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.

Sensitivity of Dynamic Response of Truss-Type Aquaculture Platform to Floating Body Arrangement

“…The demand for shellfish and algae aquatic products such as oysters is increasing with the improvement of living standards, and the increase in the output of aquatic products mainly depends on marine fisheries [1][2][3][4]. However, in recent years, the offshore environment has been severely polluted, with the excessive concentration of aquaculture leading to a gradual decline in the quality and production of aquatic products [5][6][7][8]. Therefore, moving towards the deep sea and developing new models of open marine aquaculture to accelerate large-scale food production are effective methods to address these problems.…”

Visitation patterns of two ray mesopredators at shellfish aquaculture leases in the Indian River Lagoon, Florida

“…Hard clam aquaculture has two main grow-out strategies including bagged planting, where clams are placed into 1.2 m x 1.2 m polyester mesh clam bags on the sediment surface [ 32 ] and bottom planting, where clams are able to bury and a large sheet of protective cover netting (e.g., high density polyethylene (HDPE) or chicken wire) is staked over top [ 30 ]. Given the high densities of clams within these confined areas, and the ability of whitespotted eagle rays to manipulate shellfish aquaculture gear [ 33 ], both whitespotted eagle rays ( A . narinari ) and cownose rays ( Rhinoptera spp.)…”

Diet and Feeding Ecology of the Whitespotted Eagle Ray (Aetobatus narinari) from Florida Coastal Waters Revealed via DNA Barcoding

“…; Ajemian, unpublished data). Previous research suggests that whitespotted eagle rays can consume clams and manipulate gear commonly used in hard clam aquaculture [23] and that the rays can spend extensive periods of time within range of clam leases [24]. Unfortunately, there are no data to support whether rays are indeed depredating clams from these areas or consuming other associated fauna.…”