S 6.0 



1200 



1600 



2000 



0000 

 Time of day 



0400 



0800 



1200 



FIGURE 8. — Mean stomach content weight as a percentage of Pacific 

 whiting, Merluccius productus, weight over part of a 24-h period at a 

 die! sampling location off the Washington coast in July 1967 (verti- 

 cal bars indicate the range of values in a given time period). Numbers 

 of stomachs containing food are noted above each time period, with 

 numbers of empty stomachs in parentheses. 



have similar nocturnal feeding habits, although the 

 timing of peaks in stomach fullness is not identical. 



Daily Ration 



An estimate of the average food intake by a fish pre- 

 dator is needed to evaluate predation mortality rates 

 of prey species in some models (Majkowski and 

 Waiwood 1981). Daily ration is a useful measure of 

 food intake and can be calculated from field data if 

 there exists an estimate of the gastric evacuation rate 

 (Eggers 1977; Elliott and Persson 1978). The total 

 1967 Pacific whiting data set, containing 1,749 

 stomach samples, is used here to compute daily 

 ration, since it includes a large number of samples 

 taken at many different times of day and records of 

 the number of empty stomachs present in the sam- 

 ples which is necessary for daily ration calculation. 



The Elliott and Persson (1978) model which 

 assumes an exponential, temperature-dependent 

 evacuation rate, R, is applied to the data set for the 

 daily ration computations. If stomach samples are 

 collected at intervals of t hours, the mean stomach 



content weight as a percentage of fish weight, S„ in 

 each interval i is calculated for a total of m intervals 

 over the 24-h period. According to Elliott and 

 Persson (1978) the daily ration in terms of percen- 

 tage body weight, EC,, can then be evaluated by the 

 following expression: 



XC = Rt 



2 S, (1 - e- 



m 



) = 24SR (1) 



where 5 = ES,/m. Durbin and Durbin (1980) found 

 that the relationship between R and water tempera- 

 ture T for marine fish eating small food organisms 

 was 



R= 0.4 16<? 01057 ". 



(2) 



This Equation (2) was used to calculate R here, since 

 most prey eaten by Pacific whiting in 1967 were small 

 organisms, mainly euphausiids. Some error is 

 introduced at this point because part of the diet con- 

 sisted of fish which are evacuated at a slower rate 

 than small crustaceans (Durbin et al. 1980). Water 

 temperature was assumed to be 8.2°C, the approx- 

 imate monthly mean temperature for July at 60 m 

 below surface in the Washington-Oregon coastal 

 region (Robinson 1976) where most of the fish 

 were collected. 



The calculated daily ration of Pacific whiting, using 

 Equations (1) and (2) on the 1967 data, is equal to 

 2.5% body weight/d for an average Pacific whiting 

 size of 500 mm. Although this estimate is probably 

 high, due to the use of the evacuation rate only for 

 small crustaceans, it is comparable to other 

 estimates. Daily ration estimates for a similar fish — 

 adult silver hake in the northwest Atlantic — range 

 between 0.6 and 2.7% body weight/d (Durbin et al. 

 1980; Cohen and Grosslein 1981; Pennington 1981). 

 The present estimate is reasonable but should be 

 verified further before it is used in calculating energy 

 budgets for Pacific whiting. 



Literature Cited 



Agger, P., and E. Ursin. 



1976. Food and food preference of the grey gurnard. Int. 

 Counc. Explor. Sea, ICES CM. 1976/F:41. 8 p. 

 Alton, M. S., and C. J. Blackburn. 



1972. Diel changes in the vertical distribution of the 

 euphausiids, Thysanoessa spmifera Holmes and Euphau- 

 sia ptinfica Hansen, in coastal waters of Washington. Calif. 

 Fish Game 58:179-190. 

 Alton, M. S., and M. O. Nelson. 



1970. Food of Pacific hake, Merluccius productus, in 

 Washington and northern Oregon coastal waters. In 

 Pacific hake, p. 35-42. U.S Fish Wildl. Serv., Circ. 332. 



635 



