SCIENTIFIC COMMITTEE THIRD REGULAR SESSION 13–24 August 2007 Honolulu, Hawaii, USA TRENDS IN SIZE COMPOSITION OF LONGLINE-CAUGHT ALBACORE IN THE SOUTH PACIFIC

2 Trends in size composition of longline-caught albacore in the South Pacific. Executive summary Size of albacore (nominal mean weight from logsheets and median lengths from length-frequency data supplied through observer and port sampling programmes), revealed trends in the size of albacore by domestic fleets of sub-equatorial EEZs. The impacts of domestic fisheries, changes in recruitment strength and the influence of variable oceanography are all likely to interact and influence the size of albacore captured by domestic fleets. However, trends in size data appear closely related to trends in total longline effort within individual EEZs. Not only did median size of albacore decline as many domestic fleets developed and increased in size and total effort, stabilisation or increases in median size were reported in several EEZs after reductions in domestic longline effort. In fisheries where there were increases in effort, median albacore size has continuously declined. Similar trends are also observed when the proportion of large albacore (greater than 95 cm FL and 100 cm FL) in catches are examined. Changes in median size in relation to effort suggest that management at the scale of individual EEZs (i.e. within the scope of national fisheries management agencies) can influence the size distribution of albacore (and catch rates) captured by domestic longline fisheries in the sub-equatorial WCPO. This may be a result of local populations of large albacore having longer residency times and/or shorter movement than currently assumed. An extensive albacore tagging programme is required to provide improved estimates of movement and residency of longline-vulnerable albacore than currently exist, and such a programme is currently being planned. In addition, management at the scale of sub-regions would also be likely to influence albacore size in catches and catch rates by domestic fisheries. Finally, the information presented in this paper highlights the importance of length-frequency data collected by observer programmes and port-sampling programmes in the WCPO. While size data are important inputs to stock assessment models, length-frequency data may also assist in the interpretation of the impacts of longline effort at the level of individual EEZs and sub-regions. Introduction South Pacific albacore tuna are an important component of the tuna catches in the WCP-CA (Figure 1 and Figure 2), with recent catches approaching 60,000 mt (Figure 3). Albacore are captured by longline and troll method fisheries (Figure 2 and Figure 3) with approximately 90% of recent catches attributable to longline-method fisheries. Adult albacore are …