Introduction
Chinook salmon, the king species, larger than all other salmon species is falling off the throne. Over the last century, there has been a dramatic decrease in the size of Chinook salmon (Oncorhynchus tshawytscha). This trend is not specific Chinook salmon though, this trend is being seen throughout varieties of salmon species. Specifically, Pacific Salmon (Oncorhynchus) species have demonstrated this decline in body size. In a 1980s study, it was shown three species of Pacific Salmon have decreased dramatically in size between 1951 and 1975 (Ricker 1981). While various factors could be at play resulting in the decreased length of Pacific salmon species, one major factor is the exploitation of salmon through commercial fishing practices. Fishing acts as a strong, directional selection for specific phenotypic traits. This selection force has contributed to the gradual decline in size and has showed the inadvertent evolutionary impact humans have played and continue to play.
Description of the change overtime: In the 1920s it was not uncommon to catch Chinook species weighing about 100lbs and being several feet in length, but within the last century, this size has decreased to a range of 10-50lbs, and a size of 32 inches (Hard 2008). This decline in size was originally proposed in 1920 and focused on the removal of larger, immature fish in ocean populations leading to future decline in populations (Smith 1920). As the century progressed, the decline in fish became further examined and concurrent with previous predictions. In a study done on Pink Salmon (Oncorhynchus gorbuscha), a 34% decrease in adult body weight was observed from 1950 to 1992 (McAllister et al. 1992). More recently, Kendall et al. (2009), have quantified six decades of life histories for size and maturity in Chinook Salmon (Oncorhynchus tshawytscha), finding an overall decrease in size throughout time. It has additionally been observed that the age of fishing returning to spawn is younger, which may be contributing to the decrease in size (Kendall et al. 2013).
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The two main consumers of salmon in the Pacific are orcas, and humans. The anthropogenic influence in size selective removal of salmon species has been hypothesized to contribute to the declining size of salmon species. The selective hand humans have played in animal and agricultural cultivation is not new- selective breeding is increasingly popular among farmers. Selectivity by commercial fishing practices occurs through gillnets. By creating removal mechanisms that target specific phenotypic traits one can predict a change in the frequency of that phenotype across generations. Understanding the selective pressures generated by harvesting factors can help increase our understanding on the evolution of salmon over the past decade.
Genetic Variance Among Species
In order for evolution to occur through size selective pressures, a difference in traits is required such as, those who escaped predation versus those who did not. While phenotypic diversity is something visibly seen, understanding how size selective pressures influence genetic changes is harder. Experiments surrounding aquaculture have revealed similarities among populations in hatcheries versus wild species. In one experiment, Atlantic Salmon of parr known parentage were caught, tagged and released in the Atlantic (Jonasson et al. 1997). The body weight and size of the fish returning after a winter at sea were similar to predictions to fish raised in salmon farms. This evidence helps argue a genetic basis and heritability for size and weight. Throughout this experiment it was found the heritability ranged from 0.2-0.3. This is high enough to lead to observable changes in size across generations.
Since the size of salmon is a phenotypic trait- it can be considered quantitative in nature. Quantitative traits are multi-locus, so in examining the evolution of fish, an equation that can be used to analyze the change in trait from generation to generation would be R=h2S, where R= response to selection, h2= heritability, and s= strength of selection (Law 2000). This analysis provides one the ability to determine the change in population mean across generations. Aquaculture has provided a substantial amount of literature upon the strength of selection and selective breeding showing the variance in body size in response to selective pressures.
Using quantitative genetic models, it can be predicted once the reduction to fishing mortality and selective pressures are removed, an abundance of recovery would occur, but in order to return to the pre-fishing size would take several generations without the selective pressure.
Strength Of Selection
For some salmon populations, recreational and commercial fishing practices can remove around 80-90% of individuals (Hard 2008). For those fishes being caught due to their size, this provides an increased strength of selection. In order to observe a measurable difference between generations, a large number of the population needs to be removed to decrease the successful reproduction of a specific genotype. Studies have shown the fish being removed are immature salmon. This results in the larger, immature fish being caught being unable to breed therefore removing their alleles from the population. The removal of these alleles from the population allows the fish who survived due to their smaller size being able to reproduce and pass on their alleles (Law 2000).
Quantifying the strength of selection among fish species has proved difficult due to the information needed prior to sampling such as, the parental generations size, the FI offspring size, and the selective pressures occurring in the ocean. In the 1980s one experiment focused on the size of North Sea Cod and the exploitative levels present during their life history. After the experiment, it was found the selection differential acting on the North Sea Cod entering the fishery was approximately -1cm (Law 2000). In essence, the surviving fish entering the fishery were 1cm smaller than they would have been if there was an absence of fishing. Since the 1980’s the commercial fishing industry has boomed due to increase demand which one can assume has had an impact on the strength of selection in salmon species of the Pacific.
Through The Use Of Gill Nets
Gill nets are a wall of netting that hangs in the water column, typically made of monofilament or multifilament nylon (ANOVA 2019). These nets allow for fish to get only their head through the netting, not their body. The more the fish struggles, the more tangled it becomes in the netting. As the fish tries to back out, the gills, or fins become trapped in the netting. There are variations in mesh size, length, and height. Two main types of gill nets are set gill nets and drift gill nets. Both of these netting types are used in commercial fishing practices which leads to substantial removal of salmon. Larger fish are meant to become entangled in the nets, resulting directional selection.
Fishing gears are designed to remove larger individuals and generate selection on body size. These selective pressures imposed upon salmon populations results in fish that are smaller and able to escape the meshing. While recreational fishing causes extremely slow changes in populations of salmon, the exploitation of salmon populations through commercial fishing has the ability to push larger changes in size across generations.
Implications For Future
Salmon serves as an important species in the Pacific for both ecological, and economic reasons. The decrease in salmon size, and the decrease in salmon populations will continue to affect the 137 species that rely on the nutrient’s salmon supply (Hunt 2000). Additionally, pacific salmon populations fuel a 3 billion-dollar industry. As fish size decreases, more fish will need to be caught in order to keep up with demand.
Keystone Species
Pacific Salmon serve as a keystone species in the Northwest. There are 137 different species that rely on salmon influences for their survival (Hunt 2000). Salmon play an important role in watershed- replenishing and transporting nutrients from the ocean to watersheds. While some species such as orcas, otters, bald eagles and bears rely on salmon for food supply, other salmon rely on the decomposition of dead salmon for survival. With decreasing salmon sizes and populations, this ecosystem is breaking down and will continue to do so unless action is taken. (Hunt 2000).
Economic Impacts
Pacific Salmon populations help drive a multi-billion-dollar industry in the Pacific Northwest and Alaskan region. Additionally, thousands of jobs are created due to commercial fishing practices. Despite the hypothesized correlation between commercial fishing and the salmon size reduction, no change will occur unless a clear biological threat is identified (Kendall 2013). Examples of laws that could serve to relieve the intense selective pressures would be fishery moratoria, catch limitations or creating marine reserves. These options would decrease job availability due to the decrease in catches. The best option would be implementing a law regarding size selectivity. This would ensure those who would have originally been selected against due to their increased size would now be able to breed and pass on those genes.
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