Wood et al.: Diet a of Isurus oxyrinchus in the northwest Atlantic Ocean 
83 
differences in the diets and a percentage overlap equal 
to 70.3 and the simplified Morista’s index equal to 
0.937. The contingency table analysis indicated that 
the difference between the diets was significant ac- 
cording to a chi-square test and G-statistic (PcO.OOl). 
However, a subsequent contingency table analysis with 
the grouping “other teleosts” removed from both diets 
resulted in no significant difference. 
Predator-size-prey-size relationship 
An original fork length was back-calculated for 115 
bluefish prey. The average bluefish prey length was 
66.7 cm FL (minimum size = 36.5 cm and maximum 
size = 82.0 cm). Over 96% of the bluefish found in short- 
fin mako stomachs were greater than 50.0 cm FL. 
There was no significant relationship between the 
size of bluefish prey and predator size and none of the 
quantile regressions was significant. Investigation of 
prey size indicated that shortfin makos consume inter- 
mediate-size bluefish in relative to their own body size. 
Overall, 100% of bluefish consumed fell in the range of 
0.2 to 0.5 prey-to-predator size ratio, and the majority 
(35%) were at a ratio of 0.35 (Fig. 5). 
Daily ration 
The linear relationship between mean V0 2 and swim- 
ming speed (U) was significant (P<0.05), and had 
a fairly good fit (r 2 = 0.83). The resulting regres- 
sion equation was: V0 2 = 506.42 U + 201.39. From a 
mean swimming speed of 0.5 body lengths per second 
(observed from pop-up satellite tag tracks) the active 
metabolic 0 2 consumption rate for the bioenergetics 
model was calculated to be 454.4 mg/kg/h. Adjusting 
this value to reflect the average water temperature 
in which shortfin makos are found in the western 
North Atlantic (Q 10 = 2.3), we calculated an active 
metabolic 0 2 consumption rate of 485.7 mg/kg/h. An 
oxycaloric conversion (13.6 J/mg 0 2 ) of this metabolic 
demand resulted in an estimate of 6.61 kJ/kg/h of 
food energy for a shortfin mako to maintain active 
metabolism. 
Total energy consumption increased with age until 
the onset of maturity for both sexes and slowly de- 
creased (Fig. 6 shows energy consumption following 
the growth curves as they leveled off). After females 
reach the average age of maturity (18 years) the model 
calculated an average reproductive contribution of 
86,299 KJ/yr. 
The bioenergetic demands for the shortfin mako 
were higher than previously estimated, and higher 
than observed for any other species of shark. The 
average caloric value of the shortfin mako diet was 
calculated to be 4909 kJ/kg (Table 3). In order to 
satisfy the total energy demands from the bioenerget- 
ics model, shortfin makos must consume on average 
4.48% of their body weight (BW) per day. Values of 
consumption by age ranged from 4.42-4.66 %BW/d for 
males and 4.42 to 4.56 %BW/d for females. 
CL 
O 
Number of stomachs sampled 
Figure 4 
Cumulative prey curves calculated for prey items 
found in (A) the 2001-02 diet study, and (B) in 
the historical diet (1972-83) of shortfin makos 
( Isurus oxyrinchus) in the Northwest Atlantic 
Ocean. 
The second method applied to estimate the daily ra- 
tion of the shortfin mako yielded a result very similar 
to that from the bioenergetics model. We assumed all 
but 10% of a consumed food item was evacuated after 
a period of 18-20 hours, and a corresponding range of 
evacuation rates of 0.128 to 0.115/h were calculated. 
This range of evacuation rates, in conjunction with an 
observed average stomach contents weight of 1.02 kg, 
resulted in daily ration estimates of 2.82 to 3.13 kg 
per day. Daily ration was calculated to be 4.44 to 4.93 
%BW/d for a 63.5-kg shortfin mako (the median weight 
of sharks from the 2001-02 study) according to this 
model (average of 4.68 %BW/d). 
Based on the estimates of daily ration, and the high 
proportion of bluefish in the diet, a large amount of 
