COMPARING NEKTON ASSEMBLAGES OF SUBTIDAL HABITATS IN PIPELINE CANALS TRAVERSING BRACKISH AND SALINE MARSHES IN COASTAL LOUISIANA

Subtidal habitats of pipeline canals in Louisiana brackish and saline marshes were sampled seasonally (fall, spring, and summer) between Oct ober 1991 and March 1993 with a 2-m(2) throw trap to identify dominant natant species and test hypotheses relating habitat selection to wate r depth. Densities of nekton were compared among canals and between sh allow (<1 m) and deep (greater than or equal to Im) areas within canal s to test two null hypotheses: H-1: Densities of nekton in pipeline ca nals are not related to maximum canal depth and H-2: Densities of nekt on in shallow and deep subtidal areas of canals are equal. Daggerblade grass shrimp Palaemonetes pugio, bay anchovy Anchoa mitchilli, blue c rab Callinectes sapidus, brown shrimp Penaeus aztecus, and gulf menhad en Brevoortia patronus numerically dominated nekton assemblages in bot h brackish and saline canals. Naked goby Gobiosoma bose, rainwater kil lifish Lucania parva, and gulf pipefish Syngnathus scovelli were domin ant only in brackish canals, whereas white shrimp Penaeus setiferus an d Atlantic croaker Micropogonias undulatus were abundant in saline can als only. Although variation in the abundance of most numerically domi nant species could not be related to maximum canal depth, the distribu tion of several species within pipeline canals was influenced by habit at depth and other interrelated factors. The degree of habitat segrega tion with depth was largely influenced by submerged aquatic vegetation (SAV) and salinity as well as water depth. Habitat segregation with d epth was most pronounced in brackish canals during late spring (May) w hen SAV was present. Naked goby, rainwater killifish, blue crabs, and daggerblade grass shrimp were significantly more abundant in shallow w ater (<1 m) at this time. In saline canals, most blue crabs and dagger blade grass shrimp occupied shallow habitats in March when small juven iles of these species reached peak abundance. Bay anchovy exhibited a pattern opposite that of other species. In March, bay anchovy abundanc e was positively related to maximum canal depth in brackish canals, an d densities were greater in deep than shallow areas of saline canals i n June. Salinity may have affected the distribution of freshwater spec ies (e.g., centrarchids) and limited their occurrence in saline canals . Increasing shallow subtidal habitat by backfilling canals may enhanc e the nursery habitat for some species, especially in brackish canals where the area of subtidal habitat capable of supporting SAV would be expanded.