River otters return to shorelines built on fill and riprap

River otters are being seen along the East Bay shoreline again this month, at the Albany waterfront on August 27 and off Emeryville five days earlier. Both spots are old fill and riprap pushed out into the bay, which sounds like poor otter ground until you look at what the animal actually needs.

A river otter (*Lontra canadensis*) is a long, low member of the weasel family, ten to thirty pounds, with a thick tapering tail that makes up about a third of its length. On this shoreline the animal it gets confused with is the sea otter, and the differences are easy once you know them. A river otter swims belly down, head and back breaking the surface; a sea otter floats on its back. A sea otter is several times heavier, with a short stubby tail, and almost never leaves the water. Any otter hauled out on the rocks at the Bulb or trotting along the Emeryville breakwater is a river otter. They cross land readily and will travel considerable distances overland between one body of water and the next.

They eat whatever is easiest to catch: fish first, then crabs and other crustaceans, and on occasion frogs, ducks and eggs. On brackish shorelines studied elsewhere, finfish and crustaceans make up most of the diet, and the animals take readily to man-made structure, using riprap and pilings as latrines.¹ Those latrines, piles of scat full of fish scales and crab shell on a prominent rock, are often the first sign of otters on a stretch of shore, well before anyone sees the animal.

The catch with living in salt water is the fur. An otter has no blubber; it stays warm because a dense underfur traps air, and that only works if the coat is kept clean. Otters groom after every meal, and in salt water they need fresh water to rinse with, which is why their haul-outs and dens cluster around creek mouths, seeps and freshwater ponds rather than open rocky shore.¹ Codornices Creek, the one creek in this stretch that runs year round from the Berkeley Hills to the bay, reaches the water at the Albany-Berkeley line just south of the Bulb. Most of its neighbors, Cerrito and Temescal among them, are culverted for much of their length and deliver what little September flow they carry through storm-drain outfalls. A shoreline with a living creek mouth is better otter ground than one without.

For most of the twentieth century there were no otters to see here. Fur trapping and the condition of the bay's water are the usual explanation for their absence, and it was after trapping was ended in California and water quality rules took hold that they began turning up around the estuary again, by the 2010s in places where they had been gone for generations. Cesar Chavez Park, stewarded by the Chavez Park Conservancy, sits on the Berkeley waterfront between the two places otters were seen last month, and the rocks along that shore are where to look for tracks and scat.