Juvenile salmon swimming into Puget Sound from their natal rivers depend on zooplankton. These small, hungry fish need little bites that fit in their little mouths.

When juvenile salmon get big enough to fit a herring in their mouth, it’s not always certain there will be herring around for them to eat. Herring populations, says Julie Keister, a University of Washington oceanographer, have been “a roller coaster.” A couple of years with higher-than-usual ocean temperatures seems to have boosted herring numbers, but before that there was acute worry over their decline.

Herring eat zooplankton. But at this point, it’s not known whether there’s a relationship between the health and abundance of any of the many species of zooplankton and herring populations.