Fish Community Structure and Habitat Complexity in Restored and Natural Eelgrass Meadows

Abstract

Seagrass meadows serve as nursery and permanent habitat for numerous fish species—an ecosystem function influenced by structural complexity. Rapid seagrass loss over recent decades has promoted restoration efforts aimed at recovering meadows and their ecological functions. This study assesses the restoration of habitat structural complexity and associated fish communities in two replanted eelgrass (Zostera marina) meadows on the west coast of Sweden, planted with a 2-year difference and comparing them to the inner and edge areas of natural meadows and unvegetated sandy areas. Eelgrass habitat structural complexity was assessed from seagrass samples collected to estimate shoot densities, shoot biomass, and canopy height of the meadows. Fish communities were assessed using two methods: diver-operated stereo-video transects, and traditional underwater visual census, testing the efficiency of both techniques. The two assessed methods produced similar results that sampled canopy-associated species well but were less accurate for cryptic bottom-dwelling species inside eelgrass meadows. Results indicated that structural complexity was statistically lower in the 2-year replanted area (Askerön) compared to the reference meadow, but in the 4-year replanted area (Gåsö), no significant differences were observed between the restored and reference meadows. Moreover, no differences were observed between the reference and restored meadows for canopy-associated fish species in Gåsö, indicating that the recovery of the habitat function was already underway. However, results from Askerön were inconclusive. Thus, despite rapid eelgrass growth, the recovery of ecosystem function and of the capacity to provide ecosystem services following seagrass replanting is not always immediate and may be dependent on factors such as time after restoration, environmental factors (e.g., water quality) or landscape configuration (e.g., meadow fragmentation, vicinity to natural meadows). A comprehensive understanding of how fish communities respond to seagrass restoration is necessary to effectively scale the restoration efforts.