Quarterdeck 4.1

Recent graduate
Epibenthic invertebrates and fishes of the northwestern Gulf of Mexico


by Abdulrahman Al-Jabr
The organisms that live in, on, or attached to the seafloor are called the benthos. The relationship linking benthic organisms into the epibenthic community is a common food source, derived directly or indirectly on the bottom. Therefore, these assemblages include demersal fishes that swim near the seabed and feed from it.

The data used in this study were collected from October 1988 through July 1990 on the continental shelf of the northwestern Gulf of Mexico. Fish and invertebrate samples were obtained during five cruises aboard the Texas A&M University oceanographic research vessel, Gyre. Station positions were determined when the trawl was deployed on the bottom of the contin-ental shelf.

The epibenthic organisms were collected with a nine-meter otter trawl during the first four cruises and a five-meter otter trawl during the fifth cruise. A total of 34 trawl samples from 34 stations were taken at speeds of 2.8 to 5.6 kilometers per hour with trawling periods lasting between 14 and 40 minutes at depths ranging from 13 to 400 meters.

This study presents a systematic, quantitative measurement of epibenthos structure in the northwestern Gulf of Mexico. In the past, the lack of studies in this part of the gulf has hindered attempts to assess changes in commun-ity structure that result from natural and artificial events. The main objective of this study is to contribute to our knowledge of the biology and commun-ity structure of the epibenthic assemblages (invertebrates and fishes) of the continental shelf of the northwestern Gulf of Mexico. This involves not only species identification but also understanding of population densities and group biomass, as well as diversity and similarity of species composition.
Most of the fish and invertebrates collected by the otter trawls were initially sorted on the ship and fixed in 10% buffered formaldehyde prior to their preservation in an ethanol solution. This prevents misidentification due to changes such as decoloration. The samples were stored in buckets for later sorting. Later, the organisms were washed in tap water to remove the formalin and blotted dry with paper towels for a wet "live" weight. All samples were eventually checked for their identifications, counted, and weighed to determine the biomass for each species. Following the second laboratory sorting and identification, the samples were transferred to 70% ethyl alcohol solution for preservation and storage.

In this study the average weight of the offshore trawl samples was not statistically higher than the average weight of the nearshore samples. Furthermore, the mean of the total biomass was almost the same for both regions. Despite the high diversities in most of the communities, small or large, the species in these communities were different. This suggests that in the northwestern gulf there are no continuous recurrent assemblages in time or space on the shelf.

I found no correlations between community structure and season, depth, or temperature within the seasonal and spatial scales of this study. In addition, the epibenthic assemblages of the continental shelf of the northwestern Gulf of Mexico do not experience hypoxic conditions. Finally, the northwestern Gulf of Mexico investigation showed a relatively high diversity compared with the northeastern gulf and the U.S. North Atlantic shelves. However, the northwestern gulf was less diverse than the Campeche Bank, the Red Sea, the Arabian Sea and the Arabian Gulf. Therefore, the diversity of the northwestern gulf lies between that of stable tropical shelves and those at temperate latitudes.

[Next | Previous | Contents | Home]

Oceanography, Texas A&M University
rshatto@ocean.tamu.edu

URL=http://oceanography.tamu.edu/Quarterdeck/QD4.1/Grads/aljabr.html
Updated May 27, 1996