Abstract.- In Chesapeake Bay 

 in June, the predatory lobate cteno- 

 phore Mnemiopsis leidyi and the 

 eggs of the bay anchovy Anchoa mit- 

 chilli typically reach seasonal and 

 localized abundance together. When 

 examined at small vertical (1-3 m), 

 horizontal (10-50m), and temporal 

 (6-hour) scales, the co-occurrence of 

 M. leidyi and fish eggs (32.3-74.2% 

 of which were A. mitchilli) was great- 

 est in the northern reaches of the 

 mouth of Chesapeake Bay, where the 

 water column was well mixed, than 

 in the southern reaches where the 

 water column was stratified. Stratifi- 

 cation to the south was effected by 

 the Chesapeake Bay plume. With es- 

 timates of ctenophore clearance rate 

 reported elsewhere and observed 

 densities of ctenophores and fish 

 eggs, potential predation was judged 

 to be greatest in the northern reaches 

 of the Bay mouth. The observation 

 that co-occurrence and potential pre- 

 dation are greatest in areas where 

 Chesapeake Bay water mixes with 

 coastal shelf water implies that those 

 fishes that spawn in low-salinity sur- 

 face waters of well-stratified water 

 columns may afford protection of 

 their eggs from ctenophore predation. 



Potential Predation on Fish Eggs 

 by the Lobate Ctenophore 

 Mnemiopsis leidyi Within and 

 Outside the Chesapeake Bay Plume' 



John J. Govoni 



Beaufort Laboratory, Southeast Fisheries Science Center 



National Marine Fisheries Service. NOAA, Beaufort. North Carolina 28516 



John E. Olney 



Virginia Institute of Marine Science, School of Marine Science 

 College of William and Mary. Gloucester Point, Virginia 23062 



Predation is probably the leading 

 cause of mortality for fertilized fish 

 eggs and yolksac larvae because star- 

 vation is not relevant for these early- 

 life-history stages and because the 

 short duration of egg incubation and 

 yolk absorption for most teleosts 

 limits transport to areas inimical to 

 development (Bailey and Houde 

 1989). Assessments of the impact of 

 predation on cohorts of fish eggs and 

 larvae in the ocean, however, have 

 been hindered by three problems: 

 two practical, the third inferential. 

 Eggs and larvae leave little identifi- 

 able residue in the guts of predators, 

 and, as a result, direct estimates of 

 the extent of predation are difficult. 

 Predators and prey, moreover, are 

 concentrated together in collecting 

 devices, a situation that can result in 

 artifically high feeding rates and in- 

 flated estimates of predation. Last- 

 ly, predation is often spuriously in- 

 ferred from the inverse abundance of 

 predators and prey, when presence 

 and absence may actually reflect 

 spatial and temporal segregation 

 rather than removal of prey by pred- 

 ators. Such misinterpretations result 

 from failure to consider the small- 

 scale temporal and spatial distribu- 



Manuscript accepted 12 December 1990. 

 Fishery Bulletin, U.S. 89:181-186 (1991). 



* Contribution no. 1635 of the Virginia Insti- 

 tute of Marine Science and School of Marine 

 Science, College of William and Mary. 



tion of predator and prey in differing 

 water masses (Frank and Leggett 

 1982, 1985). 



Among the known invertebrate 

 predators of fish eggs and larvae, 

 coelenterates and ctenophores are 

 likely candidates for significant pre- 

 dation because of their high rates of 

 ingestion and population growth (Al- 

 ldredge 1984, Purcell 1985, Monte- 

 leone and Duguay 1988). Lobate 

 ctenophores, in particular, are major 

 predators of small zooplankton of 

 limited mobility (Kremer 1979, Pur- 

 cell 1985, Monteleone and Duguay 

 1988). They capture prey by pump- 

 ing water past lobes lined with mucus 

 and secondary tentacles (Larson 

 1988), a feeding mechanism that is 

 seemingly well suited for the capture 

 of fish eggs. 



In Chesapeake Bay, a lobate cteno- 

 phore Mnemiopsis leidyi and the 

 eggs of the bay anchovy Anchoa mit- 

 chilli reach seasonal and localized 

 abundance together, thereby provid- 

 ing a predator and prey pair that is 

 ideal for an evaluation of potential 

 predation. Mnemiopsis leidyi is pres- 

 ent from late fall through midsum- 

 mer, and episodically explodes in 

 abundance between May and July 

 (Bishop 1967, Miller 1974, Kremer 

 and Nixon 1976, Mountford 1980). 

 Mnemiopsis leidyi can exhibit ap- 

 preciable predation on fish eggs (A. 



181 



