Relationship between spat density, food availability, and growth of spawners in cultured Mizuhopecten yessoensis in Funka Bay: concurrence with El Nino Southern Oscillation

To elucidate the factors that influence the interannual variation in the density of cultured Japanese scallop (Mizuhopecten yessoensis) spat, we analyzed the relationship between spat density (D(s)), monthly chlorophyll a (chl a) concentration, water temperature, and adductor muscle weight of spawners over 15 years (1992-2006) in Funka Bay (Japan) on the western North Pacific Ocean. The interannual variation of spat density was best explained by a campaniform model that used chl a concentration in February ([chl](Feb)) and a categorical variable that indicates whether growth of spawners in a year is low or not as independent variables (R(2) = 0.91). The gonadosomatic index increased fastest in February. Low growth years were detected as outliers in the D(s) -[chl](Feb) relationship and were characterized by an average weight of adductor muscles in February of <12 g. Therefore, food availability during gonadal development and growth conditions of spawners were the main factors determining spat density. The proportion of ovary necrosis was high in the years of low [chl](Feb) and low growth. Those years corresponded with El Nino and La Nina years, respectively. Thus, global climatic anomalies apparently affect reproduction of the scallop in Funka Bay.