examined the stomach contents of ocean pout collected 

 in Block Island Sound, in Southern New England. The 

 primary prey in this area was the amphipod, Lep- 

 tocherius pinguls, with the sand dollar being the second 

 most important prey. Tunicates were also important, 

 making up almost 10' i of the diet by weight. In the pres- 

 ent study tunicates were again found to be a reasonably 

 significant prey item but only in the Gulf of Maine. Bige- 

 low and Schroeder (1953) examined the stomachs of 

 ocean pout taken in Massachusetts Bay in 1924 and near 

 the Nantucket Lightship in 1950. They found that the 

 rather large specimen from Massachusetts Bay was full 

 of brittle stars and spider crabs while the animals taken 

 near the Lightship were full of small sea scallops. 



Dietary Overlap 



The percentage similarity in diet (Fig. 3 and text) is a 

 relative measure of overlap of the food habits, where 

 overlap is simply defined as the use of the same resource 

 by more than one predator regardless of the resource 

 abundance. In contrast, resource competition exists only 

 if the demand for prey outstrips the immediate supply 

 (Weatheriey 1963; Keast 1965, 1977; Zaret and Rand 

 1971). The index of diet overlap presented here, there- 

 fore, just highlights the potential for food resource com- 

 petition between these fish populations which could exist 

 if certain prerequisites were met, as discussed below. 



The distribution of many of these fish, especially the 

 commercially important species, has been documented 

 from the groundfish- survey data collected by the per- 

 sonnel at the Woods Hole Laboratory (Colton 1955, 1972; 

 Fritz 1965; Grosslein and Bowman 1973; Grosslein and 

 Clark 1976'). The ranges of many of these predators over- 

 lap, however, the major concentrations of the different 

 species of fish are not usually the same. Fritz (1965), for 

 example, has summarized the distribution of seven 

 gadids from the autumn groundfish survey data, 1955-61. 

 His results showed that silver hake were ubiquitous, but 

 over the 6-yr study they were most abundant off Cape 

 Cod and to the south, and also on the western and south- 

 eastern parts of Georges Bank. Atlantic cod occurred 

 north of lat. 41°00' and were abundant off Nantucket, 

 north of Cape Cod, and southeast of Nova Scotia. The 

 haddock, pollock, and white hake might be considered 

 boreal species as they, like the Atlantic cod, all occurred 

 north of lat. 41°00'. The major concentration of haddock 

 was on the northern edge of Georges Bank and on Browns 

 Bank. Pollock were abundant near Nova Scotia but also 

 occurred in moderate concentrations in the Gulf of 

 Maine. White hake were found along the northeastern 

 edge of Georges Bank and also in the Gulf of Maine. Red 

 hake occurred throughout the area ranging from Nova 

 Scotia to Cape May, N.J., being most abundant south of 



'Grosslein. M. D.. and S. H. Clark. 1976. Distribution of selected 

 fish species and status of major fisheries in the Northwest Atlan- 

 tic. Technical reference document for bilateral negotiations between 

 USA and Canada. NOAA, NMFS, Northeast Fisheries Center Lab. 

 Ref. 76-12. 171 p. 



Cape Cod. A more recent summary of the groundfish sur- 

 vey data has been prepared by Grosslein and Clark (see 

 footnote 3). This document includes both spring and 

 autumn cruise data which gives some idea of the seasonal 

 changes in the distribution of the commercially impor- 

 tant gadoids. Although certain species, such as red hake, 

 have distinct seasonal migrations, the overall distribu- 

 tion of many of these fish is reasonably constant through- 

 out the year and even from year to year. Colton (1972), 

 for example, found no major change in the general dis- 

 tribution of the haddock during the period 1950-68 

 despite changing trends in seawater temperature. From 

 this survey data it may tentatively be concluded that, on 

 a broad scale, although some spatial overlap occurs be- 

 tween many of the 15 gadiform fishes we have studied, 

 the major concentrations of each fish are usually suf- 

 ficiently distinct so that competition between the various 

 populations for the same food resource would be mini- 

 mal. On a more localized scale, however, spatial overlap 

 may occasionally be severe as has been documented by 

 Grosslein and Bowman (1973). They considered the 

 problem of bycatch in ICNAF Subareas 5 and 6 (Georges 

 Bank to Cape Hatteras), pointing out that a bottom 

 trawl fishery in these areas could not be conducted with- 

 out harvesting a substantial mixture of species. In par- 

 ticular, aggregates of red and silver hake are con- 

 sistently found in Southern New England as are mixtures 

 of Atlantic cod, haddock, and hake on Georges Bank. For 

 situations such as these, food habits studies would have 

 to be conducted on fish collected from mixed catches to 

 determine if the fish are feeding on the same prey at the 

 same time or if there is significant resource partitioning, 

 thus eliminating the potential for competition as was 

 found by Jones (1978) for some gadoid fishes in Euro- 

 pean waters. 



Even if spatial overlap were to occur on a significant 

 scale, it may further be counteracted by short-term tem- 

 poral changes in predator distribution and activity. Dai- 

 ly activity cycles may effectively function to segregate 

 predators although the predators would, at times, share 

 the same prey (Graham 1924; Rae 1967; Brunei 1972; 

 Daan 1973; Arntz 1974; Jenkins and Green 1977). 



Finally, even when the spatial and temporal distri- 

 bution of these species is accounted for, it is difficult to 

 make firm conclusions regarding resource limitation 

 without an extensive knowledge of the benthic com- 

 munity available for exploitation. StueUes such as those 

 conducted by Arntz (1971, 1973, 1974) which combine 

 food habits investigations, feeding chronology exper- 

 iments, and a quantitative evaluation of the macro- 

 benthos are needed before we can quantitatively de- 

 termine the degree to which food resource competition 

 actually exists. 



A number of nonbiological factors may also influence 

 the calculation of dietary overlap and these must be 

 accounted for when interpreting overlap data. The level 

 of identification of the prey will affect the overlap cal- 

 culations since broader taxonomic groupings actually in- 

 crease the observed degree of overlap. Moyle (1977) gave 

 an example of this when comparing the diet of sculpins 



21 



