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Reproduction

Males partake in competitive and sneaking tactics, formation of hierarchies, and non-hierarchical groupings around females who are ready to mate. Reproductive success varies more in males than females. The greater variability in male reproduction is associated with the greater average size and exaggerated shape of males. Reproductive success in females is determined by the number of eggs she lays, her body size, and the survival of the eggs, which is due in part, to the quality of the nest environment. Male spatial distribution depends on shifts in reproductive opportunities, physical traits of breeding sites, as well as the operational sex ratio (OSR) of the environment.

Hierarchy & Sneaking

Non-dominant males adopt a subordinate behavior acting as a satellite to mated pairs. During spawning, a subordinate male will move quickly into the redd and release their sperm. Nearby dominant males from other redds will also do this. Male social status is positively correlated to length and dorsal hump size. Larger females tend to spawn in shallower water, which is preferred over deeper water.

Sexual Selection

Sexual selection favors large males and females. Males choose females based on their readiness to spawn and their size. This is to maximize their breeding opportunities. Females vary their breeding rate depending on the size of the courting male, mating more quickly with larger males. This increases the likelihood that larger males will displace attending, smaller males. Males preferentially spawn with females who are red, which is the usual color of females. Even small changes in wavelength, saturation, and brightness can affect preference. Both sexually naïve males and experienced males showed the same preferences, which illustrates that preference for a red hue may be innate.

Morphological Changes

There is a dramatic sexual dimorphism at maturity. Males go through numerous morphological changes at maturation including, an increase in body depth, hump height, and snout length. Snout size also increases in females, but hump height and adipose fin length do not increase. This could mean that longer snout sizes are sexually selected, but hump height and adipose fin length are not. Females develop large gonads that are about 25% of the body mass.

Sexual Selection vs. Natural Selection

Some traits that lead to reproductive success, such as body size and sexual dimorphism can affect one’s survival. This leads to opposing pressures of natural selection and sexual selection. Larger males are favored, unless the risk of predation is very high. Sockeye salmon that die prematurely from predation are typically the larger ones in a population. This shows natural selection against large bodies. Populations with higher levels of predation tend to evolve smaller body size. Without the threat of predation, salmon that breed early in the season live longer than those that breed late in the season.

Energy Cost

Reproduction is marked by depletion in energy stores. Fat, protein, and somatic energy stores decrease from the final moments in marine migration through freshwater entry, spawning, and death. Sockeye salmon have a semelparous life cycle and do not feed during reproduction. Feeding ends once they enter into freshwater, which can be several months before spawning. Embryos are maintained with only endogenous food supplies for about 3-8 months. Reproduction in the sockeye salmon has to be accomplished with the energy stores brought to the spawning grounds. How the salmon use their energy during migration and spawning affects how successful they will be reproductively; energy used for migration cannot also be used for courtship. If they waste too much energy, they might not be able to spawn. Males must also make the decision whether to invest energy in fighting for a female or for longevity on the spawning grounds.

Parental Care

Females are responsible for parental care. They select, prepare, and defend a nest site until they die or are displaced. Males do not participate in parental care at all, and they move between females after egg deposition.

Competition

During Reproduction

Aggressive behavior displayed by dominant males is predominantly directed towards intruding dominant males. Sometimes sockeye salmon males behave aggressively towards subordinate males. These encounters are short, with the intruding male leaving after one or two aggressive interactions. Spawning females direct their aggression primarily towards intruding females or other spawning females that are close by. However, they may also direct aggression towards intruding or subordinate males. Aggressive interactions between females only last one or two charges and/or chases. The intruder retreats and the spawning female settles back in her redd. These acts of aggression are important in terms of reproductive success, because they determine the quality of the nest site the female obtains and the access to females by males.

Juvenile

Competition for food or space while the salmon are in their lake residence period can exist. This happens when there is a more populous class of young sockeye or when there are multiple classes present. It can also happen when resources are in short supply. Interspecific competition can also occur, and can lead to interactive segregation. Interspecific competition can affect the growth rates of the salmon if their access to resources is limited.

Diet

Sockeye salmon use patterns of limnetic feeding behavior, which encompasses vertical movement, schooling, diel feeding chronology, and zooplankton prey selectivity. They can change their position in the water column, timing and length of feeding, school formation, and choice of prey to minimize the likelihood of predation. This also ensures they still get the minimum amount of food necessary to survive. All of these behaviors contribute to the survivability, and therefore fitness of the salmon. Depending on location and threat of predation, the levels of aggressive feeding behavior can vary.