Sex-Specific Hatch and Growth Patterns in Young-of-the-Year Atlantic Silversides (Menidia menidia, Atherinopsidae) From Mumford Cove, Connecticut

The Atlantic silverside (Menidia menidia) is an important semelparous forage fish along the North American Atlantic coast and a well-known model for experimental studies. However, ecological and evolutionary theories, such as the adaptive significance of temperature-dependent sex determination (TSD), have yet to be tested using M. menidia juveniles from their natural habitat. Additionally, how the characteristics of survivors at the end of their growing season are shaped by growth and mortality is insufficiently understood. This study used otolith microstructure analysis to determine sex-specific size-at-age, growth, and hatch date distributions in young-of-the-year juveniles sampled monthly between October-December 2015 in Mumford Cove in northeastern Long Island Sound. I hypothesized that sexual size dimorphism was due to differences in age, with female survivors hatching earlier during the spawning season than males, because TSD in this species produces increasingly male-biased sex ratios with increasing temperatures. While females were nearly 2 cm larger in total length, I found this sexual dimorphism was not due to differences in age but differences in growth rates, as female survivors had similar hatch dates but grew on average 37% faster than males. Furthermore, realized sex ratios of fall survivors were considerably less male-biased than expected, based on monitored temperature conditions and TSD values from published common garden experiments. Selective survival of faster growing females over males may account for these more balanced sex ratios. However, growth back-calculations also revealed the existence of both slow- and fast-growing female phenotypes in the survivor population and a selective disappearance of fast-growers from the study area between October and December. The existence of different growth strategies and the presence of slow-growers into December suggest multiple modes of selection and trade-offs between slow growth and size at the time of migration