Does water-level management in marsh impoundments affect larval fish assemblages?

In coastal South Carolina, most impounded, euryhaline marshes are managed primarily for waterfowl or for fishes, and water-level management in the impoundments is central to each strategy.  Waterfowl management promotes forage and habitat for migratory birds, a goal that demands prolonged restriction of connectivity with tidal waters.  Fish management promotes healthy populations of sport fish and allows daily tidal exchange. This dual management regime has raised concerns about how water restriction in an impoundment may affect its nursery function for fishes.  Our research examined the effective diversity of early life stages, a phase that has received little attention in studies of coastal impoundments.  We hypothesized that fish management supports a more diverse seasonal assemblage of larval fishes than does waterfowl management.  We used light traps and plankton nets to sample two impoundments of each type once a month for 10 months.  We collected 85,093 fishes and identified 22 species.  Results of ANOVA analysis confirmed out prediction:  seasonal effective diversity in fish impoundments (summer mean=2.52, s.d.=0.17; winter mean=2.02, s.d.=0.57) was greater than in waterfowl impoundments (summer mean=1.27, s.d.=0.12; winter mean=1.06, s.d.=0.08).  Our findings underscore the importance of daily water exchange for promoting nursery function for fishes in managed coastal impoundments.