POPULATION HETEROGENEITY IN PACIFIC PILCHARD 



211 



McHugh's results on vertebral counts thus agree 

 with Clark's; and he concludes (p. 110)— 



Since the number of vertebrae drops very sharply with 

 decreasing latitude off Baja California, it is assumed that 

 these fish belong to a distinct race that has different tem- 

 perature requirements. 1 It is also concluded that the 

 number of vertebrae is not a particularly critical measure 

 of population segregation in pelagic spawning species. 



McHugh lint her suggests that — 



intermingling between populations may not be random 

 with respect to meristic or other characters. If this 

 is so, wandering of individuals, as shown by tag return-, 

 may not necessarily indicate movements of the population 

 as a whole. Experimental verification of these deduction- 

 is much needed . . . 



The apparent cline in growth characteristics 

 suggests the presence of intraspecific populations 

 in which there is limited intermingling. Rather 

 than a general coastwise migration pattern, a 

 series of overlapping coastal migrations of more 

 than one stock appears more consonant with 

 observed data on growth. 

 Growth characteristics determined by origin 



There is increasing reason to suppose that the 

 geographic origin of sardine populations may be 

 widely variable from season to season. Evidence 

 from spawning studies demonstrates that there 

 were, during 1949 and 1950, two centers of 

 spawning; one, early in the year, around and In 

 the south of Cedros Island off central Lower 

 California, and another, somewhat later and 

 farther offshore, oil' soul hern California. It is 

 also reported (California Cooperative Sardine Re- 

 search Program 1950, p. 37) that "In the waters 

 separating these two spawning centers very little 

 spawning has been encountered during either 

 season, and this little has been confined l<> a 

 coastal strip . . ." During these two seasons 

 there was little indication of northern spawning. 

 In 1940, however, an unusual abundance of young 

 fish of the 1939 year class was reported (Walford 

 and Mosher 1941) in north Pacific waters off 

 Oregon, Washington, and British Columbia. The 

 probable occurrence of northern spawnings of 

 lesser magnitude is suggested by the presence in 

 other seasons of small, presumably 1-ring, 

 pilchard in British Columbia waters recorded 



> From (lata obtained in the California Cooperative Sardine Research 

 Program in 1949 and 1950. the spawning requirements, so far as temperature 

 is concerned, have been closely similar in what at present appear to be the 

 two main centers of spafl nint 1 1) or! central Lower California and (2) south- 

 ern California. 



by Hart (1943b). The growth characteristics of 

 northern and southern populations may thus be. 

 early determined by their origins. 



^Vhether the populations sampled by northern 

 and southern fisheries have genetically distinct 

 components has not been demonstrated. The 

 significantly different levels of their mean growth 

 transformations may indicate only phenotypic 

 differences in growth ascribable to differing en- 

 vironmental conditions at time and place of 

 spawning or in later life or both. These differing 

 conditions of growth must largely be maintained, 

 however, in each area and in each population 

 sampled, since the mean levels of growth trans- 

 formations characteristic of each geographic region 

 have remained relatively constant and distinct as 

 shown from calculated length data for 4-. 5-, and 

 6-ring sardines of each of six year-classes over the 

 seasons of study. This appears to point to the 

 conclusion that there may be a southern spawning 

 center which contributes more heavily to the 

 southern California stocks and that although some 

 of the more southern growth types migrate into 

 central California, they are found rarely in the 

 Pacific Northwest. 



The existence of an area of intense spawning off 

 central Lower California was not realized until the 

 expanded sardine research program under the 

 California Marine Research Committee made 

 possible the colleelion of eggs and larvae ill ibis 

 spawning center as well as in the better known 

 area of more widespread spawning off southern 

 ( lalifornia. In both areas, times of spawning vary 

 between seasons and appear to be associated with 

 conditions of favorable temperature and upwelling. 

 (See California Cooperative Sardine Research 

 Program 1950. p. 39.) The varying importance of 

 the contributions of the two areas to the present 

 sardine fisheries is suggested in differing prevalence 

 of southern or of northern growth types in the 

 catches. It appears to be a reasonable hypothesis 

 that the spawning grounds off central Lower 

 California give rise to the southern component- 

 found in the catches off San Pedro (and to a lesser 

 degree in catches off central California), while the 

 large northern fish originate off southern Cali- 

 fornia and to the north in years favorable for 

 northern spawning. It is not yet known whether 

 spawning populations in the two main centers are 

 or are not distinct, i. e., genetically isolated in 

 space and time. 



258571—54— 



