For salmon, being bigger isn't always better. Being a little smaller—at least intermediate size—is more advantageous for maximum spawning in riverbeds, a University of Wyoming researcher recently discovered.
While bigger salmon can carry more eggs and move larger sediment than smaller salmon, the larger fish also take up more space in a riverbed when they build their spawning nests, or redds, in stream gravel. Hence, bigger salmon can fit fewer redds into a particular riverbed area than their intermediate-sized counterparts, which can maximize the number of eggs they can place in a riverbed.
Simply put, smaller salmon take up less space when they nest and don't have to move as many grains to lay their eggs, says Clifford Riebe, a UW assistant professor in the Department of Geology and Geophysics.
"No one has recognized this trade-off before. The trade-off has some pretty big implications for how river managers restore rivers," Riebe says. "There's a 'sweet spot' where intermediate-sized fish can install the maximum number of eggs in the riverbed."
"Our work provides these managers with a tool for making restoration projects more cost-effective," he continues. "The trade-off may also help explain why different rivers support fish of different sizes. It has long been recognized that different-sized fish spawn in different reaches of river, but there has never been a very good explanation for this."
Riebe's research, which he says will benefit the multimillion-dollar fish management industry, was published earlier this month in Water Resources Research, an interdisciplinary journal that publishes original research in the natural and social sciences of water. Riebe was among four writers of the paper, titled "Optimal Reproduction in Salmon Spawning Substrates Linked to Grain Size and Fish Length."
Leonard Sklar, an associate professor of earth and climate service at San Francisco State University; Brandon Overstreet, a UW doctoral student in water resources/environmental science and engineering, from Laramie; and John Wooster, a geomorphologist/hydrologist with the National Oceanic and Atmospheric Administration (NOAA) Fisheries, were the other contributing writers.
"Though our study focused on California, Washington and British Columbia, our results should ultimately apply more generally to understanding spawning by cutthroat trout and rainbow trout in Wyoming," Riebe says. "Managers can design their restoration projects to maximize the spawning benefit."
The study focused on pink, sockeye and Chinook salmon—which span a range of sizes from small to large—and took place in rivers and creeks in California (Shasta River) and the Pacific Northwest (South Prairie Creek in Washington state and Scotch Creek in British Columbia, Canada) during 2009 and 2010. The research goal was to understand how grain size influences spawning because conditions necessary for productive spawning were unclear. Riebe's analysis reveals that coarse substrates have been substantially undervalued as spawning habitat in previous research work.
Grain size is a fundamental regulator of the quality of salmon spawning habitat. To be suitable for spawning, substrates need to be coarse enough to prevent scouring and allow the flow of oxygen-rich water to the eggs. At the same time, substrates need to be fine enough that female salmon can move sediment to build redds and deposit their eggs within the riverbeds, Riebe says.
To create a redd, female salmon use their tails to dig a small area of gravel on the bottom of a stream or shore. They create several depressions in the gravel, which form pockets into which salmon deposit their eggs. The size of a redd depends on the size of the fish making it.
Because a redd area increases with fish length, the number of eggs a substrate can accommodate is maximized for moderate-sized fish. The previously unrecognized trade-off raises the possibility that differences in grain size help regulate river-to-river differences in salmon size, Riebe says.
"Our research suggests that the factors that influence grain size in rivers also influence fish size," he says. "And those factors are climate, geology and erosional processes."
For example, a wet climate produces finer grains while a dry climate results in coarser sediment, Riebe says.
The approach provides a tool for managing grain-size distributions to support optimal reproductive potential of fish and species resilience, Riebe says. This is valuable as millions of dollars are spent annually In the United States to revitalize salmon spawning in riverbeds. Riebe adds that the research can be useful for managing any fish that uses rivers, in the same way as salmon, for spawning.
"What our method does is give managers a method to determine what size grains are needed to bring back fish of a certain size," Riebe says.
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Riebe, C. S., L. S. Sklar, B. T. Overstreet, and J. K. Wooster (2014), "Optimal reproduction in salmon spawning substrates linked to grain size and fish length," Water Resour. Res., 50, DOI: 10.1002/2013WR014231.