Wood et al.: Diet a of Isurus oxyrinchus in the northwest Atlantic Ocean 
85 
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 
Age (years) 
Figure 6 
Growth curves for male ( ) and female ( ) shortfin mako ( Isurus oxy- 
rinchus . ), from growth equations in Natanson et al. (2006). The histogram 
represents energy costs (KJ/kg) for growth from age to age from the bio- 
energetics model derived from the 2001-02 diet data: The descending bars 
within the columns represents the cumulative frequency of these ratios. 
their diet to focus on bluefish (MacNeill et al., 2005). 
In diet studies from the eastern Atlantic off of Por- 
tugal (Maia et al., 2006), and the Southwest Atlantic 
off Argentina (Vaske- Junior and Rincon-Filho, 2003), 
bluefish were not found in the diet of shortfin makos, 
even though their distribution covers these regions and 
these sharks prey mainly on teleosts. The high concen- 
tration of bluefish in the northwest Atlantic Ocean, and 
the presence of large schools of bluefish that could be 
easily located by shortfin makos, is a likely reason for 
the predominance of bluefish in the diet. 
There have been notable changes in the perceived 
abundance of bluefish in the northwest Atlantic Ocean 
since the historical diet data were collected. Nearing 
the end of the historical sampling period bluefish were 
very abundant with a total stock biomass of 104,000 
metric tons (t) in 1982 (NEFSC 1 ). At this time they 
were the most important prey species in the shortfin 
mako diet (86.9 %V). Since the early 1980s, fishing 
mortality of age-1 bluefish has increased fourfold, and 
recruitment for age-0 fish is thought to have declined 
from 75 million to 14 million (Shepherd and Packer, 
2006). In addition, total stock biomass declined 72% 
(29,400 t) from 1982 to 1997 and was estimated to have 
increased since then to 40,000 t in 2004 (NEFSC 1 ). This 
1 NEFSC (Northeast Fisheries Science Center). 2005. 41st 
Northeast Regional Stock Assessment Workshop (41st SAW). 
41st SAW assessment report. Northeast Fish Science Center 
Reference Document 05-14, 237 p. Northeast Fisheries Sci- 
ence Center, National Marine Fisheries Service, Woods Hole 
Laboratory, 166 Water St., Woods Hole, MA 02543. 
apparent decline in bluefish abundance from historical 
levels is not reflected in the current diet of the shortfin 
mako, and bluefish still represent a very high propor- 
tion of prey consumed. 
The high numbers of bluefish in the 2001-02 diet 
indicate that even though abundance is lower than his- 
torical levels it is not limiting prey for shortfin mako. 
There appear to be suitable numbers of bluefish avail- 
able for the shortfin mako population to prey almost 
solely on this species during their seasonal residence 
off the northeast coast of the United States. It is likely 
that shortfin mako abundance in this region has de- 
clined alongside bluefish since the historical diet data 
were collected. Therefore, although there may be a 
lower abundance of prey items to feed on, the predator 
abundance is lower as well. Unfortunately, the shortfin 
mako population in the northwest Atlantic Ocean has 
never been reliably quantified. The most recent stock 
assessment for large pelagic sharks was considered 
preliminary because of limitations on both the quality 
and quantity of the data, and came up short of provid- 
ing reliable estimates. However, trends from catch-per- 
unit-of-effort indices derived from pelagic longline data 
for tuna and swordfish ( Xiphias gladius ) fisheries in the 
western North Atlantic have revealed a 43% decline 
in shortfin mako abundance since 1986 (Cortes et al., 
2007). It is possible that any increased predation pres- 
sure on the depleted bluefish population is mitigated by 
a decreased abundance of shortfin makos from histori- 
cal levels. 
It appears that the importance of bluefish in the 
shortfin mako diet has not changed since the historical 
