We report on two data sets, the first containing sporadic observations of sea otters foraging at the site from 2007 to 2017, and the second consisting of concentrated behavioral and archaeological observations at the BSC site from July 7th to 27th, 2016. Because not all otters that used the site could be individually identified, there may be data on some of the same individuals in the two datasets. We did not collect foraging data during the ethoarchaeological observations in 2016, when we concentrated on obtaining video footage of sea otters pounding mussels on emergent rocks, describing the resulting wear patterns on the rocks, and documenting the characteristic distributions and fracture patterns of the broken mussel shells that accumulated below the rocks.

Sea otter foraging observations at BSC 2007–2017

BSC is not a main foraging area for most sea otters in Elkhorn Slough (Fig. 2); however, small numbers of individuals have been observed in the area sporadically since 1998. We opportunistically observed tagged (see Methods) and untagged individuals foraging at BSC, with observation effort varying over time, and records spanning from 2007 to 2017. Observation effort at BSC was recorded only when otters were present and foraging. The increasing number of observations over time in our dataset reflects the increasing number of otters using BSC to forage as population numbers grew18. Beginning in 2007, we occasionally observed five tagged females feeding at BSC and in 2013 we began documenting foraging behaviors of untagged otters using the site as well. Foraging data from untagged individuals represented a minimum of four individuals (1 male and 3 females), but potentially up to 17 unique individuals (if each forage bout was from a different individual). In total, we recorded 629 dives from 29 forage bouts (Supplementary Data 1).

Mussels were the most common prey consumed by otters at BSC, comprising 51.2% of prey captures, followed by clams (25.4%), crabs (17%), and all other prey items (6.4%) (Supplementary Data 1; prey were not always identifiable to genus or species level). The average shell length of consumed mussels was 5.6 cm (±2.2 cm, n = 322), and the average rate of biomass intake was 6.9 grams per minute, about 25 to 75 mussels per hour (calculated from the size and number of mussels consumed and the relationship between shell length and edible biomass; see refs. 19,20). An analysis of variance showed that the proportion of mussels in the diet did not differ significantly between males and females (F 1,27 = 1.05, p-value = 0.32).

All stone-use observed at BSC was performed by female otters; however, more females than males used the site. Emergent anvil use was only observed with the consumption of mussels, and was observed in 13.8% (4 of 29) of forage bouts. Within these four bouts, emergent anvil use occurred in 79.1% of mussel captures (n = 69, SE = 20.9%), meaning that 21% of individual mussels gathered by otters were pounded on emergent anvils. In addition to emergent anvils, otters also used other mussels and empty shells as chest anvils, and we observed an otter pound a Washington clam (Saxidomus nuttalli) on a stone chest anvil. While sea otters were observed foraging at BSC during all tide levels (−0.2 to 5.6 feet), emergent anvil use was only recorded during mid-to-high tide levels (2.1 to 5.4 feet).

Ethoarchaeological study

During our ethoarchaeological study in July 2016, two tagged females with pups, three or more untagged females, and one untagged male visited the BSC site. Two of the female otters used emergent anvils, and a third female used a stone anvil on her chest. Emergent rocks were used as anvils, and one otter once used the side of a metal pipe culvert. Each otter began foraging by collecting multiple, clumped mussels during a foraging dive, usually from inside the pipes. When the otter returned to the surface, it held the clump securely on its chest and in the folds of its fur, while opening each mussel in turn with its teeth or a stone.

All observed emergent anvil use during this part of the study was mussel-pounding by two adult female sea otters (Supplementary Data 2), one of which is also listed in the foraging data collected from 2007 to 2017 (Supplementary Data 1). These sea otters held a mussel between both fore-paws and struck it rapidly and repeatedly against a rock (Video 1). Each otter would position itself either (i) floating on its stomach, side, or back, with its head and fore-paws upright (otters can achieve this posture because their skin is very loose on their body), holding the mussel up and striking downwards (Fig. 1B), or (ii) floating on its side, with the mussel then struck sideways or slightly upwards against an emergent anvil (Video 1). We observed both postures when the otters were striking at submerged anvils (Supplementary Fig. 1; Video 2), but for striking anvils at or above the water line the former was much more common (31 downward striking postures and 3 sideways striking postures; Supplementary Data 2). In each instance, the mussel was held compressed between the paws, so that each paw pushed against one of the mussel’s shells or valves. The otter continued striking until the shell was sufficiently weakened to allow the otter to use its teeth to lever the mussel open. Once a mussel was opened, the otter consumed the meat while floating on its back at the surface, close to the emergent anvil. While the sea otter ate, it performed cleaning rolls that allowed shell pieces to fall from its chest to the Slough bed (Video 3).

We coded 60 pounding series (each series involves a single, consumed mussel) from our video recordings of two adult females using emergent anvils at BSC (Supplementary Data 2). One otter struck anvils on both ridges and points above water, with a preference for points (16 of 20 series). The second otter struck only on ridges, both above and below water. Neither otter struck on rock faces. For classification of rock surfaces into ridges, points and faces, see Methods.

