605 Abstract-Longfin inshore (Loligo Seasonal and size-based predation on pealeii) and northern shortfin (lllex illecebrosus) squids are considered two species of squid by four fish predators important prey species in the North­ west Atlantic shelf ecosystem. The on the Northwest Atlantic continental shelf diets of four major squid predators, bluefish (Pomatomus saltatrix), goose­ Michelle D. Staudinger fish (Lophius americanus), silver hake (Merluccius bilinearis), and summer Department of Natural Resources Conservation flounder (Paralichthys dentatus), were 160 Holdsworth Way examined for seasonal and size-based University of Massachusetts, Amherst changes in feeding habits. Summer Amherst, Massachusetts 01003-9285 and winter, two time periods largely Email address: [email protected] absent from previous evaluations, were found to be the most impor­ tant seasons for predation on squid, and are also the periods when the majority of squid are landed by the regional fishery. Bluefish >450 mm, silver hake >300 mm, and summer Global depletion of marine predators significant predators of squid (Bow­ flounder >400 mm were all found has had dramatic effects on ecosys­ man et al., 2000). Squid represented to be significant predators of squid. tem structure and function (May et between 17% and 95% of the total These same size fish correspond to al., 1979; Jackson et al., 2001; Pauly mass consumed by these four finfish age classes currently targeted for et al., 2002). In many systems, the regionally. Dramatic changes in stock biomass expansion by management ramifications of such changes may not abundance and population structure committees. This study highlights be fully realized. Groundfish declines have occurred since specimens for the importance of understanding how have been linked to simultaneous Bowman et al.'s study were collect­ squid and predator interactions vary increases in cephalopod landings temporally and with changes in com­ ed 25-45 years ago. Exploitation of munity structure and stresses the in fifteen key Food and Agriculture squid has risen substantially, and the need for multispecies management Organization of the United Nations four noted predators have experienced in the Northwest Atlantic. (FAO) areas (Caddy and Rodhouse, severe depletions; stock biomass lev­ 1998). It is uncertain whether cepha­ els have fallen as low as 20-50% of lopod populations are experiencing their respective maximum sustain­ increased growth due to a release able yield (BMsY) limits (NOAA2). from predation (Piatkowski et al., Additionally, the size structure of 2001), or whether increased harvests predator populations within the com­ are representative of the trend toward munity has become skewed because of fishing species at lower trophic levels age-truncation. Recent evaluations of (Pauly et al., 1998). In the Northwest bluefish (Buckel et al., 1999a), goose­ Atlantic, while gadids, flatfish, and fish (Armstrong et al., 1996), silver other demersal species have been hake (Bowman, 1984), and summer reduced because of overfishing (Link flounder (Link et al., 2002) foraging and Garrison, 2002), squid have risen habits have been conducted; however, in status from a mere• bait fishery to sampling has been restricted to one one of the most economically impor­ or two seasons, primarily spring and tant stocks in the region (Cadrin and fall. Analyses that base their results Hatfieldl). on feeding habits collected during a Cephalopods have been documented single season (Buckel et al., 1999b), as principal prey for numerous species or where results are based on data of finfish, elasmobranchs, and marine that have been pooled under the as- mammals (Smale, 1996); however, in comparison to the ecological relation­ ships between fish and their preda­ tors, less is known about the ecological 1 Cadrin, S. X., and E. M. C. Hatfield. 1999. interactions between squid and their Stock assessment of inshore longfin squid Loligo pealeii. Northeast Fish. predators. In a comprehensive evalu­ Sci. Cent. Ref. Doc. 99-12, 107 p. ation of the Northwest Atlantic food 2 NOAA (National Oceanic and Atmospheric Manuscript submitted 7 November 2005 web, bluefish (Pomatomus saltatrix), Administration). 2001. Northeast to the Scientific Editor's Office goosefish (Lophius americanus), sil­ Fisheries Science Center (NEFSC). Status of fishery resources off the north­ Manuscript approved for publication ver hake (Merluccius bilinearis), and eastern United States. Website: http:// 9 March 2006 by the Scientific Editor. summer flounder (Paralichthys den­ www.nefsc.noaa.gov/sos/spsyn/species. Fish. Bull. 104:605-615 (2006). tatus) were ranked among the most html [accessed on 8 March 2006]. 