Scared strong: Enhancing oyster resilience for aquaculture and restoration by inducing oysters to grow stronger shells

Objectives

To develop a new hatchery technique that produces oysters with thicker shells that increase their survival and marketability for aquaculture and reef restoration. This research will:

1. Test whether cue exposure increases oyster survival when oysters are placed on and off bottom.
2. Compare responses of diploid oysters and triploid oysters when exposed to two different common oyster predators (blue crabs and oyster drills).
3. Assess the effectiveness of predator cue treatments across a range of environmental conditions (i.e. determine the extent changes in oyster survival and long-term growth/reproduction vary).
4. Evaluate effects of predator exposure on growth rates (i.e. time to market) and marketability (shell aesthetics) of diploid and triploid oysters.

Methodology

We will conduct a two-part experiment to induce oysters to grow thicker shells and test their survival under both on and off bottom culture conditions. Diploid and triploid oyster larvae will be obtained from the Auburn University Shellfish Laboratory and will raised as single seed as well as spat on shell following standard hatchery protocols. The Shellfish Laboratory contains a 3,000-square-foot hatchery facility that can produce up to a billion larvae annually and regularly supplies oyster farms along the Gulf coast with juvenile oysters. Within the oyster hatchery, newly settled oysters will be exposed to chemical exudates from blue crabs or oyster drills, predators that readily consume oysters, or to controls without predators. Blue crabs and oyster drills will be fed oyster tissue three times per week so that newly settled oysters receive chemical cues from actively foraging predators. After each exposure period in the hatchery, induced and noninduced oysters will be raised both on and off bottom at one of three off-bottom oyster farms and or placed on one of two natural reefs in Alabama and Mississippi. The locations of these sites encompass a gradient of salinities and distances from the main shoreline as well as other site-specific differences (e.g. predator threats) and will therefore allow us to assess the general applicability of this new technique for a variety of conditions. Prior to placement in the field and after 1, 2, 3, 6 and 12 months, oyster survival, growth and shell characteristics (e.g., strength, size, shape) will be quantified. Upon reaching adulthood, oysters will be dissected to determine their physiological state, shell and tissue growth, and the reproductive investment of diploid oysters measured.

Rationale

Commercial oyster harvesting is a fundamental component of the economy and culture of Gulf coast states. However, both Mississippi and Alabama have experienced astonishing deteriorations in the fishery. To restore oyster populations and boost production levels, states have implemented two strategies. The first strategy uses remote setting where hatchery reared oyster larvae are settled on a hard substrate (e.g. shell, crushed concrete) and then placed on bottom in a private lease or onto a natural reef. The second strategy seeks to increase oyster aquaculture production, which also typically involves farmers obtaining hatchery reared larvae that are placed in the field to grow to adulthood usually in off bottom farms. However, both strategies are plagued by mortality from predation, particularly when the oysters are juveniles. On-bottom oyster culturing techniques are especially susceptible to high predation rates whereas off-bottom culturing can become compromised when predators recruit into cages and are not removed. In fact, a recent survey from Alabama oyster farms listed high mortality, potentially from oyster drills that recruit into grow-out bags, as one of the greatest challenges that the industry faces.

We propose to increase aquaculture yields and improve the return of investment from remote setting restoration by stimulating oysters to grow thicker shells, thereby reducing their predation risk while simultaneously increasing their marketability for farmers as thicker shelled oysters are easier to transport and shuck. Previously we found that early exposure to predator cues can cause oysters to produce thicker, stronger shells and that this change lowers predation susceptibility in the laboratory. We hypothesize that exposure to these predator cues within the hatchery will induce oyster spat to grow thicker shells faster and will produce individuals that have higher survival during transport and maturation in the field.