Broodstock dynamics, fecundity, egg viability and recruitment processes in trap type estuaries

Piankatank River Oyster Broodstock studies: 2010-2011. Supported by NCBO-NOAA, in collaboration with Dr. Ryan Carnegie (VIMS), VMRC and Oyster Seed Holding LLC, Gwynns Island VA

The Piankatank River is a small sub-estuary of the Chesapeake Bay on its western shore in Virginia waters. It has been managed for several decades as a seed production river by VMRC and has not been opened for commercial fishing of market oysters. The river has a long history of seed production and fits the description, proffered by the late Jay D. Andrews, of an estuary with “trap-type” circulation that retains oyster larvae during their pelagic phase. The status of the oyster population in the river together with a comprehensive discussion of age structure, age specific mortality and substrate (shell budgets) is given in Harding et al. (2010). That text also proffers a management option based on rotational harvest that would both provide seed oysters and stabilize the shell substrate that is required to enable recruitment of the larval forms to the benthos. A notable feature of the river over the 10-year period described by Harding et al. (2010) is the high recruit to spawner ratio (R:S, commonly termed replacement ratio), approximating a value of 4.0. It is this long term running average that encouraged the use of this river in early restoration efforts (the first reef was built in the Piankatank in 1993), and effective rebuilding of the entire footprint of former reefs within the river system (reefs identified in the original Virginia Oyster Restoration Atlas).

The objectives of the current study are to:

  1. Develop a quantitative description of the population demographics in the Piankatank River as a broodstock.
  2. Develop a quantitative description of spawning periodicity, size versus fecundity, and egg viability from broodstock as defined in 1 above.
  3. Relate the findings of 1 and 2 above to disease status and observed recruitment.
  4. Relate 1 through 3 above to observed seasonal changes in environmental parameters.

Objective 1 employs methods used is stock assessment and are outlined in Harding et al. (2010). Spawning periodicity and disease status are assessed through histological examination of regular collections throughout the spawning period. Disease work is in collaboration with Dr. Ryan Carnegie at VIMS. Size versus fecundity and egg viability studies are in collaboration with Mike Congrove at Oyster Seed Holding LLC, Gwynns Island VA where experimental manipulation of  broodstock from the river are effected on a time series program throughout the summer months. Observed recruitment data is collected as part of the annual fall stock assessment and the seasonal shellstring recruitment studies (see methods in the annual reports)  The continuous environmental monitoring in the Piankatank River is described at the VECOS web site.