07 Trites FB105(2)

Diets of Steller sea lions (Eumetopias jubatus) in Southeast Alaska, 1993−1999 Item Type article Authors Trites, Andrew W.; Calkins, Donald G.; Winship, Arliss J. Download date 29/09/2021 03:04:20 Link to Item http://hdl.handle.net/1834/25538 ART & EQ UATIONS ARE LINKED 234 Abstract—The diet of Steller sea lions Diets of Steller sea lions (Eumetopias jubatus) (Eumetopias jubatus) was determined from 1494 scats (feces) collected at in Southeast Alaska, 1993−1999 breeding (rookeries) and nonbreeding (haulout) sites in Southeast Alaska from 1993 to 1999. The most common Andrew W. Trites1 prey of 61 species identified were wall- Donald G. Calkins2 eye pollock ( ), Theragra chalcogramma 1 Pacific herring (Clupea pallasii), Arliss J. Winship Pacific sand lance (Ammodytes hexa- Email address for A. W. Trites: [email protected] pterus), Pacific salmon (Salmonidae), 1 Marine Mammal Research Unit, Fisheries Centre arrowtooth flounder (Atheresthes sto- Room 247, AERL – Aquatic Ecosystems Research Laboratory mias), rockfish (Sebastes spp.), skates 2202 Main Mall, University of British Columbia (Rajidae), and cephalopods (squid Vancouver, BC, Canada V6T 1Z4 and octopus). Steller sea lion diets at the three Southeast Alaska rook- 2 Alaska Department of Fish and Game eries differed significantly from one 333 Raspberry Road another. The sea lions consumed the Anchorage, Alaska 99518-1599 most diverse range of prey catego- ries during summer, and the least diverse during fall. Diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska (Gulf of Alaska and Aleutian Islands). Dietary differences between increasing and declining populations Steller sea lion (Eumetopias jubatus) rates of decline had the lowest diversi- of Steller sea lions in Alaska correlate populations in the Aleutian Islands ties of diet. The greater the diet diver- with rates of population change, and and Gulf of Alaska began declining sity, the slower the rate of population add credence to the view that diet may in the mid-1970s and were listed as decline. Additional diet data (through have played a role in the decline of sea lions in the Gulf of Alaska and endangered under the U.S. Endan- to 2001) supported the conclusion 1 Aleutian Islands. gered Species Act in 1997 (NMFS ; that diet diversity had some influence Trites and Larkin, 1996; Loughlin, on population success (Sinclair and 1998). The cause of the population Zeppelin, 2002; Sinclair et al., 2005). decline is uncertain but may be linked Merrick et al. (1997) hypothesized to a decrease in the quantity, quality, that animals with less diverse diets or availability of prey, in turn caused may have experienced difficulty ob- either by commercial fisheries or by taining enough prey. Others have hy- a natural change in the ecosystem pothesized that consumption of larger (Alaska Sea Grant, 1993; DeMaster proportions of lower energy-dense prey and Atkinson, 2002; Trites et al., may have exacerbated the effect of 2007). Stomach contents and scat diet diversity by increasing the food analysis indicate that the diets of requirements of sea lions (Alverson, the declining population may have 1992; Rosen and Trites, 2000; Trites changed from primarily small, fatty, and Donnelly, 2003). Sea lions with schooling fishes (such as capelin (Mal- less diverse, low energy-dense diets lotus villosus) and sand lance (Ammo- may also have been more sensitive to dytes hexapterus)) in the 1950s to one changes in overall prey abundance, increasingly dominated by walleye pol- and could have theoretically incurred lock (Theragra chalcogramma), Atka higher rates of predation from killer mackerel (Pleurogrammus monopter- ygius), and flatfish (Pleuronectidae) in the 1970s, 1980s, and 1990s (Mathi- 1 NMFS (National Marine Fisheries Ser- sen et al., 1962; Thorsteinson and vice). 1992. Recovery plan for the Steller sea lion (Eumetopias jubatus), Lensink, 1962; Pitcher, 1981; Calkins 1 92 p. Prepared by the Steller Sea Lion and Goodwin ; Merrick et al., 1997; Recovery Team for the National Marine 2 Sinclair and Zeppelin, 2002). Fisheries Service, 1315 East-West High- Merrick et al. (1997) found a posi- way, Silver Spring, MD 20910-3282. 2 Manuscript submitted 10 January 2006 tive relationship between the rate of Calkins, D. G., and E. Goodwin. 