For gardeners, rabbits are a common cause of headaches, as they munch on a laundry list of vegetation, from berries and vegetables to perennials and woody plants. Aquaculturists like oyster farmers have the same problem, except not from fuzzy mammals. Marine rays are the main culprit, especially given that more than 80 percent of marine aquaculture consists of some of the rays’ favorite things to “crunch” on: bivalve mollusks.
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When culturing hard clams (Mercenaria mercenaria), the bivalves must be placed at the bottom of a marine environment where they then grow to a sellable size. Clammers use mesh netting, plastic, or wire covers to protect their clam lease, similar to using a wire fence to try to keep rabbits out of a vegetable garden. However, the effectiveness of using these methods for highly mobile marine predators like rays hadn’t fully been tested until very recently.
In a study published March 7 in the journal Aquaculture Environment Interactions, a team from Florida Atlantic University’s (FAU) Harbor Branch Oceanographic Institute and the Mote Marine Laboratory studied how the whitespotted eagle ray (Aetobatus narinari) interacted with clams enclosed in anti-predator materials. These rays are a formidable opponent with strong jaws, crushing fused teeth, and nimble pectoral fins.
In a large outdoor tank, the team used aerial and underwater videos to assess the rays’ responses to various anti-predator materials. One plot of clams were placed inside polyester mesh bags that also had a latex net coating, another under a high density polyethylene (HDPE) netting, and a third under chicken wire cover netting. The control plot of clams were unprotected.
After the completion of each trial, the team noted the number of crunched clams and how frequently the rays visited the various randomized patches. While the undersea hunters were capable of consuming clams through bags, the anti-predator treatments reduced clam mortality four- to tenfold compared to control plots where the clams were unprotected. The double-layered treatments (bags with cover netting) had the lowest clam mortality.
“Based on our findings, many of the current anti-predator grow-out strategies used in the hard clam shellfish aquaculture industry appear capable of reducing predation by large predators like whitespotted eagle rays,” said study co-author Matt Ajemian, director of the Fisheries Ecology and Conservation Lab at FAU, in a statement. “In particular, bag treatments with cover nettings achieved the highest clam survival rates, although it is important to note that this did not appear to completely deter rays from interacting with the gear.”
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The observations suggest that the rays appear to be capable of interacting with the aquaculture gear for longer periods of time, which potentially diverts them from other natural feeding habitats such as sand and mud flats.
“These habitat associations could expose these sensitive animals to other risks, although we are just beginning to understand them and admittedly have a lot more to learn,” said co-author Brianna Cahill, a research technician at Stony Brook University, in a statement. “Contrary to what we expected, rays did not prefer control plots (mimicking natural conditions) over treatment plots with anti-predator gear. This suggests a real possibility that these rays are interacting with shellfish aquaculture gear in the wild, as suggested by our clamming industry partners.”
The researchers also observed the rays interact with the treatments on the deterrents, including using their lower dental plate to dig through the sediment at the bottom of the tank to access the clams in the unprotected control plots and to move the gear.
More testing could reveal whether chicken wire, a common deterrent in Florida, is actually beneficial. Earlier studies suggest that the electric field of the metal could be detected by rays and sharks and might overstimulate them, protecting the farmed shellfish.
“Given the frequency of interactions we observed with chicken wire in our experiment, we question whether chicken wire is a deterrent, an attractant, or neutral, as it may not have a powerful enough signal to influence the rays,” said Ajemian. “Still, we have more questions than we started with, and look forward to investigating this further with other species and deterrent types.”
