Reproductive cycle and size at sexual maturity of the commercial whelk Buccinum isaotakii in Funka Bay, Hokkaido, Japan Anthony S. Ilano* P , Katsuaki Fujinaga O and Shigeru Nakao* *Laboratory of Marine Biodiversity, Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato, Hakodate 041-8611, Hokkaido, Japan. O Faculty of Social Welfare, Dohto University, 7-1 Ochiishi, Monbetsu 094-8582, Hokkaido, Japan. P Corresponding author, e-mail: anthony@¢sh.hokudai.ac.jp The reproductive cycle and size at sexual maturity of the commercial whelk Buccinum isaotakii in Funka Bay, southern Hokkaido, Japan, was examined using monthly samples of individuals collected from May 1999 until October 2000. Female and male sexual maturity was determined based on reproductive organ sizes and histological examination, and sex ratios did not di¡er from parity. There were parallel annual cycles in the size of the ovary and pallial oviduct, and an inverse cycle for the digestive gland. The testis and seminal vesicle showed inverse cycles. The peak in seminal vesicle size was at the same time as the peak in ovary and pallial oviduct. The size of the male digestive gland exhibited no evident cycle. Males were mature from May to October 1999 and from April to October 2000, and spent individuals ¢rst appeared in September 1999 and then decreased in numbers untilJune 2000. Sperm content and epithelium in seminal vesicle indicated that the copulating period started in March and lasted until August 2000. Ovarian maturation and oogenesis occurred from October 1999 onwards and peaked in January 2000, with egg laying occurring from May until September. An increase in temperature coincided with the advancement of spermatogenesis and the egg laying while the transfer of the sperm from the testis to the seminal vesicle in males and the maturation of ovaries in females coincided with a decrease in temperature. INTRODUCTION The reproductive cycles of prosobranch gastropods are more varied than other groups of molluscs and this is re£ected in their varied reproductive structures and the wide range of habitats in which they live (Fretter, 1984). In most prosobranch gastropods, gametic activity is synchronous within populations and maturation occurs in a particular season, however, for some species gameto- genesis is not synchronous amongst individuals or between sexes (Webber, 1977). Several studies of proso- branch gastropods have noted that females become sexually mature at a larger size than males (Kideys et al., 1993; Castagna & Kraeuter, 1994; Power & Keegan, 2001). Buccinids, one of the major prosobranch gastropods in the northern waters, are considered predators and carrion feeders (Himmelman & Hamel, 1993). Some of them are commercially harvested and the biology of many bucci- nids, particularly the genus Buccinum, has been studied. The Japanese whelk Buccinum isaotakii (Kira) occurs at depths of 50^300 m from north of Kashima-Nada to Hokkaido Island. In Funka Bay, southern Hokkaido, B. isaotakii has been harvested for human consumption using baited traps and gill nets over the past four decades. The minimum size is set at about 70mm in shell length and the prolonged ¢shing period (from April to November) is not set to protect the spawning period but rather because of low whelk landings and the shift of ¢shing activity to cod ¢sh during the winter months (personal communication, ¢shermen). To date, there are no published reports on the biology of Buccinum isaotakii and the lack of information on this species prevents the development of a suitable manage- ment strategy for this valuable resource. The present study examines its reproductive biology, i.e. the size at sexual maturity, the sex ratio, and the reproductive cycle as indicated by the changes in body component indices and histological observations of the gonads and seminal vesicle. Finally, we make suggestions for the management of the whelk ¢shery. MATERIALS AND METHODS The whelks were collected by ¢shermen from the mouth of Funka Bay in southern Hokkaido (Figure 1), using baited ‘shell traps’. The shell traps measured 70cm in diameter at the top, 110 cm at the bottom and 30 cm in height. Each trap was covered with 3-cm mesh gill netting. A12-cm diameter opening at the top of the trap permitted the whelks to enter. A monthly sample of whelks was taken between May 1999 and October 2000. Whelks could not be collected between December 1999 and March 2000 because of rough sea conditions and the shift in ¢shing activity to cod ¢sh. For each collection, 90 shell traps, attached at 10.5 m intervals along a 1000 m line, were deployed for 24h on the sea bottom at *70 m in depth. The Laboratory of Marine Environment and Resource Sensing at Hokkaido University provided monthly measurements of temperature and salinity. Temperature J. Mar. Biol. Ass. U.K. (2003), 83, 1287^1294 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (2003)