MEPS 560:73-86 (2016)  -  DOI: https://doi.org/10.3354/meps11871

ABSTRACT: Sea otters Enhydra lutris and sea stars both excavate clams and leave behind foraging pits. If the source of pits can be determined with confidence, they may provide information about benthic foragers without direct foraging observations. Our objectives were to determine (1) if pits can be attributed to either predator (sea otters or sea stars) using pit dimensions; (2) how pit shape changes over time; and (3) whether shell litter can be used to distinguish the relative clam predation by sea otters and sea stars. Naturally occurring pits were tagged and measured at 4 subtidal and 2 intertidal sites in Kachemak Bay, Alaska, USA, every 2 wk from May to August 2014. To determine how pit dimensions change over time, experimental sea otter and sea star pits matching published descriptions were dug at each site and measured after 2 wk. Additionally, we collected and analyzed shell litter at each site to determine the source of their mortality. Cluster analysis on pit dimensions showed that none of the measured parameters could identify pit source for the 109 pits measured. Consistent with this finding, the experimental pit types were indistinguishable after 2 wk. In contrast, shell litter proved useful in quantifying predator-specific predation. Consumption of clams by sea otters and sea stars was equal at all but one site. Only 2.4% of overall clam predation was attributed to other predators besides sea otters or sea stars. The significant sea star predation should be taken into account when making policy decisions concerning shellfish harvests and sea otter population management.

KEY WORDS: Enhydra lutris · Pycnopodia helianthoides · Clams · Foraging pits