Scientific Classification
● Kingdom: Animalia
● Phylum: Chordata
● Class: Actinopterygii
● Order: Salmoniformes
● Family: Salmonidae
● Subfamily: Salmoninae
Salmon are native to tributaries of the North Atlantic and Pacific Ocean. Many species of Salmon have been introduced into non-native environments such as the Great lakes of North America. They are intensively farmed in many parts of the world.
Typically, Salmon are anadromous. They hatch in fresh water, migrate to the ocean, then return to fresh water to reproduce. However, populations of several species are restricted to fresh water through their lives. A portion of a returning salmon run may stray and spawn in different freshwater systems, the percent of straying depends on the species of Salmon. Homing behavior has been shown to depend on olfactory memory.
The smolt body chemistry changes, allowing them to live in salt water. While a few species of Salmon remain in fresh water throughout their life cycle, the majority are anadromous and migrate to the ocean for maturation. Salmon spend about 1 to 5 years in the open ocean, where they gradually become sexually mature. The adult Salmon then return primarily to their natal streams to spawn.
Migratory Behavior & Pattern
Salmon migration is known as nocturnal selective ebb tide transport pattern of migration. Migration in fresh water predominately nocturnal. But seasonal changes cause to migration in both day and night.
They exhibit endogenous rhythm of swimming activity means moving up into water column after dusk time to sea wards. Salmons use the kinetic energy of fast-moving water column for their migration. Effect of current velocity is greater during the night and slower during the day.
During migration Salmons swim active & fast in usual water column. Behavioral transition induces from a more passive, fluvial migration to more active and sea ward-oriented migration. But in areas with strong currents their movements overridden by current induced transport.
Salmons spends significantly longer time in the river (Mean 456 ± 43h) before migrating into the coastal water. They spent (Mean 201 ± 30h) in the coastal water. [A Moore, ECE Potter, NJ Milner, S Bamber Canadian Journal of Fisheries & Aquatic Sciences 52(9),1923-1935,1995] Also, there migratory patterns were related to the water temperature. (If daily mean water temperature is about 12°C, migration was significantly lower during the day than night then if daily mean water temperature is above 12°C then no significant difference between diurnal & nocturnal migration routes)
Increasing in salinity experienced by the Salmon during their migration through the leading edge of saltwater intrusion. Also increase in salinity of the water cause to increase seaward moments & ground velocity of Salmons.
Periodicity of Migration
There is no apparent period for migration of Salmons from fresh water to salt water. But usually there are two periods of migration (December-January & March-May)
[Nina Jonsson, Bror Jonsson, Lars P Hansen Animal Behavior 40(2),313-321,199]
Migration timing period varies among rivers, local adaptations to ensure the sea entry during optimal period. Both behavioral and physiological mechanisms control the migration of salmons. Seasonal loco motor rhythm changes under behavioral and physiological circumstances. Timing of migration depends several factors such as average age of the migratory group, average body size of the migratory group and male̸ female.
Physiological Aspect of Migration
Na⁺, K⁺ and ATP-ase activity co-related with increased sea water tolerance of Salmons. Inter-strain differences in plasma growth hormones profiles could not be related to observe patterns of Na⁺, K⁺, ATP-ase and water tolerance. Differences in the timing of gill enzyme development matched observed migration pattern well. Delayed enzyme development causes to reduce the sea-water tolerance and fitness for the migration. Genetic activities of the Salmon body influence on their timing and subsequent migration. Much of genetic variations observed among the population causes to local adaptations (Morphologically, Developmentally, Physiologically and Bio-Chemically) related to the migration.
[ Christian Nielsen, Gert Holdensgaard, Hans Christian Petersen, B Th Bjornsson, SS Madsen Journal of Fish Biology 59(1),28-44,21]
Source: https://www.riversinlet.com
Chinook Salmon
source: https://oceana.org/marine-life/ocean-fishes/chinook-salmon
Chum Salmon
source: https://www.fws.gov/fisheries/freshwater-fish-of-america/chum_salmon
Sockeye Salmon
source: https://www.deschuteslandtrust.org/news/blog/2019-blog-posts/sockeye-salmon-journey
Coho Salmon
source : https://www.fisheries.noaa.gov/species/coho-salmon
Pink Salmon
source: https://www.fisheries.noaa.gov/species/pink-salmon
Survival During the Migration
Salmons expose to extreme injuries at sea entry & fjord during the migration. Survival rate depends fork length and range of the fjord. No clear diurnal/nocturnal migration pattern within the fjord but most often they enter to the fjord at night. Tidal cycle, wind-induced current and salmon own moment influence migratory speed and routes in different part of the fjord. Mortality rate is high within the estuaries and fjords than the rivers and marine areas.
Total mortality reported during early marine migration is about 8% -71% (5 km -230 km from river mouth)
[EB Thorstand, F Whoriskey, I Uglem, A Moore, AH Rikardsen, B Finstad Journal of Fish Biology 81(2), 500-542,2012]
It is obvious that early marine migration is the life stage with high mortality. Factors affecting to the mortality rate contribute to determine the abundance of spawner returns. Also, high mortality rate causes to limit the spawner production. Survival rate during the migration is affected by pollution, fish farming, sea lice, hydropower development and other anthropogenic activities.
Not only that but also, they have predatory pressure from aquatic birds and fish. Their main predatory aquatic bird is Pike [ Esox Lucius l.] Aquatic predators responsible for 56% of the observed mortality and avian predators responsible for 31% of the observed mortality.
[Niels Jepsen, Kim Aarestrup, Finn økland, Gorm Rasmussen Hydrobiologia 371,347,1998]
Factors Affecting to Migration
Mainly there are two type of factors that affect to salmon migration.
They are,
1. Man-made obstacles
Ex: Power Station Outlets, Residual Flow Stretches, Dams, Weirs, Fishways
2. Natural barriers
Ex: Water Discharge, Water Temperature, Water Velocity, Water Quality, Time of the season
Effect of these factors may differ among different river sections and sites.
[ Eva B Thorstad, Finn økland, Kim Aarestrup, Tor G Heggberget Reviews in Fish Biology & Fisheries 18(4),345-371, 2008]
Author: H.W.C.V. De Silva
