Breaking bags and crunching clams: assessing whitespotted eagle ray interactions with hard clam aquaculture gear

Abstract

Shallow coastal waters are commonly used in shellfish aquaculture for ‘grow-out’ of bivalves like the hard clam Mercenaria mercenaria. These locations have substantially higher clam densities than the surrounding environment and attract molluscivores, requiring clammers to incorporate anti-predator materials into their grow-out gear to protect their product. However, the effectiveness of these materials against larger predators like rays remains untested. Inspired by clammer reports of predator-inflicted damage to grow-out gear, we assessed the capacity of the whitespotted eagle ray Aetobatus narinari to interact with clams housed within a suite of industry standard anti-predator materials. Mesocosm experiments were conducted where rays were exposed to unprotected clams (control), clams inside polyester mesh clam bags (dipped in a latex net coating and non-dipped), and under high density polyethylene (HDPE) or chicken wire cover netting. Gear interactions were quantified from video footage throughout the course of the experiment (5 h), and clam mortality was assessed after the completion of each trial. While rays were capable of consuming clams through bags, anti-predator treatments reduced clam mortality 4-to 10-fold compared to control plots. Double-layered (i.e. bags with cover netting) treatments had the lowest clam mortality (0.6 ± 0.1%; mean ± SE), highlighting the utility of this type of protection in limiting ray impacts. Though not significantly greater, we noted relatively high levels of interactions with HDPE netting over other materials, which was facilitated by the material ensnaring the lower dental plate of the rays. Clammers should consider adopting multi-layered anti-predator gear; however, resecuring materials periodically remains imperative at reducing ray interactions.