606 



Fishery Bulletin 104(4) 



sumption that the diets collected in one season are a 

 proxy for another (Overholtz et al., 2000) could overlook 

 key periods of predation and lead to an underestimation 

 of the total predatory demand imposed on principal prey 

 resources, such as squid. 



The two primary squid species found in Northwest 

 Atlantic waters, longfin inshore iLoligo pealeii) and 

 northern shortfin illlex illecebrosus). are both highly 

 migratory, and their distribution on the shelf is tempo- 

 rally variable. Squid move between inshore (spring and 

 summer) and offshore (fall and winter) environments 

 seasonally (Macy and Brodziak, 2001). On a diurnal 

 basis they move from demersal waters during the day to 

 surface waters at night (Lange and Sissenwine, 1983). 

 The degree of vertical movement made by squid is also 

 known to vary seasonally. Activity is more pronounced 

 during warmer months when the water column is strati- 

 fied and is diminished during winter and spring when 

 shelf waters are well mixed (Hatfield and Cadrin, 2002). 

 Prey availability and distribution in the water column, 

 water temperature, and other environmental factors are 

 believed to influence diel migration patterns (Cargnelli 

 et al., 1999). Seasonal changes in squid behavior and 

 habitat use will also affect encounter rates with differ- 

 ent predators in the demersal environment. 



Knowledge of species interactions is imperative to 

 understand population dynamics and to manage stock 

 recovery (Murawski, 1991). The present study provides 

 a current assessment of the reliance on squid popula- 

 tions in the Northwest Atlantic region by four major 

 squid predators; bluefish, goosefish, silver hake, and 

 summer flounder. For each predator, ontogenetic and 

 seasonal variations in feeding patterns were evaluated. 

 Additionally, squid abundance in the demersal environ- 

 ment was related to predator diets as a mechanism for 

 diurnal and seasonal changes in predation. 



fore adequate information was not available to evaluate 

 this season. 



To compare changes in longfin squid abundance in 

 the demersal environment at different times of day and 

 between seasons, the relative masses of prerecruits (W,) 

 and recruits (W^^, ) present were estimated by using the 

 equations 



W, 



Piyear. season, time of day 



, = I 



w,p. 



t t f year, season, time of day i 



r fr 



( season, tune of day ) 



W, 



Rl year, season, tune of da' 



v,=I 



W.R, 



I 1 1 year, season, time of day i 



htst 



(1) 



(2) 



season, time of day i 



where t = the index of tows made at NMFS stations in 

 New York, New Jersey, Connecticut, Rhode 

 Island, and Massachusetts respective to each 

 year (2002, 2003), season (winter, spring, 

 fall), and time of day (day, night, dawn and 

 dusk); 

 W, = the total mass of longfin squid caught in each 

 tow; and 



P and i? = (1 - P) estimate the proportion of biomass 

 in each of two size classes, prerecruits (<80 

 mm) and recruits (>80 mm), of longfin squid 

 during each season. 

 /", = the diel correction coefficient representing 

 relative catch rates of longfin squid for each 

 size class, season, and time of day as deter- 

 mined by Hatfield and Cadrin (2002) and 

 was standardized to 1.0 during daytime for 

 all seasons. 



Materials and methods 



Estimating changes in the abundance 

 of longfin inshore squid 



Longfin and shortfin squid are regularly caught in 

 National Marine Fisheries Service (NMFS) bottom 

 trawl surveys. However, catches of shortfin squid were 

 exceptionally low over 2002 and 2003 (the time period 

 evaluated during the present study), possibly because 

 of poor recruitment. Catch data used in subsequent cal- 

 culations were provided by NMFS bottom-trawl surveys 

 (NMFS^^-SS-'-S). Sufficient information was available for 

 longfin squid only; consequently abundance estimates 

 were limited to this species. Furthermore, abundance 

 surveys were not conducted during the summer; there- 



NMFS (National Marine Fisheries Service). 2002. Fish- 

 ermen's report: bottom trawl survey. Cape Hatteras-SE 

 Georges Bank: February 5-March 2,' 2002, FRY Albatross 

 IV. 24 p. NMFS, Northeast Fisheries Science Center, 166 

 Water St., Woods Hole, MA 02.543. 



^ NMFS (National Marine Fisheries Service). 2002. Fish- 

 ermen's report: bottom trawl survey. Cape Hatteras-Gulf 

 of Maine: March 5-April 25, 2002," FRV Albatross IV. 34 

 p. NMFS, Northeast Fisheries Science Center, 166 Water 

 St., Woods Hole, MA 02543. 



5 NMFS (National Marine Fisheries Service). 2002. Fish- 

 ermen's report: bottom trawl survey. Cape Hatteras-Gulf 

 of Maine: September 4-October 25, 2002, FRV Albatross 

 IV, 31 p. NMFS, Northeast Fisheries Science Center, 166 

 Water St., Woods Hole, MA 02543. 



6 NMFS (National Marine Fisheries Service). 2003. Resource 

 survey report: bottom trawl survey. Cape Hatteras-Southern 

 New England: February 4-March 1, 2003, FRV Delaware 

 II, 19 p. NMFS, Northeast Fisheries Science Center, 166 

 Water St., Woods Hole, MA 02543. 



' NMFS (National Marine Fisheries Service). 2003. Resource 

 survey report, bottom trawl survev, Cape Hatteras-Gulf of 

 Maine: March 5, 2003-April 27, 2003, FRV Delaware II. 34 

 p. NMFS, Northeast Fisheries Science Center. 166 Water 

 St., Woods Hole, MA 02543. 



* NMFS (National Marine Fisheries Service). 2003. Resource 

 survey report: bottom trawl survey, Cape Hatteras-Gulf 

 of Maine: September 7-November 1, 2003, FRV Albatross 

 IV. 35 p. NMFS, Northeast Fisheries Science Center, 166 

 Water St., Woods Hole, MA, 02543. 



