TEMPERATURE 



The gastric evacuation rate R appears to follow an 

 exponential or power curve relationship with tem- 

 perature t (Elliott 1972): 



R 



ae" 



(1) 



Evacuation rates of brown trout that were fed small, 

 rapidly digested prey (Gammarus, Baetis, 01- 

 igochaetes, chironomids), were described by the 

 relationship 



R= 0.053 e" 



(2) 



In further experiments with other foods {Pro- 

 tonemura, Hydropsyche, Tenebrio) Elliott (1972) 

 found that the intercept (a) in Equation (2) was 

 dependent on prey type, but that the slope (6) was 

 constant and independent of prey type. Data from 

 other freshwater and marine fishes (within their pre- 

 ferred temperature range) indicate that the value of b 

 is fairly close to that found by Elliott for the brown 

 trout (Appendix Table 1; b = 0.115). At tem- 

 peratures outside the preferred range, evacuation 

 rates were depressed (i.e., Atlantic cod at 19°C, Tyler 

 1970). The value of a varies widely in different 

 studies, apparently because of the different prey 

 types used. 



FISHERY BULLETIN: VOL. 81, NO. 3 



The fastest evacuation rates have been reported for 

 a variety of small food organisms. These rates, when 

 adjusted for differences in experimental tempera- 

 ture, were fairly similar among several marine and 

 freshwater fishes: Atlantic cod, fed 0.5 g chunks of 

 shrimp {Pandalus) tails (Tyler 1970); the flounder 

 Platichthys, fed 0.1 g polychaetes (Ki^rboe 1978); 

 winter flounder Pseudopleuronectes, fed 0.5-1.0 cm 

 pieces of squid (Huebner and Langton 1982); Atlan- 

 tic menhaden, fed 80 pm diatoms (Durbin and Dur- 

 bin 1981) 1 ; brown trout, fed 1 and 15.7 mg 

 Gammarus, 0.9 and 7.8 mg Baetis, 0.33 and 3.3 mg 

 chironomids, and 29 mg oligochaetes (Elliott 1972); 

 sockeye salmon, fed small commercial pellets (Brett 

 andHiggs 1970); pumpkinseedsunfishfeddamselfly 

 larvae (Kitchell and Windell 1968); and bluegill sun- 

 fish fed 180 mg crayfish (Windell 1967) (App. Figs. 

 1,2). 



The marine species ingesting these readily digested 

 foods followed a common /?- temperature rela- 

 tionship (App. Fig. 1): 



R = 0.0406 e - 11 ". 



(3) 



The a and b values are similar to those observed by 

 Elliott (1972) for the brown trout feeding on easily 

 digested foods (0.053 and 0.112, respectively); b is 

 also similar to the overall mean value in Appendix 

 Table 1. 



APPENDIX TABLE 1.— Slope (6) of the relationship between the instantaneous 

 evacuation rate (R, per hour) and temperature (°C) for several freshwater and 

 marine fishes, where: R = ae bl . The intercept (a) varies with food type. 



1 Based on author's calculations of R. 



2 Based on our calculation of R from data in study, and assuming that evacuation is an ex- 

 ponential process. 



FOOD TYPE 



Studies have shown that there are differences in 

 evacuation rates with different food types. While 

 these results may reflect inherent differences in the 

 digestibility of the food, they may also indicate an in- 

 teraction between food type, particle size, and meal 

 size. In practice, these factors may be difficult to 

 resolve, particularly when the results of different 

 studies are being compared. 



However, certain small prey organisms were digest- 

 ed significantly more slowly than those cited above. 

 This may reflect the chemical composition of the prey. 

 For example, slower digestion of Tenebrio and Hy- 

 dropsyche by bluegill and brown trout was attributed 

 to the high fat content of these organisms (Kitchell 

 and Windell 1968; Elliott 1972). Pure fat retards 



'Gastric evacuation rate R in menhaden was estimated from the 

 feces elimination rate /?'. 



452 



