Comparing nekton assemblages of subtidal habitats in pipeline canals traversing brackish and saline marshes in coastal Louisiana

Abstract

Subtidal habitats of pipeline canals in Louisiana brackish and saline marshes were sampled seasonally (fall, spring, and summer) between October 1991 and March 1993 with a 2-m² throw trap to identify dominant natant species and test hypotheses relating habitat selection to water depth. Densities of nekton were compared among canals and between shallow (<1 m) and deep (>-1m) areas within canals to test two null hypotheses: H₁: Densities of nekton in pipeline canals are not related to maximum canal depth and H₂: Densities of nekton in shallow and deep subtidal areas of canals are equal. Daggerblade grass shrimpPalaemonetes pugio, bay anchovyAnchoa mitchilli, blue crabCallinectes sapidus, brown shrimpPenaeous aztecus, and gulf menhadenBrevoortia patronus numericallly dominated nekton assemblages in both brackish and saline canals. Naked gobyGobiosomabosc, rainwater killifishLucania parva, and gulf pipefishSyngnathus scovelli were dominant only in brackish canals, whereas white shrimpPelaeus setiferus and Atlantic croakerMicropogonias undulatus were abundant in saline canals only. Although variation in the abundance of most numerically dominant species could not be related to maximum canal depth, the distribution of several species within pipeline canals was influenced by habitat depth and other interrelated factors. The degree of habitat segregation with depth was largely influenced by submerged aquatic vegetation (SAV) and salinity as well as water depth. Habitat segregation with depth was most pronounced in brackish canals during late spring (May) when SAV was present. Naked goby, rainwater killifish, blue crabs, and daggerblade grass shrimp were significantly more abundant in shallow water (<1 m) at this time. In saline canals, most blue crabs and daggerblade grass shrimp occupied shallow habitats in March when small juveniles of these species reached peak abundance. Bay anchovy exhibited a pattern opposite that of other species. In March, bay anchovy abundance was positively related to maximum canal depth in brackish canals, and densities were greater in deep than shallow areas of saline canals in June. Salinity may have affected the distribution of freshwater species (e.g., centrarchids) and limited their occurrence in saline canals. Increasing shallow subtidal habitat by backfilling canals may enhance the nursery habitat for some species, especially in brackish canals where the area of subtidal habitat capable of supporting SAV would be expanded.