768 



Fishery Bulletin 97(4), 1999 



Feeding chronology, daily ration estimates, and 

 Impacts on bay anchovy 



Both spring- and summer-spawned YOY bluefish gut 

 fullness values varied over the diel cycle in the dif- 

 ferent geographical regions of the MAB shelf. Peaks 

 in gut fullness generally occurred at dawn, dusk, and 

 diurnal time periods, whereas gut fullness values 

 during nighttime collections were low in relation to 

 diurnal gut fullness values (Fig. 4). 



Table 7 



Size-selectivity (Chesson's a, see text for calculations) for 

 bay anchovy in YOY bluefish collected on the U.S. east 

 coast continental shelf in the autumn of 1994 and 1995. 

 Values of o=l/m (where "m" is the number of prey catego- 

 ries ) represent random feeding while values of a > 1/m or 

 a < l/m represent "selection" and "avoidance" of prey, re- 

 spectively. Values significantly different from l/m (?-test, 

 P<0.05) are indicated by a ( + ) for "selection" or (-) for 

 "avoidance". 



1994 



1995 



Bay Spring- Summer- 



anchovy spawned spawned 



size (mm) (l/m=0.25) (l/ra=0.33) 



Spring- 

 spawned Summer- 

 ( l/m =0.33) spawned 



25-34 



35-44 

 45-54 

 55-64 



0.57 (-t-) 

 0.27 

 0.14 

 0.02 (-1 



0.65 (-H) 

 0.28 

 0.07 (-) 



0.44 

 0.37 

 0.19 



Table 8 



Daily ration g/lg • d) • 100 ±SEl and weighted mean temperature at which spring- 

 and summer-spawned YOY bluefish were collected during National Marine Fisher- 

 ies Service autumn bottom trawl survey in 1994 and 1995. Bluefish juveniles were 

 collected from three geographical areas of the Middle Atlantic Bight continental 

 shelf iSNE=Southern New England, C-D=Chesapeake Bay to Delaware Bay, and 

 SOC= South of Chesapeake Bay. after Munch 1997). Daily ration was calculated 

 using the Eggers' ( 1979) approach (see text). NA=not applicable due to low number 

 of samples for diel series. 



Summer-spawned 



Year and 



geographical 



region 



Temp. 



Daily ration 



g/(g-d)- 100 

 (SE) 



Temp. 



1994 

 SNE 

 C-D 



1995 

 SNE 

 C-D 



S( )( ' 



200 18.9 



61 20.7 



8.5(3.1) 

 NA 



12.4(3.9) 



NA 

 NA 



182 

 129 



79 



177 



51 



18.8 

 20.0 



19.9 

 23.7 

 24.3 



In order to use the gut fullness data to estimate 

 bluefish daily ration, gastric evacuation rate (GER) 

 estimates were needed for bluefish at shelf water 

 temperatures. The exponential GER model ad- 

 equately described the evacuation of bay anchovy at 

 15 C from our laboratory experiment (/•-=0.87, n=22, 

 P<0.001; Fig. 5). Incorporation of the 15°C GER es- 

 timate (R^,-0.102) into the evacuation rate vs. tem- 

 perature function of Buckel and Conover ( 1996) gave 

 the following equation: fi^=0.017 e""«3^''""\ n = 16, 

 r'^=0.82 (Fig. 5). The largest deviation from the equa- 

 tion describing/?^, and temperature is from the single 

 estimate of GER at 15"'C. This is likely not due to 

 differences in experimental protocol; the 15°C GER 

 experiment was conducted identically to the experi- 

 ments described in Buckel and Conover (1996). The 

 larger deviation may result from there being only one 

 estimate of GER at 15°C whereas each GER estimate 

 for the higher temperatures represents a mean of four 

 estimates (Fig. 5). Estimates of GER from this equa- 

 tion were used to estimate bluefish daily ration. 



The daily ration of summer-spawned bluefish in 

 the SNE region in 1994 and 1995 was 8.5 and 12.4 g/ 

 (g • d)  100, respectively (Table 8). Spring-spawned 

 bluefish daily ration estimates ranged from 4.8 to 

 6.6 g-/(g  d) • 100 for the SNE and C-D region in 

 1994 and 3.7-9.0 g/(g  d) • 100 in the SNE, C-D, and 

 SOC regions of the MAB in 1995. There was insuffi- 

 cient diel records to calculate feeding rates for the 

 SOC region in 1994 or the remaining region and co- 

 hort combinations (Table 8). 



The VPA estimates of YOY 

 bluefish abundance in 1994 and 

 1995 were 24 million and 14 mil- 

 lion, respectively. These estima- 

 tions were partitioned into 

 spring- and summer-spawned 

 bluefish on the basis of relative 

 abundances of these cohorts for 

 each year (Table 9). It was esti- 

 mated that spring-spawned 

 bluefish consumed from 70 to 

 96 million bay anchovy per day 

 in 1994 and from 68 to 170 mil- 

 lion bay anchovy per day in 

 1995 during the September 

 period of their migration (Table 

 9; the range is based on the 

 lowest to highest daily ration 

 estimate). Summer-spawned 

 bluefish consumed 130 million 

 bay anchovy per day in 1994 

 and 5 million per day in 1995 

 (there is no range for summer- 

 spawned bluefish because there 



Spring-spawned 



Daily ration 



g/(g- d)  100 



(SE) 



6.6(2.5) 

 4.8(1.9) 



3.7(1.4) 

 9.0(2.9) 

 8.1 (2.8) 



