RELATION OF PHYSICAL PROCESSES TO 

 THE ALBACORE FISHERY 



It is particularly desirable to attempt to corre- 

 late the environmental conditions encountered 

 each year by albacore when they first enter the 

 region off Oregon and Washington with the subse- 

 quent distribution and numbers of fish available 

 to the fishery. Suggested relations of environ- 

 mental mechanisms to potential success of the 

 fishery (that rarely extends seaward of the plume 

 province) may then be tested in a preliminary 

 fashion. 



It appeared early in the analysis that the 

 Oregon-Washington fishery for albacore might 

 owe its degree of success to effects of the plume 

 rather than despite them — i.e., that the limit of 

 the area which contains commercial quantities of 

 albacore might be locally extended to the north 

 by these effects. Larger catches and catch rates 

 of the John N. Cobb at and within the lateral 

 limits of the plume than beyond it support the 

 proposal that numbers of available albacore gen- 

 erally are higher in the plume than beyond (table 

 3, fig. 15). 



Table 3. — Summary of albacore trolling in July by M/V 

 "John N. Cobb" off the Oregon-Washington coast, 1961-64 



(Effort Is summed (rom day of first catch) 



' WheiB S<32.2°/oo. 



= Where 5>32.2°/,„. 



' Catch per unit of effort was not measured because effort was insufBcient 

 outside the plume province after the day of flrst catch within it. On the basis 

 of 316 line-hours expended outside the plume before the day of first catch, 

 however, it can be said that the availability of albacore was tar greater within 

 the plume than outside it. 



One physical mechanism to which these con- 

 centration effects may be related is the greater 

 retention of heat within the surface layer of the 

 plume province. This retention, coupled with 

 wind-induced upwelling in the nearshore provinces, 

 produces the temperature ridge discussed earlier. 

 By extension of previous examples of albacore- 

 temperature relation (e.g. Alverson, 1961; John- 

 son, 1962) we may assume that, at any instant of 

 the fishery period, albacore will tend to concentrate 



in the region where temperature is most nearly 

 optimal and will be absent from regions where 

 surface-layer temperatures are less than 13° C. 

 From smoothed plots of albacore catch frequency 

 as a function of sea surface temperature for the 

 fishery oflF California (Glenn Flittner, personal 

 communication), 18° C. appears to be the optimal 

 temperature. This temperature is seldom exceeded 

 in the study area in July and is only barely ex- 

 ceeded in August and September. Consequently, 

 after the albacore had moved into the region, we 

 would expect the largest concentrations of fish to 

 develop in the areas with the highest water 

 temperatures or in the plume province rather 

 than in the colder waters of the nearshore or 

 offshore provinces. 



Furthermore, in the absence of other environ- 

 mental effects, relatively more albacore would be 

 expected to be in the plume than outside it in 

 years of greater temperature increase, when the 

 plume is most sharply defined; then catch rates 

 should be higher in the plume province than off- 

 shore. This expectation was not upheld, however: 

 comparison of temperature change and plume 

 intensity with average catch rates of fishing by 

 the John N. Cobb \\ithin the plume province 

 (table 3) and concurrent July- August averages of 

 catch per unit of effort for the commercial fishery 

 in the study area (fig. 16) showed that greater 

 than average numbers of albacore were available 

 in 1962 and 1963, when July temperatures were 

 lower (fig. 3) and temperature changes (table 2, 

 fig. 14) were smaller, than in 1961 and 1964. 

 Furthermore, the ratio of catch rate within the 

 plume to that beyond the plume (table 3) was 

 largest in 1963. These diflFerences in catch rates 

 indicate that albacore concentrated in the plume 

 to a greater degree when the difference in July 

 temperatures in the plume and offshore provinces 

 was least (fig. 3) . 



These results indicate that at least one variable 

 other than temperature affects the distribution of 

 albacore over the provinces. One factor is variation 

 in catch rate and its relation to abundance of 

 available albacore. If the catch rate for each 

 province adequately represents the number of 

 available albacore, then the ratio of catch rate 

 within the plume to that beyond is free of influ- 

 ence by total abundance of albacore. The agree- 

 ment of catch rates for exploratory fishing in the 

 j)lume each year with respective July and August 



522 



U.S. FISH AND WILDLIFE SERVICE 



