DIET OVERLAP BETWEEN ATLANTIC COD, GADUS MORHUA, 



SILVER HAKE, MERLUCCIUS BILINEARIS, AND 



FIFTEEN OTHER NORTHWEST ATLANTIC FINFISH 



Richard W. Langton 1 



ABSTRACT 



Diet overlap calculated as the percentage similarity between the diets of Atlantic cod, Gadus 

 morhua, silver hake, Merluccvus bilinearis, and 15 other finfish species was computed from stomach 

 contents data collected in the northwest Atlantic between Cape Hatteras, North Carolina, U.S.A., 

 and western Nova Scotia, Canada, from 1973 through 1976. Since crustaceans are preyed on by both 

 Atlantic cod and silver hake and most of the 15 other groundfish species representing members of 

 the Rajiformes, Perciformes, Gadiformes, and Pleuronectiformes, completely dissimilar diets occur 

 very rarely. Although the overlap values are quite variable, the greatest overlap, with few excep- 

 tions, occurs among the gadiform fishes themselves rather than between the gadids and species from 

 the three other ordinal taxonomic levels. Furthermore, Atlantic cod and silver hake show a size 

 dependent shift in diet (at 60-70 cm for Atlantic cod and 20-25 cm for silver hake) from crustaceans 

 to fish so that, generally, the major overlap levels are for the smaller size classes of fish. Overlap 

 levels are discussed in relation to the prey species the predators share and also in terms of their 

 usefulness in identifying potential trophic linkages between northwest Atlantic finfish. 



The traditional way of identifying fish is by 

 recognizing individual species as discrete taxo- 

 nomic units. Although the species concept is fun- 

 damental to any biological work, fishery biolo- 

 gists have been considering other means of 

 grouping species. These are usually attempts to 

 lump species in an ecological sense and they often 

 depend on the fishes' diet. These feeding niche 

 groupings may then be related to the morphol- 

 ogy and size of the fish or the prey. Food related 

 size classes for fish have, for example, been iden- 

 tified by Parker and Larkin (1959), Paloheimo 

 and Dickie (1965), and Tyler (1972). These classes 

 are referred to as threshold lengths or feeding 

 stanzas. Grouping of fish based on gut morphol- 

 ogy alone has been explored extensively for flat- 

 fish by deGroot (1971), while prey size grouping 

 was developed by Ursin (1973) and applied to 

 northwest Atlantic fish by Hahm and Langton 

 (1980 2 ). More recently, scientists on the west 

 coast of the United States have been looking at 

 Pacific fish assemblages (Gabriel and Tyler 

 1980) and have proposed the idea of an Assem- 

 blage Production Unit (Tyler et al. in press). The 



'Department of Marine Resources, Marine Resources Lab- 

 oratory, West Boothbay Harbor, ME 04575. 



2 Hahm, W., and R. Langton. 1980. Prey selection based 

 on predator/prey weight ratios for some northwest Atlantic 

 fish. Int. Counc. Explor. Sea, CM. 1980/L:62, 9 p. 



Assemblage Production Unit is defined as a geo- 

 graphically limited natural production system of 

 interacting organisms, in which all production is 

 trophically linked. 



The key to all these schemes of species indepen- 

 dent linkages is a complete understanding of 

 whatever criteria are used to group like animals. 

 In the present paper, fish predators have been 

 grouped by species in 5 cm size classes and the 

 diet of each 5 cm length group described quanti- 

 tatively as a percentage weight of the total stom- 

 ach contents for each group. Diet overlap has 

 then been calculated for each species-size class 

 combination and the overlap levels related to 

 actual diet composition. Although diet overlap 

 calculations have their limits (discussed in Lang- 

 ton and Bowman 1980), as do any other methods 

 of data reduction, this paper offers one way of 

 evaluating real and/or potential trophic linkages 

 between northwest Atlantic finfish. 



METHODS 



Stomachs were collected from both demersal 

 and pelagic fish by personnel at the Northeast 

 Fisheries Center Woods Hole Laboratory, as 

 part of a multispecies food-habit study conducted 

 from 1973 through 1976. The sampling area cov- 

 ered the continental shelf waters from Cape 

 Hatteras. N.C., to the Canadian coast of Nova 



Manuscript accepted April 1982. 



FISHERY BULLETIN: VOL. 80, NO. 4. 1982. 



745 



