I may be painting myself into the salmon-blogger corner here at Nature’s Confluence, but quite simply, I think they are fascinating, and they give us so much to ponder! So I hope others enjoy reading as much as I enjoy writing about this astounding fish. This post moves away from direct management concerns in California and talks a bit more about the big evolutionary picture of the salmon life-cycle. Spoiler alert - there are no answers here, just some ideas and lots of questions waiting to be answered.
What could possibly inspire a juvenile salmon shorter than your toothbrush to swim hundreds or thousands of miles from their home stream to the vast ocean? There are actually many analogous migrations throughout the animal world, including those of birds, butterflies, and caribou. The study of these risky, long-distance movements has resulted in theory about evolutionary ecology and the selective pressures that result in these seemingly improbable behaviors.
Some of this theory focuses on a basic question facing any mobile creature: should I stay here or should I go elsewhere? From an evolutionary perspective, when an individual makes the choice that results in many successful offspring, their lineage (and accompanying behavioral tendencies such as likelihood to go elsewhere) will become more prevalent within the population over time. Eventually their strategy will become common, and if it turns out to always be the ‘best’ choice we expect to see the strategy used by the entire species. It’s called an ‘Evolutionarily Stable Strategy’ in the academic jargon. While this conclusion stems from basic evolutionary logic (survival of the fittest), it can take us a long way toward a theory about the classic stay or go question. Since salmon have evolved to migrate to the ocean as small fish and return as sexually mature adults (which are much larger and therefore lay more and/or bigger eggs than their freshwater cousins the rainbow trout), we conclude that on average, the benefits of this trek must outweigh the costs. This includes the expense of swimming downstream, risking exposure to predators in both the river and the ocean, and then swimming back upstream against the current to a familiar creek to lay eggs and die. Because migration is quite a long and costly adventure, there must either be a gourmet buffet available in the ocean, or the quality of the resources and habitat in the streams is meager indeed.
But what if the ocean and the stream do not exist at constant states with a steady supply of food or a steady risk of predators? Will the best migration strategy (stay or go) change? What if throughout the year the stream slowly becomes a worse place for small fish to grow up? Perhaps food runs low, competitors get feisty, or the water gets hot. When we imagine a stream that gets progressively worse, we can see a situation where freshwater habitat is initially better but as it degrades the ocean becomes a better choice. In this hypothetical world, there should be an ideal time for the juvenile fish to move out and explore the immense ocean. Too early and they miss benefits in the stream. Too late and they miss benefits in the ocean. And to keep stretching our brains, what if both the stream AND the ocean are variable? And what if the relative benefits of each habitat, or the cost of migration, depends on the size or state of the individual salmon? In the real, variable world there are plenty of factors that should be considered in predicting the optimal timing for a young salmon to migrate. Understanding these factors can benefit the management of salmon stocks by identifying likely periods to protect juveniles as they move through the river system, or it can allow hatchery managers to better target releases or hatchery grown fish.
If none of this discussion of optimal timing or evolutionarily stable strategies seems like rocket science (conservation science?), how about this for adding brain-bending complexity. If we can accurately measure all the costs and benefits of the streams, rivers, estuaries, and oceans (yes, it’s unlikely, but join me in this thought exercise), then we might be able to determine the optimal time for each individual fish to migrate. But, what if the fish itself doesn’t get the memo? How is a quarter-pound fish in the high deserts of Washington going to know what benefits wait for it in the ocean on any given day? Or know how many hungry predators are lining the banks waiting for it to migrate? There are seasonal changes or cues, such as water temperature, day length, river flow, or weather patterns that might be correlated with ocean conditions or predator abundances, and thus may suggest to a juvenile salmon that it is time to leave. But are these cues reliable enough for a species to evolve consistent responses? Should they evolve consistent responses if there is a high amount of uncertainty? To anthropomorphize a little, how much uncertainty are fish comfortable with when making life-or-death decisions?
As I warned above, this blog post does not include specific answers these questions. But when we discover them, the answers will be valuable in finding new, promising ways to balance cues and habitat needs for salmon with the ever-increasing human need for freshwater. Many researchers are currently using a combination of theory, experimental manipulation, and observational data to try and improve our understanding of how salmon have evolved to make critical decisions about when (or if) they should change to the next life-stage. Perhaps you’ve had some ideas yourself?
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