Distributional ecology of zooplankton and fish larvae (spot, croaker, menhaden) in the northern Gulf of Mexico

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Biology and Fisheries

First Committee Member

Harding B. Michel, Committee Chair

Second Committee Member

Peter B. Ortner, Committee Member


Ichthyoplankton and zooplankton samples were collected during February of three years, 1980, 1981 and 1982, in the northern Gulf of Mexico. The objectives were to describe density distributional patterns of fish larvae and the zooplankton organisms which serve as prey for the larvae. Emphasis was placed on the larvae: spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus) and Gulf menhaden (Brevoortia patronus). Three transect lines were sampled: offshore from the Mississippi River delta, offshore from Cape San Blas, Florida and offshore from Galveston, Texas. Stations at 10F, 50F and 100F isobaths were sampled along each transect and samples were obtained at 0600, 1200, 1800 and 2400 hours. Three discrete depths were sampled at each station.The three target species and their zooplankton prey displayed signficant inter-annual variations, inshore-offshore gradients, and different vertical depth distributions. Menhaden larvae were more abundant than spot and croaker larvae at 5 out of 9 stations sampled. At the remaining 4 stations, the three target species displayed comparable densities. Menhaden larvae constituted 64% of total larvae at the Mississippi-10F station, 39.6% at the Mississippi-50F and 30.7% at the Mississippi-100F station. Highest densities of spot (12.5/1000 m3), croaker (9.0/1000 m3), and menhaden larvae (916.8/1000 m3) were found at the Mississippi-10F station, in the vicinity of the Mississippi River delta. Average densities of the three most abundant zooplankters: copepod nauplii (8284/m3), small copepods (8558/m3) and pelecypod larvae (1554/m3) were not distinctly high when compared with the Cape San Blas-10F inshore station. However, maximum densities of nauplii, pelecypod larvae and ciliates were encountered at the Mississippi-10F station, indicating that subregions within the Mississippi River delta with good larval feeding conditions exist. Also, the Mississippi inshore station is distinguished from the other inshore stations, by the stratified nature of its water column.Spot, croaker and menhaden larvae displayed different vertical distributions. Croaker larvae were uniformly distributed, spot larvae were more numerous at the surface during midnight, only at the Mississippi-10F station, and menhaden larvae were discernibly more numerous at the surface at noon at all the three inshore stations and at the Mississippi-50F station, where larvae occurred also at the surface during midnight. (Abstract shortened with permission of author.)


Biology, Oceanography; Agriculture, Fisheries and Aquaculture

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