FISHERY BULLETIN: VOL. 73, NO. 1 



of unfed larvae of each species were offered 

 identical concentrations of Artemia nauplii. One 

 group was handled roughly to represent the 

 physical stress associated with field capture, 

 while the other group was handled gently. The 

 roughly handled fish were chased around the tank 

 with a dip net for 10 to 30 s, captured with the 

 net, allowed to suffocate in air, and then dissected. 

 After feeding, the other fish were anesthetized 

 by carefully adding an aqueous solution of 

 MS-222 to the tank and then were dissected 

 immediately to determine the numbers of 

 nauplii in their digestive tracts. The roughly 

 handled menhaden had only 40 to 52% of the 

 Artemia numbers present in the guts of the 

 gently handled menhaden (Table 4). The loss of 

 food in menhaden larvae probably was due to 

 the stress-related defecation or regurgitation and 

 thus, may explain the consistently low quantities 

 of food observed in larval menhaden captured 

 in the estuary. Roughly handled pinfish and 

 spot larvae showed no significant decrease in gut 

 contents (Table 4). The curved digestive tract 

 of larval spot and pinfish may prevent rapid 

 passage of food, while the straight tubelike gut 

 of menhaden may permit easy loss of food. This 

 gut shape difference may account for the dif- 

 ferences we observed. 



A separate experiment was conducted to deter- 

 mine if the technique used to kill menhaden 

 larvae in the handling experiments (exposure to 

 air and suffocation versus anesthesia with 

 MS-222) influenced the amount of food remaining 

 in the gut. No difference was found. Fish killed 

 by suffocation had a mean of 19 Artemia nauplii/ 

 fish (SE = 4.7), while fish anesthetized with 

 MS-222 had a mean of 20 Artemia nauplii/fish 

 (SE = 4.2). 



EVACUATION RATES 



Estimated regression coefficients for the equa- 

 tions describing the evacuation of copepods and 

 Artemia nauplii are provided in Tables 5 and 6. 

 Certain factors may alter the reliability of our 

 estimates of evacuation rate under natural 

 estuarine conditions. Bias may result from the 

 temperature difference between estuarine waters 

 from which fish were captured (14°-15°C) and 

 the aquaria temperature during evacuation 

 experiments (16°-17°C). The effect of a 2° tem- 

 perature change on evacuation rate of larvae 



Table 5. — Linear regressions describing evacuation of copepods 

 in Atlantic menhaden, pinfish, and spot larvae. Y = A + Bt 

 where Y = logio (1 -i- mean number of copepods per larva) and 

 t = hours since feeding, n = the number of data points. 



is unknown, although a similar change signi- 

 ficantly increases the evacuation rates in some 

 juvenile marine fish (Peters and Kjelson in press). 

 Although our regression model could probably 

 be improved, the r^ values (Tables 5, 6) indi- 

 cate the model is reasonable. Initial analysis 

 included data collected until all the fish were 

 empty. This resulted in nonlinearity near the 

 end of evacuation due to bias near the end of 

 evacuation period where more weight was given 

 to the slower evacuating fish. Thus, by including 

 in the regression analysis data from only those 

 samples in which at least half of the larvae 

 contained some food, this bias was decreased 

 and the linear regression model appeared to 

 represent larvae evacuation adequately. 



INFLUENCE OF HANDLING AND 

 CAPTURE ON EVACUATION 



Evacuation experiments using Artemia nauplii 

 were performed to determine if handling and 

 capture influenced the rate of evacuation. Each 



Table 6. — Linear regressions describing evacuation of 

 Artemia nauplii in Atlantic menhaden, pinfish, and spot larvae 

 under varied handling conditions. Y = A + Bt where Y = log jq 

 (1 + mean number of Artemia per larva) and t = hours since 

 feeding, n = the number of data points. 



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