Winter mortality of young mudskipper fish: Effects of size, temperature and energy depletion

Abstract

Winter mortality can be a strong selective pressure on animals living in temperate regions, especially for young and small individuals. Therefore, understanding the mechanism for its size-dependence is important to predict population dynamics of the species in a fluctuating environment. In this study, we experimentally investigated the effects of body size, temperature and energy reserve depletion on the overwinter mortality of young mudskipper fish (Boleophthalmus pectinirostris). Small size-class fish (<44 mm in SL) died earlier than large size-class fish (≥44 mm) under both heated and unheated temperature conditions. Relatively higher energy and lipid content in live compared to dead fish and suspended mortality during the warm period under unheated conditions suggested the effects of both energy depletion and low temperature on overwinter mortality. Small size-class fish showed a relatively higher energy depletion rate under both temperature conditions probably due to their higher mass-specific metabolic rate. Since small size-class fish died earlier than large size-class fish, the body size difference in energy depletion rate may be one of the factors affecting the size-dependence of overwinter mortality. The energy depletion rate gradually decreased with the progress of experiment, suggesting that fasting-induced metabolic depression occurred. The estimated relative lipid mass decreased with the progress of experiment. The lipid depletion rate with elapsed days significantly reduced in small size-class fish under unheated conditions comparing with large size-class fish, while the rate in large size-class fish was significantly larger than that of small size-class fish in heated conditions. The difference suggests the possibility that small size-class fish switches metabolic fuels at a relatively lower temperature condition to maintain a minimum activity level for unpredictable feeding opportunities during the pre-winter season.