Seasonal fluctuation in the length-weight relationship of groupers in the Goto Islands, Japan

The length-weight relationships (LWRs) of 13 commercially important grouper species inhabiting the Goto Islands in southwestern Japan were established. The samples were collected between June 2007 and December 2013. We determined the monthly LWRs for the seven abundant species to determine whether these LWRs changed seasonally. Generalized additive models demonstrated significant differences in body weight compared to total length for all seven species during the year. Smoothing splines indicated that four species of groupers (Epinephelus awoara, E. fasciatus, E. areolatus, and Plectropomus leopardus) gained weight relative to the total length in a few months of spring to summer (April-September), corresponding to the spawning period of each species. Therefore, this increase was presumably due to the energy stored in the body for upcoming spawning events, followed by gonadal development for spawning. E. akaara decreased its weight relative to its length in September, indicating that it may exhaust energy during the spawning season and then reduce its weight. The remaining two species (E. bruneus and Hyporthodus septemfasciatus) gained weight from December to March and in October, respectively, which did not coincide with their spawning period. The increment of body weight in October for H. septemfasciatus may indicate that they stored energy in preparation for the upcoming winter because lower temperatures in winter decrease the food consumption rate. E. bruneus might be able to acquire sufficient energy during December-March despite a lower food consumption rate, possibly because of the abundance of prey inhabiting the Goto Islands.

Automated summary

The length-weight relationships of commercially important grouper species in the Goto Islands of Japan were studied. Differences in body weight relative to total length were observed for different species throughout the year, with some species gaining weight during the spawning period and others decreasing weight. These findings highlight the seasonal variations in weight of grouper species.