LAURS and LYNN: SEASONAL MIGRATION OF THUNNUS ALALUNGA 



1972 Fishing Season 



NORTH OF 

 LIFORNIA 



l0 r 1972 Fishing Season 



CALIFORNIA 



45 50 55 60 65 70 75 80 85 



FORK LENGTH IN CENTIMETERS 



FIGURE 12. — Size composition of albacore caught by U.S. 

 fishermen during 1972 north of California (upper), off California 

 (middle), and size composition of albacore caught during the 

 1972 National Marine Fisheries Service- American Fishermen's 

 Research Foundation offshore survey (lower). 



(Figure 13) show that albacore initially appeared 

 offshore near the end of May and there was an 

 abrupt increase in relative abundance in the be- 

 ginning of June. A decline in relative abundance 

 was observed offshore after 19 June as centers of 

 abundance shifted to nearshore where there was 

 an increase in early July. Within these overall 

 trends, changes in each of the three modal size 

 groups can be followed. The mid-size modal group 

 (fork length centered about 67 to 69 cm), initially 

 dominated early offshore catches and then di- 

 minished in relative importance. It formed almost 

 the entire catch of the first nearshore catches and 

 continued to dominate nearshore catches into 

 July. The large-size modal group (fork length cen- 

 tered about 82 to 85 cm) showed similar trends: a 

 rise and fall in relative abundance offshore and 

 with a subsequent shift to nearshore, but lagging 

 behind the mid-size modal group by one 10-day 

 period. The small-size modal group (fork length 

 centered about 52 and 53 cm) was dominant 

 offshore after 10 June but made little appearance 

 in the nearshore region during the survey. This 

 size group subsequently entered the nearshore 

 fishery, however, as is evident from the size com- 

 position of the 1972 fishery off California (Figure 

 12 middle). An additional geographic division in 



WEST OF I30°W 



die) and hence different from the size composition 

 of fish taken in the commercial fishery north of 

 California ( Figure 12 upper). It appears, then, that 

 albacore caught in the offshore region of high rela- 

 tive abundance south of lat. 35°N in 1972 were a 

 part of the migration of fish that reached regions 

 off California. 



We interpret the findings concerning offshore- 

 nearshore and north-south geographic variations 

 in size composition as supporting the hypotheses 

 1) that the fish which compose the fishery off 

 California are separate from those which make up 

 the fishery off the Pacific Northwest, and 2) that 

 these two groups offish follow different migration 

 routes into nearshore waters. 



Movements of Albacore by Size Groups 



The size composition data for the 1972 charter 

 vessel catch were stratified into offshore and near- 

 shore regions at long. 130°W and into one 8-day 

 and four 10-day time periods. Graphs of the strat- 

 ified data standardized by fishing effort for 1972 



I30°W 

 I 



45 50 55 60 65 70 75 80 85 90 95 100 



MAY 31- £ 

 JUNE 9 



JUNE 10- £ 

 JUNE 19 



45 50556O6S707580859095O0 



EAST OF I30°W 



[ NO CATCH ) 



(NO CATCH I 



JUNE 20- £ 

 JUNE 29 - 



A 



sk^^t^ 1 ^- 



45 50 55 60 65 70 75 80 85 90 95 100 



JUNE 30- g 



july 9 ; 



(NO EFFORT 1 

 (NO CATCH I 



FORK LENGTH (cm) 



,00 1 5552 L-H 



45 505560 65 707580 85 9095-00 



60 65 70 75 80 85 90 95 KJC 



FORK LENGTH (cm) 



FIGURE 13. — Size composition of albacore caught by American 

 Fishermen's Research Foundation charter vessels in 1972 by 

 time periods and east and west of long. 130 C W. 



815 



