HOSS ET AL.: METABOLIC RESPONSES OF SPOT AND ATLANTIC CROAKER 



< 



I 

 W 

 Ll. 



w 



UJ 



cr 

 o 



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o 



Figure 3.— Ad libitum feeding rate, growth rate 

 at ad libitum feeding rate, and routine oxygen con- 

 sumption for 4 mg dry wt spot at various 

 temperatures between 6° and 20°C. 



10. Op 

 80- 



60- 

 4 



20- 



10 

 8 

 6 



0.4 



0.2 



Spot 



• = AD LIBITUM FEEDING RATE 



A = GROWTH RATE AT AD LIBITUM FEEDING 



O - ROUTINE OXYGEN CONSUMPTION 



n 1' ^ ' 'I 



10 14 18 22 



TEMPERATURE (°C) 



transported from warm coastal waters into cold 

 estuarine waters. Atlantic croaker are capable of 

 enduring low winter temperatures with decreased 

 metabolic rates that allow for balanced energy in- 

 take. Spot, in contrast, show signs of thermal stress 

 manifested as increased respiration rate (at 10°C). 

 This increased metabolism along with no attendant 

 increase in feeding results in an energy deficit and 

 in eventual mortality of the larvae. Species specific 

 differences in the time of entry to the estuary serves 

 as ecological evidence supporting our contention 

 that spot are more susceptible to cold weather. Most 

 spot enter the estuary after the peak in Atlantic 

 croaker immigration and generally after the coldest 

 weather. 



Our findings have important implications with 

 respect to recruitment of estuarine-dependent fish 

 which spawn in the ocean during winter. It may be 

 that during severe winters, many of the larvae of 

 cold sensitive species (e.g., spot) that reach the 

 estuary early are killed by cold water temperatures 

 (10°C or less). Thus, only the late arriving larvae 

 survive to recruit into the fishery. The difference 

 in survival between severe and normal winters may 

 help to explain in part the difference between good 

 and poor year classes of certain fish. 



Acknowledgments 



We thank W. Hettler for providing eggs of labor- 

 atory spawned fish, and W. Hettler and J. Govoni 



for providing critical reviews of the manuscript. This 

 research was funded in part by a cooperative agree- 

 ment between the National Marine Fisheries Service 

 and the Department of Energy E(49-7)5. 



LITERATURE CITED 



Almatar, S. M. 



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 Brett, J. R. 



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 1981. The evolutionary development of vertebrate thermo- 

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 Dawson, C. E. 



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 Fahay, M. p. 



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 Hettler, W. F. 



1979. Modified neuston net for collecting live larval and 

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