Fish breeding facilities can have severe impacts on local aquatic systems in the form of nutrient additions. Among proposed solutions to this problem is the use of treatment wetlands where biotic and abiotic processes reduce nutrient concentrations. We asked whether organisms in the wetlands below a hatchery could be assimilating, absorbing, or sedimenting fish waste and unconsumed fish meal. The Salisbury Fish Culture Station is Vermont’s sole broodstock hatchery and a critical part of the statewide hatchery system. It produces as many as 9 million Salmonid eggs each year. The hatchery uses a combination of surface and well water sources, and water and fish waste flows out of the facility and into the 5 km of mixed wetland of Halnon Brook below. Due to a recent change in measurement protocol, the hatchery’s effluent now exceeds legal nutrient concentrations, and the facility is facing potential closure. Because Salmonids are carnivores, their tissues have comparatively elevated concentrations of the bioaccumulating stable nitrogen isotope 15N. We collected diatoms between the hatchery and outlet of the brook and examined 15N concentrations in their tissue using gas chromatography-mass spectrometry stable isotope analysis to measure the fading chemical signature of the hatchery-produced nitrogen in the water.
Results/Conclusions
We expected that nutrient uptake by wetland organisms would result in a decrease of 15N along the length of the study site. We found 42% lower 15N concentrations below the wetland than at the outflow of the fish hatchery. These data are in line with our expectations and indicate that wetland organisms are absorbing nutrient additions downstream of a hatchery. From here, further data collection will establish baselines of 15N in the water entering the hatchery and in the fish meal used, measure 15N concentrations at a smaller scale along the wetland, and use scanning electron microscopy to classify diatom taxa in the stream. Our findings offer broader implications for the management of nutrients throughout the US hatchery system. Our results may also have important consequences for the future of this hatchery as well as the lands around it. In the case that Halnon Brook is providing ecosystem services by acting as a natural treatment wetland, we recommend taking steps to include it in the hatchery’s nutrient management system.