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



waters (Warlen 1982; Warlen and Chester 1985), the 

 larvae enter estuaries where they develop into 

 juveniles. In the spawning area, water temperatures 

 are usually between 18° and 25°C (Fahay 1975; 

 Hettler and Powell 1981). The fish encounter de- 

 creasing temperatures as they move inshore to the 

 estuarine nursery areas. In the lower Newport River 

 estuary, for example, mean water temperatures be- 

 tween November and March may range from 14° 

 to 6°C with the highest temperatures during this 

 period occurring in November and the lowest in 

 January (Hoss 1974). 



Larvae of Atlantic croaker and spot were obtained 

 from both field collections and eggs spawned in the 

 laboratory. Older larvae were captured in a bridge 

 net (Hettler 1979) and held in the laboratory for no 

 more than a week prior to use. First feeding larvae 

 were obtained from spawned fish, reproduced by the 

 methods of Hettler and Powell (1981), and then were 

 reared at experimental temperatures. 



Oxygen consumption was measured with a differ- 

 ential respirometer (Umbreit et al. 1964), following 

 procedures used by Hoss, Hettler, and Coston 

 (1974). Fish were transferred to 15 mL respiration 

 flasks and, following a 2-h acclimation period, their 

 oxygen consumption was measured. Numbers of lar- 

 vae per flask varied between 1 and 30, depending 

 on the size of the larvae. Acclimation temperatures 

 were 10°, 15°, and 20°C. Notochord or standard 

 lengths and dry weights were obtained for individual 

 fish. 



The metabolic equation Q = aW^, was used to 

 describe the relation between oxygen consumption 

 and dry weight of fish acclimated at 10°, 15°, and 

 20°C. In this equation, W is the weight of the fish, 

 and a and k are constants for the species obtained 

 from least-squares regression of the log of oxygen 

 consumption on the log of weight (Winberg 1956). 

 A k value of 0.67 implies that oxygen consumption 

 varies in proportion to surface area, whereas a value 

 of 1 indicates that respiration varies in proportion 

 to weight. 



We used the metabolic equation to estimate oxy- 

 gen consumption of larvae of equal weight at differ- 

 ent temperatures. We compared larvae of 4 mg dry 

 weight because this is the realistic estimate of their 

 weight as they are transported from coastal to estu- 

 arine waters (Warlen 1982; Warlen and Chester 

 1985). 



Growth and feeding rates of small spot (^20 mm 

 SL) collected from the Newport River were calcu- 

 lated from data collected in the laboratory at several 

 temperatures. Wet weights (^15-30 mg) were re- 

 corded to the nearest milligram, and 10 fish were 



randomly assigned to 4 L test and control con- 

 tainers. Control fish were dried to determine the 

 dry/wet weight ratio, which was then used to esti- 

 mate initial dry weight of experimental fish. One 

 experiment was conducted at 6°, 8°, 10°, 12°, and 

 16°C, and two experiments were conducted at 18°C. 

 In all cases fish were fed newly hatched brine shrimp 

 several times a day to assure an ad libitum food 

 supply. After 4-6 days all food was removed; lar- 

 vae were allowed time to clear their guts and then 

 were dried and weighed. 



Growth and feeding rates were expressed as per- 

 cent of body weight per day. Growth rate was 

 calculated from the expression: 



Growth rate = 100 [(WwAVo) 



l/n 



1] 



where Wn = dry weight of all fish in a tank at day 

 n 

 Wo = estimated original dry weight of fish 

 n = number of days fed. 



Calculation of feeding rates required the assump- 

 tion of constant growth rates. Using original dry 

 weights and calculated growth rates we determined 

 the total dry weight of fish in each container at the 

 beginning of each day. Dry weights of brine shrimp 

 eaten each day divided by the calculated dry weights 

 of fish gives proportion of body weight ingested. 

 These proportions were then summarized as aver- 

 age daily percent of dry body weight ingested. 



In order to compare metabolic parameters, i.e., 

 oxygen consumption, feeding, and growth rates, the 

 following conversion factors were used: 1.0 mg dry 

 wt = 5.0 cal (Thayer et al. 1973; Paffenhoffer 1967); 

 1.0 mg O2 = 3.38 cal (Phillipson 1966) and 0.7 mg 

 O2 = 1 mL O2 at STP. One tenth calorie per fish 

 per day was added to all the rates so that measured 

 zeros could be shown on a log scale. 



RESULTS AND DISCUSSION 



The regression equations relating oxygen con- 

 sumption to weight at several temperatures are 

 shown in Table 1. Higher coefficient of determina- 

 tion (R^) values were found at higher temperatures, 

 a trend best explained by differences in the size 

 range of fish measured at different temperatures. 

 Values for k, were generally comparable to values 

 reported by other investigators for fish of a similar 

 size and at comparable temperatures— Hoss (1974), 

 pinfish; Houde and Schekter (1983), bay anchovy, 

 sea bream, and lined sole; Almatar (1984), herring; 

 and Laurence (1978), cod and haddock. 



485 



