Tony Wood – Graduate School of Oceanography , University of Rhode Island
Brad Wetherbee & Cheryl Wilga – Department of Biological Sciences, University of Rhode Island
Nancy Kohler – NOAA/NMFS/NEFSC and Francis Juanes – Fishery & Wildlife Management, UMass Amherst

Bluefish stocks along the east coast of North America have been declining for the last decade. One factor that may have contributed to the decline in bluefish populations is predator-prey interactions such as mortality due to bluefish predators or by competition for food between bluefish and other species.

Distribution of the shortfin mako shark overlaps substantially with that of bluefish and historically, bluefish have constituted a major part of the shortfin mako diet in the waters off the northeastern U.S.   For mako sharks examined between 1972 and 1979, bluefish occurred in 44% of stomachs and accounted for 78% of the overall mako diet by volume.  For makos captured near-shore, bluefish occurred in 65% of the stomachs and constituted 85% of the diet by volume.  In the early 1980’s it was estimated that mako sharks consume 4.3 to 14.5% of the available bluefish resource in the region between Cape Hatteras andGeorges Bank .   The diet of the shortfin mako in the western North Atlantic has not been quantified since 1982.  During the past several decades abundance of bluefish has declined, while mako shark abundance may have actually increased.  This study is designed to examine present day levels of predation by the mako shark on bluefish off the eastern U.S. , and to examine temporal patterns of predator-prey interactions between makos and bluefish in relation to variability in bluefish populations.

To obtain stomach contents of mako sharks we have taken advantage of the large-scale recreational fishery for mako sharks including a series of fishing tournaments conducted each summer off the U.S. east coast.  The stomach contents of several hundred mako sharks was sampled duing these tournaments.  The amount of bluefish in the diet of mako sharks was quantified using four indices of expressing diet.  The diet of mako sharks is dominated by bluefish on terms of number of prey, eight of prey, stomachs containing prey and as a cumulative index. Most (94%) of bluefish was between 50-80 cm in fork length.

Mako shark diet was expressed as:

%N  =  Total # bluefish/total # prey x 100 %W =  Total weight of bluefish/total weight of all prey x 100 %O  =  Number of stomachs with bluefish/number of stomachs with any prey x 100 IRIprey = [Number + Weight] x Occurrence

Bluefish in the diet of mako sharks expressed in the form of each index:

%N = 71.5         %O = 88.9 %W = 94.1        %IRI = 99.2

There was no correlation between size of shark and size of bluefish, although the diet of larger sharks was composed of a larger percentage of bluefish.