Hormones in Salmon

Hormones are important signaling molecules produced and secreted in the endocrine system that show interesting close similarities between humans and salmon. They are transported to target organs where they bind to their receptors and control physiological regulation and behavioral activity to maintain homeostasis via feedback mechanisms. Various hormones control freshwater and seawater adaptations to maintain water and salt balances. The juvenile imprinting migration and adult homing migration of salmon are mainly controlled by the brain (thyrotropin-releasing hormone)-pituitary (thyrotropin)-thyroid (thyroid hormones) axis and the brain (gonadotropin-releasing hormone)-pituitary (gonadotropin)-gonad (steroid hormones) axis, respectively. This chapter describes hormone species and actions, hormonal control of freshwater and seawater adaptations, and hormonal changes during juvenile imprinting migration and adult homing migration in salmon.

Memory in Salmon

It is unknown how salmon can imprint and retrieve information on their natal stream over a long period due to the lack of brain molecular markers for evaluating olfactory memory formation and retrieval. Memory in the brains of vertebrates is explained by the plasticity of the nervous system and the synaptic plasticity that promotes the ability of the chemical synapses to undergo changes in synaptic strength for long-term potentiation via the N-methyl-D-aspartate receptor, which has been identified a good molecular marker in the brain of salmon. This chapter describes the plasticity of the nervous system and synaptic plasticity, the involvement of the N-methyl-D-aspartate receptor in olfactory memory formation and retrieval in Pacific salmon, and functional magnetic resonance imaging of olfactory memory in lacustrine sockeye salmon.

Salmon Resources

Salmon are among the most important fishery resources and are produced by commercial fisheries, aquaculture, and propagation. The total production of salmon is now 4.6 million tons, among which 22% comes from commercial fishery production and 78% from aquaculture production, which is the fastest growing food production system in the world. Atlantic salmon and rainbow trout are produced by marine and freshwater aquaculture mainly in Norway and Chile, where environmental impacts are intensively investigated. The biomass of Japanese chum salmon increased steadily from 1970 to 1996 because of the successful improvement of propagation systems. However, the return rate of homing adults has decreased sharply, mainly due to recent unpredictable climate change. New trials of Japanese salmon propagation systems are being carried out by using semi-closed recirculating aquaculture systems and orally administering docosahexaenoic acid.

Behavior in Salmon

The recent rapid development of biotelemetry technologies has made it possible to continuously observe the underwater behavior of salmon in open water. Homing migratory behaviors were studied using anadromous chum salmon from the Bering Sea to Hokkaido and lacustrine sockeye salmon and masu salmon in Lake Toya. Biotelemetry results on the migratory behavior of adult chum salmon in a reconstructed reach of the Shibetsu River; the investigation of cardiac arrest during gamete release in chum salmon; the comparison of the swimming ability and upstream-migration behavior of chum salmon and masu salmon in Hokkaido, Japan; and the analysis of site fidelity and habitat use in Formosan landlocked salmon during the typhoon season in the Chichiawan stream, Taiwan were also performed. This chapter describes the homing migration of anadromous chum salmon from the Bering Sea to Hokkaido, Japan; the homing migration of lacustrine sockeye salmon and masu salmon in Lake Toya, Hokkaido, Japan; and biotelemetry research on various behaviors in salmon.

Olfaction in Salmon

Olfaction is the most primitive and important chemosensory system for detecting a wide variety of environmental changes and is involved in various functions required for survival. Salmon have an efficient olfactory system; their olfactory abilities are inferior to those of dogs but superior to those of humans. Salmon olfactory systems can discriminate seasonally and yearly stable compositions of dissolved free amino acids in their natal streams produced by biofilms in the riverbed, and salmon can imprint on single amino acids before and during smoltification and discriminate these amino acids in the spawning season several years later. This chapter describes the structure and functions of the olfactory systems in salmon, previous olfactory hypotheses for salmon homing, properties of natal stream odorants, and biochemical and molecular biological studies on salmon olfactory functions.

Salmon Species and Life Cycles

Salmon are very interesting fish with complicated life cycles. They are born and die in freshwater but can live in both freshwater and seawater and migrate between rivers and oceans. Salmon previously inhabited only cold-water regions of the Northern Hemisphere but have been transplanted to the Southern Hemisphere and are now distributed around the world, and these fish have an amazing ability to imprint and precisely home to their natal stream. This chapter describes the terminology and taxonomy of salmon and related species associated with their phylogenetic evolution, the salmon species found around the world, the life cycles of Pacific salmon with special reference to imprinting and homing migration, the migration of other important migratory fish species, and the impact of climate change on migratory fish.