Early growth and mortality of juvenile clams
Field studies and laboratory experiments probed the mystery of frequent extremely high juvenile mortality in ecologically and economically valuable clam species.
Megan Dethier, UW Friday Harbor Laboratories
Jennifer Ruesink, UW Department of Biology
Naturally seeded clams along Puget Sound often suffer extremely high mortality, which can affect the size of adult populations and harvests. Surprisingly little is known about causes of mortality and about early life stages of clams in general. This study assessed the key factors affecting the survival and growth of two common naturalized species: thick-shelled Manilla clams and thin-shelled Eastern softshell clams. Field experiments were done at eight highly diverse sites, from Willapa Bay to southern Puget Sound. Settlement, first-year mortality and growth rates were compared as salinity, temperature and acidity vary. Lab experiments helped determine how rising temperatures will impact clams and which suspected predators—worms, small snails or young crabs—already affect them.
Clams are part of a culturally and economically vital shellfish industry in Washington State that directly and indirectly employs more than 3,200 people and provides food via commercial, recreational and subsistence harvests. However, many factors threaten the survival of newly settled clams, including predation and physical conditions like temperature, salinity and pH. Understanding how these factors impact juvenile clam settlement, growth and mortality could help shellfish growers maximize the survival of their crops.
Results suggest that predation is a less important source of mortality to juvenile clams than physical stresses. While slightly warmer water may increase clam growth, there is a tipping point at which the combined conditions of low salinity and high temperature become lethal. This raises another important question about the threats to juvenile Manila clams: how vulnerable is the species to climate change?