Lake Fly Fishing Tip: Understanding Algal Blooms

By Brian Chan

Great stillwater fisheries, those that over time have proven themselves as consistent producers of big or well-conditioned trout, have several environmental features in common that sets themselves apart from the typical trout lake. These lakes almost always have extensive shoal or shallow water areas that allows for the growth of aquatic plant life that is necessary for the diverse aquatic food sources that trout eat. It is common for these fisheries to be landlocked or have only seasonally flowing inlets and outlet streams which means minimal flushing of the water. And most importantly, the water in these stillwaters is extremely nutrient rich which means the right chemical building blocks are present to grow the plants, fish food and ultimately the fish. Add a long, hot growing season and you have a fly fisher’s dream lake. Many of these special lakes are located throughout the western provinces and states. The overall quality of these fisheries is then determined by management objectives which consider stocking rates, harvest rates and specific fishing regulations.

Unfortunately, there are a few downsides to being a shallow, nutrient rich stillwater. The most common ecological events they must endure include summer and winter stratification which can play havoc with seasonal temperature and oxygen levels that at the very least impact trout feeding activity or worse their survival. Secondly, many of these water bodies support numerous species of algae which, under the right environmental conditions, can build up or explode in population and become what is known as an algae bloom. One of the most common algae encountered are those known as blue-greens. There are hundreds of species of blue-green algae but the most common ones look like slender grass clippings or tiny green balls suspended in the water or hovering in the surface film. Blue-greens are not true algae but actually a photosynthetic bacteria which belong to the family of cyanobacteria. The presence of these bacterial species can be problematic for both fish and animal life. As fly fishers it is important to understand the life history of blue-green algae because of their impact on how or where fish can live prior to and during a bloom event. There are also fishing tactics that work during the heaviest of algal concentrations.

Blue-green algal blooms typically occur during lengthy periods of hot, calm weather which often occur during the summer months of July and August. Under a bloom situation the number of algae increase dramatically which results in an almost total covering of the surface of the water. A heavy bloom closely resembles thick split pea soup in consistency and color. Photosynthetic blue-green algae need light to survive. During daylight hours blue-green algae produce oxygen so the algae accumulates as close to the lake surface as possible in order to capture the energy of the sun. Blooms can become so thick that a die-off occurs as many of the algae are unable to photosynthesize. When this happens, the decomposing algae strips oxygen from the water, which can result in a fish kill that is often referred to as a summer kill. A typical summer kill occurs when dying algal cells are blown into a shallow bay or arm of a lake. The mass of decomposing algae quickly reduces the oxygen content of the shallow water to a level that causes the fish to become disorientated and suffocate from lack of oxygen.

Blue-green algae bloom die offs are also toxic to mammals. Decomposing algal cells release neurotoxins that affect the nervous and respiratory systems and hepato-toxins that affect the liver. Livestock drinking affected water are the most common casualty from a severe blue-green algae die-off. Humans and our pets can suffer the same fate or at least get very ill from consuming enough of the contaminated water. During a bloom and subsequent die-off the water looks and smells bad. As the algal cells break down, pigments are released that often resemble streaks of aqua-blue colored paint in the surface film or along the edge of the shoreline.

In many lakes the blue-green algae will be present for the entire summer season and never reach the bloom and die off stage. Most blooms are short lived, lasting from several days to a couple of weeks before the die off is over and the water clears.

Fly Fishing Tactics

Blue-green algal growth and proliferation typically occurs during the warm, summer months on western stillwaters. The presence of algal cells in the water does not necessarily have an impact on fishing success. Aquatic insects continue with their normal development but by mid-summer most of the major emergences of mayflies, damselflies and caddisflies have occurred. On-going chironomid or midge hatches are perhaps the most significant mid-summer emergence that occur on many of our most productive waters. Often, the species of chironomids emerging are large, sometimes in excess of 5/8th’s of an inch in length. These pupae are rising vertically through the water column, however “soupy” it may appear, to complete the transformation into the adult stage. Some anglers may be under the impression that the blue-green algae completely fill the water column because they appear so thick in the first few feet of water below the surface. In fact, because they are trying to photosynthesize, the majority of algal cells accumulate within the upper 6 feet of the water column. The water below this depth range is often quite clear. We also need to remember that trout prefer to feed on the larvae and pupae either while in the bottom substrate or within a couple of feet of the lake bottom. Standard larval and pupal presentation techniques such as suspending under strike indicators or fishing floating lines and long leaders with weighted flies can be both very effective at this time of the year. Both techniques allow flies to be presented as close to the lake bottom as desired. It is always good to check your fly before each cast as it can get a coating of algae when brought up through the last several feet of water. Similarly, once your fly has reached the desired fishing depth zone give the fly line several, quick 12 inch long pulls to "clean" off any algae that may have covered the fly while it sank down through the water.

Warmer summer water temperatures in shallow lakes and the build-up of blue-green algae densities have an impact on where in the water column trout can live and feed. The upper layers of water, anywhere from 15 ft deep and less, can be too warm for trout during daylight hours. Feeding activity occurs in deeper, cooler water. This is the time when trout often switch to feeding on zooplankton such as Cladocera which are commonly known as water fleas. There are many species of Zooplankton and they can be extremely abundant in nutrient rich lakes. These small organisms are typically less than 3/32nd of an inch in length and are photosensitive. Their colors vary from translucent shades of olive to a reddish-orange. During the day they concentrate in dense layers in the deeper depth zones and at night they move up towards the surface of the lake. Trout swim through these layers or bands of zooplankton and literally breathe them into their gullet. Imitating a cloud of zooplankton would be the ultimate fly tying challenge. Instead, we try offering other food sources, such as leeches, in an attempt to entice the trout to try a bigger meal. This is one of the main reasons why leeches are so effective during the hot summer period of algal filled lakes.

Normally, leeches are found attached to the underside of rocks or woody debris along the lake bottom. They are more nocturnal in their activity levels and so are not normally seen swimming in the open water. However, they do become more active during the warm summer months and particularly during heavy algal concentrations which may make them less visible to trout. They can often be seen free swimming during daylight hours. Leech patterns fished on intermediate or fast sinking lines can be quite effective during the summer blue-green period. Patterns can be retrieved or trolled through different depth zones. Consider using bead-headed patterns tied with flashy materials and try a variety of retrieves such as several long fast strips followed by short pauses. At this time of the season it can be a case of having to attract the attention of a fish who may be concentrating on zooplankton rather than presenting a perfectly swimming leech imitation. Remember the impacts of summer stratification and the fact that the trout may be less active in their feeding activities due to overall warmer water temperatures.

While there can be some very good fishing during the presence of blue-green algae in our favorite stillwaters we should always keep in mind the impacts of playing and handling trout under less than ideal environmental conditions. Even with the best intentions of catching and releasing our fish we may be killing them due to the buildup of lactic acid, associated warm water and lower oxygen levels. Prolonged hot, sunny periods and extensive buildup of blue-green algae may be the signal to leave the trout alone until environmental conditions are better.