Six feet under – The ice

Wintertime may give young salmon a break from swift currents and keen fly fishermen, but other challenges abound.


Seks fot under – isen

Photo: Rick Cunjak, University of New Brunswick

Wild salmon in the ongoing field trials in the Sokna River have been fitted with what are known as pit-tags or Q-free tags. The tags are small, cost no more than NOK 10 each and last for years. They are, therefore, the ideal tool when you are conducting research on wild salmon. But this method only works in small rivers, because the tags send a signal that can only be detected one metre away or less. Thus, the fish and their trackers need to be in close proximity to each other.

Large rivers call for the use of radio transmitters. These transmitters are considerably more expensive, but in return can send out a signal that can be traced over several kilometres. However, the batteries only last for 14 days.This method was used during studies of wild salmon in the Orkla River in 2003.

It is important that the salmon behave as normally as possible so that researchers can compare the behaviour of salmon with radio transmitters to salmon without the added technology. Thus far, researchers have not detected any differences. The wired salmon do not seem to be aware that they are swimming around wholly in the service of science.

Research scientist Finn Økland from the Norwegian Institute for Nature Research, or NINA, holds a 12cm long salmon in his hand. Økland has the distinction of being one of the few research scientists in Norway permitted to operate on fish – a skill he is about to exercise when he inserts a radio transmitter into the abdomen of the small fish. The aim of this special operation is to provide information about how wild salmon tackle the tough winter in one of Norway’s regulated rivers.

A total of 74 young salmon will be monitored in the Orkla and Sokna rivers in a research project that is one of the first of its kind in the world.

“At the moment we don’t know very much about what actually happens with salmon that spend the winter in these rivers. The Atlantic salmon stock has declined by more than half in the past 20 years, and it continues to decline. Many die as a result of salmon lice, gyrodactilus salaris, deep-sea fishing and the like, but as a natural cause of death, winter itself is a problem.”

Exhaustion, accidents, a lack of food and low water levels can influence a salmon’s ability to survive. A limited amount of water, for example, increases the chance that the fish can be trapped in a small pool – and end up being locked in the ice.

“This is why we would like to know more – so we can manage our salmon in the best way possible,” says SINTEF research scientist Atle Harby.


The young salmon in the Orkla and Sokna rivers are monitored with Argus-eyed vigilance. The researchers have observed that the fish prefer to spend time in the parts of the river that are covered with surface ice. They have also noticed that salmon can survive in anchor ice that forms on the bottom of the river, or surrounded by frazil ice, the slushy ice crystals that form in super-cooled turbulent water. But the researchers don’t yet know if the fish are deliberately hiding in the brash ice, or if they get caught there by accident.

The fish seem to use these perilous ice locations to hide from predators – birds, mink and other fish are all potential dangers. The researchers have also found fish in other surprising places, such as in extremely shallow water or under more than a metre of thick ice. Atle Harby says one day he saw a fish under a half-metre of ice, well into the riverbank.

“I thought it was dead, but after I tapped on the ice several times, the fish suddenly moved. Presumably there must have been a channel in the ice with water into it. The next day the fish was back in the middle of the river, and the day afterwards it was on the other side of the river bank, so it was in good shape.”


Until recently, research scientists have thought fish were inactive during the winter, as a mechanism to save energy, and that the fish ate little or no food. But it now appears that this situation can vary considerably. Some fish are very active during the winter, especially after nightfall. Some of the fish also move extremely rapidly if the current is too strong or the anchor ice is too thick, while many choose to hide down in the river gravel.

Thus far, the researchers think that it isn’t just one specific incident that kills salmon, but that death results from exhaustion or a lack of food reserves. The research thus far suggests that young salmon are tough and can tolerate a host of challenges, but that too many ‘catastrophes’ during one winter can sound their death-knell. In the years ahead, the researchers hope to uncover all of the reasons behind over wintering deaths.


The scientists have now studied their elusive subjects for a year. During the remainder of 2004, the small 12cm fish and its ‘colleagues’, all fitted with radio transmitters, will provide even more information. The little fish operated on by Ørkland has been named 99 Wayne Gretzky, after a Canadian ice hockey star renowned for his precise passes and an ability to be in the right place at the right time. The little fish also positions itself well – never far from food in the energy-rich parts of the river, where the food supplies are most abundant.

The benthos, the community of plants and animals that live in the deepest part of the current, is the most important food source during the winter. It is no coincidence that Wayne has been given a Canadian name. The Trondheim research scientists co-operate closely with two other research teams in Canada. The Canadians have conducted the same kind of research as the Norwegians and have also participated in experiments here. So far, salmon in winter seem to behave the same no matter whether they are in Norwegian or Canadian rivers.


It is a difficult job for to follow little Wayne. The rivers where fish are being tracked were painstakingly mapped in advance so that the researchers have precise co-ordinates for keeping tabs on fish locations. Additionally, each fish has a number to ease in identifying exactly which fish is where, and which fish are active and which are not.

With this information, researchers can piece together the kinds of physical conditions that are important to young salmon, such as water depth, current velocity, ice thickness, and riverbed material. Bearings on the small fish are measured every six hours. The field workers wander along the riverbank, and back and forth across the ice. Taking a round of salmon bearings requires anything from 30 minutes to two hours. The fact that it is minus 10, or the darkest night of the winter is no excuse. The scientists hope, however, that their the fiveyear project will yield many rewards for all their hard work.

Christina B. Winge