80 
Fishery Bulletin 107(1) 
a shorter evacuation rate of 18-20 hours (higher end 
of the tuna scale) was used as a more realistic esti- 
mate of gastric evacuation time. This range was used 
to generate values for evacuation rate (R) by assuming 
an exponential evacuation rate according to the equa- 
tion: S t /S 0 - e~ Rt (Elliot and Persson, 1978), where S t 
and S 0 are the final and initial amounts of the prey 
item, respectively, and S t /S 0 is assumed to be 0.10 (or 
the time when 90% of the initial food has been evacu- 
ated from the stomach). With this estimate of gastric 
evacuation rate, daily ration was calculated with the 
equation: 2C(t) = 24 SR, where S is the mean weight of 
the stomach contents data over a 24-hour period. 
Results 
Diet 
The two years of seasonal sampling for the 2001-02 
diet seemed to provide a very good sample, averaging 
95 sharks per year. In total, 189 sharks (108 males 
and 81 females) were examined that ranged in size 
from 146 to 335 cm fork length (FL). The majority of 
sharks sampled (120) contained at least one prey item 
in their stomach. Overall, 63% of prey items collected 
from stomachs were at an advanced stage of digestion 
(levels 4 and 5) on the digestive state scale, and only 
4.0% were designated as levels 1 and 2 (Table 1). Any 
fresh bait that shortfin makos would have encountered 
and eaten on the day of the tournament would have still 
been fresh in the stomachs at the time of dissection. The 
low prevalence of fresh prey items in the digestive scale 
ratings would indicate that bait was not an important 
factor in the analysis. 
The historical diet data were collected over a much 
longer period (11 years) and averaged fewer sharks per 
year (27) than the 2001-02 data. Overall, 302 sharks 
ranging in size from 86 to 338.5 cm FL were sampled: 
148 males, 54 females, and 100 unsexed sharks. A high- 
er percentage of the historical shortfin makos (73.8%) 
contained at least one prey item in their stomachs. The 
size distributions of sharks sampled from the two data 
sets were similar except for the absence of sharks <140 
cm in the 2001-02 data (because of restrictions on the 
size of sharks taken at tournaments implemented after 
the historical data were collected) (Fig. 2). 
Bluefish dominated the current diet of shortfin makos, 
accounting for 71.2% of the prey by number, 92.6% by 
weight, 87.5% by occurrence, and 99.2% IRI (Table 2). 
Other observed prey items were Atlantic mackerel, two 
species of squid ( Loligo pealeii and Illex illecebrosus), 
menhaden, and Atlantic herring ( Clupea harengus). 
A graphical comparison of index calculations for the 
three subgroupings of sharks based on the 2001-02 
data illustrated the similarity in diet among groups — a 
similarity primarily due to the predominance of bluefish 
in all diets (Fig. 3, A and B). 
For the historical data, bluefish also dominated the 
diet, but to a lesser extent, accounting for 55.6% of the 
60 n 
2001-2002 
H = 189 
Q\ ON C\ On On On On 05 On + 
— d ci n ci cl m ci 
oooooooooo r ° 
^f'sOOOOCl'3-xOOOO 
~ ' ci cl cl ci cl co 
Fork length (cm) 
Figure 2 
Length-frequency distributions for the shortfin 
mako (Isurus oxyrinchus) sampled in (A) the cur- 
rent study (2001-02), and (B) in the historical 
study (1972-83). 
diet by number, 86.9% by volume, 78.5% by occurrence, 
and 97.2% IRI (Table 2). A variety of different prey 
items were found in the historical diet, mostly other 
teleosts. A comparison of prey families indicated that 
the current diet had prey from nine different families, 
plus prey from the group Crustacea. In the historical 
diet 14 different fish families of prey were found, as 
well as crustaceans, mammals, and plants (Table 2). 
Some of the specific prey items present in the histori- 
cal diet, but not found in the current diet, were saury 
( Scomberesox saurus), bullet mackerel ( Auxis rochei), 
sand lance ( Ammodytes sp.), and ocean pout ( Macrozo - 
arces americanus). 
The 2001-02 diet data appeared to be a more accu- 
rate sample of the shortfin mako diet than the histori- 
