Oyster farms don’t alter sea otter fodder

To you and I, farmed oysters are a delicacy. But for sea otters, oyster farms may not be a big deal.

That’s one finding of a team of scientists led by Brenda Konar, a marine researcher at the University of Alaska Fairbanks (UAF), and including Alaska Sea Grant-supported UAF graduate student Riley O’Neil. The researchers partnered with shellfish growers to study how their farms might impact the seafloor of Kachemak Bay, which is a breeding ground for sea otter prey like sea stars, crab, and clams. 

“We wanted to determine if the prey was similar underneath a farm versus other places,” Konar said. “Are farms good places to hang out because there is a lot of food, or better food?”

The project is part of the Mariculture Research and Restoration Consortium, a long-term program funded by the Exxon Valdez Oil Spill Trustee Council to understand broad ecosystem interactions with mariculture farms. It builds on previous Alaska Sea Grant-funded research by Konar, which found that sea otters that forage on the seafloor in the vicinity of oyster farms share similar feeding habits as those distant from the farms. These findings led researchers to a followup question: do oyster farms change the communities of animals living beneath them?

“We’re looking at how oyster farms may influence the benthic environment, specifically the communities of kelp and invertebrates that live there,” said O’Neil. “The oysters, or even the organisms growing on the gear, produce organic material that falls to the seafloor. That could provide extra food for sea otter prey.  But if too much accumulates it can also reduce oxygen in the sediments and make it harder for many bottom-dwelling organisms to survive.”

To answer their research question, the team compared oyster farms with nearby sites that shared similar environmental conditions but had no farming activity. Across three years of summer fieldwork, scuba divers swam transects along the seafloor at a total of six sites in Bootleggers, Peterson and Jakolof bays. 

The divers extracted sediment cores and recorded characteristics of the seafloor habitat like turbidity, depth and wave exposure. They also gathered sea stars, crab, clams and other invertebrates in pillow cases – a surprisingly effective method for collecting underwater animals – and brought them to the surface, where they were identified, measured and weighed before being returned to the bay.

The team had hypothesized that they might see differences between the prey communities beneath farms and those from other sites. Instead, they found that every site was unique, suggesting that local environmental conditions may play a larger role than mariculture in shaping benthic communities. 

“We didn’t see any evidence of farm influence,” remarked O’Neil. “Every site we sampled was different.”

The findings weren’t entirely unexpected, O’Neil noted, since organic material in Kachemak Bay is subject to some dynamic water conditions. “Kachemak Bay is a relatively high-energy system with strong tidal mixing,” she said. “In systems like this, a lot of organic material is likely dispersed rather than accumulating heavily under farm structures.”

These findings suggest that these oyster farms are not substantially altering the benthic prey communities that support sea otters in Kachemak Bay. Instead, natural variation among sites appears to have greater influence on seafloor communities than the presence of oyster farms. O’Neil said the results suggest mariculture is having little impact in the area. 

Konar noted the surveys offer a reminder of Kachemak Bay’s productivity. “There is a lot of life in Kachemak Bay waters,” Konar said. “I’m always amazed at the amount of seaweeds and invertebrates that thrive here.” 

The project has also led researchers in a new direction: they noticed large sea stars under the farms, which Konar said was unusual given that sea star wasting has decimated sea star populations in other areas. Konar’s team has established new transects to determine whether the size and abundance of these animals change with increasing distance from farm sites.