Oysters are different from other types of seafood such as shrimp and salmon, which require protein-based feed sources usually derived from wild-caught fish. Oysters are vegetarians and eat only plants (algae!). This means raising and feeding Barren Island Oysters doesn’t add to the overfishing of wild fish or shellfish and also uses less energy and resources than other types of seafood.
Trophic level is generally used as a way to measure how high up the food chain an animal sits; the higher the trophic number, the higher up the food chain. For example, tuna has a trophic level of about 5, meaning there are four other steps in between plankton production and tuna production. Oysters have a very low trophic number (~2), meaning they are an incredibly efficient food source! For every trophic level "up" you move up the food chain, the efficiency in transfering the embodied energy in the food is about 10%. By the time you get up to a level 5 for tuna, massive amounts of bay or ocean production are involved when you work your way backwards through the food chain - predatory fish eating medium fish eating smaller fish which eat zooplankton which finally feeds on phytoplankton. Phew!
The current problem we're facing is that worldwide, our oceans are experiencing an overall decrease in trophic levels of fish due to overfishing of higher value, predatory fish. Reducing the diversity and complexity of our marine systems puts many species at greater risk of extinction.
What does this ultimately mean? Eating an equal amount of oysters as fish higher up the food chain actually involves much less resources and is a much more efficient food source for the same amount of protein. The more we eat efficient food sources, the more sustainable we are!