78 
Fishery Bulletin 107(1) 
Table 1 
Digestive states (1-7) of prey items found in stomachs of shortfin makos (Isurus oxyrinchus) caught in fishing tournaments along 
the northeast coast of the United States, 2001-02, and the estimated digestion time (h) and the percentage of prey items found 
for each digestive state. 
Scale 
number 
Description of digestive state 
Estimated 
digestion time (h) 
Percentage 
of prey 
1 
Prey maintains original shape perfectly, skin also intact, 
pigmentation still bright. 
0 to 2 
2.4 
2 
Original shape almost completely retained, part or all of skin missing, 
pigmentation faded. 
2 to 4 
1.6 
3 
Flesh still recognizable on body, skeleton nearly complete. 
4 to 7 
15.7 
4 
Skeleton partially fragmented, flesh still attached to backbone. 
7 to 10 
27.6 
5 
Skeleton fragmented into many pieces, chunks of flesh remaining. 
10 to 14 
35.4 
6 
Prey reduced to mush consisting of flesh, skeletal fragments and scales, 
no recognizable body parts. 
14 to 18 
15.0 
7 
Opaque liquid only. 
18 + 
2.4 
same shark fishing tournaments where the 2001-02 
data were collected. 
For the 2001-02 diet data, stomachs were extracted 
on location, bagged, placed on ice, and brought to the 
laboratory for examination within 48 to 72 hours of 
catch. In the laboratory, stomachs were carefully re- 
moved from surrounding organs and cut open for ex- 
amination of the contents. Prey were identified to the 
lowest taxon possible, counted, sorted, and weighed 
individually (±0.01 g). When bluefish were found in 
stomachs, remaining bones were examined in more 
detail. In cases where one or more of five skull bones 
(maxilla, premaxilla, dentary, cleithrum, opercle) were 
found intact, and in good overall condition, these bones 
were collected for the purpose of back-calculating origi- 
nal bluefish size with a series of predictive equations 
(Wood, 2005). Unidentifiable prey items were designated 
as such and all prey items were given a value from 1 to 
7 on a scale based on state of digestion (Table 1). This 
scale of digestion was used to eliminate suspicious prey 
items that could have been bait. All prey items were 
explored for clean (knife-edge) cuts, fish hook marks, 
and imbedded fish hooks, and any items identified as 
bait were removed from the samples. Typically, Atlantic 
mackerel ( Scomber scombrus) pieces, butterfish ( Pepri - 
lus triacanthus) pieces, and bluefish are used as bait by 
fishermen, and menhaden (Brevooi'tia tyrannus ) oil and 
ground menhaden are used as chum. 
Diet 
The diet of the sharks sampled during the current study 
was quantified by three basic prey indices: percentage by 
number (%N), percentage by weight (%W), and percent- 
age by occurrence (%0) (Hyslop, 1980); and a compound 
index of relative importance (IRI) was expressed as a 
percentage (%IRI) (Cortes, 1997). 
Comparisons between the current and historical data 
were based on the index of number (%N), the index of 
weight (%W), and the index of occurrence (%0). For a 
direct quantitative comparison between the current and 
historical data, weight was used as a proxy for volume 
by assuming a constant prey density of 1.0 g = 1.0 mL. 
The %W index from the current data was compared to 
an index based on prey volume (%V) from the histori- 
cal diet data. 
Diet overlap between the current data and the his- 
torical data was examined with two measures of niche 
overlap, the percentage overlap measure and the simpli- 
fied Morisita index (Krebs, 1999). A contingency table 
(both chi-square and G statistic) based on prey numbers 
was used to investigate whether significant differences 
existed between the current and historical diets in ag- 
gregate. For the contingency table analyses, prey items 
were grouped into seven categories (Pomatomidae, Clu- 
peidae, Scombridae, other teleosts, unidentified teleosts, 
invertebrates, and mammals and elasmobranchs). 
Cumulative prey curves were generated for the 2001- 
02 and historical diet data to determine whether the 
overall shortfin mako diet was adequately represented 
by the study samples. In addition, the rate of increase 
of the last 10 points in both curves was analyzed to 
determine whether an asymptote had been reached. A 
rate of increase of less than 5.0% was used as the cut- 
off (Baremore, 2007). A jackknife estimate of species 
richness was also calculated to estimate how many prey 
species were potentially missed by sampling. The cumu- 
lative prey curves and jackknife estimate were gener- 
ated with PRIMER vers. 6.0 software (Clarke, 1993). 
Predator-size-prey-size relationship 
Measurements of bluefish bones collected from stom- 
ach contents were used to back-calculate sizes of prey 
