From 'omics to otoliths: Using responses to endocrine disrupting compounds at multiple biological scales to predict population dynamics

Background/Question/Methods

Endocrine disrupting chemicals (EDCs) interfere with endogenous hormones and cause physiological abnormalities in fishes.  Both theory and empirical data confirm that EDCs can also cause declines in fish populations. However, few studies have attempted to link complex environmental EDC mixtures with responses at multiple tiers of the biological hierarchy, including population-level effects.  Additionally, the use of environmentally relevant fishes as model species from regions with demonstrated fish population decline is lacking.  To this end, we undertook a four-tiered investigation into estrogenic and androgenic EDC effects in Menidia audens (Mississippi silverside), an atherinid distributed throughout the impacted San Francisco Bay (SFB) estuary.  Our main objective was to integrate observations at each biological scale with the goal of determining whether the reproductive health of Menidia populations is being negatively impacted by sites receiving either urban run-off and treated wastewater effluent or ranch run-off, and if so what the mechanisms of endocrine disruption may be.  

Results/Conclusions

Clear links were observed between the four biological scales.  At the ranch site, where primarily estrogenic compounds were present, males had significantly higher and females trended towards higher expression of estrogen-responsive genes.  At the urban/wastewater outfall site, which is more polluted and contaminated by both estrogens and comparatively higher concentrations of androgens, both males and females had relatively low expression of estrogen-responsive genes, and males had lower gonadal somatic index and higher incidence of severe testicular necrosis.  Notably, the proportion of females caught from the urban/wastewater outfall site throughout the spawning season over a two year period was significantly lower than the ranch site and in comparison to historical observations of Menidia sex ratios in other populations.  Menidia appear to be highly sensitive to EDCs and our results suggest that the population at the urban/wastewater site is being negatively impacted by these contaminants at several biological scales.   Results from this study were also used to parameterize a population model that predicts the conditions under which EDC exposure could contribute to population decline.