Effects of structural marsh management on fishery species and other nekton before and during a spring drawdown

We sampled experimental research areas in the Barataria Basin of Louisiana, USA to examine the effects of structural marsh management on habitat use by small nekton (>100 mm Total Length or Carapace Width). The research areas consisted of two control (unmanaged) marshes and two impounded (managed) marshes; managed areas were surrounded by levees with water-control structures constructed by the U.S. Department of Interior, National Biological Survey. We sampled nekton with 1-m² enclosure samplers in 1995 just as a drawdown was initiated (March) and after two months of drawdown (May); a drawdown is an active management technique in which water is allowed to flow out of, but not back into, the impoundment. Samples were collected randomly from all available habitat types (shallow open water, submerged aquatic vegetation (SAV), and intertidal marsh) in the managed and unmanaged areas. In March, the densities of resident taxa (e.g., Lucania parvac rainwater killifish and Palaemonetes paludosus riverine grass shrimp), which complete their life cycles within the estuary, were significantly greater in the managed areas compared to the unmanaged areas. The densities of most resident species were either similar in managed and control areas, or significantly greater in control areas during the drawdown (May). In contrast to residents species, the transient fishery species (e.g., Callinectes sapidus blue crab and Farfantepenaeus aztecus brown shrimp) reproduce outside of the marsh system and recruit to these areas as young. The densities of these transient species were significantly higher in unmanaged areas compared to managed areas during both sampling periods. We estimated standing crop (number or biomass of nekton per hectare of marsh area) by combining habitat densities with the area of different habitat types. The standing crops of transient species also were substantially greater in unmanaged than managed areas. We conclude that the restricted water exchange in marshes under structural marsh management diminishes recruitment and standing stocks of species that must migrate from coastal spawning sites to marsh nurseries. Even when water-control structures were open, the densities of these transient species were low inside managed areas. In contrast to the negative effect of management on transient species, the resident fish and crustacean populations seemed to flourish in the managed areas when a drawdown was not in effect. Following two months of a drawdown, however, the populations of residents appeared similar inside and outside managed areas. Increases in submerged aquatic vegetation (SAV) within ponds occurred outside the managed areas during the study period, but not inside managed areas. Because many resident species were closely associated with the SAV, the effect of management on SAV may have been responsible for the distribution patterns of resident species.