Stable isotope indicators of movement and residency for brown shrimp (Farfantepenaeus aztecus) in coastal Louisiana marshscapes

Abstract

Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations, we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km2 marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m−2 in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m−2 in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations. Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation and restoration efforts should focus on these areas as important for sustaining shrimp fisheries.