Use-damage on rocks at BSC

In total we mapped 421 rocks. We followed standard protocols, such as those used on prehistoric anvils21, to assess sea otter pounding use-damage. Of the 421 rocks mapped at the site, 419 were quartzite and two were concrete blocks. Seventy-seven rocks (18.3%) had macroscopically visible use-wear in the form of crushed and fractured quartz grains on abraded ridges and points, exposing visibly lighter quartz grains (Figs 3 and 4). These damaged rock ridges and points included the ones pounded by otters during our ethoarchaeological observations (Video 2). For classification of emergent anvil use-damage see the Methods. There was no significant difference in the percentage of use-worn rocks at BSC North and BSC South (χ2 (1, N = 421) = 1.68, p = 0.195) (Table 1); therefore we combined all rocks into our use-wear analysis.

Figure 4 Use-wear damage on rocks at the Bennett Slough Culverts (BSC) site. (A) Sea otter damage on the corner of a quartzite boulder at BSC South; the scale is 10 cm. (B) Emergent boulders damaged by sea otters on their upper surfaces (circled) at BSC North, with the rocks further from the water topographically higher; the water level is mid-height. (C) Emergent anvils at low tide at BSC North, with the boulders seen in (B) on the left. Mussel shell deposits are visible above and below water in the three views. Full size image

Table 1 Use-zones and total use-damaged anvils by location, Bennett Slough Culverts (BSC) site. Note that one anvil may have multiple used zones, so the number of use-zones exceeds that of anvils. Full size table

Points were significantly more damaged than ridges on the highest part of a rock, while ridges were more damaged than points on an anvil’s upper half (for division of rock surfaces into use-zones, see Methods) (χ2 (1, N = 126) = 25.532, p < 0.0001) (Table 2). Flat faces were not damaged by pounding at BSC. Thus, otters preferentially target points and ridges as pounding surfaces. The intensity of wear was not significantly different between the highest surfaces and upper, water-facing parts of the emergent anvils (two-tailed Mann-Whitney test, U = 1558.5, z = 1.59, p = 0.112). No rock had use-damage on its lower land-facing side, and only two had wear on the upper land-facing side, both on ridges. Similarly, only three rocks had use-wear on the lower water-facing side. In contrast, 50 rocks had use-damage on their upper half facing the water, and 75 on the highest point of the rock. Forty-eight of the 50 anvils (98%) with water-facing damage on their upper half also had use-wear on their uppermost surface. These use-wear patterns indicate that the pounding damage occurs from otters pounding downward onto the upper parts of anvils, from a position in the water. Consistent with our long-term data set finding that emergent anvil use by sea otters was only recorded during mid-to-high tide levels, we found that rocks closer to land had more intense use-damage. Just under a quarter (23.4%) of damaged anvils had a high use-intensity score, and indicate sea otters repeatedly target landward rocks (Table 3), which are topographically higher than rocks closer to the water and thus spend more time exposed above water throughout the tidal cycle.

Table 2 Frequency of damaged use-zones with a given morphology, Bennett Slough Culverts site. Full size table

Table 3 Frequency of damaged anvils with a given use-wear intensity at the Bennett Slough Culverts site. Full size table

Shell debris at BSC

During mussel pounding, the sea otters appear to have struck the right valve preferentially against the emergent anvils. This process resulted in the valves remaining attached to each other, with the right valve damaged or missing, and the left valve with an undamaged hinge (Fig. 5). Of 29 randomly sampled shell fragments, 18 were intact left valves with the hinge intact, and 11 were fractured right valves with no hinge. Thus, 100% of intact valves were left valves, and 100% of broken valves were right valves: the probabilities of these outcomes under a null hypothesis of equal likelihood of fracture for right or left valves are 0.000004 and 0.0005, respectively. The undamaged valves did not retain stone impact marks, and had an intact hinge and umbo, the protruding anterior part of the mussel shell (Fig. 5). The damaged right valves (both the sections still attached to undamaged left valves and the pieces removed by the otter) displayed radiating fractures, with a dominant fracture running diagonally up the shell.

Figure 5 Mussel shell breakage patterns at the Bennett Slough Culverts site. (A) Outer and (B) inner faces of each valve; (C) schematic drawing of the exterior of a mussel shell showing the typical sea otter breakage pattern (illustration by Neil Smith); (D) broken mussel shells in situ. Full size image

There are dense shell middens around BSC, most visibly on the north side, that remain underwater at all tidal heights (Supplementary Fig. 2). These enhydragenic (sea otter-derived) middens appear to be thickest around the emergent anvils between the culvert pipes, and thinnest at the culvert pipe outlets (Fig. 4C). To avoid disturbing the otters we did not excavate the middens, but by extrapolation from surface counts we estimate that there are tens to hundreds of thousands of shells present. There are also dense mussel deposits that remain despite tidal incursions at the base of heavily used anvils where surrounding rocks trap the shells. The highest density that we counted was more than 100 shells within 30 cm of an anvil (Supplementary Fig. 2B). Of the 26 anvils with shell debris present within 30 cm, 22 (84.6%) had shells only on the water-facing side of the rock. All shells that were attributable to otter consumption were mussels, although other prey such as Washington clams and crabs (multiple genera of shore crabs and cancer crabs: Cancridae, Grapsidae, and Pugettia sp.) are present in the local environment and regularly consumed by the otters.