For repeat-spawning steelhead, more than once is worth the risks
CORVALLIS, Ore. – For steelhead trout, reproductive choices represent a collection of tradeoffs – whether spawning once or doing it multiple times, no decision comes without risks and benefits.
New research by Oregon State University and Purdue University shows steelhead that spawn repeatedly have greater than double the lifetime reproductive success of fish that spawn a single time, the benefit for making the daunting journey to sea more than once.
Findings, which could lead to more effective conservation efforts, were published this week in the Proceedings of the National Academy of Sciences.
Like salmon, steelhead – Oncorhynchus mykiss – are anadromous, meaning they travel to the ocean as smolts and return to their natal streams to spawn; unlike salmon, steelhead are not limited to spawning once and then dying.
Female steelhead build gravel nests called redds into which they deposit their eggs, and males vie to fertilize them.
The research by OSU's Michael Blouin and Purdue's Mark Christie involved more than 12,000 winter-run steelhead from Oregon's Hood River, with the researchers looking at number of lifetime spawning events and age at first spawning.
"We found that repeat-spawning fish had more than 2.5 times the lifetime reproductive success of single spawning fish," said Blouin, professor of integrative biology. "However, first-time repeat spawning fish – ones having the first in a series of spawning events – had significantly lower reproductive success than single-spawning fish of the same age. That suggests repeat-spawning fish hold some energy back from reproduction to devote additional energy to continued survival."
As with salmon, steelhead that spawn only once die after doing so.
"There's a tradeoff between saving energy to successfully survive that additional migration event to the ocean and putting all of their eggs in one basket, so to speak," said Christie, assistant professor of biological sciences at Purdue and a former Oregon State postdoctoral scholar.
The risk of going to sea is illustrated by the finding that less than 3 percent of the fish in the study were found to be repeat spawners.
In addition to the trip, it's also more dangerous for a steelhead just to be in the ocean than in freshwater – but months and years at sea also allow the chance to gain size, strength and improved reproductive prowess.
"The expectation is that the more time they spend in the ocean, the higher the payoff will be to offset the risk of predation and other hazards," Christie said. "We'd expect them to come back to their spawning grounds bigger and stronger, and that's definitely what we see in male fish."
Not so with females, who showed the highest reproductive success at age 3, despite being smaller than 4- and 5-year-old fish.
Why? Because of "negative frequency dependence," Blouin said: the idea that a rare life history strategy correlates with greater fitness.
A life history strategy is how an organism parcels out energy for growth, reproduction and survivorship.
"Negative frequency dependence dictates that selection favors the rarest life history strategies," he said. "In this study, we show that the older, larger female fish have higher reproductive success when they are less common, perhaps due to decreased competition."
"Fitness tradeoffs" are the other factor at work, Blouin said.
"The costs and benefits associated with each strategy prevent any one strategy from becoming permanent," he said. "If there is too high mortality associated with going out to sea, the repeat spawning strategy would never evolve. But if it's low enough, and if there's a fitness benefit, which we've shown there is, these two life history strategies can coexist."
Findings suggest that restricting the harvest of older, larger females when their numbers are lower than average might foster negative frequency dependent processes, thus helping to maintain population diversity and reverse declines in populations.
The Oregon Department of Fish and Wildlife also collaborated on this research, which was funded by the Bonneville Power Administration and Purdue University.
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