Hydrobiologia 451: 97–111, 2001. 97 © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Numerical increases and distributional shifts of Chrysaora quinquecirrha (Desor) and Aurelia aurita (Linne)´ (Cnidaria: Scyphozoa) in the northern Gulf of Mexico W. M. Graham Dauphin Island Sea Lab and Department of Marine Science, University of South Alabama, 101 Bienville Blvd, Dauphin Island, AL, 36528, U.S.A. E-mail:
[email protected] Key words: jellyfish, medusae, Mississippi River, SEAMAP, eutrophication, hypoxia Abstract Fisheries resource trawl survey data from the National Marine Fisheries Service from a 11–13-year period to 1997 were examined to quantify numerical and distributional changes of two species of northern Gulf of Mexico scyphomedusae: the Atlantic sea nettle, Chrysaora quinquecirrha (Desor), and the moon jelly, Aurelia aurita (Linné). Trawl surveys were grouped into 10 statistical regions from Mobile Bay, Alabama to the southern extent of Texas, and extended seaward to the shelf break. Records of summertime C. quinquecirrha medusa populations show both an overall numerical increase and a distributional expansion away from shore in the down-stream productivity field of two major river system outflows: Mobile Bay and the Mississippi-Atchafalaya Rivers. In addition, there is a significant overlap between summer C. quinquecirrha and lower water column hypoxia on the Louisiana shelf. In trawl surveys from the fall, A. aurita medusae showed significant trends of numerical increase in over half of the regions analyzed. For both species, there were statistical regions of no significant change, but there were no regions that showed significant decrease in number or distribution. The relationships between natural and human-induced (e.g.