By Grace Veenstra
This article covers the 2020 study by Ulaski, Finkle and Westley on the direction and magnitude of natural selection of body size among age-classes of seaward-migrating sockeye salmon.
“Bigger Isn’t Always Better”
The idea that “bigger is better” is a prevalent one in biology. Body size has a significant impact on fitness, the ability of organisms to survive and reproduce, across all branches of life. In wolves, a larger body size gives them greater success in grappling and subduing large prey like moose or elk (MacNulty et al., 2009). In dall sheep, a larger body size gives males a reproductive advantage when they compete for the right to mate, and since adult sheep may lose up to 16% of their body mass during winter, those with a higher body mass are more likely to survive (U.S. National Park Service, 2020). However, “bigger” is not always a “better” lifestyle method for some animals, and can be actively harmful to their chances of survival.
Insects and arthropods are limited in their body size because they breathe by soaking in oxygen like a sponge. If they are too large, they cannot absorb sufficient oxygen and die of hypoxia. It was only during the Paleozoic, when oxygen concentrations reached as high as 35% during the Carboniferous period, that insects were capable of gigantism, with some dragonflies as large as seagulls (Harrison et al., 2010). Furthermore, it is costly to support a large body mass: the larger the body, the greater the energy requirements and the more food is required to sustain the animal. There are also factors of predation, where a larger body size may reduce agility, increase detection by predators, or increase costs to reproduction (Blanckenhorn, 2000). In essence, how natural selection acts on size is complex. Understanding how size correlates with survival is important, particularly when we are examining the salmon populations of Alaska.
It’s hard to understate the importance of salmon in Alaska. Salmon are a vital part of the Alaskan ecosystem and a critical resource to humans. Salmon feed hundreds of species in the oceanic, freshwater and terrestrial ecosystems, across all their stages of life. As adults they are food for bears, eagles and humans during their runs upstream, while as juveniles they are preyed upon by a range of seabirds, river otters, and marine and freshwater fish species (Ulaski et al., 2020). Furthermore, after salmon spawn and die their carcasses are a source of energy and nutrients to not only the river ecosystem — adding nitrogen and phosphorus to streams and promoting the growth of juvenile salmon — but also to forests, providing similar nutrients to trees and terrestrial habitats (NOAA, 2022). To humans, they are a vital food and economic resource, part of a large network of commercial and subsistence fisheries in Alaska, with one of the most important species among them being the sockeye salmon, Oncorhynchus nerka (Alaska Department of Fish and Game, n.d.).
When considering conservation efforts to protect salmon populations and fishery management strategies, it is important to consider as many variables as possible that contribute to the survival and existence of the salmon. Sockeye salmon are currently listed as “least concern” by the International Union for Conservation of Nature (IUCN). However, it is likely we may see their populations decline in a manner similar to the chinook salmon as climate change continues to increase temperature in lakes and streams (Alaska Fisheries Science Center, 2022). With this in mind, being able to increase growth opportunities and maximize survival may become critical for sockeye salmon populations.
Sockeye salmon are one of the most important salmon in Alaska, vital to commercial and subsistence fisheries, and an integral part of the culture and heritage of Alaska’s indigenous people (Alaska Department of Fish and Game). Understanding the salmon and their survival at all stages of life is vital not just for the benefit of the sockeye species, but also for all the animals, ecosystems, and people in Alaska and beyond who rely on these fish for their own survival and livelihoods.
A Study on Salmon, Size and Selection
A study by Ulaski, Finkle and Westley (2020) sought to explore the relationship between size and survival among several age groups of sockeye salmon in the South Olga lakes system of Kodiak Island, Alaska.
This study aimed to understand the fitness, or survival, advantages and disadvantages of body size across different ages and life histories within a diverse migratory population of sockeye salmon. Through comparing measurements of length to age in juvenile salmon, the authors sought to understand how natural selection acts on size in this migratory fish species.
Evidence
Sockeye salmon are anadromous, living in the ocean but entering fresh water to spawn. They spend their first one to four years in freshwater, feeding on zooplankton and small crustaceans, before migrating to the ocean as smolts where they then live another one to three years. Smolt-to-adult survival increases as the smolt size at the time of ocean entry increases. The higher survival rates of larger juvenile salmon are thought to arise from increased ability to escape, faster growth, and a needing less time to achieve sizes less susceptible to predation and starvation (Ulaski et al., 2020).
