Journal of Zoology. Print ISSN 0952-8369 Distribution and frequencies of shark-inflicted injuries to the endangered Hawaiian monk seal (Monachus schauinslandi) P. Bertilsson-Friedman Department of Zoology, University of New Hampshire, Durham, NH, USA Keywords Abstract Hawaiian monk seals; Monachus schauinslandi; shark-inflicted injuries; injury I analyzed the frequencies of wounds inflicted by sharks to the endangered distribution; shark interactions. Hawaiian monk seal Monachus schauinslandi at three colonies in the Northwestern Hawaiian Islands between 1990 and 2000. I applied specific criteria typical of shark Correspondence bites to all injuries to avoid bias. Pooling data from all years and the three colonies Petra Bertilsson-Friedman. Current address: revealed several patterns. Sharks injured more pups (nursing and weaned) and GPS Department, St Joseph’s College of juveniles (1–2 years old) than adults and subadults. More female pups and male Maine, 278 Whites Bridge Road, Standish, juveniles than any other size classes were injured by sharks. Almost all (97%) of ME 04084-9978, USA. the injured pups were from French Frigate Shoals. More juveniles than expected Email: [email protected] were wounded at Laysan Island and Lisianski Island. Most shark wounds were between the diaphragm and the pelvic girdle, but pups were bitten most often just Received 13 December 2004; accepted behind the pelvis compared with adults who were bitten more often near the head 23 June 2005 and neck. doi:10.1111/j.1469-7998.2006.00066.x Introduction atolls and islands. The species has declined substantially during the past several decades. Many factors have been In contrast to well-studied terrestrial mammals, little is suggested to explain the decline and lack of recovery, known about predation on marine mammals. It is challen- including predation (Taylor & Naftel, 1978; Johnson et al., ging to evaluate the significance of aquatic predation 1982; Craig & Ragen, 1999; Ragen & Lavigne, 1999). Actual as the habitat is often inaccessible, and both prey and interactions between the seals and sharks have rarely been predator are at least partially aquatic. Because predation observed, but the consequences can be investigated as may have a substantial effect on estimates of survival, wounds from shark bites are often observed (Kenyon & it is important to assess the influence of potential predators Rice, 1959; Rice, 1960; Hiruki et al., 1993; P. Bertilsson- on marine mammals and its influence on demography, Friedman, pers. obs.). Several species of sharks inhabit the particularly for endangered or otherwise vulnerable species inshore waters of NWHI and have an important role as (Sinclair & Arcese, 1995; Oosthuizen et al., 1997; Boveng high-level predators in the shallow-water communities (Par- et al., 1998; Lima, 1998; Sinclair et al., 1998; Geraci, Har- rish et al., 1984). Among the most abundant sharks are three wood & Lounsbury, 1999; Lucas & Stobo, 2000; Williams species of charcharhinids: the tiger shark Galeocerdo cuvier, et al., 2004). the Galapagos shark Charcharhinus galapagensis and the Shark predation has been suggested to potentially gray reef shark Charcharhinus amyblyrhynchos (De Crosta have substantial impacts on some pinniped populations et al., 1984). Tiger shark and Galapagos shark are both (Ainley et al., 1981; Le Boeuf, Riedman & Keyes, 1982; potential predators on monk seals as both species have been Ainley et al., 1985; Le Boeuf & Crocker, 1996; Lucas & observed attacking seals (Balazs & Whittow, 1979; Alcorn & Stobo, 2000; Klimley et al., 2001). It is suggested that Kam, 1986; P. Bertilsson-Friedman, pers. obs.). Remains of immature phocids may be more vulnerable to shark preda- seals have been found in tiger sharks (Taylor & Naftel, 1978; tion than adults (Ainley et al., 1985; Le Boeuf & Crocker, De Crosta et al., 1984), and Galapagos sharks have fre- 1996; Klimley et al., 2001), and Le Boeuf et al. (1982) quently been observed stalking and attempting to attack suggested that shark-bitten elephant seal females may have pups at French Frigate Shoals (FFS) (P. Bertilsson-Fried- reduced reproductive success. man, pers. obs.). Shark attacks can result in mortality or The endangered Hawaiian monk seal Monachus schauin- injury. Although mortality has immediate consequences to slandi primarily occupies the Northwestern Hawaiian individuals and populations, injuries can also have serious Islands (NWHI) with established breeding colonies at six detrimental effects on an animal and might be an important Journal of Zoology 268 (2006) 361–368 c 2006 The Zoological Society of London 361 Shark-inflicted injuries to the endangered Hawaiian monk seal P. Bertilsson-Friedman factor in regulating populations (Harris, 1989; Juanes & Statistical analysis Smith, 1995). I tested for differences in the frequencies of shark-inflicted I studied shark-inflicted injuries on Hawaiian monk seals injuries between islands with a one-way analysis of variance in NWHI between 1990 and 2000. Long-term monitoring (ANOVA). Annual variations between the islands were