84 
Fishery Bulletin 107(1) 
Table 3 
Percentage by weight and the energy content (kJ/kg) determined from the 2001-02 diet data for the shortfin mako ( Isurus oxy- 
rinchus) and broken down by species. All energy values were taken from Steimle and Terranova (1985). 
Prey species 
%W 
kJ/kg 
Diet contribution (kJ/kg) 
Brevoortia tyrannus 
0.68 
7500 
51.0 
Centropristis striata 2 
0.08 
4770 
3.8 
Clupea harengus 
0.35 
10,600 
37.1 
Crustaceans 7 
0.03 
4450 
1.3 
Eutliynnus pelamis 2 
0.81 
6300 
51.0 
Illex illecebrosus 
0.14 
7100 
9.9 
Loligo pealei 
0.42 
5600 
23.5 
Lopholatilus chamaeleonticeps 2 
1.07 
4770 
51.0 
Pomatomus saltatrix 
92.62 
4800 
4445.8 
Scomber scombrus 
0.93 
6000 
55.8 
Squalus acanthias 
0.02 
8600 
1.7 
Thunnus albacares 2 
2.51 
6300 
158.1 
Unidentifiable pices 3 
0.34 
5535 
18.8 
Total 
4909.0 
1 Mean energy value for Crustacea was used. 
2 Mean energy value for benthic and pelagic fish was used. 
3 Mean energy value for all fish was used. 
bluefish could be consumed annually by 
shortfin makos in the northwest Atlantic 
Ocean. The average value for daily ration 
estimated by the two methods indicates 
that shortfin makos consume roughly 
4.58% of their body weight per day. Con- 
sidering the proportion by weight of blue- 
fish in the diet (92.6%), an average shark 
(63.5 kg) could consume up to 1000 kg of 
bluefish per year (assuming a full year 
feeding cycle on bluefish). 
Discussion 
The level of top-down predation pressure 
that shortfin mako are able to exert on 
northwest Atlantic bluefish populations 
is still unclear. Quantifying this preda- 
tor-prey relationship is difficult because 
it appears to only occur seasonally off the 
northeast coast of the United States. In 
offshore regions in the northwest Atlan- 
tic where bluefish are less abundant the 
shortfin mako diet is very different; the 
sharks focus mainly on squid species and 
other more prevalent teleosts (Stillwell 
and Kohler, 1982). It is not until these 
sharks migrate inshore that they shift 
