182 



Fishery Bulletin 89(2). 1991 



MNEMIOPSIS LEIDYI DISTRIBUTION 



FISH EGG (ALL SPECIES) DISTRIBUTION 



i 1 \ rn v^ 1 1 1 i 1 — 



76°00'«J. . ^5° 50' 75° 40' 75° 30' 



Chesapeake 

 Bay Ca 



(32)»E4 



y- 



37 c 

 00' 



(28)»FT4 



Atlantic Ocean 36 _ 



00' 



1 I 



Figure 1 



Positions of stations and mean station densities (numbers/m :i ) of Mnemiopsis leidyi and fish eggs at the mouth of Chesapeake Bay. 

 Dotted line indicates position of the Chesapeake Bay plume. 



mitchilli) in the laboratory (Johnson 1987, Monteleone 

 and Duguay 1988), but while it consumes some fish lar- 

 vae in Chesapeake Bay (Burrell and Van Engel 1976), 

 its predation on fish eggs in the field is not documented. 

 Anchoa mitchilli spawns in the Bay in spring and sum- 

 mer and its eggs typically account for over 90% of all 

 fish eggs present between May and August (Olney 

 1983). 



The mouth of the Bay is characterized by water 

 masses that differ spatially in both the vertical and 

 horizontal dimensions (Boicourt et al. 1987) and pro- 

 vides hydrographic structure capable of shaping the 

 spatial distribution of planktonic animals. Its complex 

 hydrography is dominated by a buoyant plume char- 

 acterized by a horizontal scale of 10-100km, a vertical 

 scale of 5-20m, and a temporal scale of 1-10 days 

 (Boicourt et al. 1987). As a result, the small-scale ver- 

 tical and horizontal distributions of predator and prey 

 can be observed synoptically in water columns of dif- 

 ferent structure within a confined study area. 



Here we describe the small-scale spatial and temporal 

 co-occurrence of M. leidyi and fish eggs at the mouth 

 of Chesapeake Bay and assess potential predation. 



Methods 



Sampling protocol 



Three stations were allocated across the mouth of 

 Chesapeake Bay with two additional stations on the 

 continental shelf (Fig. 1) such that some stations were 



within and others outside of the typical boundaries of 

 the Chesapeake Bay plume (Boicourt et al. 1987). Each 

 station was occupied for 30 hours between 11 and 21 

 June 1985 (the sampling period at station El was in- 

 terrupted for 24 hours by vessel failure). At each sta- 

 tion, hydrographic profiles (temperature, salinity, and 

 specific gravity anomaly o t ) and plankton collections 

 at three nominal depths (surface, within the pycnocline, 

 and below the pycnocline) were obtained once at four 

 diel intervals (dawn, noon, dusk, and midnight). Fish 

 eggs and ctenophores were collected with a 1-m Tucker 

 trawl equipped with three 202-/^m mesh nets, General 

 Oceanic flow meters, and an Applied Microsystems 

 Limited temperature, salinity, and depth recorder and 

 towed at approximately lOOcm/second. Nets were 

 opened at depth and fished along a horizontal trajec- 

 tory for 30-60 seconds each; for subsurface strata, the 

 trawl was lowered while the vessel was stopped and 

 its nets were fished along a horizontal trajectory at 

 depth. The trawl was positioned at nominal depth 

 strata by the trigonometry of the warp angle and 

 length. Triplicate samples were obtained at the surface; 

 duplicate, discrete-depth samples were obtained within 

 and below the pycnocline. With these sampling pro- 

 cedures, the trawl sampled on small vertical (1-3 m) and 

 horizontal (10-50m) scales. 



All plankton collections were passed through a 

 6.4-mm mesh screen to separate ctenophores from 

 ichthyoplankton. Ctenophores retained on this screen 

 were fixed to prevent dissolution following the methods 

 of Gosner (1971), then rinsed and preserved in 5% 



