An important key to protecting salmon and trout species raised in the Midwest may come down to studying their tiny inner-ear structures, new Ohio Sea Grant research found.
Called otoliths or “ear stones,” these hard, calcium carbonate structures help fish process sound and detect predators. However, for reasons unknown to scientists, the structures can become deformed in certain species of fish, potentially affecting their hearing and, importantly, survival in the environment.
A cross-section of a fish otolith taken by researchers at Bowling Green State University. Otoliths are calcium carbonate structures found inside the heads of bony fish. Each fish has three pairs of otoliths, which vary in shape and size.
“This issue could potentially impact economic success in aquaculture and fisheries, as lower fish survival ultimately impacts the bottom line of an operation,” explained Dr. Kevin Neves, an associate teaching professor in Bowling Green State University’s Department of Biological Science who led new otolith research in a project funded by Ohio Sea Grant. “So we started looking at what is causing these otoliths to change and more importantly what the impact is.”
Otoliths are found within a fish’s cranium, where they sit on nerves that detect disturbances created by sound waves vibrating through the fish, growing throughout a fish’s lifetime in proportion to its length.
While fish normally accumulate a certain form of calcium carbonate in their otoliths called aragonite, scientists found that many fish raised in captivity under potentially high stress conditions tend to have deformed otoliths with a different form of calcium carbonate called vaterite.
To address this concern, Neves and Bowling Green researchers Drs. John Farver and Jeffrey Miner studied the presence of “vateritic” otoliths in salmonid fish species – particularly steelhead trout, which are especially prone to otolith issues. In Ohio, these fish are an important species for both aquaculture operations that raise fish for food and state hatcheries that stock millions of fish across the state each year.
“State hatcheries will release juvenile trout for fishermen to catch, and oftentimes there’s a really low return rate of these fish to streams later in life,” Neves said. “These fish with vateritic otoliths may not be able to detect sounds, which might be causing them to be more susceptible to predation in the lake before returning to streams.”
Bowling Green State University researchers studied the inner-ear structures and survival of steelhead trout, pictured here.
Looking at fish raised in both controlled, hatchery settings and out in the wild in streams, researchers conducted different experiments to examine what is causing these otoliths to change and what the impact might be. First, the team studied how protein levels and ration amounts in fish diets may affect changes in otolith composition.
“By controlling as much as we could and sampling the fish every two weeks, we developed a detailed timeline of otolith development,” Neves said.
Then, using larger-scale mesocosms, the team assessed if fish with normal otoliths might have an advantage in surviving predators compared to fish with vateritic otoliths. Researchers also collected fish from stocked rivers along Lake Erie and used genomic sequencing to analyze for the presence of vaterite. This allowed them to further gauge impacts on survival measured as a “return rate” – the percentage of fish that survived in the wild and came back to the mouths of tributaries.
“Piecing all of these bits together allows us to weave them together into a more complete picture,” Neves said.
Through the project, researchers found that fish develop vaterite on their otoliths much sooner than initially thought, and this growth varies widely among different strains of a species, likely due to genetics. The team also confirmed that having vaterite does seem to impair fish survival, Neves said, though it’s much worse for the fish if they have a vateritic otolith in one ear and an aragonitic otolith in the other.
Moving forward, findings from the study will inform both Ohio fisheries and aquaculture operations.
“We hope that our findings help both aquaculture producers and fisheries managers make decisions regarding the growth of their fish that will ultimately save them money while not unnecessarily impacting the health, survival, or well-being of the fish,” Neves said.
To learn more about this Ohio Sea Grant-funded research, contact Dr. Neves at kneves@bgsu.edu or watch his upcoming Freshwater Science webinar.
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.