Brodziak and Hendrickson: Environmental effects on survey catches of Loligo pealei and lllex illecebmsus 



21 



ently. Catches of juveniles decreased at 

 greater depths, and there was a bathy- 

 metric pattern of larger /. illecebrosus 

 with increasing depth. Although catches 

 of adults increased with depth, in con- 

 trast with L. pealei, catches of/, illece- 

 brosus adults peaked at depths of 186- 

 366 m. This depth range corresponds to 

 the convergence zone between continen- 

 tal shelf and slope waters and intersects 

 the shelf edge at roughly 150-200 m 

 (Bowman, 1977). Commercial fishery 

 catch data were consistent with this ob- 

 servation, as Lange, Ingham, and Price* 

 reported, noting that the highest catch 

 rates recorded by domestic observers in 

 the distant-water /. illecebrosus fishery 

 were generally located within several 

 miles of the shelf-slope front, at the 

 shoreward edge of the convergence zone, 

 between continental shelf and slope wa- 

 ter. Overall, these observations suggested 

 that the shelf-slope convergence zone is an impor- 

 tant habitat for adult /. illecebrosus during autumn. 

 Our analyses also demonstrated the importance 

 of diel effects on L. pealei catches. This generally 

 corroborated previous observations. Summers (1968) 

 observed that L. pealei migrated vertically and that 

 squid could be observed near the surface at night. 

 Summers (1969) and Lange and Sissenwine (1983) 

 observed that survey catches of L. pealei during the 

 day were consistently higher than catches at night. 

 Serchuk and Rathjen ( 1974) observed that 909^^ of L. 

 pealei survey catches occurred during daylight, and 

 Sissenwine and Bowman (1978) found that L. pealei 

 catches were significantly higher during day. In our 

 study, significant differences in diel effects were de- 

 tected for both juveniles and adults. Catches of L. 

 pealei juveniles and adults increased with increas- 

 ing light availability although the diel effects were 

 more pronounced for juveniles. In particular, time 

 zone I and II catches of juveniles were 92% and 54% 

 below mean catch during the day, whereas zone I and 

 II catches of adults were 66% and 17% lower. The 

 fact that the diel effect was more pronounced for ju- 

 venile L. pealei may be related to differences in feed- 

 ing behavior between juveniles and adults. L. pealei 

 hatchlings must feed near the surface until their ten- 

 tacles develop and they can capture larger prey 



-1.00 



<14.4 14.4-20.6 >20.6 



Surface temperature (C) 

 Figure 6 (continued) 



^ Lange. A.M. T.,M. C.Ingham, and C.A.Price. 1984. Distri- 

 bution of maturing lllex illecebrosus relative to the shelf-slope 

 water front of the northeastern United States. NAFO SCR Doc. 

 84/IX/109, Ser No. N906, 18 p. 



(Vecchione, 1981) whereas adults are primarily de- 

 mersal and commonly rest on the bottom (Hanlon et 

 al., 1983). Small juveniles feed primarily on crusta- 

 ceans and gradually shift to a more diverse diet of 

 crustaceans, fish, and squid as they grow and can 

 capture larger and more energetically valuable prey 

 (Vovk, 1972; Macy, 1982; Vovk, 1985; Anderson and 

 GrLswold, 1988). As a result, juveniles need to un- 

 dertake vertical migrations at night more frequently 

 to capture a sufficient amount of suitable prey. 



The fact that /. illecebrosus catches were moder- 

 ately associated with time of day was consistent with 

 Sissenwine and Bowman (1978) and Shepherd and 

 Forrester^ who found that time of day had an impor- 

 tant effect on catches of/, illecebrosus. Diel vertical 

 migrations may be related to /. illecebrosus feeding 

 activity. Vinogradov and Noskov (1979) found that 

 the feeding intensity of this species is greatest at 

 night and lowest during the day. Similar to catches 

 of L. pealei, catches of/, illecebrosus were higher 

 during day, and diel effects were more important for 

 juveniles than adults. In particular, catches of juve- 

 niles at night and those at dawn and dusk were 37% 

 and 57% below mean catch during the day. Guided 

 by a review of the NEFSC domestic sea sampling 

 program database, fishermen targeting/, illecebrosus 

 take advantage of the behavior of this species by fish- 

 ing only between dawn and dusk, when the squid 

 are available to commercial bottom trawl gear. 

 Schools of /. illecebrosus are not targeted at night 

 because they are too dispersed near the surface of 

 the water column. Instead schools of/, illecebrosus 

 are targeted beginning at dawn, when they can be 



