Research
PNW
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1995
PNW Project Overview 1995
Using Oysters to Monitor the Condition of Willapa Bay: Developing a Standard Tool for Estuaries
Principal
Investigators:
Dr. Brett
R. Dumbauld,
Washington Department of Fish and Wildlife, Nahcotta, WA
Dr. Kenneth K. Chew, University of Washington, School of Aquatic and Fisheries
Science
Awarded: $35,000
Project
Description:
Shellfish
aquaculture on the Pacific coast is a growing component of the
resource-based coastal economy. Washington State is currently one of the
biggest producers of oysters in the USA. More than half of the Pacific
oysters, Crassostrea gigas, grown in Washington State come from
Willapa Bay. The sustainability and present status of this industry,
however, is closely tied to water quality and the productivity of this
bay. Bivalves react
negatively to environmental stresses is a variety of ways including
reduced growth, changes in chemical composition, reproductive failure,
unusual shell growth, and death.
Researchers and growers utilize a measure of the “fatness” of
bivalves known as Condition Index (CI).
CI describes the volume of the shell cavity that is filled by the
body animal. The
primary goals of this project are to:
1) evaluate factors responsible for variation in CI on both a local scale (turbidity caused by burrowing shrimp, competition with other oysters, tide height) and along an estuarine gradient (salinity, nutrients, phytoplankton production and species present, sediment load);
2) evaluate the sampling procedure currently used for measuring the condition of Willapa Bay oysters, compare it with others in use, and propose changes if necessary, realizing the value of consistency and the existing long-term historical record, but also cost and statistical validity; and
3) use the results to develop a standard protocol for collecting and analyzing similar data from other resources.
Project Status:
Final
report and Master thesis by Ervin Schumacker were completed in 1999.
Preliminary findings were presented at
the following meetings and conferences:
- The joint Pacific Coast Oyster Growers Association (PCOGA), National Shellfisheries Association (NSA) meeting in Lake Chelan, Washington in September 1996,
- The University of Washington, School of Fisheries, Graduate Student Symposium in October 1996,
- The Second Annual Willapa Bay Science Conference in Long Beach Washington in April 1997,
- The joint PCOGA/NSA meeting in Newport, Oregon in September 1997.
Thesis
abstract:
This study
determined changes in CI that occurred at two experimental sites through
an eighteen-month period. Analysis of site differences for transplanted oyster
populations, if they existed, were examined as well as possible for
potential differences due to genetic lineage (natural and hatchery spawned
populations). This analysis
in combination with the work done relating condition index to the
environment served to illustrate whether oyster CI is a valid indicator of
changes in the surrounding aquatic environment.
Oyster CI responses to environmental conditions differed between the two
sites used in this study. Decreasing
oyster condition at one site was not due to lower food levels, but rather
the result of algal fouling and the corresponding decrease in food
availability cause by large mats of Enteromorpha.
Algal fouling was likely related to the warm water conditions in
the spring and summer of 1997. The consistently higher condition of oysters at the second
site may have been due in part to the near vicinity of the main channel of
the Willapa River, which feeds water and nutrients to the oysters.
Debate over the many methods to determine CI in bivalves has occurred for
years. The methods evaluated
in this research include the Westley methods, the dry shell method, and
the gravimetric method. The
gravimetric oyster CI method is able to distinguish between population and
site differences for Pacific oysters.
This method requires less time and effort when performed on
individual oyster samples in contrast to the Westley volumetric method. This study has conclusively shown that the gravimetric method
relates to the long-used Westley volumetric method with sufficient
accuracy to replace it for determination of oyster condition index of
Pacific oysters in Willapa Bay and elsewhere with the following equation:
Westley = 1.180(Gravimetric) + 0.905
This
is supported by regression analysis, with a clear r2
correlation of 0.9126 for the 504 oysters sampled over a 13-month period.
Single oyster samples rather than pooled samples should be taken to
determine condition for populations due to natural variability within
populations. Regular
inspection of natural population variability should be used to determine
appropriate sample sizes. The
results from this study suggest that twenty oyster samples are currently
most reasonable for Willapa Bay.
This
project recommends using the gravimetric method as the standard for
determining oyster CI of Pacific oysters in Washington state and elsewhere
based on its ease of use, time savings, accuracy and ability to
distinguish population and site differences for oysters.
Standardization of the condition index methodologies is critical
for comparison of studies, and should be accomplished as soon as possible
to end the proliferation of methods in scientific studies that utilize
oyster condition as an indicator of health or commercial yield.