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Foraging Interaction Between Monk Seals and Large Predatory Fish in the Northwestern Hawaiian Islands

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Foraging Interaction Between Monk Seals and Large Predatory Fish in the Northwestern Hawaiian Islands Vol. 4: 299–308, 2008 ENDANGERED SPECIES RESEARCH Published online May 16, 2008 doi: 10.3354/esr00090 Endang Species Res OPENPEN ACCESSCCESS Foraging interaction between monk seals and large predatory fish in the Northwestern Hawaiian Islands Frank A. Parrish1,*, Greg J. Marshall2, Birgit Buhleier2, George A. Antonelis1 1Pacific Islands Fisheries Science Center, National Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, Hawaii 96822-2396, USA 2National Geographic Society, 1145 17th Street N.W., Washington, DC 20036-4688, USA ABSTRACT: CRITTERCAMS were deployed on 42 Hawaiian monk seals at French Frigate Shoals, Hawaii, to assess the degree to which interaction with large predatory fish may play a role in their foraging success. Sharks, jacks, and large-bodied snappers were seen near the actively foraging seals approximately 17% of the time. Present at all but the greatest depths (>100 m), the predatory fish were most frequently observed when seals foraged at the summits of neighboring banks. Competing fish were encountered as individuals and in schools (mean 4.5, maximum 46 ind.) and often formed mixed-species assemblages that followed seals around. By traveling with foraging seals, the preda- tory fish could exploit the seals’ superior ability to flush cryptic prey from bottom cover. The probing, digging, and rock-flipping activities of seals spook prey into the open, making them available to jacks, sharks, and snappers. The jacks Caranx ignobilis and Seriola dumerili were the most success- ful at obtaining prey items flushed from cover by the seals, but carcharhinid sharks were also seen competing for prey. Some adult seals appeared to feed effectively in prey-rich areas despite the pres- ence of high densities of predatory fish. It is not known if the predatory fish have a larger impact on juvenile seals, the segment of the population with the poorest survivorship. KEY WORDS: Hawaiian monk seal · Monachus schauinslandi · Competition · Klepto-parasitism · CRITTERCAM · Endangered species · Apex predators · Sharks · Jacks Resale or republication not permitted without written consent of the publisher INTRODUCTION (Craig & Ragen 1999), and the overall poor body condi- tion of juvenile seals has been primarily attributed to The foraging success of the endangered Hawaiian starvation rather than disease (Reif et al. 2004, Aguirre monk seal Monachus schauinslandi at French Frigate et al. 2007). Shoals Atoll (FFS) has been a focus of study for 2 Recent studies have focused on various aspects of decades (e.g. DeLong et al. 1984, Goodman-Lowe monk seal foraging behavior and have included scat 1998, Parrish et al. 2002, Littnan et al. 2004, Stewart et analysis to define diet (Goodman-Lowe 1998, Longe- al. 2006). Although the largest subpopulation of monk necker et al. 2006), satellite telemetry to define oceanic seals persists at this atoll, its numbers have steadily movement, analysis of dive patterns to describe forag- declined there, following a decrease initially identified ing behavior (Stewart et al. 2006, Parrish & Abernathy in the late 1980s and early 1990s (Gilmartin et al. 1993, 2006), use of seal-mounted video cameras to under- Gilmartin & Eberhardt 1995). Subsequent demo- stand prey selection and habitat use (Parrish et al. graphic studies by Ragen & Lavigne (1999) identified 2000, 2002, 2005), and evaluation of oceanographic juvenile seals as the segment of the population with factors to assess potential influences on prey resources the lowest survival (Antonelis et al. 2006). Size at and seal survival (Schmelzer 2000, Antonelis et al. weaning was correlated with survival for most cohorts 2003, Baker et al. 2006). There is also a growing appre- *Email: [email protected] © Inter-Research 2008 · www.int-res.com 300 Endang Species Res 4: 299–308, 2008 ciation that the NWHI (Northwestern Hawaiian Such instrumentations have been shown not to com- Islands) is one of the few remaining apex-dominated promise seal survival or ability to forage (Baker & marine ecosystems (Sudekum et al. 1991, Friedlander Johanos 2002, Littnan et al. 2004). Sedated seals & DeMartini 2002) that has prompted interest in the remained awake during the entire procedure and degree of inter-specific competition for prey between returned to the water unassisted immediately after monk seals and large predatory fish of the region. The recovering from the sedative. The entire procedure unusually high number of sharks, jacks, and snappers never exceeded 60 min. Seal haul-out behavior and in the NWHI is associated with the regional absence of location was monitored every 3 h throughout each fishing and exerts uncommon top-down pressure on deployment from the northwest end of the atoll at Tern the reef ecosystem (Sudekum et al. 1991, Friedlander Island, FFS. The cameras were recovered by recaptur- & DeMartini 