Filling gaps in the biology and habitat use of two spurdog sharks (Squalus japonicus and Squalus brevirostris) in the East China Sea

Abstract

The conservation and management of sharks requires appropriate information on their biological and habitat characteristics. Data needed for the conservation and management of two dominant demersal shark species in the East China Sea, the spurdogs (Squalus japonicus and Squalus brevirostris), are inadequate. To remedy this, we analysed biological, catch and abiotic data from bottom trawl surveys conducted in 2000–05 and 2008–14 from within the East China Sea. The precaudal length of male and female S. japonicus at 50% sexual maturity was 396.1 and 459.9mm respectively, whereas for S. brevirostris it was 289.6 and 354.9mm respectively. Prey overlap was high between species, with lanternfishes the most important prey. The fractional trophic level was 4.33 for S. japonicus and 4.31 for S. brevirostris. We estimated spurdog habitat using a generalised additive model. Depth affected the presence of both sharks, with latitude also affecting the presence of S. brevirostris. These two spurdog species were spatially segregated over the continental slope, and body-length and sex-ratio distributions suggested that there was a maturity-based segregation within and among species. The results of this study improve our understanding of the population dynamics of these spurdog species, their reproduction and diet, fundamental information required for their conservation and for improved fisheries management.

High reproductive potential of the commercially exploited shark Squalus suckleyi in the major fishing grounds of the western North Pacific

“…To characterize gonadal development, the gonadosomatic index (GSI) was calculated for males and females based on GW divided by BW and multiplied by 100. Individuals were secondarily classified as immature or mature based on clasper condition and the presence of semen in males, and pregnancy status and development of the oviducal gland and ovarian oocytes in females (Yano et al ., 2020).…”

“…To characterize gonadal development, the gonadosomatic index (GSI) was calculated for males and females based on GW divided by BW and multiplied by 100. Individuals were secondarily classified as immature or mature based on clasper condition and the presence of semen in males, and pregnancy status and development of the oviducal gland and ovarian oocytes in females (Yano et al, 2020). Szöcs and Schäfer (2015) strongly recommended that proportions from counts should be analysed using a generalized linear model (GLM) because proportion data are rarely normally distributed and the use of proportions loses sample size information.…”