Trout Stream Ecology and Management

Introduction

A trout stream may be defined as a stream which contains as a dominant species some member of the Salmonidae. While trout are dominant, other fish species may be present; most common are species of minnows, suckers, and sculpins. Since trout are cold stenothermal fish, trout streams have to be cold water streams, remaining under 25 degrees Celsius throughout the year. Dissolved oxygen has to be high to support trout. Five ppm is a lower limit, but most trout streams remain near oxygen saturation. Chemistry can be quite variable; as long as the stream is not grossly polluted or receiving acid drainage, trout will survive. In general, streams with a higher conductivity (more dissolved materials) will be richer in benthos and produce faster growing trout.

Characteristic benthos of a trout stream includes:

• mayflies
• stoneflies
• caddisflies
• true flies (especially midges and blackflies)

Many other organisms are likely to be present, depending particularly on the gradient of the stream. In low gradient streams, snails, beetles and dragonflies may be abundant. Benthic insects are the principal food of trout although many of them are harvested at the stage in their life cycle where they are emerging from nymph to adult. Trout, particularly in small streams, also eat many terrestrial invertebrates: spiders, ants, leafhoppers, grasshoppers, etc. Smaller streams at the same elevation and latitude typically have more food. They both produce more benthos (g/sq m) and have a greater perimeter/volume ratio so as to intercept relatively more food of terrestrial origin.

Trout Behavior

Before considering management of a trout stream, it would be instructive to examine the behavior of trout in a stream. Perhaps the classic paper on trout behavior is by R. A. Bachman (Transactions of the American Fisheries Society, 1984. Bachman set up elevated blinds, from which he could watch and photograph brown trout in a stream. From variations in their spot pattern, he could identify individual fish. He found that the browns had a distinct home range averaging 15.6 sq m. Each home range contained a number (range 1 - 32, mean 6) of feeding stations where the trout would rest, making excursions out to feed. Home ranges could overlap; where they did, the two (sometimes more) trout used the same feeding stations, but not at the same time.

Feeding stations were created by an obstruction such as a large rock, which created an area of reduced velocity where the trout could remain with little effort. Average velocity at the fish's head was 8 cm/sec. A fish would position itself very precisly on a site; in repeated photographs of different visits, the position of the fish's eye would always be within an area 40 mm long by 20 mm wide. Fish used the same home ranges and feeding stations throughout their lives; one was followed for more than 7 years.

Within each home range was a refuge; an undercut bank or large boulder under which the trout would dive at any sign of danger. Several fish with overlapping home ranges might share the same refuge.

Fish would stay on a feeding station until a food item washed into sight. If large enough and close enough, the trout would move from the feeding station, engulf the food and drop immediately back to the feeding station. The bigger the prey item, the further from the station the fish would go to meet it. Sometimes, after a long venture, the trout would settle into a different feeding station. A smaller fish occupying that station would be displaced with a brief display. Bigger fish had smaller home ranges and utilized fewer feeding stations, largely because they were never displaced. Two fish could be in sight of one another only if the smaller fish was downstream. Since bigger fish only rose to larger items and very close small items, many food items would pass by them and be available to the smaller fish.

In summary, the wild fish operated to maximize energy gain per energy expenditure. Good home ranges typically were occupied by bigger fish; not because bigger fish took the best sites, but because fish grew faster and became bigger at the better sites. The limiting factor for trout was neither space nor drift, but rather the number of feeding sites. Growth was asymptotic; fish grew rapidly at first but gradually slowed, reaching a characteristic maximum for a given stream reach.

Bachman studied brown trout, but several other studies have shown similar behavior for rainbows.

In the light of our understanding of trout behavior, lets consider some common management strategies:

Planting hatchery-reared trout

• Even with reduced populations, the wild trout are capable of sufficient reproduction to fill the carrying capacity of the stream. Stocking hatchery fry or fingerlings adds no more fish, but may dilute the wild gene pool if any hatchery fish survive long enough to spawn.
• A natural stream has a limited production capacity. Wild trout are extremely wary and difficult to catch. Readily accessible trout streams have a far greater biomass of anglers than of fish. For the average angler to catch trout, the stream must be stocked with catchable-sized trout. (Should the average angler catch trout?)

