It is observed that larger salmon tend to be more likely to die during spawning season due to stranding in shallow waters, and predation by gulls and bears. There is a selection pressure against large body size despite the advantages it brings, such as being able to gather more territory, ability to carry more eggs.
Quinn and Buck conducted an experiment in various creeks in Alaska over the span of two years to determine the extent of size selection and sexual selection contributing to the mortality of the sockeye salmon, Oncorhynchus spp., during spawning season. The research was conducted from 1997 to 1999, and they collected data by walking by the creeks and examining the dead salmon carcass. They measured the length of the fish, their
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Larger fish tend to die stranded or bear bitten, and smaller fish tend to die from senescence or gull pecking. Bear-killed salmon tend to be predominantly male. Larger salmon have a higher probability to die of bear predation and stranding. Quinn and Buck concluded that bear predation provides selection pressure against large males. They concluded that bears might catch larger fish because they are easier to spot, and males tend to be larger than females, which might explain the overrepresentation of males in the bear killed category in many creeks. Also, male salmon tend to be wider than females of the same size, making it hard for them to maneuver in shallow waters, which causes them to be more vulnerable to bear predation than females. They also concluded that shallow river depth provides strong selection pressure against large body size as larger salmon have a higher likelihood to die from stranding. This is advantageous because older salmon tend to be larger than young salmon, and having a selection pressure against larger body size means that younger salmon will be more likely to survive during spawning season and produce higher quality