LANGTON: DIET OVERLAP BETWEEN SEVENTEEN NORTHWEST ATLANTIC EINFISII 



pandalid shrimp Dichelopandalus leptocerus, 



although a large variety of other crustaceans 

 contributed to both of the predator's diets. 



Atlantic Cod — Scup 



Overlap between the diet of Atlantic cod and 

 scup, Stenotomus chrysops (Linnaeus), is at a low 

 level (Fig. 2D), and the low values represent a 

 broad array of prey, principally crustaceans, 

 that constitutes the forage base of these two 

 predators. The only trend in these values is that 

 they are at their lowest for large Atlantic cod 

 (>80 cm) and all size classes of scup. This is the 

 result of the larger Atlantic cod's piscivorous 

 habits. 



Atlantic Cod — Butterfish 



The Atlantic cod and butterfish, Peprilustria- 

 ca nth us (Peck), show very low diet overlap levels, 

 the maximum being 19% for 11-15 cm (size class 

 3) fish of both species. There was, however, rela- 

 tively more overlap between the smaller Atlantic 

 cod (<45 cm) and all sizes of butterfish sampled 

 (Fig. 2E). 



Atlantic Cod — White Hake 



The diet of white hake, Urophycis tenuis 

 (Mitchill), shifts from crustaceans such as eu- 

 phausiids, shrimp, and mysids when they are 

 small (<=50 cm) to primarily fish for the larger 

 white hake. This parallels the change in the 

 Atlantic cod's diet with size. The result is a vary- 

 ing degree of dietary overlap across all size 

 classes of fish examined (Fig. 3A). Low levels 

 occurred between the smaller white hake and the 

 larger size classes of Atlantic cod and vice versa. 

 Intermediate levels, values in the 30-50% range, 

 are found in clusters which are the result of a 

 variety of shared prey types. Some of these val- 

 ues in the 40 and 50% range depend upon the fish 

 components in the diet. In particular, silver hake 

 and herring, Clupea harengus, together with un- 

 identified fish are the most commonly occurring 

 prey that these predators share. The greatest 

 overlaps observed were between 21-25 cm (size 

 class 5) Atlantic cod and several larger size 

 classes of white hake (36-85 cm, size classes 8- 

 17) (Fig. 3A). This high overlap is somewhat arti- 

 ficial since it is the result of unidentified fish 

 prey in both predators' diets. It does, however, 



amplify the importance of fish prey to these pred- 

 ators. 



Atlantic Cod — Red Hake 



Atlantic cod and red hake, Urophycis chuss 

 (Walbaum), have low to intermediate levels of 

 diet overlap. The lowest values occur between 

 small red hake and large Atlantic cod (Fig. 3B). 

 These small red hake prey quite heavily on crus- 

 taceans while the larger Atlantic cod have shifted 

 their habits from crustacean prey to fish. The 

 diet of red hake does, however, include more fish 

 prey as the predators themselves grow, so that 

 the overlap values between the larger size classes 

 of red hake and Atlantic cod remain at an inter- 

 mediate level. 



Atlantic Cod — Spotted Hake 



Atlantic cod and spotted hake, Urophycis regia 

 (Walbaum), have diets which overlap at low to 

 intermediate levels (Fig. 3C). The prey that they 

 have in common is primarily crustaceans but 

 may also include some fish. The cluster of inter- 

 mediate values occurring between 11-25 cm (size 

 classes 3-5) Atlantic cod and 11-30 cm (size 

 classes 3-6) spotted hake is, for example, the re- 

 sult of predation on crustaceans such as Mega- 

 nyctiphanes, Dichelopandalus, Crangon, Unci- 

 ola, and other less significant taxa, while the 

 intermediate overlap peaks between 31-35 cm 

 spotted hake and Atlantic cod are due to fish 

 predation. 



Atlantic Cod — Pollock 



Atlantic cod and pollock, Pollachius rirens 

 (Linnaeus), show low to intermediate levels of 

 d iet overlap over all size classes of both predators 

 examined. Both of these species are crustacean/ 

 fish predators, relying more heavily on fish as 

 they increase in size. For the smaller pollock, the 

 euphausiid Meganyctiphanes norvegica and the 

 shrimp Pasiphaea multidentata were the major 

 components of the diet while the Atlantic cod re- 

 lied on a much broader variety of prey. For the 

 larger fish of both species a variety of pisces were 

 included in the diet, some of which was not read- 

 ily identifiable at any lower level than simply 

 fish flesh. The problem in identifying fish re- 

 mains generated two artificial peaks in overlap 



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