POPULATION HETEROGENEITY IN PACIFIC PILCHARD 



219 



Tlio even-growth pattern of Hessle corresponds 

 to the southern growth type of the Pacific sardine 

 with a high k value, or slow deceleration in growl li. 



CONTRACTION OF FISHED STOCKS 



Recent catch data may l>c pertinent also to the 

 problem of populations. The abrupt decline in 

 landings of pilchard in recent seasons reached a 

 low of 1:50,000 tons in 1947-48, the lowest in the 

 history of the fishery on the Pacific coast since 

 192.3-24 (Anonymous 1948). To account for 

 this decline, a number of explanations have been 

 offered by pilchard investigators, all of which 

 may affect in varying degrees the fished popula- 

 tion^): (1) Lowered recruitment of young fish, 



(2) reduced availability of fish to the fisherman, 



(3) increased natural mortality, and (4) increased 

 fishing mortality. 



The decline in catrlies in central California and 

 in the Pacific Northwest have been almost entirely 

 responsible for the sudden decline in total catch 

 of the coast as a whole. In all these ports there 

 has been poor fishing since the 1945-40 season and 

 in the Northwest there were no landings dining 

 the 1(149 and L950 seasons. 



In southern California, however, the total 

 catch did not fall off conspicuously up to 1950-51, 

 and in this season San Pedro recorded the largest 

 tonnage of sardines ever taken there. The catch 

 per unit-of'-effort did fall in 1947 48 (Clark and 

 Daugherty 1950, 1952). but for no other recent 

 season has it differed greatly from previous aver- 

 age lunar months. Concentration of sardines on 

 southern grounds has been used as argument l>\ 

 representatives of the industry to obtain legal 

 extension of Monterey grounds 70 miles to tin' 

 south (Monterey Herald, May 14, 1951). 



Contraction of areas of good fishing and ap- 

 parent contraction of spawning areas may be 

 dependent on hydrographic changes, and cent cl- 

 ing of the available population in the southern 

 part of the range thus may have produced a 

 series of southern year classes. 



DISTINGUISHING SOUTHERN AND 

 NORTHERN YEAR CLASSES 



To detect early in its life in the fishery a year 

 class that is likely to be primarily southern, sonic 



measure of its relative strength in northern and 

 southern areas of catch is desirable. One measure 

 of strength of a year class in the fishery has been 

 the numbers of pilchard in the 3-ring stage caught 

 at all ports (Walford 1946b). Although avail- 

 ability and degree of recruitment at earlier ages 

 arc more variable, another measure of early 

 strength of a year class in any region is the number 

 of 1-ring or 2-ring fish caught at any port. At this 

 age presumably, migrating pelagic sardines would 

 be less widely dispersed throughout their possible 

 habitat and nearer their point of origin. 



San Pedro is considered the port nearest centers 

 of spawning. Early strength of a year class at 

 this port may be taken as a measure of spawning 

 success in the south. Figure 9 shows ratios of 

 catch per boat-month (using California catch- 

 per-unit data, Clark and Daugherty 1950) at 

 San Pedro to catch per boat-month 3 for the coast 

 as a whole for 1-ring (dot) and 2-ring (circle) 

 sardines of each year class over the period of 

 comparable sampling (1941-49). The year-class 

 strength for the Pacific coast as measured by catch 

 per boat-month at the 3-ring stage is also figured 

 (symbol x). (There is rather close correspond- 

 ence between year-class strength at 2-ring ami al 

 3-ring. For the 1047 year class, the number 

 caughl at 2-ring is figured, by symbol -• , as an 

 approximation of its probable strength in the 



following season.) 



When the ratio of San I'edro 2-ring lish In the 

 Pacific-coasf total was low, as in the 1939 year 

 class, the total year-class strength for the whole 

 coast was well above average. The L940 year 

 class was slightly above average strength; (lie 

 ratio San I'edro to Pacific coast was somewhat 

 above 1.0 at l-ring, somewhat greater at 2-ring. 

 The 2-ring age group usually dominates at San 

 I'edro. The 1941 class, however, was the domi- 

 nant age group as 3-ring at San Pedro in 1944-45, 

 showed a high ratio to the rest of (he coast, was 

 somewhat below average in year-class strength. 

 and was relatively below normal numbers in 

 northern ports throughout its life in the fishery. 

 By these criteria, the 1939 year class may be 

 called a successful year class with early strength 

 in the. northern fishery, the 1941 class relatively 



3 When estimates of total numbers of sardines caught, by year class, are 

 used instead of those adjusted for fishing effort the results corresf d closely. 



