Genetic improvement of survival in Pacific oysters to the Tomales Bay strain of OsHV-1 over two cycles of selection

Abstract Yearly mass mortalities of Pacific oysters in a major oyster-growing estuary on the West Coast of the United States (Tomales Bay, California) present a constraint to shellfish aquaculture. These mortalities have been associated with the presence of Ostreid herpesvirus 1 (OsHV-1; Tomales Bay strain) and increased seawater temperature in the estuary. In order to develop oyster families that are more tolerant to OsHV-1 than current oyster stocks, one cohort was planted in Tomales Bay per year over a three-year period, two of which were selected based on survival data collected from previous cohorts. Analyzing survival data from 23,590 oysters from 191 biparental families planted in Tomales Bay over three years, we estimated the heritability of survival on the observed and underlying liability scales to be 0.30 and 0.47, respectively. Survival breeding values increased 10.3 and 21.2 percentage points after one and two cycles of selection, respectively, suggesting that genetic selection was highly effective at producing oysters that were more tolerant of OsHV-1. The oyster families developed in this breeding program are the first in the United States specifically bred to be tolerant to OsHV-1 under elevated temperatures and will be valuable to oyster growers as well as researchers studying the genetic basis of OsHV-1 tolerance in Pacific oysters.