Biology and effects of cartilaginous fishes on fisheries in the Sagami Bay

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The total number of 21 species of cartilaginous fishes were collected from the depth below 150 m in the Sagami Bay during the course of the present study. Three triakid species were provided by fishermen. Among those species, most abundant species was Squalus mitsukurii, making up 69% of the total catch of 442 specimens captured by R/V Enoshima-maru belonging to Kanagawa Prefectural Fisheries Research Center. Other species constituted less than 10 % of the total.Shortspined dog fish, S.mitsukurii, ranged from 150 m to 550 m in depth, mainly between 300-400 m. Trend of catch per unit efforts (CUPUE) using hooked rates expressed by number caught per hundred hooks and unit net length, indicates that they made seasonally vertical migration and go into deep waters during warm seasons while they inhabit shallower depth during cold seasons. However, they were not caught in mid-winter in the coastal waters while they are captured in the seamount near the entrance of Tokyo Bay., suggesting hori … More zontal migration within the bay as well. Segregation by sizes and sexes were also suggested, judging from scarcity of males and small specimens in the catch. Predation by S.mitsukurii on commercially important fishes distributed in the Sagami Bay was also investigated using stomach contents analysis and tooth scars of damaged fishes. As a result, it is concluded that about 8 % of fishes caught by vertical loglines suffered from predation by S.mitsukurii. Age and growth studies were also conducted using annuli marked on second dorsal fin spines. Growth curves were close to those presented by the past studies in offshore area of Choshi while quire different from those in Ogasawara and Seamount of Hancock... Reproductive studies of S.mitsukurii indicate that reproductive strategy is a little different from that obtained from Choshi area. Stomach contents analysis reveals that there seemed to be a little different between regions in Sagami Bay but totally S.mitsukurii are an exclusive picivore as reported by other studies in other regions. Feeding status of seven species was analyzed using stable isotope ratio of nitrogen and carbon. C-N maps indicated that differences between species are rather smaller than differences between habitats. They also differed according to growth stage. Less