Rainbow trout (Oncorhynchus mykiss) is an incredibly important species in many freshwater systems across North America. This member of the Salmonidae family maintains an anadromous lifestyle; the majority of its wild populations venture into the Pacific Ocean to feed and mature. Once it is ready to spawn, its schools migrate into inland waters. Due to its migratory nature and its human-mediated introduction into lakes outside of its native range, some populations have become landlocked.
Named for its eye-catching, iridescent hues, rainbow trout is generally valued as a sportfish. Its adult forms tend to be heavily speckled. Sexually mature males are distinguished by their lateral, deep-red stripes. Specimens that have adapted to remaining in freshwater systems through their entire life cycle possess a more silvery color pattern and do not fully develop red stripes on their sides. Those that are used to produce fry for lake and pond stocks are usually domesticated, hatchery-derived strains.
Due to the rainbow trout’s value as a food fish, it is now frequently stocked into both recreational and aquaculture ponds and cages. Some pond owners opt to stock this fish as a secondary aquaculture crop during the cool months. Others stock trout exclusively to diversify their pond’s fish profile for winter fishing. Both natural, crude ponds and complex, high-energy systems can be used to rear this species.
Requirements for Survival in the Wild
To comprehend exactly what conditions a trout would need to survive in a makeshift pond or an artificial system, it’s necessary to look into its requirements for survival in the wild. A cold-water fish, rainbow trout favors water temperatures that range from 13 – 19˚C (55 – 66˚F). Higher temperatures compromise their capacity for feeding and may be linked to poor growth rates and the occurrence of diseases. Thus, this species is not frequently captured from ponds in summer, when temperatures may be too warm.
Another major requirement that supports the life cycle of rainbow trout is a gravelly substrate. In lakes and ponds, this allows them to search for ideal spawning sites, over which females can deposit their eggs. It must be noted that, while a gravel-based substrate is recommended for natural ponds, schools of rainbow trout can struggle to develop into self-sustaining populations outside of their natural systems.
In terms of food, wild trout are not particularly picky. Their diets are highly varied and are significantly influenced by the seasonal availability of food types. Those found in landlocked lakes may subsist on aquatic insects, fish eggs, snails, smaller fishes, and small crustaceans. They may also feed on plankton and, in cases where food is scarce, on the decomposing meat of other fish species.
General Requirements for Recreational Trout Ponds
Impulsively stocking rainbow trout into a recreational pond without going over their basic requirements can lead to disastrous consequences. Ponds that are too warm or too small can quickly lead to the demise of trout fry. These will only survive and mature in ponds that are appropriate for their cultivation. Pond size, temperature, water flow, food availability, and the presence of other fish species are just a few aspects to consider before stocking your pond with trout.
1) Pond size, depth, and stocking rates
For cold-water species like rainbow trout, ponds that are at least 8 – 12 feet (2.4 – 3.7 meters) deep and have an area of around one acre may be used. If the pond is naturally fed by a cool stream, a slightly shallower depth may be acceptable. An adequate depth should help prevent major temperature and pH fluctuations that may significantly compromise the metabolism and immune resistance of your fish.
The total volume of your pond and its rate of water flow would, of course, determine how much trout you should stock. Your intention for stocking trout and the rate at which you intend to fish or harvest them should also influence this value. Pond shape, particularly in intensive setups, can also influence trout stocking rates. In lengthy, aerated, and deep grow-out ponds, where trout are intermittently harvested as they grow, non-conventional and high-density stocking rates can be used. In earthen ponds with natural food sources, a lower number of stocked fingerlings should support the growth of larger specimens.
For example, in a one-acre pond with about 200 – 300 fingerlings of stocked trout, your fish should grow to a favorable size of about 9 ounces or more within approximately two seasons. In the same pond, a stocking rate of about 400 – 500 fingerlings would result in specimens that take twice the length of time to reach the same size. Moreover, unless daily feeding and intermittent harvests are performed, the chances of obtaining mixed sizes and lower survival rates are higher.
