ARTHUR: FOOD AND FEEDING OF LARVAL FISHES 



Size of Food 



Food particles of young anchovy larvae are not 

 selected from those near the largest ingestible size 

 as are those of young sardine larvae, though there 

 is a trend to increase particle size as larvae 

 increase in length. The correlation coefficients 

 (Figures 1, 3a) suggest that food size of sardines is 

 more controlled by larval size (0.813- or 66% of 

 variance explained) than of anchovies (0.4732 or 

 22% of variance explained). The extensive data, 

 including many older larvae, reported by Cie- 

 chomski (1967) and Rojas de Mendolia (1974) 

 indicate a sharp increase in food size between the 

 lan-al lengths of about 3 to 4 mm but relatively 

 little increase for most of the remainder of the 

 larval period. Rojas de Mendiola's data (Figure 

 3b), including food sizes of 2,088 feeding larvae 3.1 

 to 5.0 mm in length, are used to illustrate this 

 important point. These data indicate that food size 

 roughly doubles (from approximately 100 to 200 

 lim) while larvae grew from 4 to 16 mm. Assuming 

 that both larvae and food particles increased in 

 size isometrically, then their volumes increased by 

 the cube of their increase in length or width. Food 

 particles in doubling in width increased 8 times in 

 volume, while larvae increasing 4 times in length 

 increased 64 times in volume. Therefore, the 

 nutritional equivalent of a 200-/xm food particle to 

 a 16-mm larva is only one-eight of that of a 100-jum 

 particle to a 4-mm larva. Although Berner (1959) 

 measured the length rather than the width of food 

 particles and his data are not directly comparable, 

 they do indicate that while anchovy larvae in- 

 creased in length from 3 to 10 mm (an increase of 

 37 times in volume) their average food size in- 

 creased from 68 to 128 jum (an increase of only 6% 

 times in volume). 



Feeding Incidence 



Anchovy larvae also are daytime feeders 

 (Figure 4). The disparity between night and day 

 values for feeding incidence is greater for an- 

 chovies than for sardines during their youngest 

 larval stages. This difference perhaps is due to a 

 faster digestive rate for the anchovy. 



FOOD OF JACK MACKERAL LARVAE 

 Type of Food 



The jack mackerel larva first starts to feed when 



5 6 7 8 



LENGTH OF LARVAE (mm) 



Figure 4.-Diurnality of feeding incidence of northern anchovy 



larvae. 



it is about 3.25 mm long. By the end of the yolk-sac 

 stage, the jack mackerel has attained a robustness 

 which contrasts sharply with the slender early 

 larval sardine or anchovy. Its body shape, in 

 general, is more substantial and its mouth is 

 proportionately larger. No jack mackerel larva 

 was found with both yolk and ingested food 

 organisms. 



Just as for sardine and anchovy larvae, copepods 

 contributed the greatest bulk of its food (Table 3). 

 Eggs and naupliar stages, however, are much less 

 important. The "egg sacs," appearing under the 

 title of copepod eggs, were probably ingested 

 attached to adult copepods and so represented a 

 coincidental fraction of the food. Copepod nauplii 

 seemed to be significant in numbers only in larvae 

 of the smallest size group. 



Copepodid stages of copepods make up the bulk 

 of particulate food, increasingly so as the larva 

 grows older. By the time the larva is 7.0 mm long, 

 96.0% (by number) of its food is composed of 

 various species of copepods. The most significant 

 feature of the diet is the very high percentage of 

 occurrence of Microsetella norvegica, one of the 

 few planktonic species of harpacticoid copepods. 

 This probably represents a definite selection, as 

 this species of copepod, though ubiquitous, never 

 achieved numerical importance in our plankton 

 hauls. Jack mackerel (Trachurus trachurus) lar- 

 vae were reported by Sinyukova (1964) to have an 

 "inborn ability" to select two species of copepods 

 from the mass of plankton living in the Black Sea. 

 On the other hand, the respective behavior of the 

 early jack mackerel larvae and M. norvegica may 

 cause the two species to be locally aggregated, 



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