FISHERY BULLETIN: VOL. 74, NO. 2 



rates in an estuary are undoubtedly numerous; 

 therefore, it may be difficult through field mea- 

 surements to establish a relationship between 

 larval feeding rates and food abundance. For 

 example, the clumped distribution typical of zoo- 

 plankton populations may affect larval feeding 

 rates, with feeding limited primarily to those 

 periods when the fish are exposed to a dense patch 

 of copepods. 



Comparing naturally occurring mean food 

 densities with mean gut contents, to establish a re- 

 lationship between prey abundance and feeding 

 rate, presents problems if the zooplankton popula- 

 tions are not randomly distributed or if the fish 

 collected were not feeding upon the same prey 

 community sampled by the plankton net (O'Brien 

 and Vinyard 1974). Furthermore, the aggregation 

 of zooplankton discussed earlier may be important 

 in determining the rate of food consumption 

 (Schumann 1965). Ivlev (1961) indicates that 

 patchiness in the distribution of the food material 

 increases the ration by comparison with an even 

 food distribution when the average concentration 

 is the same in both cases. High consumption rates 

 by postlarval pinfish and spot may be possible only 

 when patches of copepods come within the feeding 

 range of the larvae. This hypothesis is discussed 

 by Murphy (1961). The above remarks emphasize 

 that laboratory investigations may be required in 

 understanding the relationships between feeding 

 rates and food abundance. 



Size Related Food Preferences 



Various investigators have observed selective 

 feeding by larval fish and, at times, definite pref- 

 erence for a specific food form is indicated. Much 

 of the selectivity, however, is due to the size rela- 

 tionship of the larval fish and the available 

 zooplankton (Marak 1960). Information gained 

 from our midday field samples and our laboratory 

 evacuation experiments enabled us to observe the 

 relationship between fish size and the size of prey 

 they consumed. The Wilcoxon test for paired val- 

 ues (Alder and Roessler 1964) was used to deter- 

 mine if the mean size of copepods consumed was 

 significantly different {a = 0.05) from those 

 collected in the plankton tows or provided in 

 aquaria. 



The spot collected for both field and laboratory 

 studies were significantly larger than the pinfish. 

 Both field and laboratory results indicated that 

 pinfish larvae always ate smaller copepods than 



the mean size available to them while the reverse 

 was true for spot (Table 3). Each species con- 

 sumed prey that were proportional to their size 

 with the ratio of the mean copepod length to the 

 average fish length approximately 1:35 based 

 upon laboratory measurements to 1:30 based 

 upon field data. 



The above results suggest that, as the larval 

 fish size increases, the size of the consumed prey 

 also increases. Many researchers (Blaxter and 

 Holliday 1963; Blaxter 1965; Ciechomski 1967; 

 Detwyler and Houde 1970; de Mendiola 1974; 

 Marak 1974) also have observed this relationship 

 in a variety of larval fishes. However, the mean 

 size consumed in each study by either pinfish or 

 spot varied considerably (Table 3 ). Pinfish of simi- 

 lar mean sizes (16 and 16.4 mm) fed upon 590-/u.m 

 copepods in the laboratory, but the 460-/u,m prey 

 in the field. This difference in prey size may be 

 explained by the apparent difference in the prey 

 sizes available to the fish in the two studies; 

 laboratory prey had a mean size of 663 ixm while 

 those in the field were only 515 ^tm. Spot size 

 preferences, on the other hand, are difficult to ex- 

 plain in the same manner, because spot consumed 

 larger prey in the laboratory than in the field, 

 although the prey available in the laboratory 

 were considerably smaller than those present in 

 natural waters (Table 3 ). 



Table 3. — Mean sizes of copepods eaten by larval pinfish and 

 spot in the field and laboratory compared to the mean sizes of 

 copepods present. 



Comparisons of mean size prey from plankton 

 tows to those from gut contents may be difficult 

 again due to distributional dissimilarities of both 

 predator and prey populations during feeding and 

 prey aggregation patterns (Schumann 1965; 

 O'Brien and Vinyard 1974). However, these prob- 

 lems were lessened in laboratory aquaria where 

 we were able to control the size, density, and 

 distribution of the predator-prey populations. 



Two primary factors appear to explain the in- 

 crease in prey size as larval fish size increases. 



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