DURBIN ET AL.: STOMACH CONTENTS OF HAKE AND COD 



evacuation in rainbow trout (Windell et al. 1969) as 

 well as in vertebrates other than fishes (Quigley and 

 Meschan 1941). Artificial pelleted food, which is high 

 in organic content, is also digested more slowly than 

 natural food (Windell and Norris 1969; Windell et al. 

 1969). The degree of external protection of the prey 

 can also affect the digestion rate. For example, rain- 

 bow trout digested the caddisfly Arctopsyche, and the 

 cottid Enophrys digested Colliphora larvae, more 

 slowly than other prey types, evidently because the 

 integument of the prey was resistant to the penetra- 

 tion of the gastric juices (Reimers 1957; Western 

 1971). MacDonaldetal. (1982) also reported that the 

 shell in Yoldia retarded the evacuation rate in several 

 marine fishes. 



The lowest evacuation rates which have been ob- 

 served were from fish feeding on fish flesh, usually in 

 fairly large particle and meal sizes [Atlantic cod, fed 

 whole 14 g sprats, meal sizes about 3.7% BW (Daan 

 1973), Atlantic cod, fed to satiation on 2-3 g pieces of 

 greater weever(Bagge 1977); haddock Melanogram- 

 mus aeglefinus, fed saithe, 1-7% BW (Jones 1974); 

 skipjack tuna, fed 10.2 g osmerids, 8.6% BW 

 (Magnuson 1969); Ophiocephalus, fed small fish 

 7.9% BW (Gerald 1973); sea scorpion fed to satiation 

 on 5 g pieces of greater weever (Bagge 1977); 

 northern squawfish, fed small Salmo (Steigenberger 

 and Larkin 1974); largemouth bass, fed 1.22 g em- 

 erald shiners, 2-8% BW (Beamish 1972) (App. Figs. 

 1, 2)]. These reductions in R may be significant: for 

 example, with Atlantic cod and sea scorpion, the 

 instantaneous evacuation rates for fish flesh were 

 only about one-tenth those predicted by E quation (3) 

 for easily digested foods. 



Whether these reduced evacuation rates were due 

 primarily to the food type (fish), or to the large parti- 

 cle sizes, compared with those of the small prey 

 which were digested more rapidly, cannot be deter- 

 mined from the data. 



Additionally, in a few studies, after several food 

 types were tested individually and found to have 

 similar evacuation rates, meals composed of mix- 

 tures of these food types were given (Windell 1967; 

 Elliott 1972). The evacuation rate of the mixed meal 

 was not significantly different from that of the in- 

 dividual food types. However, evacuation rate of 

 mixed meals containing food types which have in- 

 dividually different R values does not appear to have 

 been investigated. 



FOOD PARTICLE SIZE 



The exponential model predicts that the evacuation 

 rate, R, depends only on the weight of food in the 



stomach, i.e., dW/dt = —RW. Thus R should be in- 

 dependent of food particle size. A meal of a given 

 weight, composed of a number of small particles, 

 should be evacuated at the same rate as a meal of 

 equal weight, but composed of a single large particle. 

 However, if digestion occurs at the surface of par- 

 ticles, then the surface area as well as the weight or 

 volume of the food may influence digestion. A 

 surface-area dependent model predicts that small 

 particles should be digested more rapidly than large 

 particles because of their greater surface area per unit 

 volume, i.e., dW/dt = ~RW m . 



Few studies have considered the effect of particle 

 size on R. 



Elliott (1972) found no effect of food particle size on 

 the gastric evacuation rates of brown trout fed small 

 invertebrates. However, the particle sizes tested 

 were quite small and may have been below some criti- 

 cal size which has a measurable effect on R. 



The evacuation rates of Atlantic cod that were fed 

 whole fish or large pieces of fish flesh (Daan 1973; 

 Bagge 1977) were lower than those of Atlantic cod 

 fed small pieces of shrimp (Tyler 1970). However, it 

 is not clear whether this difference is an effect of food 

 particle size or food type. 



Swenson and Smith (1973) examined gastric 

 evacuation of walleye and saugerfed0.8, 1.1-1.9, and 

 3.1-5.0 g minnows. The two smallest size classes were 

 evacuated at a significantly faster rate than the 3.1- 

 5.0 g fish. However, the difference between the 

 evacuation rates was small, and the different food 

 particle sizes therefore did not have a major effect on 

 the evacuation rate. 



MEAL SIZE 



Experimental data on the effect of meal size on the 

 gastric evacuation rate is conflicting. 



The exponential model predicts that gastric 

 evacuation is not affected by meal size. Studies using 

 small prey as the food have confirmed this predic- 

 tion: Brown trout, fed 0.06-0.5% BW meals of Gam- 

 mams and 0.35-1. 49i BW of Tenebrio (Elliott 1972); 

 pumpkinseed sunfish, fed 1.2 and 2.79? BW of dam- 

 selfly naiads (Kitchell and Windell 1968); Atlantic 

 cod fed 0.25-0.78'/,' BW on shrimp tails (Tyler 1970); 

 flounder, fed variable meal sizes of polychaetes 

 (Ki^rboe 1978); and Atlantic menhaden, fed 0.7- 

 7.09? BW on the diatom Ditylum (Durbin and Durbin 

 1981). Some studies using fish flesh as the food also 

 indicated that meal size did not affect R: Sea scorpion, 

 fed 5.5-11.19? BW on greater weever (Bagge 1977); 

 skipjack tuna, fed various meal sizes averaging 8.69? 

 BW on fish (Magnuson 1969). 



453 