Advantages of planting "catchable" trout

• Fishing can be provided as needed (just before holidays) for a large concentration of tourists.
• Fishing can be maintained near population centers.
• Relatively unskilled anglers catch fish.
• The fish are caught rapidly (73% are caught within a week of release.) Because of the rapid catch, fish can be planted and harvested from waters that will be too warm for trout later in summer.
• The yield of fish caught per dollar investment is higher than any other management technique

Problems associated with planting "catchable trout"

• Most hatchery trout are weaker, having battered fins, etc. that reduce their survivability.
• They are selected for hatchery life, not wild life. Hatchery life selects for a fish which is maximally competitive. They go for any food item they see. Where a wild fish maximizes energy gain per energy expenditure, the hatchery fish burns energy pursuing food items where more energy is spent in capture than is gained. Since they are all over the stream, they often come into the territory of wild fish, so engage in far more altercations than wild fish normally would. In a natural stream, hatchery fish lose weight and ultimately die. Unfortunately, the increased hostile encounters also results in loss of weight and possible death of wild fish, even if they win every encounter.
• If any hatchery fish survive long enough to spawn, they will hybridize with the wild fish, adding genes with less survival value to the gene pool.
• Consequently, planting of hatchery "catchables" dooms that reach of stream to a "put and take" fishery.

Stream Modification           

1. Protective Devices
   1. Screens and barriers - protect migrating fish from loss in diversions (very important for fish whose life cycle includes migration.) prevent predators or competitors from entering stream - most likely to be important when the undesirable fish is a closely related exotic species. (Presence of minnows, suckers, etc. have generally been shown to have little effect on trout populations.)
   2. Spawning channels or addition of gravels - seldom needed by resident populations in natural streams - may be functional for anadromous populations, particularly downstream of dams, where stream beds are likely to be armored.
   3. Fishways or guiding devices to get migrating fish past dams, turbines, etc. - again most important for anadromous populations.
2. Devices to increase habitat diversity
   1. Current deflectors - increase scouring, form deeper pools and undercuts (only work in cooler climate, low gradient streams, where they will only be functional if availability of refuges was a limiting factor.)
   2. Cover - anchor uprooted trees, add large boulders, etc.(see parenthetical comment above)
   3. Add feeding stations (scattered small boulders added to the stream bed) may increase productivity. No point in placing them closer than 1.5 m apart - trout won't generally tolerate others feeding that close.
   4. Dams
      • In a resident trout only reach, a small dam may provide additional trout habitat.
      • Beaver re-introduction (usually in conjunction with grazing restrictions fencing all but limited water access) has produced spectacular results in semi-arid reaches.
      • Beaver re-introduction can also be spectacularly unsuccessful in low gradient, marginal streams.(An increase in ponding may raise water temperature, favoring exotic predators like smallmouth bass)
      • In a mixed species fishery, dams generally favor non-salmonids.
      • Dams create all sorts of problems for migrating (anadromous) salmonids.
      • A small, headwaters dam can maintain year-round flow in streams that otherwise might go dry in summer.
      • Dams in a main stream will silt in within 10 to 200 years.

       

3. Removal of Obstructions (Again this is primarily for anadromous fish, to provide spawning access to streams.)
   1. Sand or gravel bars built up across the stream mouth.
   2. beaver dams
   3. logjams
   4. waterfalls

   Remember, removal of obstructions may allow access to undesirable species and loss of unique native faunas (other species as well as fish).

4. Fertilization

Effects are not well established. some positive effects have been reported. Could be quite damaging to downstream lakes.

1. Predator/Competitor Control (usually with piscicidal chemicals)

2. Any use of toxins should be thoroughly evaluated on a case to case basis. Other, perhaps rare, species may be impacted either directly or through loss of food resources. DFG's blanket, state wide, environmental impact statement for piscidal use is unconscionable.

   1. Though widely practiced to eliminate non-game species, there is almost no scientific evidence supporting this, especially when the non-game fish is native to the stream.
   2. In cases where a similar, exotic species has been introduced (e.g. brook trout introduced into a stream where golden trout are native) it may be the only way to retain habitat for the native species.
   3. Introduction of an exotic predator (e.g. northern pike) may also justify piscicidal treatment.
3. Land Use Management
   This is the greatest challenge and the most important aspect to management of a trout stream:

1. Restoration