Low-Temperature Tolerance of Age-0 Atlantic Croakers: Recruitment Implications for U.S. Mid-Atlantic Estuaries

Abstract

Age-0 Atlantic croakers Micropogonias undulatus overwinter in Mid-Atlantic Bight (MAB) estuaries, where interannual variability in year-class strength is pronounced. Previous researchers have hypothesized that this variability is the result of cold-induced winter mortality of age-0 Atlantic croakers within estuarine nurseries. We tested this hypothesis by means of laboratory experiments designed to (1) estimate the effects of winter severity and duration on the probability of survival of age-0 fish, (2) determine the relationship between body size and cold tolerance, and (3) examine the effects of temperature decline rate and salinity on susceptibility to low-temperature mortality. Age-0 Atlantic croakers (15–65 mm standard length, SL) were collected from Delaware Bay during October, acclimated to 8°C in the laboratory, and then cooled (1.0°C/d) to treatment levels of 1, 3, 5, or 7°C that were designed to simulate varying levels of winter severity in MAB estuaries. Survival ranged from 0% at 1°C to 99.3% at 7°C, increasing dramatically between 3°C (1.3%) and 5°C (86.8%). Cold tolerance was size dependent, with smaller individuals surviving significantly longer than larger ones. The rate of temperature decline (1.0°C/d versus 0.2°C/d) did not affect survival at lower winter temperatures. Atlantic croakers remained intolerant of temperatures of 3°C or less even when these temperatures were approached gradually. Cold tolerance increased with increasing salinity, implying that the risk of overwinter mortality varies spatially along an estuarine salinity gradient. The mean survival duration at 3°C increased from 5.9 d at 2‰ to 14.7 d at 10‰ and to 17.1 d at 26‰. Our findings provide empirical support for the hypothesis that cold-induced overwinter mortality regulates the year-class strength and recruitment success of Atlantic croakers in the Mid-Atlantic Bight.