Compartmentalized Cages
Using compartmentalized cages also adds to the
discriminating power of the methodology. Most investigators simply dump a given
number of test animals into a bag or rigid mesh cage and make statistical comparisons of
the mean weights and lengths at the beginning and end of the test. The individual
compartments serve two functions: (1) The variance is reduced since the variance
associated with multiple measurements on the same individual is less than the variance
associated with the means of all animals at the beginning and end of the test; and (2) An
even separation of test animals assures a more even exposure to chemicals and an ability
to grow unimpeded by the proximity of an individual to its nearest neighbor. This
makes this field bioassay more like a traditional laboratory bioassay and affords more
experimental control. Combined with strategic placement along suspected chemical
gradients this approach combines the experimental control of the laboratory with the
environmental realism of the field.
|
|
|
Mussels in the 10-12 mm size range at the
beginning of a test. |
Growth of the same 18 individuals at the end of an 84-day
exposure period at a reference site. |
|
|
|
|
After
several transplant studies and using larger plastic cutlery trays with up to 44
compartments per cage, it became clear that a different cage design was
necessary; i.e., a compartmentalized and disposable mesh bag.
This would facilitate using larger bivalves and more test animals. One of the first uses of a compartmentalized mesh bag was at a controlled release
of oil in Delaware Bay. Bagged bivalves were removed at intervals to quantify the rate of accumulation and depuration of
oil. |
The cage design for a mussel deployment at 70 meters adjacent to
the diffuser of the Port Valdez Ballast Water Treatment Facility. In addition to the
compartmentalized cages, "predator mesh" was added to the outside of the
"cage" to prevent sea otter predation. This mesh has also been
effective at excluding starfish and crabs. Additionally, the
protective envelope reduces the possibility of damaging test animals
during handling.
|
