Nekton from Fishery-Independent Trawl Samples in Estuaries of the U.S. Gulf of Mexico: A Comparative Assessment of Gulf Estuarine Systems (CAGES)

Estuaries are important in supporting much of the fishery production in the Gulf of Mexico, but this support appears to vary widely among different estuarine systems.

The nekton abundance data summarized in this report are available at http://data.gcoos.org.

This cooperative study with state natural resources agencies was designed to use fishery-independent monitoring data and compare historical catches from 4.9-m and 6.1-m trawl surveys. This report provides an assessment of the abundance, length frequencies, and biomass of 14 species of fish and four species of decapod crustaceans that were either abundant in the samples or economically important. The most abundant species in the analysis include bay anchovy, Atlantic croaker, spot, and brown shrimp. Other fishery species of particular interest include Gulf menhaden, white shrimp, pink shrimp, blue crab, spotted seatrout, southern flounder, and red drum. While the years analyzed varied among states, samples from most estuaries were available and analyzed for the years 1986 to 2005. The 24 estuaries analyzed were identified using the Estuarine and Coastal Drainage Areas delineated by the U. S. Geological Survey and listed in NOAA?s Coastal Assessment Framework (http://coastalgeospatial.noaa.gov/data_gis.html).

While our goal was to provide nekton data for comparisons among all 24 estuaries, we have presented the data by state, because differences in sampling gear and protocols make comparisons among states challenging. Overall trawl size varied from 6.1 m in Texas and Florida to 4.9 m in Louisiana, Mississippi, and Alabama, and towing speeds varied from 2.2 kph in Florida to 4.8 kph in Texas and Louisiana. We converted catch to numbers per hectare swept to standardize the data and adjust for some of the gear differences among states, but effects of varying net mesh size make comparisons across state lines difficult. Size frequency distributions were different for most species among the states, suggesting that net mesh size affected abundance estimates. We converted abundance data to biomass using the size frequency data and length-weight relationships from the literature in an effort to reduce the impact of size selection. A more detailed analysis on brown shrimp, however, indicates that size selectivity still affects comparisons for many species. A comprehensive gear comparison study is needed to adequately address many of these problems.

Temporal trends in abundance and biomass within estuaries or within states are less likely to be affected by gear differences. Monthly mean values presented for each state and each estuary indicate that abundance and biomass peaks for most species are not consistent among the different estuaries. Annual variability within states was often high, and few similarities were apparent in annual trends among states.

Comparisons of abundance or biomass between estuaries should be made with care. Even within states, the selection of sampling sites may affect such comparisons. In Texas and in Florida, the randomization of sampling sites within estuaries can make these comparisons meaningful, if an adequate number of samples was collected. Within Louisiana, Mississippi, and Alabama, the location and distribution of fixed sampling stations should be considered when making comparisons among estuaries. Comparisons of nekton abundance or biomass in estuaries of different states also need to account for differences in sampling gear and size frequency distributions.

To assess variability among estuaries in supporting fishery species and other abundant nekton. It is part of a larger effort of the National Marine Fisheries Service Galveston Laboratory to develop a Comparative Assessment of Gulf Estuarine Systems (CAGES). The nekton abundance data summarized in this report are available at http://data.gcoos.org.

This cooperative study with state natural resources agencies was designed to use fishery-independent monitoring data and compare historical catches from 4.9-m and 6.1-m trawl surveys. This report provides an assessment of the abundance, length frequencies, and biomass of 14 species of fish and four species of decapod crustaceans that were either abundant in the samples or economically important. The most abundant species in the analysis include bay anchovy, Atlantic croaker, spot, and brown shrimp. Other fishery species of particular interest include Gulf menhaden, white shrimp, pink shrimp, blue crab, spotted seatrout, southern flounder, and red drum. While the years analyzed varied among states, samples from most estuaries were available and analyzed for the years 1986 to 2005. The 24 estuaries analyzed were identified using the Estuarine and Coastal Drainage Areas delineated by the U. S. Geological Survey and listed in NOAA?s Coastal Assessment Framework (http://coastalgeospatial.noaa.gov/data_gis.html).

While our goal was to provide nekton data for comparisons among all 24 estuaries, we have presented the data by state, because differences in sampling gear and protocols make comparisons among states challenging. Overall trawl size varied from 6.1 m in Texas and Florida to 4.9 m in Louisiana, Mississippi, and Alabama, and towing speeds varied from 2.2 kph in Florida to 4.8 kph in Texas and Louisiana. We converted catch to numbers per hectare swept to standardize the data and adjust for some of the gear differences among states, but effects of varying net mesh size make comparisons across state lines difficult. Size frequency distributions were different for most species among the states, suggesting that net mesh size affected abundance estimates. We converted abundance data to biomass using the size frequency data and length-weight relationships from the literature in an effort to reduce the impact of size selection. A more detailed analysis on brown shrimp, however, indicates that size selectivity still affects comparisons for many species. A comprehensive gear comparison study is needed to adequately address many of these problems.

Temporal trends in abundance and biomass within estuaries or within states are less likely to be affected by gear differences. Monthly mean values presented for each state and each estuary indicate that abundance and biomass peaks for most species are not consistent among the different estuaries. Annual variability within states was often high, and few similarities were apparent in annual trends among states.

Comparisons of abundance or biomass between estuaries should be made with care. Even within states, the selection of sampling sites may affect such comparisons. In Texas and in Florida, the randomization of sampling sites within estuaries can make these comparisons meaningful, if an adequate number of samples was collected. Within Louisiana, Mississippi, and Alabama, the location and distribution of fixed sampling stations should be considered when making comparisons among estuaries. Comparisons of nekton abundance or biomass in estuaries of different states also need to account for differences in sampling gear and size frequency distributions.

The abundance estimates included in this report are applicable only to this Technical Memorandum. The data are limited to the years between 1981 ? 2007 and no inferences should be made of the data beyond the years and the geographic regions discussed herein. The scientific views, opinions and conclusions expressed herein are solely those of the authors and do not represent the views, opinions or conclusions of NOAA and/or the Department of Commerce.

Brown, H., T.J. Minello, G.A. Matthews, M. Fisher, E.J. Anderson, R. Riedel, and D.L. Leffler. 2013. Nekton from fishery-independent trawl samples in estuaries of the U.S. Gulf of Mexico: a Comparative Assessment of Gulf Estuarine Systems (CAGES). U.S. Dept. Commerce NOAA Tech. Memo. NMFS-SEFSC-647, 269 p.

This Technical Memorandum series is used for documentation and timely communication of preliminary results, interim reports, or similar special-purpose information. Although the memoranda are not subject to complete formal review, editorial content, or detailed editing, they are expected to reflect sound professional work