KJELSON and JOHNSON: FEEDING ECOLOGY OF POSTLARVAL PINFISH AND SPOT 



tions and to the stimulation of food grasping 

 activity caused by the increased movement of 

 food in a current. 



The highly variable nature of spot feeding in the 

 laboratory also may be explained by the varied 

 current conditions within the tank itself, although 

 conditions were kept as constant as possible dur- 

 ing the two studies. Current flow may not have 

 been uniform throughout the tank, although the 

 importance of this factor upon feeding is unknown. 

 The larvae in all experiments were distributed 

 throughout the tank and did not appear to be feed- 

 ing at specific locations. The low variability in 

 feeding rate between individual fish in each exper- 

 iment, as shown by the standard errors (Table 2), 

 suggests that all individuals were feeding at a 

 similar rate even though they were dispersed 

 throughout the tank. The distribution of flow 

 across the tank vertically was not measured, al- 

 though such information would be useful (Ryland 

 1963 ). The two treatment groups of spot postlarvae 

 were from separate field collections which may 

 have altered their behavioral characteristics 

 sufficiently to produce the variable results. 



Finally, the apparent necessity for low current 

 velocity for feeding to take place may restrict 

 considerably the amount of area suitable for feed- 

 ing to be successful. This may be particularly true 

 along the channels linking the oceanic habitat to 

 that of the estuarine marsh system where our ob- 

 servations took place. The amount of protected 

 shoreline and bottom habitat characterized by low 

 current velocity along these channels is very lim- 

 ited compared to that present in the broad 

 reaches of the estuary where cordgrass (Spartina) 

 marsh shoreline and eelgrass {Zostera) beds are 

 extensive. 



Temporal Variation in 

 Food Consumption 



Considerable day-to-day variation was observed 

 in the mean number of copepods in the plankton 

 and in the larval fish collected at midday (Figure 

 1). Mean pinfish gut contents ranged from 0.4 to 38 

 copepods/fish while spot contained from 0.5 to 24 

 copepods/fish. The coefficient of variation for the 

 number of copepods per fish in single field samples 

 averaged 20% (range 7-40%) for pinfish, and 17% 

 (range 8-40%) for spot. The greatest variability 

 occurred when the average gut contents were low. 

 Copepod density also fluctuated widely fi'om 477 to 

 3,262 copepods/m^. These densities are not dis- 



z o 



z i 



< a. 



i 8 



■^ " 3 000 

 O I 



I 2 JOOO 

 z o 



i S 'OOO 



V 



21 26 



JANUARY 



IS 20 



FEBHUABT 



Figure l. — Variation in the numbers of copepods per larval 

 pinfish and spot, and copepods per cubic meter based on mid- 

 day samples in the Newport River estuary during January and 

 February 1974. 



similar from those observed during the same 

 months in the open waters of the Newport River 

 estuary (Thayer et al. 1974). The coefficient of vari- 

 ation of the copepod counts from five tows at the 

 site of larval collections was 24%. Such variation 

 is not high for field sampling and although it rep- 

 resents the variability for only a single sampling 

 date, it does suggest that the precision of the esti- 

 mate of copepod density is acceptable. 



One of our goals was to determine if the amount 

 of food present in larvae was related to copepod 

 density. In our study, the correlation coefficients 

 between copepod concentration and gut contents 

 were very low (r = +0.08 for both pinfish and spot), 

 indicating that there was no relationship. Other 

 studies on larval fish populations have shown that 

 feeding incidence may be correlated with food con- 

 centration (Berner 1959; Nakai et al. 1966; Bain- 

 bridge and Forsyth 1971), while Houde (1967) 

 found no correlation between copepod abundance 

 and feeding rate by larval walleye. 



The number of factors influencing larval feeding 



427 



