Did Maligne Lake really produce Alberta’s largest-ever rainbow trout? New scientific evidence casts serious doubt

High-elevation mountain lakes are rarely all that productive for fish. Most are cold, and remain ice-covered much longer than those at lower elevations. The growing season is short and hard. That limits the amount of food available, and it also limits when rainbow trout can spawn, which is when the water temperature reaches approximately 6°C.

Unlike fish species such as brook and lake trout that can spawn in lakes, rainbows mostly spawn in running water. They can, and sometimes do, try to spawn in springs that well up into lake beds, but their success in such places is rare. That’s why some lakes don’t have a lot of trout—they simply don’t have enough spawning habitat. Lakes with streams flowing into and out of them, on the other hand, can hold lots of trout because the tributaries provide good spawning areas.

High-elevation lakes such as Maligne remain mostly ice-covered until late May, sometimes retaining their ice surfaces into June. At lower elevations, where spring comes earlier, lakes lose their ice in late April or early May. Those lower elevation lakes are often more biologically productive, especially ones with shallow areas that warm fast. Aquatic invertebrates—food, as trout would describe them if they could—can be very abundant in some of those low-elevation lakes. That’s not the case in the higher, colder lakes.

I was skeptical all those years ago by Maligne’s sudden ability to produce a record fish

So, if you are wondering how big a fish is likely to grow in any given lake, you need to consider how many fish the lake holds, how much invertebrate food the lake can produce, and how long the growing season is. Few fish, lots of food and a long growing season equals big trout. Numerous fish, limited food and a shorter growing season equals smaller trout.

In the 1970s, Parks Canada was still stocking trout into some Jasper lakes for anglers to catch. They wanted to manage their fisheries as efficiently as possible, so they commissioned a lakes assessment by the Canadian Wildlife Service. David Donald, a CWS limnologist, was assigned the project. I was working on the wildlife component of an ecological land classification of Jasper National Park when I first met him there. He became a respected colleague, largely because of the disciplined, scientific approach he took to his work.

Donald’s Jasper study involved mapping out lakes to determine their productive areas, gill-netting to get samples of the trout populations, and examining those fish to determine their ages, growth rates and feeding habits. He then worked with the data to calculate the ideal stocking rates for each lake in order to achieve the best outcome for those who fish there. As with other Jasper anglers, I naturally took a keen interest in his research.

One of the lakes in his study was First Lake, where I’d spotted that giant rainbow. The lake had likely never held trout until the 1920s, when park staff began stocking it. All five lakes in the aptly named Valley of the Five Lakes are fed mostly by groundwater, their levels rising and falling through the seasons as the water table changes. Given the lack of spawning habitat, Parks Canada continued stocking the lakes repeatedly until the early 1970s to keep providing sport for anglers. In First Lake, those stocked fish grew fast and got  big because the lake teemed with freshwater shrimp, as well as damselfly nymphs and many big caddisfly larvae.

“The density of shrimp in First Lake to the 10-metre-depth contour, the zone typically inhabited by trout, was 2,282 per metre square,” Donald says. “Our survey of the fish food resources of 50 mountain lakes put First Lake at the top of the list. It had the highest density of Gammarus, the largest of the two species of shrimp found in these lakes. That lake was primed to produce really massive trout.”

The big rainbow called into question the scientific picture of Jasper’s lake that fisheries science had been building

Sitting at a much higher elevation than those montane lakes, Maligne has a self-sustaining fishery; the rainbows spawn in flowing water at the lake’s outlet and inlets, and the brook trout spawn in the spring upwellings. However, the shorter growing season and colder water limit its biological productivity. Donald found only 220 shrimp per square metre there. Still, Maligne’s large size makes up to some degree for the more limited food supply, giving the resident trout a lot of habitat to feed in. As a result, the lake’s rainbows can reach up to four pounds, while brookies in the seven-pound range have been caught there.

No wonder I was skeptical all those years ago by Maligne’s sudden ability to produce a record fish, a specimen four or five times larger than any other rainbow trout ever caught there. Donald was also skeptical, but he had better-informed doubts than mine, and it also mattered more to him. The big rainbow called into question the scientific picture of Jasper’s lake fisheries his studies had been building. Based on his knowledge of Maligne’s productivity, it just didn’t seem possible a record fish could have come from there. If it did, he needed to understand why.