74 



Fishery Bulletin 102(1) 



a strong role in controlling YOY recruitment levels. The 

 four highest recruitment years in our series had the lowest 

 mean subpycnocline DO levels and had distribution pat- 

 terns of SSB that differed little between the prespawning 

 April-May and spawning June-August periods (Fig. 4). Al- 

 though we do not fully understand how DO, and possibly 

 hypoxic conditions, affect migratory behavior and distribu- 

 tion patterns of bay anchovy, hypoxia in Chesapeake Bay 

 has been demonstrated in other research to affect spatial 

 and temporal patterns of fish abundance, including bay 

 anchovy (Breitburg, 1992; Keister et al., 2000). 



Our third hypothesis proposes that predation is an im- 

 portant regulator of fish recruitment in early-life stages 

 (Sissenwine, 1984; Bailey and Houde, 1989). We hypoth- 

 esize that abundant and spatially concentrated larval 

 or juvenile anchovy, as observed in the lower bay, could 

 promote early-life survival by satiating predators, even if 

 some predators migrate to areas where larval and juvenile 

 anchovy are abundant. At mesoscale distances of 10-100 

 km, distribution of predators (e.g. YOY and age-1 weakfish 

 [Cynoscion regalis] ) may be important. If the maximum 

 number of prey that can be eaten by predators is reason- 

 ably constant, the effect of predation can be density-depen- 

 satory (Hilborn and Walters, 1992), i.e. predation mortality 

 rate decreases as prey density increases. 



In support of the third hypothesis, a correspondence 

 analysis on fish species assemblages by year, season, re- 

 gion, and life stage (Jung and Houde, 2003) indicated that 

 distributions and abundances of YOY weakfish, a major 

 predator of bay anchovy in Chesapeake Bay (Hartman 

 and Brandt, 1995), and YOY bay anchovy were closely as- 

 sociated spatially, seasonally, and annually in our six-year 

 study. The major spawning area of bay anchovy is spatially 

 restricted. If predator migration to the area is limited, then 

 as the supply of larvae and juveniles increases, it may satu- 

 rate predator demand, the condition necessary for depensa- 

 tion to be important. 



It may seem contradictory to propose that density-com- 

 pensation with respect to SSB (the negative sign of j\) 

 and density-depensation with respect to AL (the second or 

 third hypothesis ) can act simultaneously during larval and 

 juvenile stages. Under this circumstance, the number of 

 surviving postlarval anchovies is hypothesized to decrease 

 because of food limitation when larval abundance is high, 

 reducing subsequent predation-related mortality rate on 

 postlarvae and small juveniles. Low abundance of anchovy 

 early-life stages will lead to the opposite effect (Fig. 7). The 

 proposed opposing responses of the early-larval and late- 

 larval-juvenile stages are explained by differences in the 

 spatial scales of distribution and densities of life stages of 

 bay anchovy (Fig. 7). The spatial scale of processes that 

 affect distributions of late-stage larvae and juveniles is 

 large compared to that for early-stage larvae because of 

 the increased dispersal and swimming ability of juveniles. 

 Comparing early-larval and late-larval-juvenile stages of 

 bay anchovy, we propose that effects of prey concentration 

 (the first hypothesis) and SSB level (density-compensa- 

 tion) act primarily on the dynamics of early-larval stages, 

 whereas predation mortality and the inhibitory effects of 

 low DO (density-depensation; the second and third hy- 



Nursery 

 Ground 



(3) Fall 



YOY recruits, 

 adults 



Late-stage larvae, 

 juveniles, some adults 

 Eggs and larvae 



Overwintering 



Recruited 



anchovy 



Adult 



Immigration from 

 tributaries'? 



Major 



Spawning Mature adults. 



/ eggs, larvae 



ground 



(1) Spring 



Adult 



Immigration from 

 ocean? 



Figure 6 



Conceptual model representing bay anchovy (Anchoa 

 mitchilli) life cycle and ontogenetic migration within 

 Chesapeake Bay, and possible immigration of adults 

 from tributaries and coastal ocean. 



potheses) are more important regulators and controllers, 

 respectively, during late-larval and juvenile stages. 



The three hypotheses that relate DO, SSB distribution, 

 and recruitment of bay anchovy are not mutually exclusive. 

 If low mean DO level is an indicator of enhanced prey pro- 

 duction and availability to larvae and juveniles, increased 

 prey productivity in the lower bay could enhance bay 

 anchovy recruitment potential by supplying enough zoo- 

 plankton prey to spawning adults, larvae, and juveniles. At 

 the same time, low mean DO in the mid-Bay could confine 

 most spawning bay anchovy to the lower bay. thus increas- 

 ing spawning and larval production there, and possibly 

 enhancing survival of juveniles by predator satiation. Ul- 

 timately, other hypotheses may provide better explanations 

 of the relationships between regional mean DO. latitudinal 

 shifts in distribution of spawners, abundances of spawners. 

 and recruitment of bay anchovy. For example, abundant 

 gelatinous organisms, such as the scyphomedusa (Chn'sa- 

 ora quinquecirra) and the lobate ctenophore \Mnemiopsis 

 leidyi), can be important predators on early-stage anchovy 

 and competitors with juveniles and adults (Purcell et al., 



