Winter severity influences spotted seatrout mortality in a southeast US estuarine system

Winterkill in spotted seatrout Cynoscion nebulosus is associated with extreme cold conditions throughout much of the species' geographic range. However, rigorous study is needed to confirm longstanding but largely untested assumptions that acute cold stress drives overwinter loss. We provide the first direct field-based estimates of spotted seatrout survival relative to the severity of cold temperatures. Spotted seatrout overwintering in North Carolina, USA, estuaries were tagged with ultrasonic transmitters and monitored during 3 consecutive and variable winters from 2009 to 2012. Fates of telemetered fish were inferred from daily movements and used in a multistate capture-recapture model to estimate instantaneous rates of natural mortality (M), fishing mortality, and emigration. Natural death was inferred for 7 fish in 2009/2010 (n = 34 telemetered spotted seatrout at risk), 6 fish in 2010/2011 (n = 9 fish at risk), and 1 fish in 2011/2012 (n = 65 fish at risk), and when estimable, weekly M ranged from 0.001 to 0.187. Daily estimates of natural mortality increased quickly with declining water temperatures (T),M=1/[1 + e(-(-0.714-0.756T))], and indicated that cold-stun deaths occurred when water temperatures were below similar to 7 degrees C. Our results provide direct evidence that winterkill in spotted seatrout at its northern limits of distribution is related to the severity of low water temperatures and demonstrate that the simultaneous monitoring of telemetry-tagged animals and abiotic conditions is an effective approach to determine lethal environmental limits.