Biomass estimate of Pacific herring, Clupea harengus pallasi, in California from the 1984-85 spawning-ground surveys

Abstract

The spawning biomass of Pacific herring, Clupea harengus pallasi, in San Francisco and Tomales Bays was estimated to be 46 000 tons and 6600 tons, respectively, during the 1984-85 season. Biomass increased 15% for San Francisco Bay and 500% for Tomales Bay over those of the previous season. The dramatic increase in the Tomales Bay biomass estimate is attributed to a change in the spawning habits of herring during the 1983-84 season, in which herring did not return to Tomales Bay as expected. This season herring returned to Tomales Bay and the biomass increased, nearly equaling the long-term average biomass of 6700 tons. In San Francisco Bay, the modest increase of 6000 tons represents the first increase in three seasons. The geographical distribution of herring spawning within San Francisco Bay has changed during the past three seasons. From 1983 to 1985, over 60% of each season's spawning escapement occurred south of Treasure Island. Prior to that, from 1973 to 1982, nearly 80% of the escapement was north of Treasure Island. The monthly distribution of herring spawning has changed little over the course of this study, except from 1980 to 1982 when there was more spawning in December than at any other time. (30pp.)