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. coastal eutrophication, fishing activity and hard substrate supplementation) ecosystem modifications are very complex in the Gulf of Mexico, and the potential impact of increased jellyfish populations in one of North America’s most valuable fishing grounds is a most critical issue. Several hypotheses are developed and discussed to guide future research efforts in the Gulf of Mexico. Introduction Increased jellyfish production in marine ecosys- tems is perhaps a symptom of larger ecosystem de- The role of jellyfish in long-term ecosystem change gradation due to coastal eutrophication and over- is receiving increased attention. A number of mar- fishing (Caddy, 1993; Mills, 1995). During ‘bloom’ ine ecosystems, identified by Mills (2001) have either events, jellyfish are capable of exerting considerable documented or suspected cases of long-term eco- control over the flow of energy and nutrients through logical variations that involve jellyfish populations. the ecosystem due to extremely high consumption However, systematically collected, long-term data- rates (Purcell, 1989, 1992, 1997). As such, coastal sets involving jellyfish numbers or biomass are rare. seas with a high degree of susceptibility to eutrophic- Among the notable cases where ecological change is ation and with high fisheries yields should be closely best documented with respect to jellyfish are the Black watched for similar ecological change. Yet, again, the and Azov Seas (Kideys, 1994; Kovalev & Piontkovski, availability of existing long-term data-sets involving 1998; Shiganova, 1998; Purcell et al., 2001), the Ber- jellyfish numbers is rare. ing Sea (Brodeur et al., 1999) and the Mediterranean The northern Gulf of Mexico continental shelf is Sea (Goy et al., 1989). among the most productive and highly fished regions 98 of North America. Pulsed delivery of nutrients to the The goal of the present study is to analyze more Gulf of Mexico through the Mississippi River Delta than 10 years of data from large-scale trawling efforts and numerous other river-dominated estuaries of the in the northern Gulf of Mexico. These fishery resource northern Gulf account for a cumulative regional estu- survey data will be used to assess long-term vari- arine surface area of 30 000 km2. The drainage area ations in two important jellyfish species: the Atlantic emptying into the Gulf of Mexico is over 4 mil- sea nettle, Chrysaora quinquecirrha (Desor), and the lion km2 or approximately 55% of the conterminous moon jellyfish, Aurelia aurita (Linné). Similar ana- United States with the Mississippi River and Mo- lyses of fisheries trawl data have been used previously bile Bay estuary discharging the 1st and 4th largest to identify long-term changes of jellyfish populations volumes, respectively. To complicate ecological vari- in the Bering Sea (Brodeur et al., 1999). ations, the Gulf of Mexico yields about 1/3 of the total United States fishery production and supports the largest fishery by volume in North America in the Methods planktivorous Gulf Menhaden, Brevoortia patronus. Fluvial discharge of nutrients directly onto the SEAMAP trawl data-set shelf is responsible for the high production rates (Lohrenz et al., 1997). Rivers and estuaries of the The data presented in this analysis are from the United northern Gulf of Mexico, from the Florida panhandle States National Marine Fisheries Service (NMFS) to southern Texas (Fig. 1), discharge the greatest Southeast Area Monitoring and Assessment Program volume of water during winter and spring months. (SEAMAP) managed through the Southeast Fisheries Low estuarine residence times due to high freshwater Science Center in St. Petersburg, Florida. The spe- discharge and typically shallow estuarine geomorpho- cific subset of data used for this study were collected logy rapidly displace nutrients and production out onto as part of the twice-yearly shrimp/groundfish surveys the shelf (Pennock et al., 1999). The greatest sea- from 1985 to 1997 in the northern Gulf of Mexico sonal primary production rates in the northern Gulf of between Mobile Bay, Alabama and the southern bor- Mexico are associated with the Mississippi-Atchafalya der