FISHERY BULLETIN: VOL. 81, NO. 3 



continuous, the estimates of C, will not be seriously 

 biased, provided that stomach samples are collected 

 at intervals of 3 h or less. 



The total daily ration can also be calculated in a 

 single step from 



C= 24 RS 



(2) 



where S equals the mean stomach content weight 

 over the 24-h period (Elliott and Persson 1978). 

 However, in order to investigate diel feeding 

 periodicity, it is necessary to calculate ingestion dur- 

 ing time intervals <24 h, using Equation (1). For the 

 present analysis the 24-h day was divided into eight 

 consecutive 3-h time periods; data collected within 

 each of these periods were arbitrarily assigned to the 

 midpoint of that time period. Ingestion between that 

 time and the midpoint of the next time interval was 

 then calculated using Equation (1), where t = 3 h. 



In the Elliott and Persson model, R is assumed to be 

 exponential (i.e., a constant proportion of the 

 stomach content is evacuated per unit time), and un- 

 affected by fish size, food size, meal size, and the fre- 

 quency of feeding. R is affected by food type, 

 however, and increases with increasing temperature, 

 usually following an exponential or power curve. Gas- 

 tric evacuation is assumed to begin immediately after 

 the food is ingested, without an appreciable time 

 lag. 



The most appropriate values of R to be used in this 

 analysis were determined from a literature review 

 (Appendix 1). The general relationship between R 

 and temperature (t) is that R = ae 1 " (Elliott 1972), 

 where a and b are constants. The slope (6) of this 

 relationship appears to be fairly constant for dif- 

 ferent prey types and fish species (b = 0.115, App. 

 Table 1) but the intercept {a) may change significant- 

 ly according to the type of food. 



For several marine fishes that were fed small prey, 

 the relationship between/? and temperature (°C) was 



R= 0.0406 e 0111 ' 



(3) 



(App. Fig. 1). The data indicated that fish prey are 

 digested more slowly than small prey types, however. 

 This effect has not been clearly defined, but the max- 

 imum range in R that has been observed within a 

 single fish species was in the Atlantic cod, where the 

 exponential evacuation rate for fish flesh (based on 

 our calculation of data from Bagge 1977) was about 

 107( of that for a crustacean prey, shrimp tails (Tyler 

 1970) (App. Fig. 1). A complicating factor is that the 

 food particle sizes in those studies using fish as prey 

 were much larger than in studies using other prey 



types, and the effect of large particle size on the 

 evacuation rate is poorly known. Thus we are not pres- 

 ently able to determine whether the reduced evacua- 

 tion rates observed for fish prey are principally due to 

 the prey type (fish flesh) or to the comparatively large 

 particle sizes used in these studies. A further prob- 

 lem is that we lack information on digestive rates for 

 many important prey species of marine fishes. 



Because of these limitations to our knowledge of the 

 rates at which different prey species are evacuated, 

 the stomach contents of the Atlantic cod and silver 

 hake have been grouped into two categories in the 

 present study: "fish prey" and "all other prey." Most 

 of the "other prey" were small organisms, and we 

 used Equation (3) to estimate R for these prey. 

 Because of the uncertainty concerning the value of a 

 for fish prey, we made two estimates ofR for this food 

 type: first, where a in Equation (3) = 0.0406, and sec- 

 ond where a was 107c of this value, i.e., a = 0.00406. 

 These estimates should represent upper and lower 

 limits to the true value of R for fish prey. The tem- 

 perature for which R was calculated was the mean 

 temperature at which each fish species and size class 

 was collected (see Tables 1, 6, 7). 



Description of the Data Set 



The survey area from which stomach samples were 

 taken extends from the offshore waters of Cape Hat- 

 teras to western Nova Scotia, and is divided into five 

 geographic regions (Fig. 1). Stomach content data 

 gathered during spring and fall cruises (Table 1) dur- 

 ing the years 1973-76 were analyzed. Details of the 

 sampling procedure and methods of stomach content 

 analysis are given in Langton et al. (1980). Sampling 

 continued throughout the 24-h day and was designed 

 to provide broad coverage over a wide geographic 

 area rather than intensive surveys within small 

 regions. In order to define the food web, 100 

 stomachs (50 young-of-the-year and 50 adult fish) 

 were to be collected per geographic region per cruise 

 from each of 17 selected fish species. At each station 

 no more than 10 stomachs per species were to be 

 sampled. The same species was not to be sampled 

 from consecutive stations unless it appeared that, 

 because of low abundance, the desired number of fish 

 could not be collected using the normal sampling 

 scheme. In this case the fish were collected as needed 

 to fill the quota for the geographic area. Young-of- 

 the-year fish were preserved whole in 10% Formalin, 4 

 after slitting the gut cavity to ensure quick penetra- 



4 Ref'erence to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



438 



