Book Description

Oregon's commercial fishery for albacore tuna (Thunnus alalunga) is large, valuable, and variable. Little is known, however, about the oceanographic factors which determine abundance and distribution of albacore. Primarily using logbook data contributed by fishermen, this study compares albacore catches with selected ocean conditions. The study is limited to troll-caught fish taken during the summer of 1969 off the Pacific Northwest. The location and success of commercial albacore catches were analyzed with respect to the sea-surface temperature field. Also examined was the relationship between fish catches and sea-surface temperature, mixed-layer depth, and water color. The 1969 albacore season off the Pacific Northwest lasted for 67 days, from 12 July to 16 September. Nearly 30 million pounds of albacore, worth $6.7 million, were landed in Oregon ports, making 1969 the second best season in history. The first commercial quantities of albacore were caught about 135 n mi west of Coos Bay. During the rest of July the fish moved north somewhat but stayed well offshore. During August the fishery spread out considerably, moving as far north as Vancouver Island. In September fishing activity shifted south and inshore. Off the Pacific Northwest no albacore were taken from waters cooler than 54F(12.2C) or warmer than 66F(18.9C). Most fish were taken from the sea-surface temperature range of 59. 5-63.5F(15.3-17.5C). July fish favored 61.5-63.5F(16.4-17.5C) water. By contrast, August fish favored 59.5-63.5F(15.3-17.5C) water while September fish favored 59.5-61.5F(15.3-16.4C) water. Catch-per-unit- effort data did not indicate as marked a decline in availability at cooler temperatures as did gross catch. Mixed-layer depths associated with favorable albacore catches, 56-135 ft(17.1-41.1 m), were somewhat deeper than those determined by previous investigations. Water-color results showed that most albacore were caught in blue oceanic water. Other factors associated with good fishing included favorable wind and sea conditions, areas with considerable temperature structure, plentiful food supply, presence of "tuna birds", and stable, moderately developed upwelling situation.