2002, Parrish & Boland 2004, DeMartini & ing each seal; the time between deployment and Friedlander 2006). Data obtained from diet studies of recapture ranged between 1 and 10 d following instru- the monk seal (Goodman-Lowe 1998), sharks mentation. (DeCrosta et al. 1984), and jacks (Sudekum et al. 1991) Seal dive depths were digitally logged every 10 s indicate that these predators feed in similar habitats on throughout the camera deployment. The video cam- the same prey types and thus, at times, may compete eras were programmed to record segments of images for the same resources. Inter-specific competition is and sound for 6 min each daylight hour (1.5 min were difficult to document for highly mobile species that are recorded every 15 min, or 3 min every 30 min). A sea concealed from view underwater. Seal-mounted video water conductivity and depth/pressure switch (<1 m) cameras such as National Geographic Television’s was used to cease recording when the seal was on the CRITTERCAM (Marshall 1998) affords one method to beach or at the surface to maximize collection of address the topic. Eight years of research using CRIT- images during foraging activity. If the seal surfaced TERCAMs on monk seals at French Frigate Shoals to while the camera was still recording, the segment was identify various aspects of seal foraging behavior has interrupted and the tape saved for future recording generated a sizable sample of video recordings of segments. The total surveillance time for the seals interactions between seals and predatory fish. In the ranged from 36 to 72 h depending on the size of the present study, we investigate the level of apparent tape cartridge used and the amount of time the seal inter-specific competition among seals and predatory spent at sea. Five seals were fitted with night vision fish by defining the frequency of interactions, identify- CRITTERCAMs, and the sampling was limited to evening ing the size and species involved, and determining the hours only. The night vision systems were able to ‘see’ habitat in which the interactions occurred. the bottom area extending 3 m in front of the seal (Par- rish et al. 2002). Scoring of data. Video images were reviewed and MATERIALS AND METHODS scored into a set of standardized variables entered into a relational database. For each video segment, the Study site. FFS is a crescent-shaped atoll bounding a depth, type of seal behavior (e.g. swimming, resting, lagoon with a barrier reef running along its northern bottom-searching), type of habitat (e.g. atoll, bank, perimeter and diagonally through its center (Fig. 1). open ocean), feeding events, and the maximum num- The sand islets located along this diagonal barrier reef ber of predators present were recorded. The forward- are the only land that monk seals have within a 60 mile facing mount of the camera could not document the radius on which to haul out. During summer and fall presence of ‘following’ predators, and thus the count of months of 1995 to 2002, 42 monk seals (adults and fish predators is unavoidably biased low. Side-to-side juveniles) were captured, sedated with diazepam, and motion of the seal’s head as it passed over the bottom instrumented with programmable digital video cam- was clearly swimming behavior. Segments showing eras (CRITTERCAM: National Geographic Missions Pro- the seal motionless on the sea floor, often under a gram, Wild Insight Venus UTPR [Underwater Timed ledge or in a cave, were classified as resting behavior. Picture Recorder]) using 10-min epoxy to glue the units Bottom-searching behavior was quantified for each to the dorsal pelage (Parrish et al. 2000). The camera segment by scoring the number of times that the seal was oriented such that the field of view was forward, probed the substrate with its muzzle (standardized for with the seal’s head just visible at the bottom of the observation time) while moving along the sea floor. image (Fig. 2). The size of the instrument deployed The maximum number of predatory fish was the great- depended on the size and maturity of the seal est number present on screen at any one time during (Table 1), with the smaller instruments attached to the video segment. Behaviors of the predatory fish juvenile seals. The camera units also included very were binned into 4 different classes: ‘escort’ — the fish high frequency radio tags and time depth recorders. were clearly swimming in association with the seal, Parrish et al.: Interactions of monk seals and large predatory fish 301 Fig. 1. Hawaiian Archipelago with the French Frigate Shoals region expanded to show the atoll (dark grey) and neighboring banks (light grey). Contour depths in m visible either in front or peripherally; ‘in-face’ — the RESULTS predatory fish nosed in close to the mouth of the seal while the seal probed the bottom for food; ‘feeding’ — A total of 3192 video segments were collected com- fish was consuming a prey item; and ‘stealing’ — fish prising 69 h of underwater surveillance with a mean of either took or attempted to take a prey item from the 1.7 h (±0.9 h SD) per seal.
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