NOAA/Saltonstall-Kennedy Grant Program: Manila Clam Mortality Investigations

Purpose or Scope:

One of the most serious constraints to industry growth on the west coast of the United States, according to clam producers, is the occurrence of sporadic mortality and poor growth, due to unknown causes, and mainly occurring in fall and winter months. Freezing damage is believed to be the cause of manila clam mortalities in British Columbia.
No highly pathogenic infectious diseases of Manila clams are known to occur on the west coast of North America and there had been limited evaluation of the problem, particularly in the U.S. The Manila clam is also widely cultured in Europe where it is extremely desirable, but production has been limited by a bacterial infection known as brown ring disease (BRD). While BRD has not been reported from the U.S. west coast, it is important to exclude BRD and other pathogens, which could become established.

Approach:

The overall goal of the project was to initiate the establishment of production standards and a health baseline for intensive clam production on the west coast of the United States. The following tasks were completed in order to implement these objectives and were fulfilled, with small modifications, necessitated by results obtained during the study and additional input from shellfish producers during the study.

  1. Cooperative industry clam survival and performance study.
  2. Survey of clam health and disease in Washington, Oregon and California.
  3. Evaluation of short-term freezing and freshwater exposure effects on clams.
  4. Clam mortality response team.

The project staff, along with associated scientists from the University of Washington responded to several clam mortality events during the study.

Progress and Results:

The field survey documented wide variations in the survival and growth of clams at the experimental sites. Growth was variable between different clam groups but yearling clams more than doubled in average weight. Air temperatures, and water column and sediment pore water temperatures were nominal during the winter 1999 and 2000 seasons and were well within the normal ambient range of Manila clams.

For the west coast evaluation of health and condition of manila clams; 1,184 adult and seed clams were examined from sites in Washington, Oregon and California. No diseases considered reportable by the Organization Internationale Epizooties (O.I.E.) were found, and there were no significant infectious diseases of manila clams identified during the course of this project.

Extensive evaluation of the tolerance of more than 15 populations of clams to low salinity demonstrated that all clams tested can tolerate a salinity of 15 psu, that none can tolerate 10 psu in the long term and that 12.5 psu is a marginal salinity. Certain clam populations have a high proportion of individuals that can withstand 12.5 psu while others have few or no individuals that show long-term tolerance to this salinity. Importantly, certain populations of clams have a survival response that allows them to tolerate lethal low salinities for as long as four weeks.

Evaluation of three case samples of opportunity of report clam mortalities was conducted during the study. These studies found hydrogen sulfide bacteria associated with those clams located very deep in the beach and which may have had a reduced clam shelf life. In another case, impacted material in the stomachs and a pathological reaction of the clam digestive organs was a significant finding.

Further needed work identified during this project includes the following: (1) a brood stock selection program, (2) more detailed evaluation of the effects of water temperature on low salinity tolerance of manila clams and (3) continued evaluation of clam mortality events.

Currently, one peer-reviewed publication has resulted from this project. In addition, a number of presentations about project results were made at regional meetings.

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