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New Study Sheds Light on Seals as Magnets for Marine Algae

Two images of harbor seals with thick mats of algae on their backs.
Researchers studying pinnipeds (seals and sea lions) have long noted marine algae sometimes growing on their fur. But few had studied the phenomenon. Now, a new study led by Dr. Floyd Hayes found that at least eight genera of photosynthetic algae attach to six different pinniped species.

Floyd Hayes / Pacific Union College

March 2022 - Researchers studying pinnipeds (seals and sea lions) have long noted marine algae sometimes growing on their fur. But few had studied the phenomenon. Dr. Floyd Hayes, struck by a few seals in Bodega Bay with large mats of algae on their backs, decided to investigate. He and researchers from the San Francisco Bay Area Network (SFAN) and Point Reyes National Seashore recently published their findings in the journal Pacific Science. Their paper includes (1) a literature review, (2) summaries of central California harbor seal data collected from a radio telemetry study by S Allen and during field surveys by the the SFAN Pinniped Monitoring Program, and (3) additional anecdotal harbor seal observations.

The researchers concluded that at least eight genera of photosynthetic algae attach to six different pinniped species. Sea lettuce was the most common of the green and brown algal species observed on seals' backs, typically from the backs of their heads to their tails. The pinniped species affected include northern fur seals, gray seals, monk seals, and most commonly, harbor seals. The algae clearly benefit from hitching a ride on pinnipeds—it exposes them to nutrients and helps them avoid predators. For pinnipeds, though, the algae are likely unhelpful. Mats of growth on their fur make the seals less streamlined as they move through water.

Nonetheless, field observers noted algal growth on seals every month of the year except August, with fewer algae-covered seals seen from June-October. Those months coincide with and immediately follow the seals' annual molt, when they shed and regrow their fur.

An analysis of this study’s field data indicates that algal growth occurs primarily on adult females, and even more so on those in estuaries. Females tend to spend more time in the water while pregnant and attending small pups, which might facilitate algal growth. Overall, the proportion of seals with algal growth was small. Less than 0.13% of seals throughout central California were noted as having algae on their fur during 2017 breeding season field surveys. However, the frequency was significantly greater at estuarine versus coastal harbor seal sites. During surveys of radio-tagged seals in Drakes Estero in the 1980s, 40% of tagged seals had algal growth. It's possible that the tags may have provided added substrate since encrusting on tags was especially noted on several of the seals.

This preliminary study provides a basis for future studies to explore additional dimensions of this curious phenomenon. For example, do seal foraging locations within estuaries influence the presence of algae? And what organisms make their homes on the algae itself?

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Golden Gate National Recreation Area, Point Reyes National Seashore

Last updated: April 28, 2022