FISHERY BULLETIN: VOL. 79, NO. 4 



30 m of water along the eastern side of Johns Bay 

 (Figure 1). Two or three 15-min trawls were made 

 during each sampling trip to obtain a sufficient 

 number offish. I measured temperature and salin- 

 ity at the surface ( 1 m and at a depth of 30 m ) using 

 a Beckman"* RS5-3 salinometer. 



Immediately after capture I sorted the trawl 

 catch by species. Total length (TL to nearest mil- 

 limeter), weight (to nearest gram), sex, and 

 maturity determinations were made for each 

 specimen. By cutting at the esophagus and pyloric 

 constriction stomachs were removed from a 

 maximum of 20 specimens ( >15 cm TL) of each 

 species (a subsample of the total size range), fixed 

 in 10% Formalin, later preserved in 709^ iso- 

 propanol, and then, contents were sorted and iden- 

 tified to the lowest possible taxon. Prey items were 

 damp dried on bibulous paper, and the number of 

 individuals and total wet weight ( to nearest 0.01 g) 

 of each prey category were recorded. Total weight 



■"Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



JOHNS BAY 

 GULF OF MAINE 



JOHNS BAY 



s> 



p 



included shell weight for mollusks, crustaceans, 

 and echinoderms. 



I determined the contribution of different prey 

 categories to the diet of a fish species by three 

 methods: 1) the percentage weight of a prey cate- 

 gory (pooled) to the weight of the total stomach 

 contents, 2) the percentage abundance of indi- 

 viduals of a prey category to the total number of 

 individual prey in the stomachs, and 3) the 

 percentage frequency of occurrence of the number 

 of stomachs in which a prey category occurred to 

 the total number of stomachs examined. Berg 

 (1979) discussed the limitations of using any 

 single measure to evaluate the importance of a 

 food taxon. Therefore, an index of relative impor- 

 tance, modified from Pinkas et al. ( 1971), has been 

 calculated since it incorporates all three measures 

 and gives a better assessment of the dietary im- 

 portance of a prey group. The formula used is as 

 follows: IRI = iN + W)F, where N = numerical 

 percentage, W = weight percentage, F = percent- 

 age frequency of occurrence, and IRI = index of 

 relative importance. The original formulation 

 proposed by Pinkas et al. (1971) used volumetric 

 percentage instead of percentage weight. 



I calculated niche overlap using the formula 

 proposed by Pianka (1973): 



A„- 



[IPihPjh] 



Figure l. — Location of trawling area (closed bo.x) and benthic 

 sampling sites t transects A-C, stations 1-3 ) in Johns Bay, Maine. 



where A,, is the overlap of species 7 on species i\ 

 Plf^ is the proportion (percentage weight) of a par- 

 ticular food h {h = l...,s) in the diet of species i; 

 and pj/^ is the proportion of the same food h in the 

 diet of species J. Values for the overlap index may 

 vary between 0, if no overlap occurs, and 1 for 

 complete overlap. A value >0.3 is significant and 

 ones >0.7 are considered high (Keast 1978). 



The division of principal prey among the pred- 

 ators was examined by means of a partition plot. I 

 defined principal prey as those with an IRI >100 

 because this emphasized the major food sources of 

 each predator. Principal prey accounted for 73.3- 

 94.7% by number of the food items in predator 

 diets. The partition plot facilitates the calculation 

 of the percentage reoccurrence of prey in more 

 than one predator, which is empirically defined as 

 the number of reoccurrences observed divided by 

 the total number of reoccurrences possible in the 

 plot, multiplied by 100 (Tyler 1972). One reoccur- 

 rence is defined as the presence of a prey in two 



776 



