Abstract:
The migrations, distribution, and feeding of American shad, Alosa
sapidissima (Wilson), were studied from February 1980 through January
1981 in the Columbia River Estuary, an important rearing zone for
young-of-the-year shad. Sampling was performed with purse seine,
beach seine, and otter trawl.
Adult shad and large numbers of juveniles from the 1979 spawning
(age group I) entered the estuary from the ocean in May 1980. Adults
continued on their upstream migration while age group I juveniles
appeared to congregate in the estuary, joining those (age group I) that
had over-wintered there. Many young-of-the-year shad (age group 0)
reached the estuary on their seaward migration in September 1980, during
which time most of the age group I juveniles migrated back to the ocean.
By January 1981, large numbers of young-of-the-year shad migrated to
the ocean, although a few remained in the estuary.
The stomachs of 26 adult shad and 503 juveniles were analyzed.
Adults did not appear to feed during their upstream migration, but
juvenile shad fed extensively. Diet varied with season, gear type, and
salinity zone. Calanoid copepods were important throughout the year,
but Corophium salmonis and Neomysis mercedis were important during
fall, winter, and spring. Freshwater prey were consumed during summer
and fall. The most important of these were Daphnia spp., Chironomidae
larvae and pupae, and Trichoptera adults for shad feeding in shallow
water near beaches.
