Fish kill dead zones in the gulf are mainly created by the nitrogen and phosphorus enrichment of the Lower Mississippi River. Some species of fish exhibit mass simultaneous mortality as part of their natural life cycle. Fish kill due to spawning fatalities can occur when fish are exhausted from spawning activities such as courtship, nest building, and the release of eggs or milt (sperm). Fish are generally weaker after spawning and are less resilient than usual to smaller changes in the environment. Examples include the Atlantic salmon and the Sockeye salmon where many of the females routinely die immediately after spawning. A
Sockeye salmon in freshwater lakes. The majority of sockeye spawn in rivers near lakes and juveniles will spend one to two years in the lake before migrating to the ocean, although some populations will migrate to saltwater in their first year. Adult sockeye will spend two to three years in the ocean before returning to freshwater. Females will spawn in 3–5 redds over a period of several days. The eggs usually hatch within six to nine weeks and the fry typically rear in lakes before migrating to the ocean. Males partake in competitive and sneaking tactics, formation of hierarchies, and non-hierarchical groupings