BREEDING COMPETITION IN A PACIFIC SALMON (COHO: ONCORHYNCHUS KISUTCH): MEASURES OF NATURAL AND SEXUAL SELECTION

Area of Science:

• Evolutionary Biology
• Animal Behavior
• Ichthyology

Background:

• Pacific salmon exhibit sexual dimorphism driven by natural selection in females and sexual selection in males.
• Female salmon compete for oviposition territories, while males compete for fertilization opportunities.

Purpose of the Study:

• To quantify direct selection intensities on phenotypic characters during breeding in coho salmon (Oncorhynchus kisutch).
• To examine how competition and breeding density influence natural and sexual selection in salmon.

Main Methods:

• Coho salmon (wild and hatchery-raised) were bred in experimental arenas to measure breeding success.
• Seven phenotypic characters, including secondary sexual traits, were measured.
• Selection differentials and opportunities for selection were quantified under varying densities and competition levels.

Main Results:

• Without competition, natural selection favored female body size for egg production; no significant selection was detected in males.
• Under competition, natural selection favored female body size and caudal-peduncle depth, enhancing egg production and nest survival.
• Competition significantly increased the opportunity for selection in males (52-fold) compared to females (6-fold).
• Sexual selection favored male body size and hooked snout length, crucial for accessing spawning opportunities.
• Increased breeding density reduced large male dominance, favoring smaller "sneaker" males, suggesting disruptive selection.

Conclusions:

• Female reproductive success is linked to body size and tail depth, influenced by natural selection for nesting.
• Male reproductive success is driven by body size and snout morphology under sexual selection for fertilization.
• Breeding density and operational sex ratio critically affect male mating strategies, potentially leading to the evolution of alternative reproductive tactics in salmon.