Read the first entries in this series, Sea Otters on the Island of the Blue Dolphins 1, 2 and 3
Sea otter scientists observe foraging sea otters on the teaming shores of San Nicolas Island, CA. Photo by G. Bentall
My eye is pressed to the rubber ring on the eyepiece of a telescope. I am waiting for a sea otter, one of 150 or so now living in the waters around San Nicolas Island, to surface from a foraging dive. My footing is weathered sandstone and my company, at the sea’s edge, a raucous, redolent spectacle of elephant seals, sea lions, cormorants, and brown pelicans fills my ears and nose. But it’s one sea otter—and the ring of bubbles where I expect her to surface— that holds my attention.
For a sea otter scientist making real-time foraging observations of wild sea otters on a remote island, larger-sized prey are always appreciated and are sometimes the source of jubilation. Large prey—big or bigger than the width of three sea otter paws (15 cm)—are easier to see, count and, most importantly, identify. But I venture most spotters would admit, whatever the species, large prey are a thrill to see as a sea otter first breaks the surface.
A foraging sea otter with a giant rock scallop meal (photographer unknown)So on this day on a rocky shore of San Nicolas Island—even though over twenty years having elapsed since my first observations of sea otter foraging—my awe is still fresh and bright when she surfaces with a giant bivalve between her paws. It is roughly the size of her head and looks every bit like an algae-crusted rock to the untrained eye. She hefts the closed shell and slams it down on a large, smooth rock on her belly. While cryptic in appearance, the large size, texture, and heft (as observers we can guess this by her effort in lifting and cracking it) are unmistakable. This is a giant rock scallop, Crassadoma giganteus, a massive bivalve that cements its right (lower) valve to the substrate as an adult. These giant scallops are aptly named as they can grow to a diameter of 25 centimeters. She will likely have found this living rock by probing crevices with her sensitive paws and whiskers. Once located these scallops (like their cousins the abalone) must be dislodged but, unlike abalone, they require more work at the surface before a meal is to be enjoyed.
A living giant rock scallop (Crassadoma giganteus) attached to rock substrate. As I watch this female sea otter work to breach the shell, I use her paws to estimate the size of this prey and record this as a 4c—every bit of four “paw-widths” or 20 centimeters. Once opened, this is a substantial meal for her, and she gives me a flash of the white porcelain-like interior of the shell as she cleans away the last tidbits. Our data sheet shows that it has taken her six dives to dislodge and retrieve this scallop, and ten minutes to crack, open and eat it, after which she immediately dives to hunt for another. The hunger of sea otters is endless.
This prey observation was exciting for reasons beyond the spectacle of it. In the days of my graduate research on the island back in 2003 and 2004 San Nic’s sea otters (affectionately, SNotters) retrieved a single species—specifically the red urchin, Mesocentrotus franciscanus—59% of their foraging dives and were part of a study of sea otter behavioral ecology that defined our understanding of the adaptability of sea otters to changing availability of their prey (link to PNAS paper). Sea otters were sparse at San Nic then and provided an opportunity for comparison to the much more crowded and long-established sea otters of the central California coast. One of their most prominent features in this study, was the lack of diversity in their diet, and this has become an earmark of a sea otter population living well below the habitat’s carrying capacity. Long-term research here provides an exciting opportunity to witness dietary diversification (OK, that’s jargon: different types and frequency of prey) as this insular population grows and adapts.
Relative sea otter density during 3 years of fall and spring surveys taken around San Nicolas Island, California, from A, 2014 through 2016; and B, 2017 through 2019. Relative densities were spatially modeled using kernel density methods on the cumulative sea otters counted per hectare across six surveys in each comparison period. Kelp canopy images taken during the fall of the 3 respective years are merged and displayed for reference. (Yee et al. 2020)The what, when, and how many of giant rock scallop prey are notable, but so too is the where. After decades of holding steady at the southwest end and south shore of the island, sea otters have begun to explore and settle new locations (Yee et al. 2020). As I stood upon this same rock on the island’s east end back in 2004, I often wished to see a raft of grizzled faces. Today my forager is one among forty or so resting, grooming, and foraging here. It inspires me in a way that appeases my disappointment over the stalled range expansion on California’s mainland (USGS Sothern Sea Otter Census Report 2019).
During three days spent perched here, I have observed only a handful of the red urchin prey once so common. At this new location the otters surprise me with lots of giant rock scallops, wavy turban snails, and even abalone. Also surprising is the frequency of their use of the rock tool during these foraging bouts, a behavior once not especially common for here compared to other California population (see Fujii et al. 2015). As our team scans the east end kelp with binoculars, it seems every other forager is banging away on a rock anvil.
Change and adaptation—whatever the time scale— are the essence of ecology and evolution. It is a privilege to witness the process unfolding in my lifetime, the island’s community rebuilding after an extinction like a microcosm of the history of the sea otter on a global scale. San Nicolas Island resonates with such stories—the fall and rise of the elephant seal, abalone, island fox, the tragic stories of Juana Maria and her Nicoleño family and ancestors. To stand on a rocky bluff watching a hungry sea otter contend with a scallop’s resistance to being eaten, is to be a character in the progression of all their stories.
Be a sea otter foraging scientist and watch this video by Marge Brigadier!