Recruitment

In addition to monitoring adult populations of oysters, biologists study oyster spat settlement on deployed spat arrays.  When oyster larvae settles from the water column and has attached to a permanent fixture it can be called a spat, a juvenile oyster.  Deciding when an oyster stops being a spat and is now an adult is tricky, since some, especially precocious males, will develop gonads within a very short time period at sizes as small as an inch.

To estimate oyster settlement, three spat traps are placed on monitored oyster reefs. Each spat traps consist of a PVC pipe that is constructed in the shape of a T and drilled to fit cut galvanized wire strung with oyster shells. Oyster shell has been observed as a favored substrate for oyster spat.

Once a month, researchers retrieve traps and deploy new traps. All samples are taken back to the lab and, if any are present, spat are counted on all shells, providing a comparison of oyster recruitment rate among sites. The number of oyster larvae settling on our traps is an indication of the health of the estuary being monitored and perhaps an indication of how many adults are present.

Oyster spat survivorship is dependent on many factors, some being salinity and temperature influences, competition between other spat/barnacles/other sessile organisms, presence of biofilm, algae, and sedimentation. As oysters settle onto other oyster shell and other hard substrates, they will often have to outcompete other organisms for space. The presence of biofilm on substrates will also hinder the ability for oyster larvae to settle. In areas that are particularly muddy, sedimentation can increase mortality of oyster spat by smothering them. Pulses of freshwater can be helpful to maintaining oyster health, however, when the pulses become large and are sustained it can become detrimental to oysters and spat.