The Secret Life of the Sea Trout

While not normally thought of as a dodgy character, the sea trout, a distant cousin of the Norwegian Salmon has a lot of secrets. Researcher Jan G. Davidson and his graduate students are on the case and trying to decipher the development and habits of the sea trout and in the process hopefully discover why there has been a 60% reduction in the numbers of sea trout in Norway. Similar to their salmon cousins, the sea trout or Salmo trutta L, begin their life in fresh water. Eggs are laid in rivers and streams in nests known as “redds.”  For about two years the young fish go through significant physiological changes, with the end result being the ability to live in salt water. This is known as the smolt stage of development. Once the smolts are able to live in salt water, they are considered anadromous, which means they leave the freshwater where they were hatched and make their way to the open ocean.  The benefit is that there is more food in the open sea and more opportunities to breed. The mystery surrounding the sea trout is that while they have the ability to become anadromous, they don’t always develop fully into ocean fish.  Davidsen and his students are trying to figure out why some sea trout do not adapt and go to the open ocean. Davidson believes that some environmental or physiological change has to occur for the sea trout to develop into an ocean fish, but so far he has been unable to figure out exactly what and when this change occurs. As with anything, location matters. Fish that mature in the fjords are likely very different from fish that are raised in farms in other areas. Davidsen used high tech listening stations that involved fitting 50 sea trout smolts with 2 cm long cylindrical acoustic tags about the width of a pencil, which the researchers did in May of 2014. The tags can be monitored by a series of 43 listening stations that Davidsen and colleagues put out in the Søa River and in nearby Hemnefjord and Snillfjord in February. Much like a grocery scanner, the stations record the traffic of fish that pass by the sensors. Preliminary data is showing researchers that some fish spend more time in the fresh waters of the fjords while others quickly move out to sea. Davidsen believes that the data collected can explain some of this. "Salmon lice is one major cause," he says. "But I also believe there are other causes. It might be that there has been a reduction in prey for sea trout, especially for sea trout that prefer smaller fish. We know there has been a collapse in some of these species. Or it might be that the temperature has changed," because of global warming. The data collected from the listening stations placed throughout the seat trout’s habitat will continue to be analyzed and Davidsen hopes that the data will provide answers to the decline in the sea trout population in central Norway.