A male Sockeye Salmon (Oncorhynchus nerka) fends off an intruding male with the gape threat 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.

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.

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.

A Sockeye Salmon (Oncorhynchus nerka) spawning pair Underwater view in an Alaskan stream during the summer.

Chum Salmon (Oncorhynchus keta) spawning pair underwater view in an Alaskan stream during summer.

Sockeye Salmon (Oncorhynchus nerka) spawning pair positioned over a redd that is under construction in an Alaskan stream during early summer.

Female Sockeye Salmon (Oncorhynchus nerka) attempting to oust a sneaker male by using the gape threat in an underwater view in an Alaskan stream during the summer.

Sockeye Salmon (Oncorhynchus nerka) spawning pair positioned over a redd that is under construction in an Alaskan stream during early summer.

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.

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

Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) male crosses over a female as part of courtship in an Alaskan stream during the summer; Alaska, United States of America

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

Mature Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka) male in nuptial (breeding) coloration 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. Two competing males are lurking nearby, one is visible at the right edge of the frame and the other, a jack or precocial male, is visible...

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

Female Sockeye Salmon, also known as Red Salmon (Oncorhynchus nerka), covers her redd immediately after spawning as the male guards in an Alaskan stream during the summer. Sperm that has not drifted downstream clouds the water above the redd; Alaska,...

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

Anticipating spawning, male Coho Salmon, also known as Silver Salmon (Oncorhynchus kisutch) approach a female probing her redd 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

A male Pink Salmon (Oncorhynchus gorbuscha) among others of the same species and Dolly Varden Charr (southern form; Salvelinus malma lordi) form a dense school at the bottom of a deep pool during their spawning migration in an Alaskan stream during...

Three male Sockeye Salmon (Oncorhynchus nerka) competing to spawn with one female during early summer in an Alaskan stream.