“The Pink Effect”

All natural ecosystems are difficult to study, but oceans—which are vast, largely out of sight, and often frustratingly remote—might be some of the hardest. Salmon scientist Greg Ruggerone and other researchers have been using a kind of workaround by taking advantage of a natural pattern in pink salmon populations.

In parts of their range, pink salmon are far more abundant during odd years than during even years. This see-sawing pattern is unique, and creates a sort of experimental “control” to help scientists understand changes they’re observing in North Pacific marine species. Researchers can study conditions in years when pink salmon are especially numerous and compare them to years when their numbers are lower.

Ruggerone has been applying this technique for two decades. More than 20 years ago, he was studying scales from wild sockeye salmon, which, like tree rings, are marked with a fish’s growth rate and age. He noticed that growth patterns on these scales were different in odd and even years and that they mirrored the up-down patterns of pink salmon populations: When pinks were especially abundant, sockeye salmon didn’t grow as much, suggesting that these fish were competing for food.

In the years since, Ruggerone and others have found this pink salmon “fingerprint” in numerous marine species across ocean food webs. Ruggerone and oceanographer Sonia Batten, for example, compared 15 years of plankton data with pink salmon abundances and found that during years when pink salmon populations were up, large zooplankton—the choice food for many species—were scarce, while populations of phytoplankton, which are consumed by zooplankton, shot up. It appeared that pink salmon were essentially high-grading the ocean pasture, leaving crumbs for other species.

Ruggerone and other researchers have seen similar patterns in other marine creatures. Some seabirds produce fewer chicks in years with abundant pink salmon. And the “pink effect” seems to be at play in mortality of endangered killer whales off British Columbia and Washington State as well.

These findings (and many others showing similar patterns) led Ruggerone to conclude that pink salmon can cause food web-scale effects, triggering what amounts to a “trophic cascade,” whereby these fish, as predators, have domino effects across different parts of the ocean food system.

While directly observing competition for food among salmon is hard to do, Ruggerone and others believe that the “pink effect” is having far-reaching consequences for other salmon, other types of marine creatures, and for ocean food webs.

--Miranda Weiss