Seasonal Changes in Water Chemistry
Trout are cold-blooded and their metabolism is directly related to water temperature and associated dissolved oxygen levels. During the open-water period there are three limnological events that occur. These determine where in the lake the trout can live. At ice-off the entire water column is cold. But on close examination we find the bottom layers of water are actually a few degrees warmer than the surface waters. The benthic water is warmer due to heat released during the decomposition process of plant matter, which occurred through the winter months. Some of these lakes are covered with ice and snow for up to six months a year. Two major mixing events occur each year in almost all small trout lakes in the northern hemisphere. Spring turnover typically occurs within a few days to a couple of weeks after ice-off. Warmer air temperatures slowly warm the cooler upper layers of water until the temperature of the entire water column is similar. Water is most dense at 39 degrees Fahrenheit and at that point mixes easily with a little assistance from the wind. A lake in turnover becomes quite turbid or murky as detritus is swept off the lake bottom and mixed throughout the water column. Spring turnover is an important event as it ensures the entire water column is saturated with oxygen. However, fishing during a turnover event is generally not very productive, as the overall water chemistry is poor and the fish generally become dour. It generally takes three-to five days to complete the turnover and for the water to clear. The completion of turnover signals the start of the spring fishing season as continually warming water initiates a parade of aquatic insect emergences.
Long, hot summers are a key ingredient in producing a quality stillwater trout fishery. Photosynthetic energy drives the aquatic ecosystem by stimulating phytoplankton and zooplankton growth. In addition, it produces aquatic plant production, and the development and maturation of insect populations. Under optimum conditions this all translates into significant fish growth. However, trout are cold-blooded and their metabolisms are affected by water temperature and related changes to water chemistry.
By early summer, the upper layers of water in many small lakes begin to warm and become less dense than the colder, deeper water. In other words, the energy from the sun penetrates to a certain depth and beyond that the water stays much cooler. A temperature gradient known as a thermocline develops where the warmer upper layers, known as the epilimnion, meet the colder, deeper water, known as the hypolimnion. Thermoclines are invisible and typically establish between 15 and 25 feet below the surface of a lake. They are a barrier to mixing. Therefore, summer winds continually warm and circulate the epilimnion while the deeper hypolimnetic water below remains unmixed. Ongoing bacterial decomposition occurring within the benthic or bottom areas of the deepwater zone results in decreasing oxygen levels below the thermocline. In many of the most productive small trout lakes there may not be enough oxygen below the thermocline to support fish life. In very hot weather trout leave the warm shallow shoal areas and sit just above the thermocline where the water is colder and well oxygenated. Trout suspended along the thermocline show up well on a properly calibrated fish-finder. Trout often come back onto the shoals to feed at night when water temperatures are cooler and oxygen levels hit the appropriate concentrations. That is why we experience some great late-night summer fishing in water often less than 10 feet deep.
By early fall the temperature differences between the upper and lower layers of water is reduced and the thermocline breaks down. By mid-fall, air temperatures will have cooled upper water layers significantly, and once the entire water column is isothermal, or of similar temperature, and as close to 39 degrees Fahrenheit as possible, the entire lake will mix with the aid of a good wind. As in the spring, the water becomes murky for a few days and then clears. Fall turnover is critical to the winter survival of trout, as this late-season mixing action ensures the entire lake is well oxygenated prior to freeze-up.
Although short in duration, late fall fishing provides some of the most exciting action of the year. Trout feed very aggressively before the onset of ice cover. In a nutshell, it is shallow water fishing for some of the biggest trout of the season. The fish are looking for just about any food item that moves, but imitations of leeches, scuds, damselfly and dragonfly nymphs, and chironomid larvae top the grocery list.
Seasonal Changes in Water Chemistry
Having a good understanding of where trout live and feed in a lake makes it much easier to determine which fly lines should be in the stillwater tackle bag. A floating line can be used extensively because so much feeding occurs in water less than eight meters deep. Floating lines with varying leader lengths can be fished with or without strike indicators. The choice of setup is dependent on the food source being imitated and the specific depth zones being targeted. An intermediate sinking or a slow sinking line is another must-have for the serious lake fly fisher. These lines can be fished in shallow water. And, by varying the wait time before initiating a retrieve, these cover of any part of the depth zone being fished. Fast sinking lines from Type 3 to Type 7 allow coverage of the steeper drop-offs and also any fishing situation encountered in the deepwater zone. Another excellent lake-fishing line is an emerger tip, meaning a line with short, five-foot-long intermediate sinking tips. These mini sink-tips are great for fishing chironomid pupae, mayfly nymphs, damselfly nymphs and emerging caddis pupae.
A good background in basic structure and the biological processes that occur each year in our favorite fishing lakes makes catching fish, including large ones, a lot easier. And knowing the various aquatic food sources that trout eat, and their life-cycles, is paramount to success.Knowing these things arms you with the knowledge you need to present the correct flies to trout, at the proper depths, in any season.