FISHERY BULLETIN: VOL. 86, NO. 3 



Table 1.— Metabolic equations relating oxygen consumption to 

 body size. Q = aW* where Q = oxygen consumption (piL Oj  

 h"'), a and k are regression coefficients, W is dry wt in mg, N is 

 tfie number of observations, T the temperature, and Rj the coef- 

 ficient of determination. 



Comparing between measured oxygen consump- 

 tion rates at 10° and 15°C and those predicted from 

 Van't Hoff's equation (Vernberg and Vernberg 

 1972), we conclude that Atlantic croaker show no 

 sign of regulating their oxygen consumption as 

 water temperature is decreased. The difference be- 

 tween oxygen consumption rates based on Q^q 

 values of 2 and 3 (Fig. 2) is an expected range. For 

 every 10°C change in temperature, the rate of a 

 chemical reaction typically changes by a factor of 

 2 to 3. A Qio value of appreciably <2 or more than 



a- 



z 

 O 



h- 

 o. 



ID 

 CO 



z 

 o 

 o 



X 



o 



20 

 15 



10 

 8 



Spot 



-'i 



10 15 



20 



TEMPERATURE (°C) 



Figure 2.— Oxygen consumption rates for 4 mg Atlantic croaker 

 (A) and spot (B) at three temperatures estimated from equations 

 in Table 1. The bars indicate standard errors from regressions in 

 Table 1. Broken lines are estimates of the rates expected based 

 on Van't Hoff's equation, the rate measured at 20°C, a Q^ of 2 

 (upper line) and a Qjq of 3 (lower line). 



3, indicates that some process other than a chemical 

 one is involved (e.g., a change in cell membrane 

 permeability). A Qjq of one indicates temperature 

 independence (Vernberg and Vernberg 1972). Our 

 conclusion that Atlantic croaker did not display 

 thermal stress is based on the fact that measured 

 respiration rates at reduced temperatures (10° and 

 15°C) were within the range expected (Fig. 2A). 



For spot a decrease in temperature from 20° to 

 15°C resulted in a decrease in oxygen consumption 

 of approximately the amount expected for a Qjq of 

 3. A further decrease in temperature to 10°C, how- 

 ever, caused an increase in the respiration rate. The 

 changes in oxygen consumption at low temperatures 

 could be interpreted either as adaptive, i.e., main- 

 taining a high metabolic rate even at the lower tem- 

 perature, or inadaptive, i.e., a metabolic breakdown. 



Based on feeding, growth, and survival data, how- 

 ever, we think the increase in respiration by spot 

 at 10° C is a result of cold stress, not adaptation. 

 In Figure 3 we present three measures of metabolic 

 rate, ad libitum feeding rate, maximum growth rate, 

 and routine oxygen consumption for spot, all as a 

 function of temperature. Feeding, growth, and oxy- 

 gen consumption rates decrease with decreasing 

 temperature and the rates are similar over a limited 

 range of the conditions tested (Fig. 3). The rates of 

 decline in feeding and growth from 18° to 12°C ap- 

 proximates that of oxygen consumption from 20° 

 to 15° C. At lower temperatures stress appears to 

 become important. For example, it was not possi- 

 ble to measure growth at 10° C or below because 

 only a fraction of the larvae survived. At 8° and 

 10° C some of the larvae did not eat and at 6°C none 

 of them did. This agrees with Dawson (1958) who 

 concluded that the lethal minimum temperature for 

 spot is in the 4.0°-5.0°C range and probably varies 

 with size. The intersection of ad libitum feeding rate 

 and routine oxygen consumption occurs at approx- 

 imately 10°C (Fig. 3). At this temperature there is 

 just enough energy available for routine metabolism. 

 Below this temperature there is not enough energy 

 available even at the ad libitum feeding rate to 

 maintain the larvae, and spot held at this 

 temperature for any length of time would be unlikely 

 to survive. 



We conclude from our data on metabolic responses 

 to temperature that spot and Atlantic croaker lar- 

 vae differ in their response to cold temperatures 

 which prevail at the time of their recruitment to the 

 estuary and that this difference may have impor- 

 tant implications for their survival. Both species 

 spawn in warm waters of the continental shelf 

 where the eggs hatch. As the larvae grow they are 



486 



