Status of Stocks & Multispecies Assessment Program
Salmon Bycatch Patterns in the Eastern Bering Sea Pollock Trawl Fisheries (cont.)
Figure 4. Chinook salmon proportions at length by sex for the A season (above) (Jan-May, 57% females from 84,099 samples; top panel) and B season (below) (June-Dec, 55% females from 66,361 samples; bottom panel) as taken in the pollock fishery, 1998-2006 combined.
The sex ratio of Chinook salmon bycatch in the pollock fishery favors females over males, particularly in the size range greater than 55 cm (Fig. 4). Chinook less than that size tend to be males more than females, particularly during the summer and fall (B season). Chum salmon have an opposite pattern with more males overall and with females appearing smaller than males (Fig. 5 below).
Over time, the trends in these observed sex ratios have remained fairly consistent (Fig. 6 below). These patterns likely relate to the fact that female Chinook tend to mature at older ages than male Chinook salmon which return to spawning grounds earlier than the females.
Analyses such as these provide insights on the question on what factors affect bycatch levels of salmon in the pollock fishery. It appears that increases in the bycatch of salmon in recent years are due to factors other than changes in the way that pollock fishing is conducted. Environmental conditions may have altered the distribution of salmon relative to the pollock fishery. For example, observations from inter-annual surveys by the BASIS program have shown significant increases in chum salmon in their survey area.
Figure 5. Chum salmon proportions at length by sex for the B season (June-Dec, 44% females from 287,933 samples) as taken in the pollock fishery, 1998-2006 combined. Chum salmon are much less prevalent (~1% of total chum catch) in A season, hence length frequency samples from those months are omitted.
Figure 6. Sex ratios for Chinook and chum salmon over time. A and B seasons are shown for Chinook since there are significant catches in each of these seasons, chum salmon are primarily taken incidentally during the summer-fall (B) season.
Information from this study provides additional insight on patterns of bycatch by age, region and season. These results, combined with improved information on stock of origin and predictions of salmon abundance will provide managers with improved scientific advice for developing and evaluating management measures to reduce salmon bycatch in BSAI trawl fisheries.
