LANOTON: DIET OVERLAP BETWEEN SEVENTEEN NORTHWEST ATLANTIC FINEISH 



between Atlantic cod of size class 5 and pollock of 

 size classes 10 and 14 as seen in Figure 3D. 



Atlantic Cod — Haddock 



Haddock, Melanogrammus aeglefinus (Lin- 

 naeus), is primarily benthic in its feeding habits 

 with the result that its diet is similar to Atlantic 

 cod's only when the Atlantic cod are also feeding 

 on the benthos. Consequently, the degree of diet 

 overlap between these two predators is highest 

 for the smaller animals, as can be seen in Figure 

 3E. The diversity of prey that both these preda- 

 tors consume reduces the computed overlap val- 

 ues. The relatively low maximum values, 47% 

 being the highest for 16-20 cm (size class 4) Atlan- 

 tic cod and 11-15 cm (size class 3) haddock, make 

 it difficult to identify any particular species of 

 prey that gives rise to the observed intermediate 

 levels of overlap. 



Atlantic Cod — Ocean Pout 



Ocean pout, Macrozoarces a mericanus( Schnei- 

 der), are fairly specific in their predatory habits, 

 and these habits do not overlap with those of 

 Atlantic cod to any extent (Fig. 3F). A single 

 prey species, the sand dollar, Echinarachnius 

 pa rma, accounts for most of the diet of ocean pout 

 although amphipods were also present in many 

 of the fish stomachs examined. 



Atlantic Cod — American Plaice 



The diets of Atlantic cod and American plaice, 

 Hippoglossoides platessoides (Fabricius), over- 

 lap at. quite low levels, the highest value being 

 33% for 26-30 cm (size class 6) Atlantic cod and 

 21-25 cm (size class 5) American plaice. Ameri- 

 can plaice prey on a variety of benthic animals 

 but, as they grow larger, they rely more on echin- 

 oderms than crustaceans and polychaetes. This 

 is reflected in the diet overlap plot (Fig. 4A); the 

 larger size classes of American plaice (>45 cm) 

 have extremely small overlap with Atlantic cod 

 because of predation on the sand dollar. 



Atlantic Cod — Witch Flounder 



Little diet overlap occurs between Atlantic cod 

 and witch flounder, Glyptocephalus eynoglossus 

 (Linnaeus), with all of the calculated values be- 

 ing 30% or less (Fig. 4B). Witch flounder are 

 benthic predators with polychaete worms being 



of major importance as prey although they do 

 consume crustaceans and other invertebrates. It 

 is the crustacean component of the diet which 

 accounts for these low levels of overlap with the 

 Atlantic cod. 



Atlantic Cod — Yellowtail Flounder 



The diets of Atlantic cod and yellowtail floun- 

 der, Limanda ferruginea (Storer), overlap at 

 generally low levels (Fig. 4C). There is only one 

 cluster of intermediate levels involving yellow- 

 tail flounder from a single size class (class 3, 11- 

 15 cm) which reflects the occurrence of pandalid 

 shrimp Dichelopandalus leptocerus in both pred- 

 ators' diets. As with many other species, a reduc- 

 tion in the level of overlap occurs as the disparity 

 in fish size increases. For yellowtail flounder this 

 is quite apparent when compared with the larger 

 Atlantic cod because the large Atlantic cod are 

 primarily piscivorous. However, this reduction 

 is not as noticeable for large yellowtail flounder 

 and small Atlantic cod since the benthic habits of 

 yellowtail flounder change little as the fish in- 

 crease in size. 



Atlantic Cod — Fourspot Flounder 



Atlantic cod and fourspot flounder, Paralich- 

 thys oblongus (Mitchill), show low and intermedi- 

 ate levels of diet overlap (Fig. 4D) which is pri- 

 marily a result of predation on crustaceans. The 

 single most important crustacean prey was the 

 pandalid shrimp Dichelopandalus leptocerus 

 which makes up from 17% to 30% of the diet of 21- 

 30 cm fourspot flounder and 3% to 40% of the diet 

 of 6-45 cm Atlantic cod. 



Silver Hake — Little Skate 



The pattern of diet overlap between silver 

 hake, Merluccius bilinearis (Mitchill), and little 

 skate is shown in Figure 5A. Generally, overlap 

 levels are low but medium levels range up to a 

 high of 44% between small silver hake 1-15 cm 

 (size classes 1-3) and little skate 11-45 cm total 

 length (size classes 3-9). This degree of overlap 

 can be attributed to the crustaceans in each of 

 these predators' diets with the sand shrimp, 

 Crangon septemspinosa, being of particular im- 

 portance. For larger silver hake the diet overlap 

 with little skate is insignificant since these 

 larger hake prey on fish while little skate of all 

 sizes prey primarily on benthic crustaceans. 



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