POPULATION* HETEROGENEITY IN' PACIFIC PILCHARD 



209 



indicator, as suggested for vertebral counts in 

 European pilchard (Ruivo 1950) to separate 

 homogeneous populations of certain fishing areas 

 without implying genetic significance. The re- 

 sults of the covariance tests apply only to mean 

 growth curves, and at present it seems probable 

 that if significant difference were found in the 

 growth characteristic k in certain individuals or 

 segments of the population it would indicate 

 genotypic difference. 



There is some evidence thai deviations from 

 the straight-line regression for each area are 

 greater for mean observed lengths of a year class 

 in successive seasons of catch than for mean 

 calculated lengths of a year class in one season. 

 To compare transformations of mean observed 

 lengths with those of mean calculated lengths is 

 rather difficult since observed lengths arc not 

 obtained until the scales have one ring, or the fish 

 are in their second year of life, when they have 

 accomplished much of their growth: whereas, 

 calculated lengths begin at I -ring, and this first 

 growth increment as already noted shows great 

 variability. In San Pedro growth curves for 

 the 1942 class, however, variance of mean observed 

 lengths alone about the regression is 10 times the 

 variance for calculated lengths only. Such ir- 

 regularities suggest that greater shifts in popula- 

 tions of a given year class may occur from season 

 to season than within one season. Small devia- 

 tions from the transformations of mean calculated 

 lengths of the 4-, 5- and 6-ring pilchard of each 

 year class indicate that within a single fishing 

 season the populations at each port are more 

 homogeneous. 



North-south migration of larger fish 



Tagging of fish along the Pacific coast by some 

 of the agencies engaged in pilchard research 

 gives evidence of extensive migrations (Hart 

 1943a). It is the larger fish within a group tagged 

 in the south that are caught first farthest to the 

 north (Clark and Jansseo 1945, pp. 19-20). 

 That some of the larger pilchard cover great 

 distances is likewise indicated in the apparent 

 shift at older ages in central California and San 

 Pedro toward the level of regression of northern 

 growth types as shown in transformations of the 

 9-year averages of observed lengths (fig. 1). 



If, however, the migration course for the entire 

 population were of great seasonal regularity from 



south to north and return, one would expect with 

 increasing age of fish a tendency toward homo- 

 geneity of growth types along the coast. If 

 populations of older fish became more homogeneous 

 in their growth characteristics, a given year class 

 a i 5- or 6-rings would show a tendency for the 

 level of northern and southern growth types to 

 shift toward a single norm of oceanic migratory 

 sardines common to all areas of the fishery, or 

 at least to shift toward a level and slope, other 

 than the one consistently associated with a 

 particular area of catch. This shift does not so 

 far occur in transformations of mean calculated 

 length data. At 1-, 5-, and 6-rings the levels of 

 these regressions remain distinct for northern 

 and southern sardines (cf. fig. 4). 



Complete intermixture and homogeneity in 

 populations of adult pilchard in different regions 

 of catch is not evidenced from available data on 

 mean calculated lengths. The distinct levels of 

 the growth transformations which are maintained 

 in northern and southern populations, however, 

 do not controvert the evidence from tagging thai 

 there is migration of larger lisb toward tin' north. 

 As is shown from t :i ltlti 1 1 lt results, growth data also 

 indicate migration of northern pilchard into south- 

 ern waters. That larger lisb of older ages enter 

 the southern catches is demonstrated not only 

 from the 9-year averages of observed length 

 data (fig. 1). but likewise for a single year class 

 as it passes through the fishery (cf. table I), 

 for San Pedro the increase in mean observed 



lengths for certain individual year classes at 5- 

 ring and o|,|er is reflected in a sharp rise at this 

 stage in their length-on-time curves of growth. 

 Similarly, an apparent departure from an ex- 

 pected asymptote of length is observable in 

 Phillips' (1948, p. 7) curve of average observed 

 length of sardines at each age over a 6-year period 

 at San Pedro. Such irregularity in the San Pedro 

 growth curves must be explained by an influx 

 onto southern fishing grounds of large, old fish 

 differing in their growth pattern from the smaller 

 lisb up to 4-ring age which are caught, in San 

 Pedro. Conversely, in certain other year classes, 

 there has been an actual decrease in observed 

 lengths as a year class reached older ages and 

 was caught off San Pedro. This indicates an 

 influx of smaller fish of more southern growth 

 characteristics onto these grounds. The extent of 

 the seasonal north-south migration and the 



