A tool for estimating sustainable oyster biomass and shell resources

Biological Reference Points (BRPs) provide guidance for management actions. There are no extant and agreed upon BRP’s for oyster management in Virginia. Harvest is not generally managed by quota developed from stock assessment. Harvest is managed by a mixture of seasonal and area closure, daily time and gear restrictions (effort control), and per day bushel limits.  The development of BRP’s is a useful exercise in consideration of future management actions; however, a consideration of recent historical context is also important. 

BMSY, the biomass that supports MSY, maximum sustainable yield, is typically estimated at one half of carrying capacity. At carrying capacity yield is effectively zero. Unfortunately we have no estimates of current potential carrying capacity of either the Chesapeake Bay as a whole or specific subestuaries/regions for oysters. We require sustainable BRPs for both live oysters and shell, and that natural mortality must be sustained to maintain the shell (habitat). Over longer time frames (hundreds to thousands of years) shell production must be adequate to maintain accretion and reef topography in the face of sea level rise.

What are reasonable BRPs for oyster management?  Any management plan must accommodate a disease-driven, truncated life expectancy and inter-annual variability in recruitment and survival. While modest single year losses in habitat may be tolerated prolonged shell loss cannot be. The options are either to set very conservative BRPs in order to preserve shell at the risk of lost production in a harvest scenario, accept loss of shell in the setting of BRPs with an accompanying commitment to shell repletion (assuming adequate quantities of shell and funds are available to make up the deficit), or develop an interactive management tool that adaptively manages in real or near real time as survey and fishery dependent data become available. To this end we have developed a simple virtual population estimator in the form of an EXCEL program that allows input and manipulation of recruitment and age specific values of M. A copy can be downloaded from this website. The output is a population demographic in numbers and biomass, together with serial contribution of shell to the accreting reef structure, offset by shell loss to degradation. Rate functions of growth, biomass estimation from length, shell production and more are taken from long-term James River assessment data, and shell loss to taphonomy is set at a single 30% per year. The tool is set as a 30-year progression with years 1 through 20 having user determined recruitment and years 21-30 as a “decay” period when no recruitment occurs. Shell accretion is one of the outputs. We require a no net loss of shell habitat as a BRP for shell. The relevant output in the EXCEL tool is accretion rate – is it positive or negative? If the accretion rate is zero then the accompanying value of mortality, M, sets the no fishing BRP (in the case of an exploited resource) or a minimal equilibrium for a restoration project, in that M is adequate to supply shell to offset taphonomic loss. If M is below this value then accretion occurs in an undisturbed population. Fishing is permissible until (F+M) reaches the BRP equivalent to zero accretion rate. There is, however, a long term modifier to this scenario that requires attention in that a no net loss of shell or zero (not negative) accretion rate does not address the requirement to match sea level rise (if for example the desire is to maintain the reef relative to that moving standard) and/or sedimentation rates (reviewed in detail by Donoghue 1990, from 1 mm/y to as high as 10 mm/y in the upper Chesapeake Bay). Finally, and not included in the EXCEL tool, is the requirement to consider the sediment: shell ratio in accreting reef structure.  This is variable, but proportionately increases accretion. Smith et al. (2003), citing Cuthbertson (1988) as the original source, describe the sediment – shell mix as a “matrix of < 75% loose shell”.  DeAlteris, (1988) comments that “The void space in an oyster shell reef is approximately 50% depending on the shell size. This space may be filled with fecal deposits that contribute to reef growth.”  Thus if the long term accretion rate to match sea level rise is 4.55 mm/y (taken from Mann et al. (2009) using a relative sea level rate of 3.5 mm/y corrected for 30% taphonomic loss), but the sediment: shell ratio in content of the reef substrate is 50:50, then the shell accretion rate at equilibrium with sea level rise is 2.275mm/y. Assigning appropriate values to these current “estimates” is a future research need.

The suite of graphs allows the user to observe the interactions of the inputs on the time course of data resulting from the varying imputs. Go ahead, change a few inputs - and watch.

Link to download EXCEL tool (.xlsx)

Link to download EXCEL tool (.xls)

If you have trouble downloading either file, please contact kreay@vims.edu

Link to Section 7 - Research Needs