KJELSON and JOHNSON: FEEDING ECOLOGY OF POSTLARVAL PINFISH AND SPOT 



First, mouth size usually increases as the length 

 of larvae increases. This relationship has been 

 documented for larval fish of various species by 

 Marak (1960), Blaxter (1965), Ciechomski (1967), 

 Detwyler and Houde (1970), and Shiroto (1970). A 

 few body measurements of pinfish and spot post- 

 larvae showed that the gape of the mouth in- 

 creased as the size of the fish increased. Pinfish of 

 16 mm TL were estimated to have a mouth gape 

 of 1.43 mm, while spot of 1.6 mm had a gape of 

 1.70 mm. The larger gape in spot may explain, in 

 part, their consumption of larger prey. Sec- 

 ondly, swimming speed also increases with an in- 

 crease in the fish's body size (Houde 1969; 

 Hoagman 1974); hence, the large spot may be 

 capable of capturing larger copepods. 



Although this study emphasized the food size 

 preferences of postlarval pinfish and spot, a topic 

 of potential importance in the selective nature of 

 larval fish feeding deals with the selection of 

 specific species of copepods. We did not compare 

 the copepod taxa in the digestive tracts with 

 those found in the plankton tows, but such effort 

 should provide valuable information, because 

 copepod species differences in swimming speed, 

 vertical position in the water, and aggregation 

 behavior may be very important in determining 

 the type of prey available to and finally consumed 

 by larval fish. However, the dominant genera 

 present in the estuary during the study period 

 were Centropages, Temora, Acartia, and Euter- 

 pina, common forms in the Beaufort area during 

 winter and early spring (Thayer et al. 1974). 

 Marak (1960) and Ciechomski (1967) attempted to 

 assess the selectivity of larval fish for individual 

 species of copepods, but did not observe any such 

 preferences. 



The size differences in spot and pinfish that we 

 observed in the Newport River estuary may be 

 due either to dissimilar spawning times, different 

 growth rates, or both. Observations made in 

 another North Carolina estuary (the White Oak 

 River estuary) during 1969 indicated that es- 

 tuarine spot and pinfish larval populations dur- 

 ing January and February differed in size and 

 that spot were significantly larger than pinfish 

 (R. M. Lewis, pers. commun., Atlantic Estuarine 

 Fisheries Center, Beaufort, N.C.); spot average 

 18.0 mm in length while pinfish were 15.5 

 mm. Thus, there appears to be consistancy in the 

 size differences observed in these two species dur- 

 ing their influx into North Carolina estuarine 

 waters. 



Evacuation Rates 



Regression coefficients for the equations de- 

 scribing the evacuation of copepods by larval 

 pinfish and spot are shown in Table 4. The 

 coefficients differ significantly from those calcu- 

 lated earlier (Kjelson et al. 1975). Copepod evacu- 

 ation in our previous study was determined using 

 fish collected in the estuary, placing them in a 

 food-free environment, and observing evacuation. 

 Those fish contained limited amounts of food at 

 the beginning of the experiments apparently due 

 to a low rate of feeding just prior to capture. Also, 

 there was a 2°C difference between estuarine and 

 laboratory water temperatures, and this may 

 have altered the evacuation rates. 



In an effort to measure the evacuation through 

 a wide range of gut quantities and thus, hope- 

 fully, achieve a better description of evacuation, 

 our present study used fish that initially had 

 their guts full of copepods (21-57 copepods/fish) as 

 determined from sacrificing 20 fish of each species 

 at the beginning of the experiment. In addition, 

 the possible stress of transport and rapid temper- 

 ature changes in the earlier study were elimi- 

 nated by using fish that had been acclimated to 

 laboratory temperatures and that were fed in the 

 laboratory. 



The regression coefficients (slopes) achieved 

 from our present study (Table 4) were sig- 

 nificantly different from and approximately twice 

 those found during the 1972-73 evacuation exper- 

 iments. We consider the estimates of evacuation 

 rates in the present study to be more representa- 

 tive of natural evacuation because the techniques 

 used in measuring evacuation were more refined 

 than in the earlier study. 



The experimental temperatures, although dif- 

 ferent for the two species, were within the normal 

 range for larvae immigrating into North Car- 

 olina estuaries. The larger negative slope in 

 the regression model for spot compared with that 

 for pinfish (Table 4) is probably due in part to the 

 temperature differences (12°C for pinfish and 



Table 4. — Linear regressions describing evacuation of copepods 

 in pinfish and spot larvae. Y =A + Bt where Y = logio(l + mean 

 number of copepods per larva) and t = hours since feeding, n = 

 number of data points. 



•P<0.01 



429 



