10 



Fishery Bulletin 97(1), 1999 



76 



44 



42 - 



40 



38 



and surface and bottom tem- 

 perature on survey catches to 

 determine whether the habitat 

 preferences of juveniles and 

 adults overlap. Potential effects 

 of different categories of depth, 

 time of day, and surface tem- 

 perature and bottom tempera- 

 ture on mean catch per tow 

 were tested for juveniles and 

 adults of both species, and the 

 potential impacts on the ratio 

 of juvenile to adult catches were 

 also examined. These analyses 

 also suggested whether environ- 

 mental effects were similar for 

 these sympatric species, be- 

 cause the degree of spatial over- 

 lap between their geographic 

 distributions is at a maximum 

 during autumn. 



Last, we evaluated the rela- 

 tive magnitude of diel effects on 

 juvenile and adult squid catches 

 in relation to survey design and 

 fluctuations in annual squid 

 abundance. Previous bottom 

 trawl studies have shown that 

 diel effects on catches of L. 

 pealei and /. illecebrosus can be 

 important (Roper and Young, 

 1975; Sissenwine and Bowman, 

 1978; Lange and Sissenwine, 

 1983; Arkhipkin and Fedulov, 

 1986; Shepherd and Forrester^) 

 because greater catchability oc- 

 curs during the day. As a result 

 of diel vertical migrations, 



night catches of these two species can be biased low 

 in relation to day catches. To account for diel effects 

 on minimum swept-area estimates of L. pealei biom- 

 ass and stock size, diel correction factors have been 

 used to adjust nighttime bottom trawl catches to 

 daytime equivalents (Lange and Sissenwine, 1980, 

 1983). However, these correction factors were devel- 

 oped for total numbers of squid and were not size 

 specific, even though Sissenwine and Bowman ( 1978) 

 noted that a sixfold difference between the diel cor- 

 rection factor for weight and numbers of L. pealei 

 suggested differential vertical migration by size. In 

 this study, we applied a general linear model to de- 



74 



72 



70 



68 



66 



36 



Offshore depth strata 

 surveys, 1963-94. 



Figure 1 



for autumn Northeast Fisheries Science Center bottom trawl 



^ Shepherd, G., and J. Forrester 1987. Diurnal variation in 

 catchability during bottom trawl surveys off the Northeastern 

 United States. ICES Council Meeting i987/B;44 ( mimeo), 15 p. 



termine size-specific diel correction factors for L. 

 pealei and /. illecebrosus that accounted for poten- 

 tial effects of survey design and fluctuations in an- 

 nual abundance. When diel effects were significant, 

 correction factors were determined to standardize 

 nighttime catches to daytime units. 



Materials and methods 



Survey data 



Research survey data were analyzed from NEFSC 

 autumn bottom trawl surveys conducted during 

 1967-94 between Cape Hatteras, North Carolina, 

 and the Gulf of Maine. In general, autumn surveys 

 were conducted from mid-September through mid- 



