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Fishery Bulletin 104(4) 



cruises conducted by the NMFS during these two periods 

 were the primary data source for these prior analyses. 

 The present study indicates that these two seasons are 

 not wholly representative of total predation pressure 

 on squid; indeed, winter and summer may be primary 

 periods for predation on squid. Consequently, previous 

 estimates of the total predatory demand imposed on 

 squid populations by these predators may have been 

 underestimated (Buckel et al., 1999b; Overholtz et al., 

 20001. It should be noted that in order to collect ade- 

 quate numbers of fish during all four seasons, fishery- 

 dependent sources were used. However, nonrandom sam- 

 pling could have influenced seasonal differences seen in 

 the present study. 



It is likely that heightened predation on squid dur- 

 ing the winter season reflects an increase in habitat 

 overlap by both predator and prey. All species of squid 

 and fish evaluated in this study are known to retreat 

 to warmer waters of the outer shelf and slope during 

 the colder months of the year. This movement may 

 act to concentrate fish and squid and lead to elevated 

 predator-prey encounter rates. Predicted nighttime 

 and crepuscular catches during winter indicate that 

 longfin squid are at higher concentrations in the de- 

 mersal environment than at other seasons and reflect 

 diminished vertical migrations by squid (Hatfield and 

 Cadrin, 2002). Increased residence time in bottom wa- 

 ters may make squid more susceptible to demersal 

 predators and may explain their marked increase in 

 silver hake and summer flounder diets during winter. 

 Although the present study has made contributions to 

 increase knowledge of predation rates during summer, 

 data on squid abundance levels were insufficient to 

 make inferences about predator interactions during 

 this season. 



Historically, longfin squid seasonal biomass on the 

 shelf has been estimated to be at its maximum during 

 the fall (Cargneli et al., 1999). This trend was reflected 

 in catch results for prerecruits during the time period 

 evaluated; however, this supposed increase in availabil- 

 ity was not reflected in predator diets. Instead, sum- 

 mer flounder and bluefish both exhibited diminishing 

 reliance on squid and increased predation on similar 

 piscine prey (Staudinger, 2004). It is unclear whether 

 predator and squid size relationships or predator prefer- 

 ence is mediating bluefish and summer flounder forag- 

 ing behavior; therefore further studies are warranted. 

 Alternatively, it has been acknowledged that the NMFS 

 fall bottom-trawl survey has better coverage of the shelf 

 than the winter and spring surveys, as well as the 

 greatest overlap with the distribution range of longfin 

 squid. "^ Both of these factors could influence seasonal 

 historical abundance estimates and inaccurately rep- 



i« NEFSC (Northeast Fisheries Science Center). 2002. Report 

 of the 34th northeast regional stock assessment workshop 

 (34th SAW); stock assessment review committee (SARC) 

 consensus summary of assessments. Northeast Fish. Sci. 

 Cent. Ref. Doc. 02-06, 3.56 p. National Marine Fisheries 

 Service, 166 Water St., Woods Hole, MA 02543. 



resent the fall population in relation to other times of 

 year. 



Size-based shifts in predation by fish 



Ontogenetic shifts in feeding commonly take place in 

 the early life stages of a fish. However, for many spe- 

 cies, less is known about progressive shifts through the 

 adult stages of growth (Gerking, 1994). Bluefish showed 

 a dramatic shift in diet at lengths of approximately 

 450 mm, transitioning from a diet primarily of piscine 

 prey to that of squid. Longfin squid was the dominant 

 cephalopod identified in the diets across all sizes of 

 bluefish; however, shortfin squid was found to increase 

 in importance in large bluefish. It is at this larger (551-1- 

 mm) size that bluefish begin to inhabit outer-shelf and 

 offshore waters (Fahay et al., 1999). Because shortfin 

 squid are more pelagic than longfin squid (Brodziak 

 and Hendrickson, 1999), this change in diet may reflect 

 an ontogenetic shift towards offshore food resources as 

 bluefish mature. 



Silver hake also exhibited a strong and positive in- 

 crease in predation on squid with increasing size. Sig- 

 nificant differences were observed among size class- 

 es — the greatest amount of squid being consumed by 

 fish 300 mm and greater. The size at which silver hake 

 were found to begin feeding on squid is similar to that 

 found in previous studies (Garrison and Link, 2000). 

 However, the relative proportions consumed by large 

 hake were found to be nearly double those previously 

 reported. It is possible that the findings of the present 

 study represent an expanded view of the silver hake 

 diet at larger body sizes because of intensified sampling 

 over all seasonal periods. Alternatively, increased squid 

 consumption could reflect decadal differences in squid 

 availability. 



Summer flounder spend the juvenile stage of their 

 lives in bays and estuaries. Link et al. (2002) reported 

 squid in the diets of individuals smaller than those 

 examined in the present study. Therefore, the initial 

 shift toward squid by young flounder as they transition 

 from inshore to offshore waters may not have been real- 

 ized. In fish that were sampled on the continental shelf, 

 squid were found to be an important constituent of diets 

 across all body sizes. Although a statistically significant 

 shift among size classes was not detected, predation on 

 squid was observed to peak in medium-size fish. This 

 finding is similar to that of Bowman et al. (2000), who 

 determined squid consumption peaked between 500 and 

 600 mm and then declined again for larger fish. 



Bowman et al. (2000) reported cephalopods as a major 

 constituent (>20':'( ) of the goosefish diet in the southern 

 New England region. In a study conducted by Arm- 

 strong et al. (1996), longfin squid were found in small 

 goosefish (<400 mm TL) stomachs during summer and 

 represented approximately 10% of the diet. In contrast, 

 the present study found that squid contributed trivial 

 amounts to the diet of goosefish across all size classes 

 and seasons. It is unclear why such vast discrepancies 

 were observed between these studies. One possible ex- 



