Date of Award

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Aspects of the early life history of 6 species of sciaenids that spawned in west Louisiana shelf waters from December 1981 to April 1982 were studied. This research documented the distribution of larvae and tested a transport hypothesis that detailed advective processes which affected their distribution and, ultimately, recruitment into estuaries in southwest Louisiana, and provided information on larva age and growth. Ranked in order of abundance the six sciaenid species collected were: sand seatrout (Cynoscion arenarius); Atlantic croaker (Micropogonias undulatus); spot (Leiostomus xanthurus); black drum (Pogonias cromis); southern kingfish (Menticirrhus americanus; and banded drum (Larimus fasciatus). Atlantic croaker, spot and black drum were most abundant early in December to February while sand seatrout were most abundant in April. Larva density was highest in April and high densities were associated with a freshet of water on the shelf that probably issued from the Atchafalaya River east of the study area. Most larvae were captured in water less than 40 m deep even though some were present to the shelf break (115m). Spawning by these sciaenid species took place over the continental shelf and the larvae were transported into near-shore coastal waters. Transport rates and larva transit times have been roughly quantified. The density-distribution of sciaenid larvae and a suite of environmental variables collected concurrently were analyzed statistically by using canonical correlation analysis. Among the environmental variables examined were micronutrients (nitrate, silicate, phosphate etc.), temperature, salinity, chlorophyll a and zooplankton biomass. In general, sciaenid larvae were most highly correlated with increased zooplankton biomass, but several broad associations between larva density-distribution and environmental variables were identified. The age and growth rate of the larval Atlantic croaker were estimated from growth increments in saccular otoliths. Examination of the data suggested that the growth increments were daily and were used to generate the linear growth equation L(,(t)) = 0.189 (t) + 0.634 mm TL which described larvae growth from an estimated 40 to 80 days old. Two distinct morphological types (morphs) of larval sand seatrout were identified. The morphs were distinguished by 3 pigment differences and nearly 100% separation was possible. The two morphs also exhibited differences in body depth and larva growth rates; differences were tested statistically with analysis of covariance. The morphs were separated in horizontal space and little overlap occurred in their density-distribution.

Pages

201

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