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



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o 



Discussion 



We found that L. pealei was consistently 

 associated with all the environmental 

 factors examined. The habitat associa- 

 tions of L. pealei with depth, bottom tem- 

 perature, and surface temperature indi- 

 cated that these factors were important 

 determinants of its autumn distribution. 

 The consistent association of L. pealei 

 with time of day appeared to be a conse- 

 quence of the behavioral ecology of the 

 species as it moves upward in the water 

 column during the night to avoid preda- 

 tion or to acquire prey. 



In comparison with L. pealei, autumn 

 survey catches of /. illecebrosus were 

 much lower and associations with envi- 

 ronmental factors were inconsistent. For 

 many of the years examined, /. illece- 

 brosus catches were not associated with 

 depth, temperature, or time of day. lllex 

 illecebrosus feeds opportunistically in continen- 

 tal shelf waters from Newfoundland to Cape 

 Hatteras, during summer and autumn, prior to 

 undertaking a lengthy offshore migration to 

 spawning areas south of Cape Hatteras (O'Dor 

 and Dawe, in press). Therefore, the lack of con- 

 sistent habitat associations may be partly due 

 to incomplete survey coverage of/, illecebrosus 

 habitat during autumn and a lack of availabil- 

 ity of this species to the bottom trawl survey 

 gear. For example, the timing of the offshore 

 and southward migration of/, illecebrosus may 

 precede rather than follow the timing of the 

 autumn survey in some years. In addition, few 

 stations are sampled in the autumn habitat of 

 this species, at the shoreward edge of the con- 

 vergence zone, and these low sample sizes may 

 make it difficult to detect habitat associations. 

 Nevertheless, /. illecebrosus catches were sig- 

 nificantly associated with depth, time of day, 

 and surface temperature in approximately half 

 of the years examined. Depth and surface tem- 

 perature may be determinants of preferred 

 habitat of /. Illecebrosus; however the impor- 

 tance of these factors varied between years. The 

 significant association of/, illecebrosus catches 

 with daylight indicated that the species under- 

 takes vertical migrations similar to those of L. pealei, 

 but with less regularity as might be expected of an 

 ommastrephid (Roper and Young, 1975). 



Depth had an important effect on the magnitude 

 of juvenile and adult L. pealei catches. This associa- 

 tion with depth corroborated previous studies. 



0.50 



c 10.2 10.2-12.9 >12.9 



Bottom temperature (C) 

 Figure 5 (continued) 



Serchuk and Rathjen (1974) examined the distribu- 

 tion and relative abundance of L. pealei and found 

 that the highest catches were at depths less than 

 100 m during autumn. Vovk ( 1978) reported that the 

 primary depth range of L. pealei was 50-100 m dur- 

 ing September-November, and Lange and Sissen- 