of Texas. Though SEAMAP shrimp/groundfish River system and Mobile Bay estuary outflows; pro- surveys were initiated in 1982, I have omitted 1982– ductivity rates are depressed accordingly with the 1984 entirely from analysis because of inconsistent lower-discharge estuaries of Texas (reviewed in Pen- coverage. nock et al., 1999). Zooplankton grazing and secondary The SEAMAP shrimp/groundfish survey protocol production are intense at the coastal transition zone is detailed in Stuntz et al. (1985). In summary, surveys (Dagg & Whitledge, 1991; Dagg, 1995; Ortner & over the entire sampling area were divided into 11 stat- Dagg, 1995) and trophic transfer of this energy to istical regions. Only NMFS designated regions 11 and fish is highly coupled to estuarine delivery in the Gulf 13 through 21 were used; regions 1–10 east of this area (Deegan et al., 1986). The alternate pathway of en- did not have shrimp/groundfish survey data, and re- ergy to gelatinous zooplankton predators has thus far gion 12 was only occasionally surveyed. Surveys were received little, if any, attention. conducted twice-annually: once in the summer (May– The historical lack of interest in the ecological role July) and once in the fall (October–November). Trawl of jellyfish is surprising. The Gulf of Mexico supports sites (typically 30–50 trawls per statistical region per among the greatest diversity of pelagic cnidarians in survey) were randomly located within each region in the world. Over 115 epipelagic species were listed 5 depth strata. The location of stations within depth by Phillips (1971) including 16 species of Scypho- strata caused sampling frequency to be higher near the zoa and Cubozoa. Given that Phillips’ (1971) synopsis coast but consistent between regions and years. An is nearly 30 years old and lacks deep-water inform- example of station density and arrangement (for fall ation, certainly this number severely under-estimates 1991) is given in Figure 1. A preliminary analysis of the real diversity. Given such diversity and potential variance was performed as a check on the randomness importance of jellyfish in this very productive sys- of distribution within and between statistical regions tem, long-term data on jellyfish variations are crucial and depth strata in order to avoid errors in the inter- for realizing current or future ecological changes as pretation of the data. Because of this, I eliminated identified by Caddy (1993) and Mills (1995). statistical region 12 entirely from the study, and the years 1985–1986 from analysis of the summer trawls. 99 Figure 1. Map of study region in the northern Gulf of Mexico. Boxes indicate 10 statistical regions of the SEAMAP sampling program. Points within the boxes are an example of station distribution for a single trawl series in the Fall of 1991 (446 stations). The 20 m and 40 m isobaths are indicated with heavy lines. Table 1. Summary of SEAMAP trawls conducted in the northern sizes large enough to be captured in the trawl net- Gulf of Mexico from 1985 to 1997. Chrysaora quinquecirrha me- dusae were analyzed only from summer trawls and Aurelia aurita ting, and they are hardy enough to be retained without medusae were analyzed from fall trawls so much damage that they are unrecognizable or un- countable. Chrysaora quinquecirrha medusae occur Year No. Trawls Collected during summer months in the Gulf of Mexico with Summer Fall peak abundance usually in June–July (Burke, 1975, 1985 ··· 404 1976). Some reporting of ‘Dactylometra quinquecir- 1986 ··· 236 rha’ from Texas regions has been combined with the 1987 615 365 C. quinquecirrha since these are widely considered 1988 500 701 to be the same species. Aurelia aurita medusae occur 1989 359 631 during the fall months with peak abundance usually in 1990 432 451 October–November (Burke, 1975, 1976). Therefore, 1991 433 446 analysis of C. quinquecirrha is limited to the summer 1992 420 364 surveys (1987–1997) and A. aurita is limited to fall 1993 454 402 surveys (1985–1997). As seen in the trawl summary of 1994 483 380 Table 1, over 10 000 individual trawls were included in 1995 363 337 the present analysis. 1996 388 438 Numerical trawl data are reported here as a stand- 1997 365 377 ardized catch.