On The Trails: Capelin

This capelin has just arrived on its spawning beach. (Photo by Bob Armstrong)

By Mary F. Willson

A horde of capelin swept into Auke Bay this winter, happily feasted on by seals and sea lions and diving birds.  Seeing a video of that horde prompted me to learn more about those little fish; here’s a small summary.

Capelin (Mallotus villosus) are small, schooling fish that live in cold Arctic and subarctic waters of both Atlantic and Pacific Oceans. The Atlantic and Pacific populations are sometimes divided into two species. Mature individuals reach a length of five to eight or 10 inches. Males tend to be bigger than females. They forage chiefly on zooplankton such as copepods and krill, choosing small ones at first and then larger ones as they grow toward maturity. 

They ordinarily reach sexual maturity at two or three years old in Alaska, but recently warm temperatures have allowed some to mature at age one. Under some circumstances, maturity is reported to be delayed until four or five years old. Females may mature a bit sooner than males. Spawning may occur from late spring (commonly) to late summer, depending on location and conditions, often at night. Locally, a naturalist observed them spawning in May along the lower reaches of the Mendenhall River. They come onto beaches in huge numbers, even jumping up onto the land. Males may come in just ahead of females. The spawning action occurs among the wet sediments of the beach. A male presses closely against the female, releasing his milt as she releases her eggs. Sometimes two males may surround a female. The eggs stick to the sand or gravel and usually hatch in two to three weeks. Males usually die after spawning, but females can sometimes spawn again in another year. Beach spawners often get stranded when the waves recede, leaving a beach littered with dead and dying fish. 

In some regions (not yet documented for Alaska), capelin also spawn on the seabed in water as deep as a hundred meters or more, with approximately equal success. In the colder water there, eggs take longer to hatch — commonly four weeks or more. Interestingly, egg-cannibalism is more common in the deeper waters, with many of the adults doing it, during spawning and especially post-spawning. Nevertheless, it usually only accounts for a small percentage of the eggs. Post-spawning mortality is even higher in deep water than on the beaches.

These eagles have a capelin feast at their feet. (Photo by Bob Armstrong)

All those dead and dying little fish on the beaches attract lots of predators and scavengers — ravens, eagles, gulls, and so forth. In the deeper waters, there are many willing consumers, from seals and whales to salmon, cod, and halibut to diving seabirds (e.g., gannets, murres). 

Capelin are a bonanza for other creatures not only in their dying, but also when they swim in huge swarms — in one case, Atlantic cod ate over 10 million capelin in one episode. When a capelin population crashes, perhaps due to over-fishing or low food availability, the populations of predators may crash too — that happened in the Atlantic some years ago, still taking years to recover.

There are a few other fish species that have the odd custom of using beaches for spawning. Surf smelt spawn in the surf zone along the Pacific coasts from Alaska to southern California. More famous are the grunions of southern California and Baja California. Grunion females come onto a beach on a high tide and dig their tails into the sand. Males wrap around the females to fertilize the eggs, which are laid under the sand, hatching in a couple of weeks.

Thanks to J. Vollenweider for helpful consultation.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.