Effect of tow duration on estimations of CPUE and abundance of the grenadier Coelorinchus jordani (Gadiformes, Macrouridae)

The effect of tow duration on estimates of catch per unit of swept area (CPUE) and the abundance of the grenadier Coelorinchus jordani was investigated in Kagoshima Bay, southern Japan. A total of 305 tows were completed using a trawl net with mesh sizes of 37.9 mm and 20.2 mm at the body and cod-end, respectively. Towing was conducted at pre-determined warp lengths for each station and for preset tow durations of 10 or 20 min. Effective tow durations (actual fishing times) were computed from the trawl depth profiles derived from compact temperature-depth loggers that were attached to the net. All hauls were sorted by species, and the total numbers and weights of the catches were recorded. Estimations of CPUE and biomass were conducted for C. jordani, one of the most abundant fish species in Kagoshima Bay. Effective tow durations varied; on average, they were >50% longer than the preset times. CPUE and biomass estimations for C. jordani were significantly influenced by the tow duration. Estimates from 10-min tows were approximately three times higher than estimates from 20-min tows. The total catch per haul and log-transformed 10:20-min preset CPUE ratios were positively correlated, indicating that haul size might influence CPUE for C. jordani. Consequently, the influence of other species on CPUE and biomass estimations for a particular species in multi-species fisheries cannot be ignored. Coefficients of variations for CPUE and biomass estimations from both preset and effective tow durations were not significantly different. However, estimations that were based on the effective tow durations showed slightly lower coefficients of variations (CVs), and the effective 10- and 20-min tow durations were equally efficient for estimating the CPUE and abundance of C. jordani.

► We examine effect of tow duration on estimates of abundance for Coelorinchus jordani in Kagoshima Bay, Japan. ► Tow durations were preset for 10 and 20 min and effective tow durations computed from trawl depth profiles. ► Effective tow durations were 50% longer than preset times and CPUE and biomass estimations for C. jordani are significantly influenced by tow duration. ► Total catch per haul and log-transformed 10:20-min preset CPUE ratios were positively correlated, hence haul size may influence CPUE for C. jordani.