676 
Fishery Bulletin 95(4), 1 997 
were similar to those estimated for similar dates in 
1992 (Fig. 5, A and B). A comparison of these field 
consumption rate estimates with estimates from 
bioenergetic models (Steinberg, 1994; Hartman and 
Brandt, 1995b) is dealt with elsewhere (Steinberg 
and Conover 1 ). 
There are several possible explanations for the 
relatively low estimates of daily ration in early July 
1993. First, these are point estimates of feeding rate 
that may not reflect average daily feeding over longer 
periods (see Smagula and Adleman, 1982; Trudel and 
Boisclair, 1993). Although Trudel and Boisclair ( 1993) 
found low day-to-day variation (7.0-16.3%) of daily 
ration for minnows in a field study, Smagula and 
Adelman (1982) found large day-to-day variation (30— 
40%) in daily ration estimates of piscivorous large- 
mouth bass in the laboratory. 
Alternatively, other factors known to affect fish 
consumption rates include temperature, fish size, 
prey availability, prey biomass, prey type and size 
composition, and risk of predation. Both tempera- 
tures and bluefish sizes were similar in 1992 and 
1993 (Tables 1-4). Prey abundance was not recorded 
during diel collections in 1992 and thus cannot be 
compared, but there were differences in the types 
and sizes of prey consumed by bluefish in these years 
(Table 4). The much larger clupeid species were a 
dominant part of bluefish diet in 1992 but were not 
a dominant prey in 1993. This finding reflects 
riverwide relative abundance estimates from a sepa- 
1 Steinberg, N. D., and D, O. Conover. 1997. Young-of-the-year 
bluefish ( Pomatomus saltatrix) consumption in the Hudson 
River estuary: a bioenergetic modeling approach. Marine Sci- 
ences Research Center, State University of New York, Stony 
Brook, NY 11794-5000. Manuscr. in prep. 
rate beach-seine study (Buckel, 1997). In July 1992, 
clupeids, striped bass, and bay anchovy represented 
90% of the available forage fish. Striped bass alone 
represented 63% of the available forage fish at this 
time in 1993. Moreover, striped bass and bay anchovy 
were larger for a given bluefish size in 1992 than in 
1993. A combination of the size and type of prey avail- 
able, along with the possible behavior differences 
between the prey (e.g. clupeids are more pelagic and 
less refuge oriented), may have caused the large dif- 
ferences in daily ration in July. We have no informa- 
tion on relative abundances of predators of juvenile 
bluefish in July 1992 and 1993 and therefore cannot 
discount predation risk as a potential mechanism 
explaining differences in daily ration in July. 
Diet analysis 
Diets of YOY bluefish in 1992 and 1993 were domi- 
nated by fish prey, confirming past studies in the 
Hudson River estuary that have shown YOY blue- 
fish to be largely piscivorous (Texas Instruments, 
1976; Juanes et al., 1993). Diets ofYOY bluefish were 
dominated by important anadromous resource spe- 
cies: clupeids in 1992 and striped bass in 1993. 
Interannual variation in diet was also observed by 
Friedland et al. (1988) in their study ofYOY blue- 
fish in a New Jersey marine embayment. Because 
YOY bluefish are believed to be opportunistic preda- 
tors (Friedland et al., 1988; Juanes et al., 1993), the 
diet differences we observed appear to be a result of 
the availability of different prey types in 1992 and 
1993 (see riverwide abundances above). However, our 
diet data may be biased because spatial coverage was 
limited to the Croton Point region of the Hudson River. 
Table 4 
Percentages of fish with empty stomachs, prey types, mean prey size, and mean values of temperature and gut fullness for diel 
collections in 1992 and 1993 (beach-seine and the 15-16 July 1993 surface trawl collection (ST)). Prey types are C = clupeids; 
SB = striped bass; BA = bay anchovy; and AS = Atlantic silversides. Prey types are listed in their order of importance in bluefish 
diet on each date. Mean prey sizes follow the order of prey type. 
Year 
Date 
% with empty 
stomach 
Prey type 
Mean prey 
size (mm) 
Mean temp 
(°C) 
Mean gut fullness 
(g of prey/g of bluefish) (%) 
1992 
16-17 June 
29.6 
C, SB, BA 
42.9, 32.9, 27.3 
25.6 
4.31 
28-29 July 
28.9 
C 
45.7 
25.3 
3.40 
13-14 Aug 
20.8 
C, SB, AS 
50.9, 56.1, 68.3 
25.0 
3.26 
26-27 Aug 
20.0 
SB, C, AS 
68, 52, 45.2 
26.7 
2.27 
19-20 Sept 
12.1 
BA, C 
43.2, 60.2 
22.4 
3.62 
1993 
7-8 July 
30.6 
SB, BA 
26.3, 16.5 
26.9 
1.52 
15-16 July (ST) 
24.5 
BA 
20.4 
25.6 
1.69 
20-21 July 
37.2 
SB, BA 
39.4, 24.5 
26.1 
2.13 
4—5 Aug 
27.7 
SB, AS, C, BA 
52.5, 47.0, 57.3, 35.7 
25.9 
2.08 
18-19 Aug 
24.0 
SB, AS, BA 
68.3, 61.5, 40.4 
24.8 
2.10 
