Buckel et al.: Foraging habits of Pomatomus saltatnx 



771 



Table 9 



Estimated spring- and summer-spawned bluefish daily consumption of bay anchovy on the U.S. east coast continental shelf in the 

 autumn of 1994 and 1995. We used mean size of bluefish, daily ration estimates, proportion of bay anchovy in the diet, and mean 

 bay anchovy size that were measured for each cohort in this study. Estimates of the numbers of YOY bluefish for 1994 and 1995 

 are from virtual population analysis (VPA) performed by NEFSC (see Footnote 1 in main text); we partitioned the numbers 

 between spring- and summer- spawned bluefish using relative abundances of each cohort from the NEFSC-NMFS autumn ground- 

 fish survey cruises. 



Cohort 



Biomass of 

 Bluefish bluefish cohort 

 numbers (10* kg) 



Amount of 



bay anchovy 



consumed daily 



(kg/d; low ration ) 



Amount of 



bay anchovy 



consumed daily 



(kg/d; high ration) 



Number of 



bay anchovy 



consumed daily 



(no./d; low) 



Number of 



bay anchovy 



consumed daily 



(no./d; high) 



34,000 



110,000 



70 X 106 

 130 X 106 



68 X 106 

 5 X 106 



96 X 106 



170 X 106 



Bank in 1994. Morris (1984) also found gadids in 

 stomach contents of adult bluefish. This finding is 

 of interest because there is a large effort to under- 

 stand the factors that regulate fish populations on 

 Georges Bank (Peterson and Powell, 1991). The bio- 

 mass of skates (Rajidae) and dogfish sharks 

 (Squalidae) has increased in this area of the shelf 

 and in some locations makes up the largest part of 

 fish biomass (Overholtz et al., 1991). However, carti- 

 laginous fishes were not found in the diet of adult 

 bluefish. Bluefish predation will have no direct ef- 

 fect in regulating the population sizes of these fishes. 

 However, bluefish may play a role in the recovery of 

 commercially important groundfish species on 

 Georges Bank. Because bluefish distribution is 

 closely linked with temperature (Munch, 1997), an 

 increased warming trend may allow a larger propor- 

 tion of the bluefish population to extend northward 

 onto Georges Bank. Ware and McFarlane (1995) 

 found that Pacific hake (Merlucciiis productus) bio- 

 mass and hake predation on herring (Clupea 

 harengus) increased with recent increases in tem- 

 perature off the west coast of Vancouver Island. They 

 concluded that this recent increased predation ex- 

 plains recent declines in the herring stock. The im- 

 portance of bluefish predation to recovery of ground- 

 fish species warrants further investigation. 



Net feeding 



Postcapture net-feeding can bias diet indices and gut 

 fullness level estimates. Net feeding is known to oc- 



cur in a variety of Pacific midwater fish (Lancraft 

 and Robison, 1980). There was little evidence that 

 net feeding biased our estimates of bluefish diet given 

 the similarities between percent of fresh versus di- 

 gested prey; however, any net feeding would lead to 

 biased estimates of gut fullness level and inflated 

 estimates of consumption rate. Lancraft and Robison 

 (1980) found that larger fish were more likely to in- 

 gest artificial prey. The larger spring-spawned co- 

 hort had more fresh prey in its diet than the smaller 

 summer-spawned bluefish which may be a result of 

 a higher incidence of net feeding by this cohort. 



Prey-type selectivity 



Spring-spawned YOY bluefish selected for bay an- 

 chovy over all other prey tjrpes in both 1994 and 1995. 

 Butterfish, squid, and "other" potential prey were 

 avoided. This is most likely due to both the relative 

 abundances of these prey as well as interspecific size 

 differences. Bay anchovy tended to be the smallest 

 prey available; the only smaller prey of spring- 

 spawned bluefish were invertebrates such as amphi- 

 pods (ranging from 3 to 15 mm TL). In the Hudson 

 River estuary, the prey with the highest relative 

 abundance were selected for, whereas the prey with 

 the lowest abundance were selected against (Buckel 

 et al., 1999). A second possible explanation for 

 the strong selection of bay anchovy could be sam- 

 pling bias. Chesson's ( 1978) index assumes that prey 

 abundance is large in relation to the amount con- 

 sumed (likely true for the shelf) and that the 



