Adaptive and plastic variation in growth and maturation of the cutlassfish Trichiurus japonicus in the subtropical Pacific Ocean

1 Institute of Oceanography National Taiwan University No. 1, Sec. 4, Roosevelt Road Taipei 10617, Taiwan 2 Department of Biological Sciences University of Bergen P.O. Box 7803 5020 Bergen, Norway Abstract—We investigated variation in growth and maturation in response to 1) spatial variation in climate and 2) exploitation of the subtropical cutlassfish Trichiurus japonicus, an important fishery species whose population ecology is virtually unknown. Individuals of this cutlassfish species were sampled monthly at 2 primary landing sites: Kengfang, in northeast (NE) Taiwan, and Tsukuan, in southwest (SW) Taiwan, during 2013–2015. Habitat temperatures were about 1–4°C lower at the NE site than at the SW site, and the length at age of adult fish had an inverse pattern with temperature (NE lengths >SW lengths at age). The probabilistic maturation reaction norms did not differ significantly between the 2 areas, but ages and lengths at maturation were higher for males from NE than for males from SW. Differences in asymptotic lengths (NE>SW) and growth coefficients (NE<SW), together with narrower length distribution at the SW site than at the NE site, indicate potentially different mortality between the areas (NE<SW). These results indicate plastic variation in maturation and potentially adaptive variation in growth for this species that are related to the differences in temperature and mortality between the areas, although reliable proxies for mortality are lacking. The observed life-history variation was consistent with the temperature–size rule that organisms tend to grow faster, mature earlier, and reach smaller asymptotic sizes at warmer temperatures and may indicate an adaptive divergence of cutlassfish populations.