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Predation and Competition
Some of the more common pests are mussels and barnacles. The most common and dangerous
predators are crabs, oyster drills and blister worms.
Green Mussels
Green mussels grow faster than oysters and compete with them for space, food and
oxygen. Mussels are filter feeders like oysters. Green mussels can be controlled
by air-drying if they are less than 5 cm. Larger specimens are removed by hand picking.
Barnacles
Barnacles spawn about the same time as oysters and are particularly troublesome
when they set on oyster cultch. Oyster larvae will not set on cultch covered with
barnacles. Cultch should be put out after the barnacle setting season is over. Barnacle
fouling is worse in higher salinity areas. If barnacles foul market-size oysters,
they have to be scraped off before the oysters are sold.
Crabs
Crabs are the most dangerous predators of spat. Even relatively small crabs are
able to break the fragile shells of spat to get at the flesh. Crabs are frequently
abundant in the immediate vicinity of spat trays, particularly if the trays are
at the water surface. The most effective control measure is by screening. Screens
should be fine enough to exclude even small crabs. The screen material should be
thick enough to prevent crabs from cutting through it.
Starfish
Starfish are a major predator of bottom-cultured oysters. They attack oysters of
all sizes. The starfish grasps the oyster with its five arms and pries open the
shell using its sucker ‘feet’. The stomach is extruded into the oyster
to digest its flesh. Starfish are not a problem for any type of off-bottom culture
(raft, longline etc.).
Oyster Drills
Oyster drills are snails, which drill a small hole through the oyster shell using
a rasping device (radula). The snail’s proboscis is inserted through
the hole into the oyster’s flesh and the meat digested and sucked into the
drill’s stomach. Drills are not a serious danger to oysters grown by hanging
culture methods, but they can seriously affect bottom-cultured animals.
Blister Worms
Blister worms bore through the shell. They do not prey on the oyster and a low rate
of infestation is not harmful. However, in cases of heavy infestation, the shell
becomes thin and easy to break. If the worm penetrates to the inside of the shell,
the oyster tries to cover up the hole by depositing shell, hence the term ‘blister’
worm. Growth is slowed and, in extreme cases, mortality will result. The easiest
way to control blister worms is by air-drying the oysters for 24-48 hrs. Blister
worms are normally a problem only in areas of high salinity.
Others
The alga Gracilaria sp. grows profusely on oyster cages and affects the
now of water into the oyster trays. Balanus amphitrite is a fouler
that settles on the wooden structures, trays and oysters. It competes for food with
the oysters. Large-scale mortalities of oysters due to the diseases caused by the
fungus Perkinsus marinus, and the protozoan parasite Minchinia nelsoni,
have been reported from temperate countries.
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