Effects of time of solar day, jigging method and jigging depth on catch rates and size of Gould's squid, Nototodarus gouldi (McCoy), in southeastern Australian waters

Three squid-jigging experiments were conducted from 29 December 1980 to 12 April 1981 on FV Hoyo Maru 81. The aim was to determine the effects of time of night or day and of depth of setting jigs controlled by jig-line length on catch rates and mantle length of Gould's squid taken by machine-jigging and hand-jigging methods. Each machine had two jig-lines with 25 jigs on each jig-line and each hand-jigging fisher used one line with a single jig. Catch per unit effort (CPUE) expressed as weight of squid per machine-hour or weight of squid per hand-jigging fisher-hour was higher for the hand-jigging method than for the machine-jigging method at all times over the 24-h period. CPUEs were higher during the night than during the day for both methods, and, apart from lower CPUEs during the first 2 h of the night, CPUEs were steady through the night. CPUEs for hand-jigging from the surface to > 80 m depth exceeded those for machine-jigging in the depth-range 0-80 m and CPUEs for machine-jigging declined as the depth-range was reduced from 0-80 m to 0-40 m. Mean mantle lengths of both female and male squid caught by hand-jigging were greater than those caught by machine-jigging at all times and all depths. Mean length for machine-jigging declined as depth-range was reduced from 0-80 m to 0-40 m. Diel vertical migration of squid, whereby small squid tend to rise higher in the water column than large squid during darkness, is advanced as a hypothesis to explain these patterns. Other factors that could have contributed to the pattern of catches include time out of the water by jigs not fishing between jigging cycles, gear competition between jigs, and short-term localised depletion of squid in the immediate vicinity of the jigs. (C) 1998 Elsevier Science B.V.