KJELSON ET AL.: FEEDING ECOLOGY OF POSTLARVAL FISHES 



Prey size is an important factor in determining 

 the individuals selected by planktivorous fish 

 (Ivlev 1961; Brooks and Dodson 1965; Kjelson 

 1971). The larval fish we studied appeared to 

 restrict the majority of their feeding to items 

 of a size ranging between 300 and 1,200 jum. 

 Our observations of the mean length of ingested 

 copepods showed that the larger menhaden 

 larvae (26-31 mm, x = 29 mm TL) ingested 

 750-fj.m copepods with an estimated copepod wet 

 weight of 0.04 mg, while the smaller spot (17- 

 22 mm, x = 19 mm TL) and pinfish larvae (16- 

 20 mm, X = 18 mm TL) fed upon 600-/jm copepods 

 with an estimated wet weight of 0.03 mg. Small 

 zooplankters such as copepod nauplii, barnacle 

 larvae, or small adult copepods such as Oithona 

 (all present in the plankton tows) were rarely 

 found in gut contents. Copepods larger than 

 1.2 mm were in the plankton, but were rarely 

 consumed. Perhaps copepods were the only food 

 items of the appropriate size present in sufficient 

 abundance. Had we collected smaller larvae, it is 

 possible food preferences may have been for 

 smaller food items such as copepod nauplii and 

 copepodites and adults of small-sized species as 

 well as phytoplankton. May (1970) stressed the 

 fact that larval fish require progressively larger 

 prey as they grow. However, since larvae smaller 

 than the size we collected are rarely found in 

 the Newport River estuary, we feel our data 

 indicates that smaller planktonic forms are rela- 

 tively unimportant to the larval fish studied in 

 this estuary. 



Thayer et al. (1974) found that as a yearly 

 average, copepods represented 81% of the zoo- 

 plankton numbers and 85% of the zooplankton 

 biomass retained by a No. 10 mesh plankton 

 net. Since larval fishes enter the Newport River 

 estuary during winter and spring, the con- 

 sumption of copepods by these three larval species 

 may, in part, explain the decrease in copepod 

 abundance observed by Thayer et al. (1974) dur- 

 ing this period. They noted that the four copepod 

 taxa utilized by these larvae decreased from a 

 mean of 81% of the copepod biomass during March 

 1970 and 1971 to a mean of 48% of the biomass 

 during the summer. 



FEEDING CHRONOLOGY 

 AND INTENSITY 



All three larval fishes had the highest food 

 content in their digestive tracts during daylight 



hours (Figures 1-3). Periodicity of gastrointestinal 

 contents indicates that each population begins 

 feeding near dawn and reaches a maximum gut 

 fullness near midday. The rapid single increase 

 in the gut content of the three species indicates 

 they have one major burst of feeding activity 

 per day (Figures 1-3). Other studies (Blaxter 

 1965; Schumann 1965; Braum 1967; June and 

 Carlson 1971) have shown that larval fish 

 generally do not have food within their digestive 

 tracts when captured at night, suggesting that 

 larval fish do not feed at low light intensities. 

 Considerable variation was observed in the 

 amounts of food present in larval guts (Figures 

 1-3). The variation is probably due to differences 

 in prey abundance or capture and handling 

 techniques, although other factors such as fish 

 size and copepod size may also be important. 

 During our 24 h sampling the variation in 

 numbers of copepods in individual fish was high 

 at some times and low at others. The ratio of 

 the standard error of the estimate to the mean 

 varied from 4 to 48% for spot with a mean of 

 21%; for menhaden the ratio varied from to 

 100% with a mean of 40%' ; and for pinfish it varied 

 from to 100% with a mean of 43%. Spot larvae 



•- 40 



o 



t 35 



2 30 



a. 

 </» 



Q 



o 



2; 25 



a. 

 O 

 u 



O 20 



i '* 



z 

 < 



u> 



s \0 



0400 0800 1200 1600 2000 2400 0400 

 TrME OF DAY 



Figure 1. — Variation in diel cycle of gastrointestinal contents 

 in postlarval spot. 



139 



