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Fishery Bulletin 107(1) 
sampling; however, there has been an apparent decrease 
in species diversity in the diet. The cumulative prey 
curves indicate that the 2001-02 diet was well sampled, 
but more sampling was needed to better represent the 
historical diet. Additionally, the jackknife estimates of 
species richness indicate that 36 species would be rep- 
resented in a fully sampled historical diet, and only 16 
in the current diet. It is possible these results are an 
artifact of sampling. The total number of shortfin ma- 
kos examined and the number of years over which the 
data were collected were both greater for the historical 
sampling, which likely affected prey diversity. On the 
other hand, some of the shift observed in the diet diver- 
sity over the past few decades could be due to temporal 
changes in the prey community structure of the north- 
west Atlantic Ocean. This ecosystem has experienced 
significant fluctuations in the relative abundance and 
biomass of many fish and invertebrate species (Over- 
holtz et ah, 2000). In addition, the community is now 
dominated by pelagic finfish such as Atlantic mackerel 
and Atlantic herring whose large concentrations draw a 
variety of piscivorous predators, such as bluefish (Over- 
holtz et al., 2000). Large predatory schools of bluefish 
feeding on abundant pelagic finfish would themselves 
be easy prey for shortfin makos. 
The schooling nature of bluefish is the likely explana- 
tion for their high concentration in the shortfin mako 
diet. Adult bluefish feed and also spawn in large schools 
as they migrate up the northeast coast of the United 
States in the spring and early summer (Juanes et al., 
1996; Salerno et al., 2001). The focus of shortfin mako 
predation seems to be these adult bluefish. The length- 
frequency distribution of bluefish prey found in short- 
fin mako stomachs revealed that the majority (96%) 
of individuals fell in the 50- to 90-cm-FL size range. 
Bluefish at this size are around 2 years old and are 
likely mature individuals (Juanes et al., 1996; Salerno 
et al., 2001). The large feeding and spawning aggrega- 
tions of these adult bluefish would be very easy to find 
and target for shortfin makos in the region. Intense 
predation on these large schools could potentially have 
a regulatory effect on bluefish abundance in the north- 
west Atlantic Ocean. 
In order to quantify the level of shortfin mako 
predation on the bluefish population a reliable esti- 
mate of daily ration was needed. Recently available 
information on the metabolism, average swimming 
speed, and growth rate of the shortfin mako has al- 
lowed the development of a good bioenergetics model. 
The resulting estimates of daily ration are notably 
higher than those of many other elasmobranch spe- 
cies, which rarely exceed 3.0% BW/d (Wetherbee and 
Cortes, 2004). The highest published rate of consump- 
tion observed for an obligate ram ventilating shark 
was 3.54 %BW/d for juvenile scalloped hammerheads 
( Sphyrna lewini) (Bush and Holland, 2002). The most 
abundant pelagic shark in the North Atlantic, the blue 
shark, has a daily ration of approximately 1% BW/d, 
which is significantly less than that of the shortfin 
mako (Schindler et al., 2002). The high metabolic 
and high consumption rate of the shortfin mako can 
be attributed to its ability to thermoregulate. The 
endothermic capability of the shortfin mako increases 
its aerobic capacity, resulting in a higher metabolism 
and increased energy demand. 
The estimates of daily ration from this study provide 
a means to quantify predation on bluefish on an indi- 
vidual predator basis. Because there is no estimate of 
the shortfin mako population size, a relevant exercise 
is to backcalculate the number of sharks it would take 
to match the fishing pressure. Bax (1998) determined 
that predation by fish can range from 2-35 times the 
loss to fisheries. However, we assumed that short- 
fin mako predation on bluefish was set equal to the 
amount of bluefish taken by fisheries in 2002. The 
total bluefish catch (commercial+recreational) in 2002 
was 11,566 t (NEFSC 1 ). Taking an average value of 
the daily ration estimates, it was determined that an 
average shortfin mako (63.5 kg) consumes up to 1000 
kg of bluefish per year. For this estimate it is assumed 
that shortfin makos are feeding on bluefish all year 
long, which may not be the case. If shortfin makos 
spend around 6 months off the northeast coast of the 
United States. (May to October), that period results 
in around 180 days of intense predation on bluefish. 
During this feeding season an average shark would 
consume roughly 500 kg of bluefish. At this rate of 
consumption it would have taken only 23,132 sharks to 
equal the take of the fisheries in 2002. These are very 
simple calculations but they serve to illustrate that 
the level of predation by shortin mako on bluefish is 
likely much greater than the impact of the fisheries. If 
true, this would not be a unique case. Multiple studies 
have shown that predation mortality on a variety of 
important prey species exceeds fishing mortality, and 
in some cases even exceeds maximum sustainable yield 
of the prey population (Christensen, 1996; Bax, 1998; 
Overholtz et al., 2000). 
The most important factor often attributed to the de- 
cline of bluefish stocks in the northwest Atlantic Ocean 
is fishing pressure (Shepherd and Packer, 2006), but 
it is evident that predation should not be disregarded. 
Bluefish mortality as a result of predation could ex- 
ceed the loss to these fisheries, as has been shown in 
other predator-prey systems (Bax, 1998; Overholtz et 
al., 2000). It is becoming increasingly apparent that 
depressed fish stocks are very vulnerable to predation, 
but the mechanisms driving this vulnerability are still 
unclear. In recent studies there have been deeper probes 
into trophic interactions, such as efforts to quantify 
prey vulnerability to predation (Bundy and Fanning, 
2005; Overholtz, 2006). The exact predator-prey dy- 
namics that exist between shortfin makos and bluefish 
are still unclear; however, it is likely that predation 
has played a more important role in the decline of the 
northwest Atlantic Ocean bluefish population than pre- 
viously thought. 
Adding predation as a variable into the management 
of northwest Atlantic Ocean bluefish would increase the 
difficulty of an already complicated task. The highly 
