TUCSON — When Bryan Black accepted a post-doctoral research position at Oregon State University to study how fish grow, he hoped to apply his background in forest ecology to marine life.
Black, a dendrochronologist, or tree-ring scientist, brought techniques from the land to the sea, using samples from fish, clams and coral to build timelines and draw conclusions about the environment and climate in water forms.
Now an associate professor at the University of Arizona, Black has compiled a 500-year chronology that tracks fish growth in relation to environmental changes. He took his experience with tree rings and tracked the fish that formed the rings – what he called “fish rings.”
“That’s the approach we want to bring to fish and clams,” Black said. “A big part of the work is can we build these fish chronologies and have the equivalent of tree-ring chronologies, just underwater.”
Black applied tree-ring techniques to marine life that, like trees, form growth rings, helping to fill in gaps in climate records before humans tracked environmental conditions such as of ocean temperature and El Niño events.
“We’re getting these perfectly dated chronologies to address questions in the marine environment just as we do on land,” Black said, “such as the interactions between the marine and the terrestrial: Is sharing are they common patterns, and what does this mean for future climate impacts and extremes?”
Although he focuses on marine life in the north Pacific, Black has studied fish rings from around the world and hopes to bring his work closer to home by examining fish from the Arizona and Colorado Rivers. .
How are fish like trees and how can you count their ‘rings’?
Nature has a record-keeping role of Earth’s past, long before humans invented tools to document climate conditions. In the early 1900s, Flagstaff astronomers discovered that trees could act as climate historians, and in his work, Black learned fish played a similar role.
Astronomer Andrew Ellicott Douglass realized that trees could give scientists a glimpse into the past, discovering a correlation between the annual rings formed by trees and sunspot cycles.
He founded the field of dendrochronology and the Laboratory of Tree-Ring Research at the University of Arizona in 1937. The lab pioneered tree-ring analysis for decades, allowing ecologists like Black to observe phenomena such as of drought and forest fires over the centuries.
Trees increase their trunk circumference as they grow. They begin to form new cells during the spring when conditions are favorable, creating early wood. In summer, growth slows down and creates smaller and denser latewood cells.
The contrast between fast and slow growth creates a pattern of light and dark rings on the tree trunk. A pair of light and dark rings usually represents a year in the life of a tree.
Dendrochronologists count these rings and can assign a ring to a particular year. This helps them determine the age of a tree and make observations about the environment they live in each year.
To ensure that they are accurate in dating trees, dendrochronologists cross-date wood samples using various techniques, such as comparing samples from multiple trees in a particular that place. This helps scientists evaluate their work and make broader trend observations about weather, temperature or even whether a forest fire may have occurred.
Black applied tree-ring techniques to fish, clams and corals, which form similar annual rings as they grow.
“I had no idea that fish lived that long or developed rings but I was intrigued by it,” Black said. “Anything that forms rings triggers scientists to have this wonderful toolkit for handling growth rate data that can be applied to other systems.”
Because trees can live for centuries in one location, they make easy test subjects when looking for long-term data. But fish can swim thousands of miles with varying lifespans, so to study fish rings and make environmental observations, Black had to find more specific, sitting candidates.
Not all fish have bones – sharks have skeletons made of cartilage – and only bony fish form fish rings. Black takes the otolith, an ear stone or ear bone, to study the rings.
Marine species must be long-lived to create decades of rings and stay close to home for most of their lives. Migratory fish will be affected by different climates, which will hinder wider environmental analysis.
Black often studies rockfish, a group of fish belonging to the Scorpaenidae family. They can live upwards of 200 years, travel long distances and have bony skeletons, making them ideal candidates for fish ring studies.
Otoliths are formed in the inner ear of fish with layers of calcium carbonate as the fish grows. Researchers use a similar method of counting and cross-referencing to estimate the age of the fish and the climate it experienced in certain years.
“These rockfish match tree-ring chronologies on land because the same atmospheric processes that drive productivity in the ocean also affect drought on land,” Black said. “So we see that trees and fish are related to each other, applying the same exact techniques.”
Landscape history: What an Arizona rock sample has taught scientists about climate change 250 million years ago
How fish rings can lead to climate observations
Like tree rings, fish rings can record events such as prolonged drought, increased rainfall, high temperatures and wildfires. Warming greenhouse gases warm the atmosphere, producing extremes, and the rings help scientists make climate observations about the past, present and future.
Fish rings vary in thickness depending on how productive the fish are growing in a given year. Narrow rings form in low productivity individuals, and thick rings form when a fish grows well.
Cool temperatures in the California Current, the cold-water ocean off the west coast of North America, contribute to a productive year for rockfish. This happens when water outside the surface sea is replaced by deep, cold water rich in nutrients during a phenomenon called upwelling.
When upwelling occurs, rockfish develop thicker rings, and scientists can record lower temperatures in that particular year.
Black hopes that if scientists retrieve a rockfish from the North Pacific in the future, they will be able to observe the 2023-2024 El Niñoevent. El Niño brings warm water into the California Current, preventing the ascent of cold, nutrient-dense water.
“I bet if we go out and collect in the future, we’ll know it’s probably going to be a narrow year for these fish in the California Current,” Black said. “Really narrow rings form in low productivity years to coincide with large El Niñoevents.”
Using dendrochronology techniques, Black created chronologies of climate changes in the northern Pacific over the past 500 years. This helps shed light on climate events before people began recording them in the 1800s.
The climate naturally varies with decades of cold and heat, but since 20th century, he observed “unprecedented” warming.
“It’s another way for us to see what the natural patterns of variation are, how they’re changing and what that means for the future,” Black said. “Volatility is increasing, and the fish and the trees give us the first indication that this is happening and provide a longer context to show that what we are seeing today is unusual.”
Sign up for AZ Climate: The Republic’s weekly environment and climate newsletter delivers stories like this every Tuesday.
Applying science to inland fish and rivers
Dendrochronology can be applied to fish and marine life around the world if they form rings over long periods of time in a stable location. Black has worked with scientists from the north Atlantic off Iceland and Norway to the Benguela Current off Africa.
Black hopes to expand his climate chronologies and create a 1,000-year climate history of the northern Pacific using clam rings. In partnership with fisheries, he hopes to collect clams that lived centuries ago to make these observations.
He believes that creating fish chronologies is a “shortcut” to observing climate variability, because it would take decades to collect and measure fish to come up with an average growth rate.
Although his work focused primarily on marine life in the ocean, this analysis is also possible on fresh-water marine life.
Black hopes to work with the Arizona Museum of Natural History to examine fish around the state, especially how invasive species affect ecosystems, how they grow and their population structure.
As the Colorado River changes after being dammed and continues to change due to drought, he plans to study what observations can be made of the river’s marine life using dendrochronology techniques.
Hayleigh Evans covers environmental issues for The Arizona Republic and azcentral.com. Send tips or questions to [email protected].
Environmental coverage on azcentral.com and The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Sign up for AZ Climate, our weekly environmental newsletter, and follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook and Instagram.
You can support environmental journalism in Arizona by subscribing to azcentral today.