data collected by National Marine Fisheries Service analyzed using the w2 test for independence and simple linear (NMFS) provide reliable demographic data of the species, regressions with arcsine transformation. Injury frequency and allow for tracking individual seals through time. The was calculated as the number of seals injured divided by the results can serve as a foundation for an index of shark attack total number of seals identified at a given site in a given year. effects on monk seal population. My objectives were to Both the total identified including and excluding pups was investigate patterns in the frequency, severity and body used as the denominator in calculating injury frequency, as location of shark wounds on monk seals according to annual fluctuations in pup production can introduce great colony, size class and sex. variability. Comparisons between Lisianski Island and the other two islands excluded data from 1991 and 1994. Methods I used a two-way ANOVA and the w2 test of indepen- dence (Zar, 1999) to examine differences in the distribution Study area of injuries among size classes [pup, juvenile, subadult and adult size (following Stone, 1984)]. Weaned pups and nur- Data on injuries to Hawaiian monk seals were collected sing pups were combined into a single category. Paired t- from 1990 until 2000 at three of the six Hawaiian monk seal tests were used to examine differences among the sexes. To colonies in NWHI. FFS (231450N, 1661100W) had the examine patterns in the distribution of shark-inflicted in- largest population of Hawaiian monk seals with at least juries on the seals’ bodies, and the severity of these injuries, 345 seals (including 67 pups) in 2000, Laysan Island I used w2 goodness of fit and test of independence analyses (251460N, 1711460W) had 315 individuals (43 pups) in 2000, (Zar, 1999). Sequential Bonferroni adjustments for multiple and in the same year Lisianski Island (261020N, 1741000W) comparisons were used for all repeated tests. had 204 individual seals (20 pups) (Johanos & Baker, 2002). Data on injured seals were collected every year (except that there were no data for Lisianski in 1991 and 1994) Results in conjunction with standardized population assessment I used 315 shark-inflicted injuries in my analyses (FFS (Johanos & Baker, 2004). n=191, Laysan Island n=85, Lisianski Island n=39). Another 143 injuries, described at the time of data collection Analyses of injuries as possibly shark inflicted, were not included in the analysis because they did not meet the criteria I applied. It is I reviewed all records of injuries to monk seals that may important to note that observations of the seals are not year have been caused by sharks between 1990 and 2000. Using round and consequently some injuries go unnoticed, and criteria modified after Long & Jones (1996), I included an also that an unknown number of shark-bitten seals never injury as a positive shark injury if either (a) a shark attack make it back to shore. Consequently, my analyses consider was witnessed or (b) a living or dead seal showed injuries only a minimum estimate of seals injured by sharks during with characteristics of a shark bite: oval or crescent arcs, the 11-year study. triangular punctures, jagged serrations or flipper amputa- tions. I excluded injuries that did not fit these criteria from the analysis. Furthermore, data on all subsequent injuries Colony comparisons were excluded if an animal had been previously injured by a Overall, there was no significant difference in the rates of shark in the same year to avoid bias. I did not include shark-inflicted injuries among the three colonies (P40.05) wounds inflicted by the small (o50 cm) ectoparasitic cookie (Table 1). There was, however, a significant increase in the cutter shark Isistius brasiliensis as this small deep-sea shark percentage of shark-injured animals over the years at FFS only takes small circular pieces of flesh (usually o3–7 cm (P=0.011, r2 =0.532). There was no significant change in diameter) from its victims (Jones, 1971; Jahn & Haedrich, the percentage of animals injured by sharks over the years 1987), and this study focused on wounds from large sharks. for the other islands, although there was a small increase in Injuries included in the analysis were new for each year, the percentage of injured animals at Lisianski Island when and severity standards [levels between one and three, with pups were excluded (P=0.053, r2 =0.439). three being the most severe (life threatening)] for all injuries were applied as developed by NMFS in 2000 and described Distribution of injuries in Johanos & Baker (2002). I used methods developed by Long et al. (1996) to determine whether shark-inflicted The observed number of juveniles and pups with shark- injuries occurred more often on any part of a seal’s body. inflicted wounds deviated significantly from the proportion For this analysis, I divided the body into four regions: snout available in the population (w2 =19.81, Po0.001, d.f.=3; to shoulder girdle, shoulder girdle to diaphragm, diaphragm w2 =23.58, Po0.001, d.f.