In order to understand how smolt size is selected across age groups, Ulaski et al. looked at the length of juveniles upon entering the ocean, which fell into three age groups: “age 0”, “age 1”, and “age 2”, which reflected how many full years they had been in freshwater. To determine smolt age, the researchers used scale samples. The researchers also used archived scale samples of returning adult sockeye salmon to reconstruct the size of those adults as smolts, which they were able to do thanks to the close relationship between the scale size and length of the salmon. This was used to compare the survival of the different age smolts entering the ocean at different times to the estimated smolt length of the adults returning to spawn. In other words, determining which smolt sizes had survived.
Using the aforementioned measurements of smolt age, size, and survival, the researchers determined the intensity and direction of natural selection acting on body size at the time of ocean entry of salmon smolts. If more surviving smolts were found to have larger body sizes, then natural selection was directed towards a larger size. If 90% of the smolts were said to be “large” versus, say, 60%, then we’d say there that the natural selection was of greater intensity. With this in mind, the researchers calculated a ‘selection differential’ that compared the average length of outward migrating smolts to the reconstructed smolt length of surviving, returning adults.
As seen in Figure 2, a positive SSD (strength of selection) indicates that natural selection favors larger individuals, while a negative value indicates that the selection favors smaller individuals. To determine if the measured selection on sockeye salmon smolt size was large in comparison to other organisms, the researchers compared their measurements to a global database of selection differentials. This helped determine the relative magnitude of selection, to see how strongly the size of the smolts was being influenced by natural selection. Using the global database, if the measured strength of selection was in the 90th percentile, then selection was considered strong. Indeed, nearly all the measurements taken from the sockeye smolts indicate strong selection for size relative to other species. Interestingly, the authors found that while “age 0” and “age 2” sockeye smolts experienced selection that favored larger body sizes, the “age 1” smolts experienced selection favoring smaller body sizes.
Ultimately, the study’s conclusions corroborated the idea that, in general, smolts of larger average size tend to have higher survival rates. However, they also showed that “bigger doesn’t always mean better”, since the age-1 smolts experienced selection for smaller sizes.
Smaller Size, So What?
Overall, the findings of the study corroborate that larger smolts may have a survival advantage over their smaller counterparts, but what does this inverted selection mean for the age-1 smolts?
Well, the different levels of survival of smolts could be due to sized-based consumption by predators. This could be a result of physical limitations due to the predators’ own size, the hunting preferences of predators, or smaller smolts having an increased ability to escape. For instance, the researchers note bird predation can select for large and intermediate sized prey, and harbor seals have been found to prefer consuming large-bodied smolts (Ulaski et al., 2020). This would make large age-1 smolts more vulnerable to this type of predation, with size-selective survival resulting due to the relative levels of mortality by different predators.
The researchers do note that there is “limited evidence for the selection against large juvenile salmonids,” but give an example of a previous 2004 study by Carlson et al. that found selection could favor small, fast growing trout (Ulaski et al., 2020). There is further ambiguity since it is difficult to determine whether negative selection is a result of the size of age-1 smolts, ocean-entry timing or an interaction of the two.
The size of juvenile salmon is intrinsically tied to when they enter the ocean, which in turn can strongly impact early marine survival. Early-entry smolts are smaller since they have not spent as much time feeding in the freshwater streams, while late-entry smolts are larger thanks to spending more time growing in the relative safety of their freshwater habitat. In the marine environment, an even greater range of factors influence the survival of smolt size, some connected to size and some connected to the time of ocean entry. Environmental factors like spring upwelling and freshwater discharge alters the zooplankton that smolts feed off of, which could lead to less predation risk or increased size disparity among smolts (Ulaski et al., 2020).
This presents a rather perplexing confounding variable, since the marine environment is critical to salmon survival, but it is also extremely challenging to identify the mechanisms underpinning size-selective survival for salmon as they enter the ocean.
Where Do We Go From Here?
Size-dependent survival is a topic of interest and concern for conservation efforts, hatchery operations and fisheries alike. To effectively help conserve and manage fish populations and the fisheries industry, we must understand how survival of salmon may change as larger fish are routinely caught and climate shifts alter growing conditions. We also must understand how the survival of different age smolts is impacted by size, which is important to hatchery operations that dictate when smolts are released to the ocean.
While this study illustrated that survival can favor smaller or larger freshwater sockeye smolts depending on their age, the authors noted that they cannot precisely determine why this is the case. There are many interlocking factors such as ocean entry time or differential predation that may be responsible for the different directions of selection between ages.
