Natural History
Oysters use changes in the environment to know when it’s time to start the spawning process. An increase in water temperatures along with a shift in salinity levels let the oyster know that spring...
Category: Natural History of Oysters
Oysters use changes in the environment to know when it’s time to start the spawning process. An increase in water temperatures along with a shift in salinity levels let the oyster know that spring is around the corner, and that it’s time to begin spawning. The process begins by creating a gonad, which can take about two months to fully develop before being released into the water. Once one oyster releases its gonad, others in the area will follow. When an oyster detects sperm in the water, they know it’s time to release their own as well. When spawning, an adult female oyster can release around five to eight million eggs at once. When these eggs get fertilized they will float around in the water column for the next two weeks, developing into a larvae and looking for food. After about two weeks they develop an appendage, that they use to maneuver around looking for a suitable hard surface to attach themselves to. After about a year they will be considered a juvenile, and after 3 they are considered an adult.
The Pacific oyster, equipped with a strong shell, sharp edges, and a firm grip on the substrate faces a wide variety of predators, pests, and diseases. Oyster larvae are the most vulnerable to predation; targeted by crab, shrimp, sea stars, flatworms, and filter feeders. Rock crabs are a common, but not serious threat. They mostly will target free floating larvae and young oysters, crushing the shell to expose the meat. Shrimp also pose a threat to oysters, although not always a physical threat. Mud-burrowing shrimp dig beds and stir up sediment to feed, the softening of the bed that occurs causes oysters to be buried and smothered in the loosened material. A more serious threat to oysters both young and old, are sea stars and their relatives. Using their suctioned appendages, these predators are able to exert over ten pounds of pressure to pry open an oyster’s shell and begin digesting it. The greatest threat to the adult oysters is that presented by the drill snail. Initially introduced by mistake with the first plantings of Japanese Pacific oysters, the drill snail thrives in oyster beds, using its rough tongue and acids to “drill” through the shell it can kill or damage the body of the oyster.
Outside of predation, the Pacific oyster is also exposed to a host of other threats in the water. Parasites can enter through the oysters filtering system and damage the internal tissue and organs. Microorganisms like boring sponges and sea worms weaken the outside of the shell of the oyster, increasing vulnerability to predators. Bacterium, planktonic organisms, and other disease-causing organisms can also enter an oyster through the water. Red tide carries multiple dinoflagellates that can be extremely toxic to both the oyster and to those who consume it. Poor circulation in the water increases the likelihood of the presence of infection agents, but often times the oyster is able to recover and clean itself of the toxins over time.
The oyster has one predator to fear more than all the others combined – humans. Overharvesting, pollution, climate change and many other human caused occurrences are affecting oyster populations drastically. The Olympia oyster (Ostrea Lurida) could at one time be found growing up and down the coast of North America, filtering the water and keeping the ecosystems in which it lived balanced. Due to overharvesting and destruction of their natural habitats, these oysters now exist at 5% of their historical population. Olympias are typically grown for their meat, but in Washington state they are currently being raised for restoration projects on a larger scale than for consumption. Pacifics hold the majority in terms of world-scale oyster farming, as well as in Washington. They are resilient and grow faster than Olympias or the coveted Kumamoto. Kumamotos are a common “beginners oyster” due to their sweet flavor, small size and dense texture. Kumos are also cultivated in Washington, but less than Pacifics because they are more picky in terms of their habitat. Generally an optimal habitat for oyster growth will have cold water temperatures, with salinity levels that aren’t too high or low. Oysters also need a hard surface to attach themselves to, usually old oyster shells.
Aquaculture grown oysters are grown in a recreation a wild oyster’s ideal growing environment. Unlike other aquaculture farms, no feed needs to be added to the water in oyster farms as they will filter their nutrients from the water around them. Filtering around 50-60 gallons of water per day, oyster aquaculture can actually increase water quality over time. Water quality is the most important factor in oyster cultivation and without good, clean, nutrient-rich water it is hard to produce a marketable oyster.
Most oyster farmers buy their seed from a producer. Seed production typically occurs in a hatchery tank that hosts ideal nutrients, water temperature, and other growing conditions for an oyster larvae. Once larvae are more suited to be introduced to an exposed aquaculture environment, producers will ship out the larvae to farmers. “Sexless” oysters, or triploid oysters, can be produced in hatcheries by the way of chemical exposure and atmospheric pressure. These are often more desirable for farmers as they neither develop sexual organs or have a spawning season, all their energy is spent on feeding and growth.
Farmers having recently purchased seed will often “plant” the oysters in floating nurseries, or systems that allows the young oyster to mature before being introduced to the bottom. Bottom culture is a practice in which farmers introduce oysters who have matured in hatcheries and nurseries to the bottom where they continue to grow to market size in natural conditions but have a greater chance for survival than a wild oyster would. While better suited for survival than a wild oyster, they are still vulnerable to predation and, in the case of loose bed sediment, burial and suffocation. Another problem rises in the harvesting of bottom culture. Harvesting by hand at low tide is a labor intensive technique. Long tongs are used from boats to scoop up oysters, but this is also hard physical work with a low yield. The dominant method for harvesting involves a boat dragging a basket along the bottom to efficiently collect the oysters. This method, known as dredging, will destroy the natural environment in which wild oysters grow. It is most effective on flat bottomland in cultivated areas.
Bottom culture presents dangers and complications like predation and overcrowding of the bed. These complications in cultivation can be tackled by off-bottom culture. In off-bottom culture, cages with multiple levels of trays hold bags of oysters one to two feet off the seafloor in a nutrient-rich area of the water column. Another technique involves driving oyster-seeded stakes into the ground, holding the oysters off the seafloor. This technique can be applied in areas where the bottomland may not be suitable for bed cultivation. Off of the bottom the oysters grow faster, compete less for food and space, are protected from predators, and do not face the risk of being buried during the tide change. Off-bottom culture also provides easier, and less destructive methods of extracting the oysters, little to no damage is done to the habitat by the recollection of the materials during the harvest. Another increasingly popular method combines both bottom and off-bottom culture: growing oysters in bags and then transferring them to the bottom. This ensures that the oyster is mature enough to be protected from predators and also allows the oyster to develop outside of the bag and thicken its shell in response to the natural conditions of the environment.
Japanese aquaculture techniques have made a great impact on the oyster industry with new, creative approaches to dealing with common problems and difficulties. Longline culture is an off-bottom culture method created by the Japanese. Ropes are seeded with oyster spat and secured about a foot off the bottom. Oysters grow on the rope with exposure to the natural conditions of the water, but are also removed from the bottom and the complications it may present. When it comes time to harvest, farmers simply detach the ropes, haul them in, and harvest the oysters from the rope. Another aquaculture method originating from Japan is suspended culture. In this technique, mesh “lantern nets” full of young oysters are suspended in temperate water channels full of nutrients. When it comes time to harvest, the floating nets are hauled out of the water onto a boat. This method is applicable even in areas where bottom culture is not viable and opens up new possibilities for where oyster cultivation can occur.
