FISHERY BULLETIN: VOL. 81, NO. 3 



However, some authors have suggested that in their 

 studies (where fish flesh was the food), evacuation 

 was slower for large meals than for small (Steigen- 

 berger and Larkin 1974; Jones 1974). These authors 

 found that significant time lags, on the order of hours, 

 elapsed between ingestion and the onset of gastric 

 evacuation. Similar lags have been reported by other 

 investigators who used fish as the prey type (Daan 

 1973; Gerald 1973). Some question therefore 

 remains as to whether meal size had a direct effect on 

 R in these studies, or whether the primary effect of in- 

 creasing meal size was to cause a progressive in- 

 crease in the time lag before the onset of gastric 

 evacuation. 



EFFECT OF MULTIPLE MEALS 



Most digestion rate studies have examined stomach 

 evacuation rates following a single meal. However, in 

 nature most fish do not normally feed in this manner, 

 but rather feed on a more or less continuous, or a 

 periodic, basis. In this situation the time for food to 

 pass through the stomach and the relation between 

 the amount of food in the stomach and evacuation 

 rates will be more complex. 



Elliott (1972) fed brown trout three meals, 6 h 

 apart. He then determined the stomach contents 4 h 

 after the last meal. This amount was in good agree- 

 ment with that calculated from the exponential mod- 

 el, and Elliott concluded that multiple meals, and the 

 presence of food already in the stomach, did not af- 

 fect R or the exponential model of gastric 

 evacuation. 



Tyler (1970) fed Atlantic cod three meals, 24 h 



apart at 5°C. He too used an exponential model to 

 predict the amount of food remaining in the stomach 

 after the third meal. The actual amount was slightly, 

 but significantly, lower (by about 7%) than the pre- 

 dicted. He concluded that, overall, the fit was 

 adequate. 



In the studies of Ki«Jrboe (1978) and Huebner and 

 Langton (1982), individual fish were fed a number of 

 meals in sequence. Although the data showed a 

 significant degree of variability, reflecting differen- 

 ces in the voluntary food intake of the fish, the mean 

 values followed a common exponential relationship, 

 which implies that multiple meals did not affect R. 



CONCLUSIONS 



In summarizing this brief discussion, several con- 

 clusions can be drawn: 



1 ) The exponential model of gastric evacuation pro- 

 vides a good fit to most experimental data, and also 

 provides good estimates of ingestion rate when used 

 in the Elliott and Persson (1978) model. 



2) The two factors which are known to most strongly 

 influence the instantaneous gastric evacuation rate R 

 are temperature and food type. Multiple meals do not 

 affect the value of R. The available evidence in- 

 dicates that particle size and meal size probably do 

 not affect/? (at least with small prey items), but these 

 questions need further investigation. 



3) The slopes (b) of the /^-temperature re- 

 lationships among several marine and freshwater 

 fishes were similar, although the intercepts (a) varied 

 according to the type of food. 



454 