For example, migratory time can play a significant role in survival, influencing a variety of factors that may select for size. Migratory time can affect the availability of food, the presence of different types of predators, and result in temperature differences based on time of year. A future study could explore some of these elements that influence sockeye smolt size within or between age classes. It’s also important to note that this data is approximately 30 years old, and things have changed since then.
In the end, the potential paths a future study could take could all contribute to a better understanding of sockeye salmon, whether that is regarding their survival, direct influences on size, or what forms natural selection takes on this trait of the salmon.
Further Reading
Ulaski, M. E., Finkle, H., Beaudreau, A. H., & Westley, P. a. H. (2021). Climate and conspecific density inform phenotypic forecasting of juvenile Pacific salmon body size. Freshwater Biology, 67(2), 404–415. doi: 10.1111/fwb.13850
This study looked at how changes in climate and population density may impact the survival of sockeye salmon smolts. Their predictions of smolt length showed variable responses to increasing temperatures and different population densities, suggesting that responses are population-specific and that local habitat conditions may filter out large-scale climate shifts.
Richardson, N., Beaudreau, A. H., Wipfli, M. S., & Finkle, H. (2016). Prey partitioning and use of insects by juvenile sockeye salmon and a potential competitor, threespine stickleback, in Afognak Lake, Alaska. Ecology of Freshwater Fish, 26(4), 586–601. doi: 10.1111/eff.12302
This study looked at the seasonal variation in diet and interspecies interactions between sockeye salmon and threespine sticklebacks in shallow nursery ponds in Alaska. They found significant differences in diet and a thin separation of habitat, with adult aquatic insects making up roughly 70% of all juvenile sockeye salmon diets by weight.
Sparks, M. M., Westley, P. a. H., Falke, J. A., & Quinn, T. P. (2017). Thermal adaptation and phenotypic plasticity in a warming world: Insights from common garden experiments on Alaskan sockeye salmon. Global Change Biology, 23(12), 5203–5217. doi: 10.1111/gcb.13782
Presently, an important question is how populations of coldwater-dependent fishes will respond to rapidly warming water temperatures. This study was testing for thermal adaptation and phenotypic plasticity in sockeye salmon, measuring various physiological variables between two populations that overlapped in spawn time but have different temperature environments. Their results ultimately indicated no local adaptation, but they did detect the presence of plasticity in contrasting conditions.
Cunningham, C. J., Westley, P. a. H., & Adkison, M. D. (2018). Signals of large scale climate drivers, hatchery enhancement, and marine factors in Yukon River Chinook salmon survival revealed with a Bayesian life history model. Global Change Biology, 24(9), 4399–4416. doi: 10.1111/gcb.14315
This study estimated the effect of factors in marine and freshwater environments on Chinook salmon survival, using data including river breakup, wintertime ocean temperatures, and bycatch mortality. Conducting analysis with a Bayesian life history model, the study suggested that mortality at sea is the primary driver of population dynamics of Chinook salmon populations.
References
Alaska Department of Fish and Game. (n.d.). Sockeye Salmon Species Profile. https://www.adfg.alaska.gov/index.cfm?adfg=sockeyesalmon.printerfriendly
Alaska Fisheries Science Center (2022). What’s Behind Chinook and Chum Salmon Declines in Alaska? NOAA Fisheries. https://www.fisheries.noaa.gov/feature-story/whats-behind-chinook-and-chum-salmon-declines-alaska
Blanckenhorn, W. U. (2000). The Evolution of Body Size: What Keeps Organisms Small? The Quarterly Review of Biology, 75(4), 385–407. doi: 10.1086/393620
Harrison, J. F., Kaiser, A., & VandenBrooks, J. M. (2010). Atmospheric oxygen level and the evolution of insect body size. Proceedings of the Royal Society B: Biological Sciences, 277(1690), 1937–1946. doi: 10.1098/rspb.2010.0001
MacNulty, D. R., Smith, D. W., Mech, L. D., & Eberly, L. E. (2009). Body size and predatory performance in wolves: is bigger better? Journal of Animal Ecology, 78(3), 532–539. doi: 10.1111/j.1365-2656.2008.01517.x
NOAA. (2022). Sockeye Salmon. https://www.fisheries.noaa.gov/species/sockeye-salmon
Ulaski, M., Finkle, H., & Westley, H. (2020). Direction and magnitude of natural selection on body size differ among age‐classes of seaward‐migrating Pacific salmon. Evolutionary Applications, 13(8), 2000–2013. doi: 10.1111/eva.12957
U.S. National Park Service. (2020). Dall Sheep. https://www.nps.gov/articles/about-dall-sheep.htm