We are determining the effects of hypoxia (low dissolved oxygen) on reproduction of fish (Atlantic croaker) throughout the northern Gulf of Mexico to accurately model population outcomes and to assess the model’s capabilities as a resource management tool.
Why We Care
The long term effects of the increase in seasonal coastal hypoxia in the northern Gulf of Mexico (nGOM) on the growth, reproduction, and behavior of marine organisms are largely unknown. Atlantic croaker (Micropogonias undulates, fish family Sciaenidae or drums) represents dwellers of the water column and serves as a biological model for upper trophic levels in the coastal region likely impacted by hypoxia.
Recent findings suggest that exposure to stressors like hypoxia can decrease fish reproductive output and could possibly lead to dramatic population declines. Decreased reproductive output in individual Atlantic croaker has been documented at several hypoxic sites in the nGOM.
What We Are Doing
Evidence from individual Atlantic croaker suggests that persistent seasonal hypoxia decreases their reproductive output throughout the nGOM and results in long-term declines in population size. The objectives of the project are:
- Population modeling: Refine the existing population dynamic models and compare them to the reproductive effects of hypoxia on croaker fish ranging from individuals to populations.
- Behavioral movement modeling: Quantify possible exposure scenarios of individual croaker based on various behavioral movements.
- Spatial extent: Determine the spatial extent (geographic range) of reproductive impairment of croaker in the nGOM.
- Exposure: Estimate the duration of hypoxia exposure of croaker in the nGOM and its relationship to reproductive impairment.
- Translation to management: Provide resource managers with the results of the modeling predictions and assist them in interpreting the findings for incorporation into their management decisions.
Atlantic croaker will be collected during the reproductive season (in September) and also when the hypoxic zone is extensive (in July) from 4 normal oxygen level (reference) sites east of the Mississippi delta and from 10 sites that span the entire spatial extent of the hypoxic region. The collected fish will be measured for a suite of reproductive biomarkers. Movement of the croaker in the hypoxic nGOM region will be investigated using acoustic transmitter tags and hypoxia exposure will be assessed by measurement of body tissue and organ samples. The duration of hypoxia necessary to impair reproduction and potential interactive effects of hypoxia with an additional environmental stressor, a PCB (polychlorinated biphenyl) pollutant, will be investigated in controlled laboratory studies.
This project is part of the CSCOR Northern Gulf of Mexico Ecosystems and Hypoxia Assessment Program (NGOMEX). The project is led by Dr. Peter Thomas of the University of Texas Marine Science Institute, in partnership with Dr. Kevin Craig (NOAA) and Dr. Kenny Rose (Louisiana State University).
What We Are Finding
Building on prior research findings of endocrine and reproductive disruption, results from this project indicate that hypoxia in the northern Gulf of Mexico is resulting in widespread masculinization of female croaker. Nearly 1 out of 5 croaker ovaries collected from the hypoxic zone had male reproductive cells and most were smaller, less developed, and produced fewer viable eggs. Efforts are underway to develop an individual based population model to determine the long-term population impacts of hypoxia on croaker under varying levels of hypoxic area (i.e., small, moderate, or severe). Analysis of fisheries data indicate that there is high spatial overlap between croaker and other species in this region, suggesting that the sublethal impacts found in croaker likely extend to other bottom-dwelling species. Related results by the research team found the same effects in mantis shrimp and dragonets in Tokyo Bay.
Benefits of Our Work
Using Atlantic croaker as a model species, this project is advancing our understanding of the impacts of hypoxia and is providing the foundation for the inclusion of sublethal effects into fisheries management efforts. Additional studies will be required to determine if reproductive impacts also occur in other species in the northern Gulf, but the detailed understanding developed through this project will allow for targeted and efficient assessments. Results from this study are also being combined with results from other NGOMEX projects to provide an ecosystem-scale understanding of hypoxia impacts that will allow for refinements to nutrient management efforts focused on reducing the size of the hypoxic zone. This project supports Action 5 of the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force’s 2008 Action Plan.