as those stocks “which occur mostly in the area north of the 20 degrees north parallel”.
Blue shark comprise a single stock in the North Pacific, and occur widely from the Equator to at least 57 degrees N. They mate in subtropical and tropical waters during the summer, after which females migrate northwards, giving birth in the following year between 30-40N. The area south of 20 degrees N is an important part of the blue shark distribution in the North Pacific, particularly for adults. Furthermore, the area may be part of the breeding ground, and/or postbreeding area for pregnant females.
Catch and CPUE data from several fisheries and research cruises were examined for indications of relative distribution of blue shark north and south of 20 degrees N. Japanese research cruise data indicated that blue shark CPUE is higher in the northern area. Other data sets examined – Chinese Taipei large-scale tuna longline and Hawaii-based deep-set longline – indicated similar levels of CPUE in both northern and southern areas. It is acknowledged that such spatial comparisons may be confounded by possibly different depth distributions that blue shark occupy in the northern temperate and southern tropical regions of the North Pacific, and the depths that longline gear fish in these regions. Nevertheless, it is clear from the available data that the tropical region of the North Pacific south of 20 degrees N is an important component of the blue shark distribution. This is also supported by conventional and electronic tagging data.
The question: do blue shark occur mostly north of 20 degrees N? – is difficult to answer scientifically because of the qualitative nature of the question. Based on nominal CPUE spatial comparisons, we would judge that blue shark has a tropical component at 0-20 degrees N similar to some already designated northern stocks. Comparisons of swordfish, albacore and blue shark nominal CPUE in the Hawaii-based deep-set longline fishery show similar ratios north and south of 20 degrees N for all three species, indicating that, at least in the area of this fishery, swordfish, albacore and blue shark all have significant parts of their distributions south of 20 degrees N.
In order to provide more specific advice, the Commission needs to clarify, and ideally quantify, what is meant by mostly north of 20 degrees N. If this can be done, indicators of the spatial distribution of candidate northern stocks, or indeed existing northern stocks, could be more objectively evaluated. Recommendations for further research that could improve the indicators of spatial distribution of blue shark include:
• The further collection and analysis of observer data for longliners fishing in the North Pacific;
• The development of spatially-structured population models;
• The collection and analysis of electronic tagging data to estimate patterns of vertical habitat use in the North Pacific and;
• Analyses to estimate effective effort and standardised CPUE that simultaneously take into account patterns of blue shark habitat use, and the fishing depth and other characteristics of longline gear.
SC14 is invited to note the information currently available to evaluate blue shark spatial distribution in the North Pacific, and to provide relevant advice to WCPFC14 on the question of its potential designation as a northern stock.