Clam leases are designated aquatic areas in which individuals or businesses can cultivate and harvest clams of all sizes, from littlenecks to chowders. However, unwanted marine intruders such as rays often endanger clam production or aquaculture.
For instance, in the Indian River Lagoon along Florida’s Atlantic coast, clam fisherman have frequently reported seeing rays and suspect that their interactions could lead to damaged aquaculture gear and crushed clams.
Now, a team of researchers led by Florida Atlantic University (FAU) has used passive acoustic telemetry to better understand the interactions between two highly mobile species of rays – the whitespotted eagle rays (Aetobatus narinari) and the cownose rays (Rhinoptera spp.) – over two years in two clam leases and four other sites in Sebastian, including the Sebastian Inlet and the Saint Sebastian River mouth.
“We wanted to understand how often and when rays visited these clam leases, how long they stayed there, and if it was seasonal or year-round,” said senior author Matt Ajemian, an associate research professor of Fisheries Ecology and Conservation at FAU. “We tagged them, set them on their way, and logged 17,014 unique visits to the leases derived from 38 different rays.”
The investigation revealed that, although rays spent even more time in the clam leases sites than fisherman reported, these places were not necessarily where they preferred to roam. In fact, white spotted eagle rays spent only 6.2 percent of their time near clam leases, and cownose rays 13.2 percent.
Nevertheless, both species exhibited longer visits at clam lease sites than at other locations, with the longest continuous duration spent at the northern and southern clam leases being 387.5 and 207.1 minutes, respectively. The extended periods of time spent in these areas suggest that they may possibly interact with and even forage upon clams.
“Since 84 percent of all visits were from whitespotted eagle rays and their visits were significantly longer at night, this information suggests that observed interactions with the clam leases are potentially underestimated, given most clamming operations occur during daytime,” said lead author Brianna Cahill, a graduate student in Marine Science and Oceanography at FAU.
“Results from our study justify the need to continue monitoring mobile predators in the region, including more studies to assess their behaviors, such as foraging at the clam lease sites.”
However, since both ray species tend to be highly mobile, their impact on clams may be limited. “Additional good news for clammers is that rays did not use the clam lease sites year-round, and their visits varied seasonally, with substantially fewer detections and visits during the summer months. This suggests that clammers need only deploy anti-predator protections against rays, if needed, for a portion of the year,” Ajemian explained.
These findings highlight the need to clarify if specific clam leases are situated within rays’ natural foraging habitats and if rays are actively interacting with the clams or are attracted by other organisms nearby.
“It’s possible that rays may be feeding on other organisms that are attracted to the clams in the clam lease sites, as bivalve aquaculture farms have been known to change community structure and attract a wide variety of predatory snails, which may be an alternative attractant for cownose and whitespotted eagle rays given their diets,” Cahill concluded.
The study is published in the journal PLoS ONE.