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Fishery Bulletin 102(1) 



applied to evaluate reliability of the regression model 

 (Belsley et al„ 1980; SAS, 1989). 



Results 



Environmental factors 



Stream flows from the Susquehanna River (Table 3) 

 varied annually and seasonally. Freshwater stream 

 flows were higher in 1996 and 1998 than in other 

 years. Baywide mean values of water temperature, 

 salinity, and DO concentration, averaged from surface 

 to bottom, varied annually, seasonally, and regionally 

 (Table 1 ). Annually, mean temperature was highest in 

 1995 and lowest in 1997. Mean salinity was highest 

 in 1995 and lowest in 1996. Mean DO concentration 

 was highest in 1996 and lowest in 2000. Regionally, 

 salinity was more variable than temperature and 

 DO concentration. Seasonally, temperature and DO 

 concentration were more variable than salinity. Tem- 

 perature was highest in the June-August period, the 

 spawning season of bay anchovy. Seasonally, salinity 

 increased progressively from April-May to October. 

 Mean DO concentration was consistently lowest in 

 June-August. 



Trends in abundance and recruitment 



Estimates of bay anchovy abundance reported in our 

 study are for the entire mainstem of Chesapeake Bay. 

 The estimated recruitment levels (baywide abundance 

 of YOY bay anchovy >30 mm TL in October) varied 

 ninefold and were low in 1995 and 1996 (47.5 ±16.6 

 and 30.6 ±8.6xl0 9 individuals) but much higher in 

 1997-2000 (99.6 ±12.4 to 264.8 ±32.6xl0 9 ). Baywide 

 estimates of bay anchovy biomass for individuals >30 

 mm TL increased from April to October in each year 

 (Table 4). October baywide biomass varied sevenfold 

 from 27.1 ±5.5 x 10 3 to 192.9 ±20.4 x 10 3 tons and was 

 highest in 1998 and lowest in 1996. 



Estimated spawning stock biomass (SSB) in 

 April-May was lowest in 1995 (3.3 ±1.1 x 10 3 tons), 

 and highest in 1997 (20.1 ±5.3 x 10 3 tons), indicating 

 sixfold variability. SSB in June-August was lowest 



in 1996 (2.4 ±0.2 x 10 3 tons), and highest in 1997 (21.1 

 ±2.3 x 10 3 tons). The SSBs in April-May and June-August 

 did not show any obvious relationship to YOY abundance 

 (recruitment) in October. 



Ontogenetic migration 



The length-specific mean locations (latitudes of occur- 

 rence ) of bay anchovy revealed an apparent ontogenetic 

 migration. Small juveniles of bay anchovy tended to move 

 upbay and were located primarily upbay until they were 

 approximately 45 mm TL, after which they began to move 

 downbay (Fig. 2). In April-May, age-1 bay anchovy <60 mm 

 TL, consisting of individuals recruited from the previous 

 year, varied annually in their mean latitude of occurrence, 

 whereas large (sage 1, a60 mm TL) bay anchovy had 

 relatively stable locations near the boundary between the 

 lower and middle bay regions, centered at latitude 37°40'N 

 (Fig. 2A). Compared to April-May, age-l+ bay anchovy in 

 June-August were more variable in their annual mean 

 locations, but both YOY and adult bay anchovy tended to 

 occur upbay of latitude 38°00'N, except in year 2000 (Fig. 

 2B). In 1997 and 1999, when annual mean temperatures 

 were lowest (Table 1), YOY bay anchovy were too small 

 to be sampled by the MWT in June-August and are not 

 represented in Figure 2B. In October, mean latitudes of 

 occurrence (Fig. 2C) indicated a consistent distribution 

 pattern and an apparent ontogenetic migration by YOY 

 anchovy. The most probable explanation for the observed 

 latitudinal distributions was that small YOY bay anchovy 

 tended to move upbay initially, but then downbay at about 

 45 mm TL. Distribution of age-l-t- individuals in October 

 was variable. 



The SSB of bay anchovy (excludes YOY) from 1995 to 

 2000 was centered near 38°00'N in April-August except 

 in June-August of 1995 and 1996, when the SSB was 

 centered farther upbay (Fig. 3A). In 2000, the migration 

 pattern differed from other years. Spawning bay anchovy 

 in 2000 were located farther downbay in July than in April 

 (Fig. 3A). The April-May location of prespawning SSB was 

 mostly explained by the mean flow of the Susquehanna 

 River from June of the previous year to February of the 

 current year (r 2 =0.94, P=0.0012; Fig. 3B ). But, in June-Au- 

 gust, the mean location of spawning fish was more strongly 

 and significantly related to the subpycnocline-layer mean 



