Hobbies And Interests
Osmotic Adaptations
Fish that migrate between the sea and freshwater rivers need to adjust to different saline levels. Marine fish lose fluid to their environment and compensate for this natural osmotic action by continually drinking sea water. These fish then expel the salt via highly concentrated urine. The body fluids of freshwater fish have a higher concentration of minerals than their environment, and water continually seeps into these fish, which expel the excessive fluids through large amounts of diluted urine. Migratory fish species must adapt to a totally different osmotic environment each time they pass from marine water into freshwater and back.
Tracking
Scientists are continually discovering additional secrets about migratory fish species. Advances in technology allow researchers to track migratory fish by transmitters which are typically attached to the body of the fish, but may also be placed in the body cavity. Transmitters are acoustic devices which emit ultrasound pulses. These pulses are subsequently converted into sounds that people can detect. Scientists may follow these fish in vessels, but ongoing advances in technology allow the signals to be tracked from longer distances.
Navigation Signals
Salmon are able to recall the scent of the river from which they were hatched. These fish migrate across the vast expanses of the ocean, homing in on the river mouth through which they emerged as young fish. Most salmon will make this upstream migration only once in their lives once they reach sexual maturity. The journey upstream can take many months to complete. In some instances, the fish need to leap over rapids to arrive at the spawning streams. Salmon do not feed once they enter the river, and the vast majority die from slow starvation and exhaustion after they have spawned.
Crossing Oceans
Select fish species will cross oceans to spawn. One such species is the bluefin tuna, while another is the European eel. Young European eels swim upriver, where they mature after a period of years. Once these eels are sexually active, they swim back down the river and out to sea, where they begin a 3,700-mile migration from Europe to the Sargasso Sea in the North Atlantic. Here the eels lay their eggs 1,000 feet below the ocean surface. Larvae that hatch from these eggs are carried by the currents back to Europe, a journey which normally takes two years.