Tidally restricted salt marshes invaded by Phragmites australis reduce habitat quality for resident fish populations

Background/Question/Methods

Roads, bridges, and other impoundments constructed across salt marshes that restrict tidal flow can decrease habitat value for nekton and facilitate invasion by non-native plants including Phragmites australis. Introduced P. australis raises the elevation of the marsh surface and fills in small marsh depressions, which can adversely affect fish by reducing access to intertidal habitat essential for feeding, reproduction, and refuge. The goal of this study was to assess the impact of P. australis on the condition of resident fish populations (Fundulus heteroclitus) at four tidally restricted and four tidally restored salt marshes relative to reference (unrestricted) marshes in the Northeast. We used physiological indicators of fish condition including lipid reserves, recent daily growth rate, age, parasite load, female gravidity status, and a common morphological indicator (Fulton’s K) to assess impacts to fish health. We collected 3,017 fish during summer and fall from 2010-2011. Using petroleum ether and a Soxhlet apparatus, we removed and quantified storage lipids from 1,920 fish. We removed sagittal otoliths from 960 fish, recorded age, and calculated recent daily growth rate from the marginal ten increments using image analysis software and a Zeiss Stereo Microscope.

Results/Conclusions

Using fish length as a covariate, we found that Fundulus heteroclitus residing in tidally restricted marshes exhibit significant reductions in lipid reserves (p=0.0481), contain lower proportions of gravid females (p=0.0025), and have higher incidences of parasitism (p=0.0001) relative to fish in adjacent reference salt marshes. Parasitized fish exhibit significantly reduced lipid reserves in comparison to unparasitized fish (p=0.0181) and gravid females have significantly lower lipid reserves than non-gravid fish (p<0.0001). When parasitized and gravid individuals were removed from the analysis the significant difference between tidally restricted and reference marsh fish remained (p=0.0013). Unparasitized, non-gravid fish in tidally restored marshes were equivalent to fish in reference marshes, with similar lipid reserves (p=0.2445), gravidity (p=0.5358), and parasite load (p=0.3486). Fish in all marshes (regardless of restriction status) exhibited similar growth rates (p>0.05) and morphology (Fulton’s K; p>0.05). Reference marshes adjacent to tidally restored sites contained the highest abundance of young fish (ages 0-1) while tidally restricted marshes contained the lowest. Overall, results indicate that tidal restrictions have reduced the quality of habitat for resident fish populations and that tidal restoration efforts have been successful at re-establishing the ecological function of salt marshes in the Northeast.