Some of the local planktonic crustaceans that the murrelets are feeding on
are most likely euphausids, a shrimp-like critter of which we have a couple
of species. The gammarid amphipod (a.k.a. beach hopper) Cyphocaris is large
enough to also be prey, and it can occur in dense patches; these are a type
of beach hopper, but are known for being in open water. They're bright
orange, and can be seen in the water if sufficiently aggregated.

For a picture, go to http://dnr.metrokc.gov/wlr/waterres/marine/inverts.htm
and scroll down to the Arthropods and Cyphocaris challengeri.

For more on euphausids, check
http://www.pac.dfo-mpo.gc.ca/ops/fm/shellfish/Euphausiid/biology_euphausiid.
htm

I suspect the bonaparte's gulls feed on them as well.

Kevin Li
King County Environmental Lab
Seattle, WA
http://dnr.metrokc.gov/wlr/envlab/index.htm