THEILACKER: FEEDING AND GROWTH OF NORTHERN ANCHOVY 



tinuous feeders, and at the high prey concentration, 

 high consumption rates reduced the gut residence 

 time and decreased digestion (assimilation; Tables 

 10, 11). At the lower prey concentration, the slower 

 digestion time and increase in assimilation was not 

 sufficient to compensate for the reduced consump- 

 tion, and daily increase in weight, 15%, was less 

 than the weight increase on the high-density diet, 

 21%. This result is similar to that in the recent study 

 on assimilation by Pacific herring larvae fed Brachi- 

 onus and Artemia (Boehlert and Yoklavich 1984). 

 Using radioisotope tracers, Boehlert and Yoklavich 

 found decreased assimilation at high food densities, 

 but overall the larvae had a greater total energy gain 

 at the high food densities because the higher food 

 consumption more than compensated for the de- 

 creased assimilation. 



Assimilation estimates, given here for northern 

 anchovy larvae, fed two rotifer diets, averaged 53 

 and 79% and are somewhat higher than rates of 

 39-68%, depending on prey density, given by Boeh- 

 lert and Yoklavich (1984) for Pacific herring. The 

 assimilation estimates for northern anchovy larvae 

 may not be reliable because the estimate assumes 

 that weight-specific metabolic rates of fish fed both 

 diets were equal. In addition, I made no attempt to 

 correct for activity or to partition the portion of food 

 assimilated into parts lost as feces and urine. Buck- 

 ley and Dillman (1982) developed a technique to 

 measure nitrogenous wastes of larval flounder. 

 Assimilation efficiencies of 65-75% are commonly 

 used in calculations of larval fish growth efficiencies 

 (Ware 1975). 



Estimates of gross growth efficiencies are not 

 compromised by the above concerns. Larval north- 

 ern anchovy gross-growth efficiencies increased 

 with age, indicating that an increasing fraction of 

 the calories consumed was translated into growth 

 (Tables 10, 11). It is unrealistic that growth effi- 

 ciencies would continue to increase. Brett and 

 Groves (1979) suggested that for older fish growth 

 rates become asymptotic with time. Growth effi- 

 ciencies given here (24-46%) are consistent with 

 growth efficiencies reported for other larval fish 

 species (14-41% in Theilacker and Dorsey 1980) fed 

 1,000 or more prey per liter; reported efficiencies 

 are high and extremely variable. 



Direct observations of larval stomach contents 

 showed that some rotifer-fed larvae fed at three 

 times the average rate (C^^^; Fig. 2). These fish 

 may be the successful ones that survive in the field. 

 Their high feeding rates would yield faster growth 

 through the vulnerable larval stage. Feeding inten- 

 sity also was highly variable for larvae eating cope- 



pods (Fig. 6), but the average number of copepods 

 eaten was less than the average number of rotifers 

 eaten (Table 2). 



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TIME(hours) 



Figure 6.— Number of copepod nauplii observed in stom- 

 achs of 4.0-5.0 mm northern anchovy fed 2 copepods/mL. 

 Each point is one larva. 



Northern anchovy raised on copepods ate mainly 

 Gymnodinium, augmenting their diet with copepod 

 nauplii (Table 12). The stomachs of 96% of the 5-d- 

 old larvae were fuR with Gymnodinium cells (Table 

 6). Fish were obtaining 60-90% of their daily caloric 

 intake from Gymnodinium. This is evident by com- 

 paring the daily caloric intake (FJ for day 5-6 lar- 

 vae of equal size or weight that were fed copepods 

 (Table 12) with those fed rotifers (Tables 10, 11). 

 Consumption of 2-4 cells/minute can account for this 

 energy input (4.2 x 10"^ call Gymnodinium cell 

 [Vlymen 1977]), and successful feeding acts of this 

 magnitude have been directly observed by Hunter 

 (1981). Lasker and Zweifel (1978) developed a model 

 (a modified version of Vlymen's [1977] model) to 

 describe survival at sea in areas of various concen- 

 trations and proportions of large and small prey and 

 concluded that large prey made very little contribu- 

 tion to survival of first-feeding larvae when suffi- 

 cient (40 mL) small prey were available. As observed 

 here, the ingestion of 10-20 copepod nauplii/day in 

 addition to small Gymnodinium cells resulted in a 

 growth rate of 0.35 mm/day, which is comparable 

 to a rate of 0.37 mm/day reported for wild north- 

 ern anchovy of similar age (Methot and Kramer 

 1979), and lends additional credence to Lasker and 

 Zweifel' s (1978) hypothesis that survival of northern 

 anchovy depends on patchy (layered) distributions 

 of small, abundant prey like Gymnodinium. 



For all diets, an equivalent number of small prey. 



225 



