Behaviourally mediated thermal experience in relation to final oocyte maturation by free-swimming barfin flounder (Verasper moseri)

The reproductive performance of barfin flounder (Verasper moseri), a multi-batch spawner spawning during February-April (boreal winter) in the wild, is particularly sensitive to temperature perturbations. Final oocyte maturation (FOM) of captive flounder occurred only when post-vitellogenesis fish experienced a slowly-elevated water temperature regime (SETR) (from 2 °C to 6 °C over a month). We used electronic tags to show how post-vitellogenesis flounder experience SETR during its pre-spawning migration, during their southward migration, to a location about 750 km from the summer feeding grounds in the coastal waters of Hokkaido (CWH; 40.2-43.4°N). From 2010 through 2012, we tagged a total of 208 V. moseri in early December at the feeding grounds of the CWH, during in which vitellogenin incorporation into oocyte was almost completed. One hundred and two tags were recovered and the data from 98 were downloaded. All of the tagged females moved offshore by the end of January. Fish spent more time in cold and deeper waters (3-4 °C, 351-380 m) in January than they did in December (6-8 °C, 33-59 m), but experienced warmer temperatures (5.1-5.7 °C, 367-396 m) in February and March around the spawning period. All tagged fish experienced SETR and then experienced the onset of SETR in January, although SETR of tagged fish have large variations among individuals. A comparison between depth-temperature data recorded by data-logger and oceanic horizontal temperature conditions suggests that fish did not experience warm surface water of the Tsugaru Warm Current, a branch of the Kuroshio that flows into the Pacific Ocean via the Tsugaru Strait, and instead were in the colder Oyashio water by migrating deep and finally experiencing warmer water in February and March. In conclusion, SETR appears to be one of the exogenous factors that induces FOM in the wild.