An analysis of environmental effects on survey catches of squids Loligo pealei and Illex illecebrosus in the northwest Atlantic

An analysis of environmental effects on autumn survey catches of two commer cially exploited squid species, Loligo pealei and Illex illecebrosus, was c onducted. Research survey data collected during 1967-94 were used to determ ine the significance and relative importance of average depth of tow, time of day, bottom temperature, and surface temperature on bottom trawl catches of L. pealei, a neritic species, and I. illecebrosus, an oceanic species. We examined habitat associations of both species by using randomization met hods and found that L. pealei was consistently associated with all of the e nvironmental factors examined. In comparison with L. pealei, catches of I. illecebrosus were much lower and associations with environmental factors we re inconsistent. We also examined whether environmental conditions affected catches of juvenile and adult squid differentially. Depth had an important effect on the magnitude of juvenile and adult L. pealei catches, with the ratio of juvenile to adult catches decreasing with depth. Depth had a simil ar, but less pronounced, effect on I. illecebrosus catches. Time of day als o affected L. pealei and I. illecebrosus catches. Catches of both species w ere lowest at night and diel effects were more pronounced for juveniles tha n for adults. Bottom and surface temperatures had a substantial effect on c atches of juvenile and adult L. pealei but had a variable influence on I. i llecebrosus catches. The joint effects of depth stratification, time of day , and annual squid abundance on survey catches were also analyzed to determ ine correction factors for diel differences in catchability of juvenile and adult squid. Significant diel differences in catchability were detected fo r juvenile and adult L. pealei and for juvenile I. illecebrosus and diel co rrection factors were determined for survey catches of these size categorie s. In contrast, significant diel differences in catchability of adult I. il lecebrosus were not detected.