FISHERY BULLETIN: VOL. 80, NO. 4 



both haddock and Atlantic cod from the same 

 trawl haul preyed on juvenile Sebastes, they were 

 preying on different-sized juveniles and, in gen- 

 eral, Jones observed that Atlantic cod tended to 

 consume larger prey than haddock of the same 

 size. This type of detailed stomach contents anal- 

 ysis, together with observations on fish feeding 

 behavior in the laboratory and in situ, is the type 

 of biological information necessary for accu- 

 rately defining what constitutes an ecologically 

 homogeneous system, or, more importantly, an 

 energetically coupled unit within the system. 



There are some general patterns to the overlap 

 values which may indicate real, or at least poten- 

 tial, trophic linkages. In comparing Atlantic cod 

 with six other gadids, for example, the overlap 

 levels generally are at their lowest between the 

 larger cod (greater than size class 10, >50 cm) 

 and the smaller size classes of the other preda- 

 tors (Fig. 3). This reflects the shift in the Atlantic 

 cod's feeding habits from being primarily a crus- 

 tacean predator to a piscivore with an increase in 

 body size. In effect, Atlantic cod occupy differ- 

 ent, size-specific, feeding niches which corre- 

 spond to these other predators only when both 

 species are small and more dependent on crusta- 

 cean prey. For silver hake (Fig. 6) the pattern is 

 similar but silver hake are a smaller fish than 

 Atlantic cod and switch to a piscivorous habit at 

 a smaller size. In addition to this pattern, there is 

 also a noticeable shift in overlap between silver 

 hake, the three other hake species, and pollock. 

 The overlap with the smaller size classes of silver 

 hake is low or even decreases slightly as the four 

 other gadid species increase in size. Conversely, 

 overlap is highest for the larger silver hake and 

 these four gadids. This is a result of predation on 

 many different species of crustaceans by all 

 these predators and a shift towards fish preda- 

 tion as they grow. 



In comparing both Atlantic cod and silver 

 hake with the one rajiform and the four perci- 

 form fish species, the overall pattern of diet over- 

 lap is the same. The larger Atlantic cod and silver 

 hake show a decreasing level of overlap with the 

 smaller size classes of these other finfish preda- 

 tors (Figs. 2, 5). Atlantic cod and silver hake do 

 not show a similar pattern of diet overlap when 

 compared with the four pleuronectiform species 

 of finfish (Figs. 4, 7). With the three pleuronectid 

 species (Fig. 4A-C) the highest levels of overlap 

 with Atlantic cod occur between the smaller size 

 classes of all three species while the lowest levels 

 occur between the larger size classes. This is 



similar to changes in overlap observed for the 

 perciform species. For the one bothid (Fig. 4D) a 

 size-dependent shift in overlap is not readily 

 apparent. The overlap values for silver hake and 

 flatfishes are at a maximum, albeit low overall, 

 for the smaller individuals of the three pleuro- 

 nectids (Fig. 7A-C) but, like the Atlantic cod, are 

 fairly constant over all size classes of the one 

 bothid species examined (Fig. 7D). This pattern 

 of diet overlap with the pleuronectids may be at- 

 tributed to the crustacean/fish shift in diet for 

 the Atlantic cod and silver hake, and either little 

 change in the flatfish diet or, as with American 

 plaice, a change in diet to one that does not in- 

 clude much, if any, fish prey. 



Atlantic cod and silver hake are crustacean/ 

 fish predators with a size-dependent shift in 

 predation from crustaceans to fish as these pred- 

 ators grow. In the present data the shift to fish 

 predation for Atlantic cod occurs at about 60-70 

 cm and for silver hake at about 20-25 cm. These 

 sizes are not absolute and depend very much on 

 the availability of prey. Daan (1973), for exam- 

 ple, observed a difference in North Sea cod feed- 

 ing habits when comparing samples from the 

 northern and southern North Sea. Crustaceans 

 predominated in the stomachs of the larger speci- 

 mens from the southern region while their 

 northern counterparts had already shifted over 

 to a piscivorous habit. In the northwest Atlantic 

 a similar, but less obvious, shift was observed in 

 the diet of cod when compared over a 10-yr peri- 

 od (Grosslein et al. 1980). When the finfish bio- 

 mass was low (1973-76 vs. 1963-66), crustaceans 

 were slightly more important as prey although 

 the general impression resulting from studying 

 the two data sets was a fairly constant pattern of 

 predation over time. The diets of Atlantic cod, 

 and presumably most of these other predators, 

 are fairly stable although there is an apparent 

 degree of fine tuning that depends upon the avail- 

 ability of prey and other controlling factors in 

 the environment. 



In summary, assuming that the diets of Atlan- 

 tic cod and silver hake are reasonably stable over 

 time, and that the same is true for the 15 other 

 predators examined, then the pattern of overlap 

 described above suggests that the greatest over- 

 all potential for interaction exists between the 

 smaller stages of the two gadids and the other 

 predators. Furthermore, the greatest overlap, 

 with few exceptions, occurs among the gadiform 

 fishes themselves rather than between the 

 gadids and the other ordinal taxonomic levels. 



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