Abstract
Southern flounder (Paralichthys lethostigma) exhibit environmental sex determination (ESD), where environmental factors can influence phenotypic sex during early juvenile development but only in the presumed XX female genotype. Warm and cold temperatures masculinize fish with mid-range conditions producing at most 50% females. Due to sexually dimorphic growth, southern flounder fisheries are dependent upon larger females. Wild populations could be at risk of masculinization from ESD due to globally increasing water temperatures. We evaluated the effects of habitat and temperature on wild populations of juvenile southern flounder in North Carolina, USA. While northern habitats averaged temperatures near 23 °C and produced the greatest proportion of females, more southerly habitats exhibited warmer temperatures (>27 °C) and consistently produced male-biased sex ratios (up to 94% male). Rearing flounder in the laboratory under temperature regimes mimicking those of natural habitats recapitulated sex ratio differences observed across the wild populations, providing strong evidence that temperature is a key factor influencing sex ratios in nursery habitats. These studies provide evidence of habitat conditions interacting with ESD to affect a key demographic parameter in an economically important fishery. The temperature ranges that yield male-biased sex ratios are within the scope of predicted increases in ocean temperature under climate change.
Highlights
Fish are strongly influenced by climate variability with regards to reproduction, productivity, food availability, and recruitment[1]
To enable characterization of nursery habitat influences on sex determination, we developed and validated a molecular biomarker technique based on gonadal expression of genes involved in sexual differentiation that allows us to sex individuals at much smaller sizes[37]
Temperature loggers were placed in representative North Carolina (NC) juvenile southern flounder nursery habitats across four years: 2014 (11 probes), 2015 (13 probes), 2016 (14 probes) and 2017 (13 probes) (Fig. 1)
Summary
Fish are strongly influenced by climate variability with regards to reproduction, productivity, food availability, and recruitment[1]. No studies have investigated the effects of environment on juvenile sex ratios of southern flounder populations or any Paralichthids in the wild If these fish are exposed to unfavorable conditions during the sex determination window, wild stocks could be at risk of masculinization which would negatively impact an already declining female-dependent fishery. It is currently unknown if wild juvenile southern flounder exhibit annual or geographic variation in sex ratios. It has been suggested that studies where temperatures are held constant may not be ideal for modeling impacts of natural environments[39]
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