184 



Fishery Bulletin 89[2), 1991 



i 



Q- 



10 



15 



in evaluating predation is apparent 

 across the mouth of Chesapeake Bay. 

 Potential predation in the southern 

 reaches where the Chesapeake Bay 

 plume overlays coastal shelf water was 

 low because of the relative lack of ver- 

 tical co-occurrence there. In the north- 

 ern reaches where the water column 

 was well mixed, M. leidyi and fish eggs 

 co-occurred in a more or less well-mixed 

 water column, and as a result our 

 estimates of potential predation were 

 high. 



The application of parameter esti- 

 mates derived from laboratory preda- 

 tion experiments to the evaluation of 

 the impact of gelatinous planktivores on 

 their prey in nature, an approach that 

 avoids field sampling errors, has other 

 pitfalls (Purcell 1985). These problems 

 relate to the unrealistic confines of ex- 

 perimental vessels, which constrain 

 movement and small-scale hydrodynam- 

 ics, and to unnaturally high experimen- 

 tal densities of predator and prey (Sulli- 

 van and Reeve 1982, de Lafontaine and 

 Leggett 1988). The result is often arti- 

 ficially low estimates of clearance rate, 

 values that are then used as functions 

 in mathematical operations that range 

 from simple multiplication of clearance 

 rate and predator density (e.g., Reeve 

 et al. 1978) to complex models that in- 

 volve the swimming and foraging velo- 

 cities and ambit geometries of motile 

 predators and prey, and the turbulence 

 of the environment in which they are 

 embedded (e.g., Bailey and Batty 1983, 

 Rothschild and Osborn 1988, Evans 

 1989). The simple approximation used 

 herein was justified, in part, by the be- 

 havior of M. leidyi feeding on immobile 

 fish eggs. Lobate ctenophores feed as 

 a moving pump, pumping water con- 

 tinuously through mucus- and tentacle- 

 lined lobes, while either swimming ver- 

 tically or hovering (Larson 1988), and 

 changing position in response to low prey density 

 (Reeve et al. 1978). While the geometry of the pred- 

 atory field of M. leidyi is unknown, we assume, given 

 forage velocities of from 1-3 mm/second for its con- 

 gener M. mccradyi (Larson 1987), that it encounters 

 new water continuously. Although the gut capacity of 

 lobate ctenophores is small, M. leidyi egests super- 

 fluous food in a mucus bolus when its gut is full and 



EBB 



FLOOD 



MIDNIGHT DAWN 



EBB 



NOON 



FLOOD 



DUSK 



EBB 



MIDNIGHT 



10 



10 



10 



15 



Cteno- 

 phores 



1 j>.h 



Fish Eggs 



C! 



a 



73 



10 



100/10 



100/10 



— f -f- 



100 10 



1 I ' 



100/10 



u 



100 



NUMBERS • m" 



Figure 2 



Temporal hydrographic sections (temperature, salinity, sigma-t) and densities 

 (numbers/m 3 ) of Mn^miopsis leidyi and fish eggs at station El at the mouth 

 of Chesapeake Bay. Vessel failure caused a 24-hour interruption in sampling 

 between noon and dusk. 



continues feeding; egested fish eggs, embedded in this 

 bolus, are either dead or moribund (Johnson 1987). 



The observation that co-occurrence of M. leidyi and 

 fish eggs, and consequently potential predation, is 

 greatest in areas where Chesapeake Bay water mixes 

 with coastal shelf water, coupled with the observation 

 that M. leidyi are more abundant in regions of higher 

 salinity within other estuaries, implies that those fishes 



