Fall is unquestionably my favorite time to be drifting in my boat fishing loch style. In this post I'll take you through how the anatomy of a lake forces a resurgence in trout activity as water temperatures cool. Fall is often a time when the largest fish of the year show themselves in the shallows where they are most easily available to the fly angler. For example, the fish below came from a Wyoming lake a few weeks ago all between 4 and 13 feet in depth.
Typically most mid-elevation lakes (at least in the western United States) are what limnologists refer to as dimictic. This means they have two periods, spring and fall, when they become isothermal (one temperature) and they are prone to mixing during these times because of the water current that wind creates. During summer, the water in a lake typically stratifies into 3 layers defined below. This stratification occurs because of the different densities of water when water is at different temperatures.
Epilimnion: this is the warm surface of the lake where light penetrates easily. By definition, it remains relatively isothermal and thus the wind can continue to mix this layer. As phytoplankton or zooplankton are produced in this layer but die without being eaten, nutrients sink to the bottom of the lake and are sequestered until the beginning of turnover in the fall.
Thermocline: this layer is where light penetration fades and water temperatures rapidly drop as a result.
Hypolimnion: this layer is below the thermocline and is once again somewhat isothermal. In productive lakes this layer can often become hypoxic (devoid of sufficient oxygen) because of detritus decomposition and lack of photosynthesis. Fish may be limited by oxygen demands to the thermocline or epilimnion if it remains cold enough for comfort.
As an example of this stratification I've included a graph of August temperatures on Dillon Reservoir, CO where I've been conducting my Master's research. Notice the clearly defined water layers.
As lakes move into fall the epilimnion and thermocline begin to cool to the point where the entire lake becomes isothermal as seen by the straight line in the November graph below.
At this point water begins to mix and nutrients lost from the surface layers during the summer are brought toward the surface again often resulting in a bloom of zooplankton which is not lost on the trout. Once the entire lake becomes isothermal around 40 degrees fahrenheit, wind can readily mix the water which reoxygenates the water column going into winter. This is a critical process in lakes that are prone to winterkill. In shallow productive lakes the water can become quite turbid at this time and the fishing can turn off. This effect is less pronounced in large deeper lakes where the transition happens in spatially separate areas of the lake. You can usually find clear water and feeding trout somewhere int he lake in this situation if you have mobility.
As surface waters cool, trout activity picks up. In many lakes, epilimnion temperatures become too warm for comfort or even survival for trout during the summer. As the temperature drops back into the low 60's and 50's their metabolism responds dramatically and their demand for consumption increases. The conundrum is that most of the main insect hatches have waned at this point other than in some lakes which have multiple broods of callibaetis. The trout must focus on other food year round food sources like leeches, scuds, zooplankton, etc. Trout usually move into the shallows as temperatures cool since these are usually the richest areas in lakes with a functioning littoral zone. Fluctuating reservoirs don't often fit this description when they are steep sided but trout still utilize the food congregating nature of the water's edge. Trout also become more opportunistic during this period since they are not seeing scores of one food item but usually must make use of any food item they come in contact with. This is when the fly angler should put the dainty wet flies and midge tip aside and pull out the buggers and heavy tippet. This is also why I believe fall provides the perfect recipe for the fly angler. It's a time when trout need to eat but there is less to eat than during the spring and summer hatches. So go show the trout a few flies they are desperate to chomp before your lake freezes this fall. I think you'll be happy with the result.