Reanalyses of Gulf of Mexico fisheries data: Landings can be misleading in assessments of fisheries and fisheries ecosystems

doi:10.1073/pnas.0704354105

Reanalyses of Gulf of Mexico fisheries data: Landings can be misleading in assessments of fisheries and fisheries ecosystems

*Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803-7503; and
^‡School of Aquatic and Fisheries Sciences, University of Washington, Box 355020, Seattle, WA 98195

Edited by John J. Magnuson, University of Wisconsin, Madison, WI, and accepted by the Editorial Board December 27, 2007 (received for review May 9, 2007)

Abstract

We used two high profile articles as cases to demonstrate that use of fishery landings data can lead to faulty interpretations about the condition of fishery ecosystems. One case uses the mean trophic level index and its changes, and the other uses estimates of fishery collapses. In earlier analyses by other authors, marine ecosystems in the Gulf of Mexico (GOM) and U.S. Atlantic Ocean south of Chesapeake Bay were deemed to be severely overfished and the food webs badly deteriorated using these criteria. In our reanalyses, the low mean trophic level index for the GOM actually resulted from large catches of two groups of low trophic level species, menhaden and shrimp, and the mean trophic level was slowly increasing rather than decreasing. Commercial targeting and high landings of shrimps and menhaden, especially in the GOM, drove the index as previously calculated. Reanalyses of fishery collapses incorporating criteria that included targeting, variability in fishing effort, and market forces discovered many false cases of collapse based simply upon a decline of catches to 10% of previous maximum levels. Consequently, we suggest that the low mean trophic level index calculated in the earlier article for the GOM did not reflect the overall condition of the fishery ecosystem, and that the 10% rule for collapse should not be interpreted out of context in the GOM or elsewhere. In both cases, problems lay in the assumption that commercial landings data alone adequately reflect the fish populations and communities.

Footnotes

^†To whom correspondence should be addressed. E-mail: jhcowan{at}lsu.edu
Author contributions: K.d.M., J.H.C., T.E.E., and R.H. analyzed data; and K.d.M., J.H.C., and T.E.E. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.J.M. is a guest editor invited by the Editorial Board.
Freely available online through the PNAS open access option.