Employing statistical models and Geographic Information Systems (GIS) to investigate the effect of changing marsh edge on the biomass of estuarine nekton in Barataria Bay, Louisiana, USA
Marsh wetlands are thought to function as nursery habitats for post-larval and juvenile fishes, providing both refuge from predation and increased foraging opportunities. In Louisiana, natural and anthropogenic disturbances have resulted in the continued loss of marsh vegetation (~43km2 year-1). As the loss of marsh appeared to have a null effect on fishery landings, a hypothesis evolved describing a positive effect on nekton production; marsh edge distance (edge) would temporarily increase during marsh degradation, providing a short-term increase in marsh access for organisms. After these ephemeral benefits, it is thought that populations may decline or collapse due to the reduction of nursery habitats. Investigating if the system is still experiencing the “benefits” of increased edge is instrumental in determining if Louisiana coastal fisheries could be in danger of a decline in fisheries production.
Here we used GIS and fisheries data to analyze the relationship between marsh habitats and estuarine nekton. Using land/water raster image data, we tested a widely accepted hypothesis that the distance of edge will reach an apex during marsh degradation. We then applied a number of statistical analyses using GIS and fishery data to test previously held conceptual models that correlate increasing marsh habitats and estuarine nekton.
The findings of this study empirically corroborates the theory that the linear distance of marsh edge follows a dome-shaped curve as marsh degradation increases or continues through time. The maximum value of edge was achieved in 1985, 10 years prior to previous estimates. While the habitat trend has been supported through this empirical investigation, the relationship of habitat to fish and shellfish species is less clear. Even though the system has experienced a drastic decrease in marsh edge distance and marsh area, nekton biomass appears resilient to these changes. The statistical analysis between six species and marsh habitat features showed limited to absent significant results. We hypothesize that the resilience seen in many of these estuarine species could be derived from their adaptations to dynamic ecosystems over thousands of years. Even though the delta cycle has effectively stopped in coastal Louisiana, it is clear that habitat change is still occurring, albeit at potentially higher rates because of the synergistic effects of multiple perturbations. Estuarine species may already be well adapted for a constantly changing ecosystem, and in this case, land loss may not affect nekton as severely as once hypothesized.