Scientists have found that sockeye eye salmon use cues from the Earth’s magnetic field (geomagnetic field) to help navigate from their ocean habitat back to the Fraser River to spawn.
The salmon spawning run is an amazing example of animal migration; each year millions of fish travel thousands of kilometers between feeding sites in the ocean and spawning sites in freshwater rivers. Of the five Pacific salmon species in Canada, sockeye is the most commercially valuable and the second most abundant species. In B.C, sockeye salmon are known for their iconic spawning colours, a deep red body and green head, and the migration of millions of fish during the spawning run.
Sockeye salmon are an anadromous species; the adults live in the ocean while the young live in freshwater rivers and streams. For example , in the headwater streams of the Fraser River, sockeye salmon hatch and grow.
For up to two years the salmon will grow and slowly migrate downstream to the river mouth, or estuary, where the Fraser River meets the Pacific Ocean. During this migration the fish undergo incredible physiological changes that allow them to transition from living in freshwater to surviving in salt water, this process is known as smoltification. Putman and colleagues hypothesize that during smoltification the magnetic field signature of the Fraser River mouth is ‘imprinted’ on the young salmon and that they use this magnetic signature to find their way back after spending up to three years in the Pacific Ocean. After the adult salmon have arrived at the mouth of the Fraser River, they rely on olfactory cues (chemical cues, like smell) to navigate to their original natal stream, where they were born.
Puttman and colleagues tested this hypothesis by monitoring how the salmon navigated around Vancouver Island on their way back to the Fraser River. The fish must either go north around the Island to the Queen Charlotte Strait, or south to the Strait of Juan de Fuca in order to reach the mouth of the Fraser River.
The Earth’s magnetic field varies in strength and position to a small degree over time and this variation influences the magnetic signature of the Fraser River mouth. Putman compared the magnetic signature of the north and south Vancouver Island passage and the Fraser River to fishery (catch statistics) and sea surface temperature data. From this comparison Putman found that the magnetic field explains about 16% of the variation in the salmon’s ability to navigate to the mouth of the Fraser River.
He showed that the sockeye salmon used the passage around Vancouver Island that had a magnetic signature most similar to that of the Fraser River mouth when the salmon first migrated to sea. In other words, a two year old salmon leaving the Fraser River in 2004 would ‘imprint’ on the 2004 magnetic signature. When the adult fish returned to spawn in 2006 (at 4 years old) it would use the passage around Vancouver Island that had a magnetic signature most similar to that of the Fraser River in 2004. A diagram below explains the migration cycle.
Many animals, including sea turtles and seals, have been hypothesized to use the Earth’s magnetic field to aid in navigation. The authors in the study described above are the first to find empirical evidence to support magnetic navigation in fish.
Further evidence supporting the hypothesis that salmon utilize magnetic navigation comes from a laboratory study on the cellular response of rainbow trout cells to changes in the magnetic field. Click below to watch cells from a rainbow trout responding to magnetic field changes.
Putman and colleagues hope that by untangling the mystery of how salmon navigate back to their natal rivers they can assist fisheries managers in their efforts to conserve the Pacific sockeye salmon.
Pacific Salmon Threats
During the last two decades fewer sockeye salmon have returned to the Fraser River to spawn. Of those fish that did return, many died before spawning. Disease, increased sea surface temperature, changes in salinity, habitat destruction, decreased water quality, and barriers to migration (dams), may all be contributing to the observed decline in sockeye salmon. Over-harvesting of sockeye salmon continues to be a cause for concern as the number of fish that return to spawn each year dwindles. Trout Unlimited outlines some of the key issues regarding salmon harvesting on the West Coast, click here to read the full article. A thorough assessment of the challenges facing salmon, and particularly sockeye salmon, in one watershed of Northern British Columbia can be found here.
Pacific salmon are often considered the flagship species for conservation efforts along the entire North Pacific Rim. In case you were wondering “why salmon??”check out this article by the Wild Salmon Centre. Click here to read the full essay.
How do you think this new discovery will help conservation efforts of sockeye salmon in the Pacific Ocean?