1988. to the Scientific Edfitor’s Office. population change and the diversity Investigation of the declining sea lion population in the Gulf of Alaska, 76 p. of summer Steller sea lion diets in the Manuscript accepted: 6 October 2006 Unpublished report. Alaska Department by the Scientific Editor. declining population during the early of Fish and Game, 333 Raspberry Road, Fish. Bull. 105:234–248 (2007).234 1990s. Regions that had the highest Anchorage, AK 99518-1599. PREFLIGHT GOOD TO GO Trites et al.: Diets of Eumetopias jubatus in Southeast Alaska 235 whales if they had to forage for longer periods of time. Population trends in Southeast Alaska have been opposite to those observed in the Gulf of Alaska (Trites and Lar- kin, 1996; Calkins et al., 1999; Pitcher et al., 2007). The robustness of the Southeast population compared to the other regions of Alaska may reflect a difference in diet. One explanation for this finding is that Steller sea lions in Southeast Alaska eat a wider range of prey and therefore have a more diverse diet. Another is that low energy-density prey (such as pollock) do not comprise a significant portion of the sea lion diet in Southeast Alaska. Our goal was to determine the diets of Steller sea lions in Southeast Alaska. We sought to test two hypotheses: 1) diet in Southeast Alaska is the most diverse of all regions inhabited by Steller sea lions; and 2) pollock is not an important prey species in Southeast Alaska. We also wanted to document prey associa- tions and seasonal changes in diet. Materials and methods There are three major breeding areas (rookeries) and over 45 major nonbreeding areas (haulouts) in Southeast Alaska. We collected 1494 scats from 12 haulouts and all three rookeries from 1993 through 1999 (Fig. 1). Some areas, such as the Forrester rookery, Figure 1 were sampled every year, and others Major rookeries (White Sisters, Hazy Island, and Forrester Island) and haul- were sampled less frequently (Table 1). outs (all other sites) of Steller sea lions (Eumetopias jubatus) in Southeast We grouped our analyses into rookeries Alaska during 1993−99. Labeled sites indicate where scats were collected. and haulouts, and then into subgroups by sample size, location, and frequency of sampling. Haulouts consisted of 12 nonbreeding sites shaking of the jars ensured that the scats broke down in the inside protected waters of Southeast Alaska (Fig. and formed a uniform slurry at the bottom of the jar. 1). Rookeries consisted of the three breeding areas in Volume was recorded from graduated markings on each Southeast Alaska (Forrester Island, Hazy Island, and jar. An elutriator removed most of the water-soluble White Sisters Islands). elements (Bigg and Olesiuk, 1990) before the remaining Scats were generally collected opportunistically, when sample was washed through a fine mesh screen. rookeries and haulouts were disturbed in order to count Prey species were identified at Pacific IDentifications pups or for other research purposes. Each scat was Inc. (Victoria, BC) from cleaned and dried hard parts; placed in a zip-lock plastic bag and frozen in a 5-gal- the types of hard parts that were present and the spe- lon plastic bucket before it was shipped to the Food cies from which they came were also noted. Prey hard and Energy Consumption Laboratory at the Vancouver parts recovered from scats were compared with hard Aquarium Marine Science Centre for cleaning. Only parts from a reference collection of identified skeletal scats that were big enough or solid enough to likely and nonskeletal hard parts. Otoliths and all other hard contain prey remains were collected, and only one scat parts were identified to the lowest possible taxon. Hard was collected from any group of scats if there was any parts that were digested beyond recognition or were doubt about whether the scat came from more than one not diagnostic for prey taxa were not included in our Steller sea lion. Each thawed scat was transferred to a analysis (e.g., ribs). Some recovered structures, such plastic jar and soaked in water for 1−6 days. Periodic as otoliths or squid beaks, could be used to estimate 236 Fishery Bulletin 105(2) the type and number of prey consumed, but other hard Scats that were empty or contained prey that could parts, such as scales, teeth, branchials, and gill rak- not be identified with certainty were not analyzed. ers, could only be used to quantify the type of prey These represented few of the scats collected (56 of 1494, consumed. 4%). Unrecognizable hard parts could have been from Table 1 Total number of Steller sea lion (Eumetopias jubatus) scats collected in Southeast Alaska during 1993−99 by year, location, and month. Note that North Rocks, Cape Horn Rocks, and Sea Lion Rocks are part of the Forrester Island rookery complex. Year Location Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total 1993 Benjamin Is. 0 0 0 0 0 0 0 0 0 0 28 0 28 1993 Brothers 0 0 0 0 0 0 0 0 0 7 27 0 34 1993 Cape Horn Rocks 0 0 0 0 0 5 0 0 0 0 0 0 5 1993 Hazy 0 0 0 0 0 27 0 0 0 0 0 0 27 1993 North Rocks 0 0 0 0 0 8 0 0 0 0 0 0 8 1993 Pt. League 0 0 0 0 0 0 0 0 0 0 38 0 38 1993 Sail Is.

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