If herring is a keystone species then what does that make the plankton that herring feed on. I say it's more important because without it there is no herring.
If anything a variety of forage species are keystone food web critters like sandlance, oolichan, opal squid, dungeness crab, herring, juvenile rockfish...ect. I fished my whole life for Salmon and herring are far from the only food found in their guts. They will eat what is available. In some instances they eat pellets their whole life. I even seen a picture of a fillet knife one had swallowed!