FISHERY BULLETIN: VOL. 71, NO. 2 



is thus associated with a decreased availability 

 of saury in near-surface waters where jig 

 fishing for albacore is most effective. 



The biomass of fishes, shrimps and squids 

 per 1,000 m^ of water filtered in nighttime 

 midwater trawl tows is shown in Figure 21. 

 Note that the diameter of the circles is pro- 

 portional to the logarithm of total biomass of 

 the tows. The highest catches were usually 

 composed of the shrimp Sergestes similis in 

 plume waters. Fishes often predominated 

 catches, but when they did, the total biomass 

 was generally intermediate or low. Squids 

 were usually present but composed a relatively 

 small portion of the biomass. 



A comparison of midwater trawl catches 

 and tuna stomach contents reveals some obvious 

 differences. Saury was rarely if ever caught 

 in trawls but was common as forage especially 

 during the first part of the cruise. Squids 

 sometimes predominated in albacore caught 

 offshore but were unimportant components 

 in the trawl collections, though no trawl was 

 towed in the offshore region where squids 

 were common in stomachs on 30 July. Few 

 midwater trawl and albacore catch stations 

 coincided exactly. Nevertheless the correlation 

 between albacore catches and midwater trawl 

 catches sometimes appeared negative. For 

 example, lowest biomass of micronekton (Figure 

 21) and euphausiids (not shown) occurred in 

 the region of high albacore catch rates (5.0 

 and 2.9 in Figure 19). Perhaps such a negative 

 relationship was caused by the grazing of 

 albacore or their carnivorous prey. 



DISCUSSION 



Our present state of knowledge about the 

 behavior of albacore and the influence of the 

 ocean environment is inadequate to explain 

 the drastic fluctuations of catches by jig boats 

 that occurred off Oregon in 1970. The large 

 concentration of albacore located by the jig 

 fleet in July 1970 off the Columbia River was 

 very vulnerable to surface jigs and record 

 catches were made. In contrast, boats that 

 trolled through this same area, and Oregon 

 waters that had been productive in past years, 

 often reported "no fish days" in August. 



Oceanographic changes that correlate with 

 these catch variations were not obvious. Winds 

 were variable during the entire summer with 

 the prevailing northwesterlies being interrupted 

 occasionally by weak frontal systems and con- 

 comittant southerly winds (Figure 3). The first 

 large peak in catches per day was during a 

 period of strong northerlies; the second peak, 

 during calm sea conditions. Sea-surface tem- 

 peratures remained within the "preferred" 

 thermal range, 14.4°-16.1°C (Alverson, 1961), 

 over much of the region during August, usually 

 the best month for albacore fishing. Mixed layer 

 depths also did not change appreciably and 

 15 °C water was usually found to 20-40 m 

 offshore of coastal waters. 



The fact that bait boats had good fishing 

 after the demise of the jig fishery suggests 

 that albacore were present off Oregon but 

 not susceptible to capture by jig fishing from 

 August through mid-October. This can be 

 explained by changes in the vertical distribu- 

 tion and behavior of albacore. Surface-towed 

 jigs are most effective on fish feeding near 

 the surface, whereas bait boats, by chumming 

 an area, are better able to attract fish from 

 subsurface schools. 



Because albacore was feeding mainly on 

 surface schools of saury during the second 

 peak in jig boat success and because a signi- 

 ficant positive relationship existed between 

 albacore catches and feeding on saury, it is 

 postulated that the collapse of the jig fishery 

 was related to a change in the availability of 

 the saury, the major prey of the albacore during 

 the preceding period in 1970, and usually 

 the most important food of albacore off Oregon 

 in other years (Pearcy, unpubl. data). Saury 

 could have been grazed down, emigrated out 

 of the area, or descended into deeper water. 

 Any of these could have induced the large 

 concentration of albacore to migrate into deeper 

 water where better feeding conditions existed 

 and where they would be relatively inaccessible 

 to surface trolled jigs. 



During late August we repeated a cruise 

 in the same region as the 27 July-2 August 

 cruise, off northern Oregon and the mouth 

 of the Columbia River. Nineteen albacore were 

 caught on jigs. Anchovy larvae, small rock- 



502 



