Water Temperatures

Parker Jefferson, One Cowichan, January 21, 2015

Spawning cohoI have had my feet in the river almost every day for the past few weeks, enjoying our great winter weather and river conditions while pursuing the elusive winter steelhead.  The river is very busy again this year with anglers travelling from all over the Island as well as the Lower Mainland and beyond to try to catch and release one of these magnificent creatures.  BC Outdoors magazine just named the Cowichan one of the top ten steelhead rivers in the province.

On a recent drift of the upper river I was pleasantly surprised to see several hundred coho salmon still in the river spawning, while trout swam just behind them feasting on any stray eggs that floated by.  It is getting very late in the season for coho to be spawning and this is an indication that we have had a very good run this year.

Recreational angling generates more money for the BC economy that fish farming, commercial fishing and fish processing combined.  Recreational angling is a sustainable source of revenue for our local economy and that is one of the many reasons why we should make sure our river and the fish in it remain healthy.

At the January Stewardship Roundtable meeting we learned about some important research into summer water temperatures and how they affect salmon and trout done in the Somenos watershed.  Remote sensors were used to monitor temperature at regular intervals in Bing, Averill and Richards’s creeks as well as at various depths in Somenos Lake.  Salmon fry were monitored at all the sites to see how they were affected by temperature changes over the summer. 

Researcher David Preikshot provided some interesting facts about the effects of water temperature on fish in his presentation.  He said the optimum temperature for growth in salmon fry is about 12 degrees C.  At that point, the dissolved oxygen in the water matches the metabolic demand of the fish.  As the water warms above 12 degrees in the summer two things happen, the metabolic oxygen demand of the fish increases while the ability of the warming water to hold dissolved oxygen decreases.  This slows growth in the fry, which is crucial for their survival.  As the water warms it becomes less and less suitable habitat for the fish until it reaches about 24 degrees where it becomes lethal.

Swim counts done last summer in the Cowichan River revealed how the warming water caused all of the fish in the river to seek cool water refuges.  There are several springs and small spring fed creeks in and around the river that create cool water areas.  During the peak of the summer heat in August, the surface of the lake and river were very near the lethal 24-degree temperature and no fish were ever seen outside of the cool water refuges they had found.  The problem is that the cool spring water has very low dissolved oxygen levels and the fish have to venture into the warm water to breathe, performing a dance for survival in the mixing currents.

Rodger Hunter of the Cowichan Watershed Board stated that the raising of the weir and increasing summer water storage is their first priority.  They will be working with the CVRD board and Catalyst to finalize technical plans for this large infrastructure project.  We know we need to store more water in the summer to ensure we have enough in the fall months and now we know we need to be concerned about the temperature of the water entering the river in mid summer as well.

The current weir skims off the top of the water column and sends it into the river.  This upper layer of the lake is the warmest water and it can approach the lethal temperature of 24 degrees C in July and August.  We are considering plans for a new weir to include some sort of pump or siphon system to gather water from the cool deeper layers in the lake to provide cooler and more suitable water temperatures for the fish in the river over the summer.  As the days shorten in September the temperatures in the lake cool quickly and are not a problem for fish. 

It is very important for us to support scientific research in our community.  This is especially important today as our federal government is reducing budgets for aquatic research and has closed libraries where aquatic research information was kept.  We are facing many climate-related challenges and scientific research is vital for our decision making process.  Sharing information and coordinating our activities will continue to ensure that our stewardship groups, community leaders and business leaders know the right steps to take to protect our watershed and build resilience to climate change.

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