Numerical increases and distributional shifts of Chrysaora quinquecirrha (Desor) and Aurelia aurita (Linne) (Cnidaria : Scyphozoa) in the northern Gulf of Mexico

Fisheries resource trawl survey data from the National Marine Fisheries Ser vice from a 11-13-year period to 1997 were examined to quantify numerical a nd distributional changes of two species of northern Gulf of Mexico scyphom edusae: the Atlantic sea nettle, Chrysaora quinquecirrha (Desor), and the m oon jelly, Aurelia aurita (Linne). Trawl surveys were grouped into 10 stati stical regions from Mobile Bay, Alabama to the southern extent of Texas, an d extended seaward to the shelf break. Records of summertime C. quinquecirr ha medusa populations show both an overall numerical increase and a distrib utional 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. quin quecirrha and lower water column hypoxia on the Louisiana shelf. In trawl s urveys from the fall, A. aurita medusae showed significant trends of numeri cal increase in over half of the regions analyzed. For both species, there were statistical regions of no significant change, but there were no region s that showed significant decrease in number or distribution. The relations hips between natural and human-induced (e.g. coastal eutrophication, fishin g activity and hard substrate supplementation) ecosystem modifications are very complex in the Gulf of Mexico, and the potential impact of increased j ellyfish populations in one of North America's most valuable fishing ground s is a most critical issue. Several hypotheses are developed and discussed to guide future research efforts in the Gulf of Mexico.