606 Fishery Bulletin 104(4) sumption that the diets collected in one season are a fore adequate information was not available to evaluate proxy for another (Overholtz et al., 2000) could overlook this season. key periods of predation and lead to an underestimation To compare changes in longfin squid abundance in of the total predatory demand imposed on principal prey the demersal environment at different times of day and resources, such as squid. between seasons, the relative masses of prerecruits (WP) The two primary squid species found in Northwest and recruits (WR) present were estimated by using the Atlantic waters, longfin inshore (Loligo pealeii) and equations northern shortfin (lllex illecebrosus), are both highly migratory, and their distribution on the shelf is tempo­ W _ ~ W,P,(year,s ason,timeofdny) ( ) rally variable. Squid move between inshore (spring and P(year,season,timeof day) - k f. 1 summer) and offshore (fall and winter) environments t t(season,timeofday) seasonally (Macy and Brodziak, 2001). On a diurnal basis they move from demersal waters during the day to ~ W, t(year,s ason,timeofday) surface waters at night (Lange and Sissenwine, 1983). WR(year,season,timeofday) = k f. (2) The degree of vertical movement made by squid is also t t(season,timeofday) 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 where t = the index of tows made at NMFS stations in shelf waters are well mixed (Hatfield and Cadrin, 2002). New York, New Jersey, Connecticut, Rhode Prey availability and distribution in the water column, Island, and Massachusetts respective to each water temperature, and other environmental factors are year (2002, 2003), season (winter, spring, believed to influence diel migration patterns (Cargnelli fall), and time of day (day, night, dawn and et al., 1999). Seasonal changes in squid behavior and dusk); habitat use will also affect encounter rates with differ­ W1 = the total mass of longfin squid caught in each ent predators in the demersal environment. tow; and Knowledge of species interactions is imperative to P and R = (1 - P) estimate the proportion of biomass understand population dynamics and to manage stock in each of two size classes, prerecruits (:s:80 recovery (Murawski, 1991). The present study provides mm) and recruits (>80 mm), of longfin squid a current assessment of the reliance on squid popula­ during each season. tions in the Northwest Atlantic region by four major { 1 = the diel correction coefficient representing squid predators; bluefish, goosefish, silver hake, and relative catch rates of longfin squid for each summer flounder. For each predator, ontogenetic and size class, season, and time of day as deter­ seasonal variations in feeding patterns were evaluated. mined by Hatfield and Cadrin (2002) and Additionally, squid abundance in the demersal environ­ was standardized to 1.0 during daytime for ment was related to predator diets as a mechanism for all seasons. diurnal and seasonal changes in predation. Materials and methods 4 NMFS (National Marine Fisheries Service). 2002. Fish­ ermen's report: bottom trawl survey, Cape Hatteras-Gulf Estimating changes in the abundance of Maine: March 5-April 25, 2002, FRV Albatross IV, 34 of longfin inshore squid p. NMFS, Northeast Fisheries Science Center, 166 Water St., Woods Hole, MA 02543. Longfin and shortfin squid are regularly caught in 5 NMFS (National Marine Fisheries Service). 2002. Fish­ National Marine Fisheries Service (NMFS) bottom ermen's report: bottom trawl survey, Cape Hatteras-Gulf of Maine: September 4-0ctober 25, 2002, FRV Albatross trawl surveys. However, catches of shortfin squid were IV, 31 p. NMFS, Northeast Fisheries Science Center, 166 exceptionally low over 2002 and 2003 (the time period Water St., Woods Hole, MA 02543. evaluated during the present study), possibly because 6 NMFS (National Marine Fisheries Service). 2003. Resource of poor recruitment. Catch data used in subsequent cal­ survey report: bottom trawl survey, Cape Hatteras-Southern culations were provided by NMFS bottom-trawl surveys New England: February 4-March 1, 2003, FRV Delaware (NMFS3·4·5,6,7,8). Sufficient information was available for II, 19 p. NMFS, Northeast Fisheries Science Center, 166 Water St., Woods Hole, MA 02543. longfin squid only; consequently abundance estimates 7 NMFS (National Marine Fisheries Service). 2003. Resource were limited to this species. Furthermore, abundance survey report, bottom trawl survey, Cape Hatteras-Gulf of surveys were not conducted during the summer; there- Maine: March 5, 2003-April27, 2003, FRV Delaware II, 34 p. NMFS, Northeast Fisheries Science Center, 166 Water St., Woods Hole, MA 02543. 3 NMFS (National Marine Fisheries Service). 2002. Fish­ 8 NMFS (National