STOMACH CONTENTS OF SILVER HAKE, MERLUCCIUS 



BILINEARIS, AND ATLANTIC COD, GADUS MORHUA, AND 



ESTIMATION OF THEIR DAILY RATIONS 



E. G. Durbin, 1 A. G. Durbin, 1 R. W. Langton, 2 and R. E. Bowman' 



ABSTRACT 



The model of Elliott and Persson was used to estimate the daily ration of silver hake, Merluccius bilinearis, 

 and Atlantic cod, Gadus morhua, collected in the western North Atlantic between Cape Hatteras and Nova 

 Scotia during the years 1973-76. The model required field measurements of the weight of food in the 

 stomachs during consecutive 3-h periods over 24 h, and laboratory estimates of the exponential gastric 

 evacuation rate. The silver hake and Atlantic cod were each grouped into two size classes for analysis (<20 

 cm and > 20 cm, and<30 cm and >30 cm, respectively). Upper and lower daily ration estimates were 3.2 and 

 2.9% body weight (BW) per day for hake <20 cm, 2.2 and 0.8% BW per day for hake >20 cm, and 1.5 and 

 0.97c BW per day forcod>30 cm. There were insufficient small cod to estimate daily ration. These ration es- 

 timates are intermediate between two previous estimates for silver hake and Atlantic cod on Georges Bank 

 obtained by different methods. 



With the increasing interest in multispecies manage- 

 ment and total ecosystem management, it is essential 

 to understand the role of fish predators within the 

 ecosystem. As a part of this, it is necessary to deter- 

 mine the feeding habits and the daily ration of the 

 major species. Here we estimate the daily ration of 

 silver hake, Merluccius bilinearis, and Atlantic cod, 

 (todus morhua, in the northwest Atlantic, based on 

 stomach samples collected by the Northeast 

 Fisheries Center, National Marine Fisheries Service. 

 The model of Elliott and Persson (1978) was used to 

 investigate diel feeding periodicity and to estimate 

 the daily ration, based on field measurements of 

 stomach contents, and laboratory estimates of the 

 exponential evacuation rate R. 



METHODS 



ferent models to estimate daily ration in fishes, El- 

 liott and Persson (1978) demonstrated that their 

 model, which assumes exponential evacuation, pro- 

 vided accurate estimates of ingestion, whereas models 

 which assumed a constant (linear) gastric evacuation 

 rate (Bajkov 1935 and derivations thereof) significant- 

 ly underestimated ingestion. Since recent comments 

 support the validity of the exponential model for the 

 field estimation of daily ration on the basis of 

 stomach contents (Cochran 1979; Elliott 1979; 

 Eggers 1979), this approach has been adopted for 

 the present analysis. 

 In the Elliott and Persson model, the consumption 

 of food (C t ) by a fish over the time interval t to t, is 

 calculated from the amount of food in the stomach at 

 time t (S ), the amount in the stomach at time t, (S t ), 

 and the instantaneous evacuation rate R: 



Method of Estimating Daily 

 Food Consumption 



Most recent studies have concluded that gastric 

 evacuation is best described by a curvilinear function 

 such as an exponential curve (Tyler 1970; Brett and 

 Higgs 1970; Elliott 1972; Kitfrboe 1978; Persson 

 1979; but see Jobling 1981). In an evaluation of dif- 



' Graduate School of Oceanography, University of Rhode Island, 

 Kingston, RI 02881. 



2 State of Maine, Department of Marine Resources, Marine Re- 

 sources Laboratory, West Boothbay Harbor, ME 04575. 



'Northeast Fisheries Center Woods Hole Laboratory, National 

 Marine Fisheries Service, NOAA, Woods Hole, MA 02543. 



Manuscript accepted February 1983. 

 FISHERY BULLETIN: VOL. 81, NO. 3, 1983. 



c, 



(S, - S„e-"i) Rt 



-Ht 



(1) 



To apply the model, a sample offish is collected from 

 the field at intervals of t hours for at least 24 h, and 

 the mean stomach content weight is used to estimate 

 S () and S, for each time interval. The estimates of C, 

 calculated for each time interval t are then summed 

 to give the total daily ration. 



The model assumes that the fish feed continuously, 

 at a constant rate, during time interval t. The cumu- 

 lative amount of food consumed (C,) therefore in- 

 creases linearly during time t. However, Elliott and 

 Persson (1978) showed that even if feeding is not 



437 



