Education on Trout

I shamelessly ripped this article off a website for a fish hatchery in Ontario. It's a good read and applies directly to our small, deep lake:

CLIMATE CHANGE PUTTING LAKE TROUT AT RISK

By Steve Lawrence, OMNR Sr. Fish & Wildlife Technician

Climate change and global warming are constantly in the news these days. Melting polar ice, rising ocean water levels, drought, increase in severe weather like hurricanes and tornadoes seem to be everywhere in the news. These effects are not something we can ignore, as we found out last August when a rash of tornadoes ripped through the area. We can’t help but notice horrifying events like that but the effects of climate change are not only dramatic, they are also subtle but no less deadly.

Our environment has been finely tuned by many thousands of years of gradual change. When you throw a monkey wrench into the system like rapid global warming caused by human excess and abuse, our fish and wildlife suffer. Creatures adapted to live in extreme environments in many cases are most susceptible. Polar bears and lake trout, adapted to live in extremely cold environments become the virtual “canary in the coal mine.”

Lake trout are a species of the ice age and their life history is dependent on pristine, cold water environments. In this area which is considered the southern portion of their range, lake trout exist tenuously even under normal climatic conditions. Increasing temperatures in the last 25 years are negatively affecting lake trout habitat and biological processes.

Global warming affects lake trout in two subtle but significant ways. Increased heat units pumped into our local lake trout lakes can obviously reduce cold water habitat but warmer water can also disrupt the basic reproductive cycle of the species by reducing survival and recruitment of young fish. Lake trout have adapted to spawning around mid October. Historically the water temperatures at this time of year were 10°C (50°F). Today when lake trout spawn in mid October the water temperature in some lakes is as high as 14°C (57°F). So why don’t lake trout just spawn later in the fall? It would seem that the lake trout is hard wired to spawn at the same time regardless of water temperature and consequently this could be their evolutionary downfall.

Why could a minor change of 4 degrees be significant? In the perfect lake trout world, eggs are deposited in mid October, develop slowly over the course of the winter and hatch in early spring. Lake trout egg development is determined by the amount of heat units they receive from the moment they are laid. Research has shown that it takes 490 accumulated degree-days until the eggs hatch into sac fry. The sac fry live in a semi-dormant state for approximately 6 weeks in the substrate as they absorb the energy stored in their yolk sac. When the yolk sac is used up the fry swim up from the shoals and they must actively feed or starve to death.

Historically, under normal water temperature conditions, lake trout fry hatch in February or early March and swim up in late March and early April when the lakes are starting to break up. Food supplies at this time of year are abundant. However, increase the water temperature by just 2 degrees and the effects on survival are drastic. This slight increase in water temperature during egg incubation results in the lake trout hatching significantly earlier. Under these conditions the lake trout will hatch from late December into January, up to 4 to 6 weeks earlier then normal. The sac fry then absorb their yolk sac reserves, swim up in February into a sterile environment and consequently starve to death. Water temperature observations between the 1980’s and today indicate that many of our local lake trout fisheries may not have produced many successful year classes in the last 25 years.

Some lakes are more susceptible to heat retention then others. Lakes with a small surface area to water volume ratio are most affected. Our smaller, deeper lakes suffer the most from these effects. As our warmer climate pumps heat into these lakes they accumulate more and more heat units and in effect become huge thermos bottles of heated water. The thermal inertia that builds in the lakes is very hard to remove and it can take several consecutive below average temperature seasons to develop conditions suitable for successful lake trout recruitment.

It’s not all bad news for our lake trout lakes. Some of our lakes are doing just fine, but these lakes generally have large areas of shallower water that cool rapidly in the fall. If the wind keeps these lakes open later in the fall because of their size even more heat is removed. For many years Fisheries Managers have accused anglers, who would complain about poor fishing, of just catching too many lake trout but if the truth were known we would all have to take some of the blame for the loss of our valuable lake trout resources.