KLEIBER and BAKER: INTERACTION BETWEEN NORTH PACIFIC ALBACORE FISHERIES 



fJL = In 



100 



100 -X 



where X is the answer to the above question. The 

 monthly migration coefficients were then ob- 

 tained by scahng the distribution of intensity 

 over months so that the average was equal to |jl. 

 The migration coefficients for the 10 cm size 

 classes were then assigned to the smaller (5 cm) 

 size classes used in the model, and the coefficients 

 smoothed over size to soften discontinuities. 



The pattern of movement represented by the 

 migration coefficients can be summarized as fol- 

 lows: For immature fish (<85 cm) in the zones 

 north of 25° north the pattern is vigorous seasonal 

 movement toward the east in the summer and 

 toward the west at other times. New recruits, 

 which appear in the southern zones, migrate 

 mainly northward throughout the year and are 

 entrained in the east-west excursions of the 

 northern zones. Mature fish (>85 cm) accumulate 

 in the southern zones with brief movements 

 northward in April and May. 



RESULTS 



When we ran the model with input values esti- 

 mated as described above and with the effort of all 

 fleets set at nominal levels, that is, the average 

 seasonal and geographic pattern and magnitude 

 of effort for the 1970s with the pattern repeated 

 year after year, we found that after 10 years of 

 simulation the seasonal and geographic pattern 

 and magnitude of catch closely repeated itself 

 year after year. Therefore in making comparisons 

 of model results under different conditions, we 

 allowed the model to run at least 10 years under 

 a given repetitive annual regime before recording 

 the catch results during 1 year of simulation. 



In using the preliminary catchability values in 

 the model, we found that the predicted catches 

 were too low and the exploitation rate achieved 

 (2.6%) was less than half the exploitation rate in 

 the cohort analysis, which estimated those catch- 

 ability values (6.3%). This is because the cohort 

 analysis could not deal with geographic and sea- 

 sonal variability. The fleets were presumed to be 

 harvesting the ocean-wide population rather 

 than the fish in a localized area and time as in the 

 simulation model. We therefore scaled the catcha- 

 bilities of each fleet upward to make the annual 

 catches in number in the model (after 10 years of 

 simulation) close to the real average annual 

 catches (Kleiber and Baker fn. 2). With the cor- 



rected catchabilities, an exploitation rate of 5.1% 

 was achieved and we took the results in this case 

 to be our nominal (control) results (Fig. 2, 

 Table 1). 



We then made runs in which the original sea- 

 sonal and geographic pattern of effort was main- 

 tained but the magnitude of effort of one of the 

 fleets was either doubled or halved. We could 

 then compare the annual catch of each fleet under 

 the changed (experimental) conditions with the 

 annual catch under nominal conditions. 



Table 1, — Average albacore annual (1970-80) catch in numbers 

 and metric kilotons (kt) by baitboat, longline, and U.S. fleets plus 

 annual catch from model after at least 10 years of simulation under 

 nominal conditions and under various conditions of altered fishing 

 effort. 



The catch-at-size for the three fleets under 

 nominal and experimental conditions is plotted in 

 Figures 4 to 6. Changes in effort in one fleet ap- 

 pear to have little effect on the size distribution in 

 the catch of any of the fleets. 



The effect on amount caught is another matter. 

 Total catch of all sizes, both in numbers and in 

 weight, is given in Table 1. We obtained catch in 

 weight by converting the number caught in each 

 length category to weight using the length- 

 weight relationship of Clemens (1961) and then 

 by summing over length categories. The effects 

 are summarized in Tables 2 and 3 where the 

 change from nominal catch for each fleet is given 

 for each experimental treatment. By far the 

 largest effect of a change in effort of any fleet is 

 the eflect on its own catch. A doubling of the bait- 

 boat eflbrt causes the largest between-fleet effect, 

 which is a 7.5% depression of the longline catch in 

 weight, a loss of approximately 700 t (Table 3). A 

 similar loss to the baitboat fleet, due to doubling 

 of U.S. eff'ort, is only a 1.3% decrease in the bait- 

 boat catch (Table 3). 



We tested the sensitivity of our results to the 



707 



