According 'to Lgvo-S (1929), pilchards feed on diatoms and dinoflagel- 

 lates, as v/ell as upon copepods and othdr zooplankton. Moreover, the amount 

 of food ingested is positively correltited with the abundance of surface 

 plankton in the surrounding water. This in turn is controlled by tempera- 

 tures j "low temperatures associated virith upvelling of subsurface v/aters 

 favor the grovrth of diatoms, which in turn attract the sardine" (lewis, 

 op. cit., p. 179). This being true, it is reasonable to expect grovrth 

 in abundance of diatoms to favor growth of pilchards. 



For study of this question. Dr. \f. E. Allen has kindly made available 

 statistics on relative diatom abundance, which he has observed for several , / 

 years at the end of the Scripps Institution pier at La Jolla and at Heuneme.-' 

 Numbers of all species of diatoms per liter per vreek were svimned by months; 

 and the monthly totals siiramed in various groupings of months. No significant 

 correlation bet^reen anomalies in these totals and anomalies in growth was 

 found, no matter what grouping of. months v^as used. Hov;^ever, the niunber of 

 weeks in the year that the count of diatoms per liter exceeded 1,000, or 

 2,000 or 5,000, gave measures 6f diatom persistence , all of which shov/ed a 

 positive correlation v/ith growth. As with the temperature and salinity data-, 

 these measures were summed by various combinations of months. The most satis- 

 factory colrrelation was given by the niomber of weeks between June and the 

 following May, inclusive, that the count exceeded 2,000 per liter. Devi- 

 ations from the norm of that statistic taken over the period 1932-1938 are 

 given in the seventh curve of figurc'6. The period used, June through the 

 following May, lags two monthSv^behind that used for \7ind data, and may in- 

 dicate the time required for the actich set up by the northiifest wljids to 

 affect the supply of diatoms. 



Although the abundance of diatoms thus appears to affect grovfth directly, 

 it is a sustained abimdance of these plants above a certain minimal level 

 that is most effective in promoting growth, rather than sporadic flowerings 

 of great abundance interspersed with periods of scarcity. 



The relations betvfeen grcftvth and other factors shovm in figiire 6 might; 

 not necessarily mean that pilchards normal]^ spend the grov^ing part of their 

 first year of life near the locality where these data were collected. How- 

 ever, second and later growth increments v/ere found not to be correlated 

 with wind force, surface salinities, and temperatures measured at La Jolla. 

 This may be explained in part by the fact that pilchards migrate successively 

 greater distances as they grow larger, the extent of their migrations prob- 

 ably being a function of size more than of age. Consequently, the history 

 of members of the same year class becomes less uniform as the fish grow older, 

 though they probably never leave the Pacific coast save for soavirard migrations 

 of not more than 300 miles. This is all consistent with the conclusion that 

 the bulk of pilchards in a year class do normally spend the first year of 

 life in or neaf southern California, and that the subsequent history of the 

 fish becomes so diverse that year-round conditions in any one locality can 

 no longer reflect growth among the whole population of that year class. 



-' For the method of sampling, see Allen, 1936. 



114 



