Shark Predation on Hawaiian Monk Seals - Pacific Islands Fisheries ...

pups in daylight hours (NMFS, 2003; NMFS, 2004; Harting et al., in prep.). Additional
observations of shark activity were also recorded in 1997–1999 and 2004–2007.
Observed Trends in <strong>Shark</strong> <strong>Predation</strong>
R. Dean Grubbs and C. Meyer summarized information about Galapagos and tiger shark
movement and behavior, especially at FFS, at Workshop II (detailed in Section I). The
following provides additional information about the species’ behavior at FFS.
Galapagos sharks have been implicated as monk seal pup predators as derived from direct
observation; gut contents of tiger sharks containing seals of unknown age suggest their possible
involvement. Galapagos sharks are the only shark species observed by HMRSP to be near
pupping sites charging, chasing, biting and killing preweaned pups. (NMFS, 2003; NMFS,
2004; NMFS, 2005; NMFS, in prep.; Harting et al., in prep.; NMFS, unpubl. data). Based on
tagging and other distinguishing features of individual sharks during the peak times of shark
predation (1998), a minimum of 14 identified Galapagos sharks were observed patrolling or
pursuing preweaned pups at Trig Islet (NMFS, 2009). One of the previously sighted and tagged
Galapagos sharks from 1998 was again sighted around Trig Islet during the 1999 pupping
season and was identified with the killing of a preweaned pup that year (Harting et al., in
prep.).
Globally, Galapagos sharks are resident insular, typically found in depths of 150 m. They feed
primarily on cephalopods, elasmobranchs, and reef, demersal and bottom fishes (Vatter, 2003).
However, it has been reported that pinnipeds have been included in the Galapagos shark diet
(Compagno, 1984). Shallow water predation by sharks, especially during dusk and nocturnal
hours, has also been described in the literature (Castro, 1983). Results from insular shark
surveys in the NWHI at 10 major shallow reefs (< 40 m) (2000–2002, 2004, 2006, 2008)
indicate that Galapagos sharks are not commonly found in shallow areas (NMFS, 2009b).
Together, these findings suggest preying on pinnipeds at shallow depths is not common but it is
within the behavioral repertoire of the shark species.
Tiger sharks are opportunistic predators that consume a diverse array of taxa including marine
mammals (Meyer et al., 2009). A clear ontogenetic shift in diet occurs in both increasing prey
diversity and increasing prey size as shark size increases for Galapagos and tiger sharks
(Wetherbee et al., 1996; Meyer et al., 2009). Gut contents of 3 tiger sharks out of 23 caught in
1977 at FFS contained monk seals of unknown age; no seal remains were found in the gut
contents of the 2 Galapagos sharks caught (Taylor and Naftel, 1978). No seal remains were
found in the gut contents of 4 tiger sharks and 11 Galapagos sharks caught in a 1998 study
(Vatter, 2003).
Other trends noticed in shark predation patterns at FFS include: (1) a general decline from the
1997–1999 peak to 6–11 pups lost per year, (2) a shift in shark activity from centering around
Trig and Whaleskate Islets to additional islets within the atoll (East, Gin and Round Islets) (Fig.
5), and (3) an increasing predation focus on the youngest pups (Fig. 6). The decline in shark
predation and greater distribution of predation may be a result of a combination of any of the
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