Fisheries biology of the red sea urchin, Pseudocentrotus depressus, in coastal waters around Nagasaki Prefecture

The Red Sea Urchin, Pseudocentrotus depressus, is an important sea urchin in coastal waters around Nagasaki Prefecture. However, knowledge of fisheries biology of the species have been beggarly and the practices used to manage fisheries have been inadequate. This study was carried out in order to clarify basic knowledge for biology of the species and actual situation of red sea urchin fishery, which are necessary for management of this resource. The results of the study are summarized below: 1) Age Determination by Rotula Growth rings were clearly visible in rotula of red sea urchin by heating at 350 degC for 10-180 min. After confirming a single formation of annulus in a year from December to January, I confirmed that growth rings in rotula were useful for age determination of red sea urchin. 2) Growth in Coastal Waters around Nagasaki Prefecture The growths of red sea urchin were studied by measuring the length of rotula annulus. Specimens were collected from 2005 to 2007 in five areas of local populations. The test diameter in a given year from the birth was back calculated. The von Bertalanffy's growth equations were then derived as follow: Kamoise: TD=74.79(1-exp(-0.415(n+0.218))) Ishida: TD=66.32(1-exp(-0.489(n-0.107))) Nakano: TD=66.02(1-exp(-0.489(n-0.375))) Mie: TD=53.91(1-exp(-0.562(n+0.151))) Fukue: TD=59.63(1-exp(-0.599(n-0.145))) TD: Test diameter. 3) Maturation Seasonal changes of gonad index (GI) and gonadal histology were examined to clarify the annual reproductive cycle of red sea urchin. Samples were collected in shallow waters at depth of 3-5 m in Hirado, Nagasaki Prefecture, from May 2002 to March 2005. The GI increased from March and attained a high value from May to October and decreased from November to January. Histological observations of the gonad revealed that oogenesis and spermatogenesis started in September, and then the ovary and testis were filled with mature ova and spermatozoa, respectively, from November to January. Spent gonads were observed from November to March. These results suggested that spawning season lasted from November to January. The biological minimum size was 24 mm in test diameter, corresponding to two years of age for wild populations, and one year of age for hatchery-raised released populations, respectively. 4) Actual situation catch Questionnaire to fisheries cooperative association I carried out a questionnaire survey at fisheries cooperative associations which exist in coastal waters around Nagasaki prefecture to obtain information on the actual fisheries of red sea urchin. The result showed that red sea urchin was caught in all waters off Nagasaki prefecture other than the Ohmura bay and Ariake Bay. The total catch of red sea urchin is 9,917 kg in coastal waters around Nagasaki prefecture. Hokusho area and Iki area accounted for 81 percent of the total catch of red sea urchin in Nagasaki prefecture. Characteristics of landed urchin Test diameter, gonad weight, gonad index, color in the gonad, spine color and age composition of red sea urchin were surveyed from 2005 to 2007 in coastal waters in five areas of local populations. Mean test diameter is 49-68 mm. Mean gonad weight is 4-13 g. Mean gonad index (GI) is 9-13. Significant differences were detected in size and GI among five areas. The color of the gonad turned worse with aging. Most landed ages were three or four years old. 5) Stock assessment The stocks in the Kamoise, Isida and Nakano area were assessed using dynamic pool model, using population parameters obtained in this study. The results show that this stock is being under-fishing. 6) Effective utilization of resources It was suggested that red sea urchin resources in coastal waters around Nagasaki prefecture were low use resources. For the effective utilization of resources, I think that coherent system from fishery to the processing and distribution is necessary. In recent years an environmental change by the water temperature rise is remarkable. Because the decrease of red sea urchin was detected where the algae was declined, it is necessary for effective utilization of the red sea urchin resources, to recover the algae ground, to control amount of herbivore and algae.