Effect of Egg Sampling Efficiency on Estimates of Historic Striped Bass Egg Production in the Savannah River Estuary, Georgia-South Carolina

The striped bass (Morone saxatilis) population of the Savannah River estuary declined in the 1980s, likely because of the operation of a tide gate that increased salinity on spawning grounds and accelerated seaward transport of eggs and larvae. Following discovery of this negative effect, periodic egg sampling monitored striped bass reproductive effort and documented a 96% decline in egg density (n/100m3) from pre-tide gate levels. The decline in egg density was concomitant with a similar decline in the adult striped bass population. An intensive stocking program eventually restored the adult population, but reproductive output remained low through the 1990s. Previous estimates of egg density allowed only relative comparisons between areas and/or years. Estimates of actual egg production for the system have not been attempted but would be helpful in understanding reproductive levels needed to set recovery goals (i.e., to pre-tide gate levels). Recent estimates of sampling efficiency now make back‑calculation of egg production possible for this system. We used these estimators to back-calculate a minimum level of egg production at two historic spawning areas to 1978. Estimated minimum egg abundance the year before tide gate operation (1978) was 220 million eggs. After tide gate installation and operation, estimated annual egg abundance was variable, peaking in 1986 (486 million) but declined to as low as 4.5 million by 1998 (1990-1998 average: 33.4 ± 22.0 million SD). In 1999 and 2000, however, the minimum estimated egg production was over 60 million each year (60.3 and 63.1 million eggs, respectively). With continued recovery of the adult population, egg abundance should continue to rise and may eventually return to levels estimated to exist prior to the decline.

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