A female Sockeye Salmon (Oncorhynchus nerka) fends off an intruding female with the gape threat in an underwater view in an Alaskan stream during early summer.

Alpha Sockeye Salmon (Oncorhynchus nerka) male chasing away a rival in an Alaskan stream during early summer.

A male Sockeye Salmon (Oncorhyncus nerka) holds position over a redd site while the female is away in an underwater view in an Alaskan stream during early summer.

While her alpha male guards downstream, this female Sockeye Salmon (Oncorhynchus nerka) uses her anal fin to probe her redd in an Alaskan stream during summer. Underwater view in an Alaskan stream during the summer.

Male Sockeye Salmon (Oncorhynchus nerka) threatens another male by raising his formidable jaws with this tilted head posture, underwater view in an Alaskan stream during early summer; Cordova, Alaska, United States of America

A female Sockeye Salmon (Oncorhynchus nerka) is positioned over her redd while competing jacks fight over her downstream, Alaska

A male Sockeye Salmon (Oncorhynchus nerka) holds a position high over a redd site as the female approaches from downstream in an Alaskan stream during early summer.

A female Sockeye Salmon (Oncorhynchus nerka) is positioned over her redd while competing jacks fight over her downstream, Alaska

Mature male Chum Salmon (Oncorhynchus keta) size each other while competing over a female in an Alaskan stream during the summer.

Male Sockeye Salmon (Oncorhynchus nerka) threatens another male by raising his formidable jaws with this tilted head posture in an Alaskan stream during early summer.

Male Sockeye Salmon (Oncorhynchus nerka) threatens another male by raising his formidable jaws with this tilted head posture in an Alaskan stream during early summer.

Coho Salmon (Oncorhynchus kisutch) spawning pair in an Alaskan stream during autumn, Southcentral Alaska

Incidental tree branch bites can result from the aggressive behavior of male Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) competing to spawn in an Alaskan stream during the summer; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) spawning in an Alaskan stream during the summer. The small male is a jack; Alaska, United States of America

Mature male Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) in an Alaskan stream during the summer, South-central Alaska; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) spawning pair in an Alaskan stream during the summer; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) male threatening a challenger by also raising his snout in an Alaskan stream during the summer; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) spawning pair in an Alaskan stream during the summer; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) spawning pair in an Alaskan stream during the summer; Alaska, United States of America

A Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) pair in the act of spawning in an Alaskan stream during the summer; Alaska, United States of America

A female Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) attacking a female who is excavating a redd that is too close to hers in an Alaskan stream during the summer; Alaska, United States of America

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) spawning pair in an Alaska stream during the summer, South-central Alaska; Alaska, United States of America

Coho Salmon, also known as Silver Salmon (Oncorhynchus kisutch) crosses over the tail end of a female as part of courtship in an Alaskan stream during the summer; Alaska, United States of America

Aggressive behavior of male Sockeye Salmon (Oncorhynchus nerka) competing for the same female, underwater view in an Alaskan stream during autumn.