64 



Fishery Bulletin 102(1) 



39°N 



38°N 



^vt^w 



N 37°N 



77°W 



76°W 



Figure 1 



Chesapeake Bay and stations sampled by the midwater trawl in the 1995-2000 surveys. 

 Horizontal lines indicate boundaries of three designated regions. 



We evaluated environmental factors, spatial distribution 

 of spawning stock biomass (SSB), and possible ontogenetic 

 migrations of prerecruits (Dovel, 1971; Loos and Perry. 

 1991; Wang and Houde, 1995; Kimura et al, 2000) with 

 respect to bay anchovy recruitment variability. Our objec- 

 tives were 1) to estimate annual and regional variability 

 in bay anchovy recruitment. 2) to evaluate effects of hy- 

 drological conditions (mainly, freshwater input, and dis- 

 solved oxygen concentration) on stage-specific distribution, 

 ontogenetic migration, and recruitment, and 3) to identify 

 mechanisms and describe patterns or trends in the bay 

 anchovy recruitment process. Data were obtained in a 

 six-year, multidisciplinary research program conducted 

 throughout Chesapeake Bay. 



Materials and methods 



Study area 



Chesapeake Bay is a coastal plain estuary of partially mixed 

 fresh water and sea water. Its 320-km mainstem varies in 

 width from about 6 to 50 km (Fig. 1 ). The Bay is shallow; 

 less than 10' r of its area is >18 m deep and approximately 

 50' i is <6 m deep. More than 809& of the freshwater entering 

 the bay is from tributaries on its northern and western sides 



(Chesapeake Bay Program 1 ). Salinity grades from near-full 

 seawater at the mouth of the bay to freshwater near its 

 head. Water temperatures reach 28-30°C in mid summer, 

 and fall to 1^°C in late winter (Murdy et al, 1997 ). Despite 

 shallow depth, the bay usually has a strongly developed 

 pycnocline, and has seasonally strong vertical gradients in 

 temperature, salinity, and dissolved oxygen. 



Surveys 



Trawl surveys were conducted three times annually over 

 the entire bay (April-May, June-August, and October). 

 1995-2000 (Table l.Fig. 1). Midwater-trawl (MWT) fish col- 

 lections 2 were made on transects in three regions: the lower 

 bay (37°05'N-37°55'N), middle bay (37°55'N-38°45'N I, and 

 upper bay (38°45'N-39°25'N). As defined, the lower bay 

 contains 51% of water volume, the middle bay 32^ .and the 

 upper bay 17^ (Fig. 1). The number of midwater trawl sta- 



Chesapeake Bay Program. 2000. Chesapeake Bay: Introduc- 

 tion to an ecosystem. U.S. Environmental Protection Agency, 

 publ. EPA 903-R-00-001. 30 p. EPA. 410 Severn Ave, Suite 109, 

 Annapolis. MD 21403. 



Trophic interactions in estuarine systems, midwater trawl sur- 

 vey. University of Maryland Center for Environmental Sci- 

 ence, Chesapeake Biological Laboratory, http://www.ch.esa 

 peake.org/ ties/mwt laccessed 15 October 20031. 