2) Water temperature
The water temperature in a rainbow trout pond should rarely, if ever, exceed 21˚C (70˚F). Higher temperatures may be lethal for this cold-water species as it can increase their susceptibility to a host of parasites and diseases. It also negatively affects their metabolic rate, preventing them from efficiently digesting food. While a few hours of exposure to moderately warm temperatures may be tolerated by healthy trout, temperatures above 26˚C (78.8˚F) are considered lethal.
In response to rising temperatures, you’ll find that trout tend to retreat to the deeper sections of a pond. They will rarely rise to depths of less than 3 feet (91 cm) during particularly warm days. Keep in mind, also, that higher temperatures will lead to a slower rate of growth. For ideal growth rates, aim to stock trout in ponds where the water remains within the range of 13 – 18˚C (55 – 64˚F) for the majority of the year (at least through fall and spring).
In the winter, rainbow trout should have no issues surviving in adequately deep ponds that eventually become covered in a surface layer of ice. They can withstand water temperatures that dip to around 0 – 2˚C (32 – 35.6˚F). During this time, they will retreat to the bottom layers of a pond, where the water may be a few degrees warmer than the layer right beneath the ice. Rainbow trout may enter a dormant period, during which they conserve their energy and may stop consuming food. Growth may resume once temperatures rise to more than 4˚C (39.2˚F).
3) Water quality
Water quality should be optimized prior to stocking trout. This may mean freeing your pond of all potential sources of pollutants, amending its pH levels, and ensuring that it has sufficient concentrations of dissolved oxygen. Optimizing water quality may be challenging in natural, earthen ponds, where a degree of sediment suspension and uncontrolled populations of plankton and algae may alter conditions. In lined aquaculture ponds that allow for a full or partial drain and the replenishment of clean water, the maintenance of preferred conditions would be more straightforward.
Dissolved oxygen (DO) is one of the most important parameters to watch for when rearing rainbow trout. Ideally, the DO concentration should be maintained at or above 5 ppm at all times. When rearing the initial developmental stages of fry, higher concentrations are recommended. Adults can tolerate slightly lower levels, especially those observed during and after feeding times, but they may struggle to survive through prolonged drops. While pond vegetation and movement from a source stream can aid in maintaining DO levels, pond owners usually provide supplementary aeration.
The pH level of your pond water is another crucial parameter for trout growth. This should be kept at around 6.5 – 9 throughout the growth period. Drops in pH, which are often associated with leached nutrients draining into natural ponds, can be amended with the addition of lime or other alkaline substances. Apart from a markedly low pH, harmful gases released by decaying materials may also negatively affect trout growth. The inflow of new water, while a partial drain takes place, may aid in the removal of these gases.
4) Presence of other fish
Other commercially valuable types of freshwater fish may be stocked into rainbow trout ponds if they share similar water preferences. Note, however, that other fish species will understandably have slightly varied requirements. Moreover, they may compete with rainbow trout for resources or they may perceive them as either predators or prey. Before stocking other species alongside your trout, consider how their presence may alter the food chain or the overall ecology of your pond.
If your pond is intended for personal recreational use, you need not be too concerned about how other species may impact the growth rate of rainbow trout. It does, however, pay to be wise about stocking times. For example, smaller species, like minnows, may be stocked prior to or while introducing trout fingerlings into the pond. This should give them time to generate self-sustaining populations (as a forage base) that are more likely to support trout growth over a longer period of time.
5) Food sources
Food sources for trout will significantly differ depending on the type of pond into which they will be stocked. In natural ponds, trout may feed on smaller fish, amphibians, and a wealth of other freshwater critters. In aquaculture ponds, they are usually supported with a specialized formulation of pellets. Some small-scale trout farmers may opt to supplement an existing forage base with fish feeds.
In unfertilized ponds, higher growth rates are usually observed with a daily feeding routine. Note that, without the provision of fish feeds, growth rates may be exceedingly slow – even if rainbow trout are stocked at conservative densities. The amount of feeds necessary for rapid growth would depend on water temperature and a manageable feeding schedule for your desired trout length or weight.
Stocking Rainbow Trout
Stocking schedules would depend on the size of the fish, seasonal temperatures in your area, and the type of fish pond. For fish-out ponds, which are typically stocked on a daily to weekly basis with catch-size trout, stocking should ideally be restricted to the cooler months of the year. For winter fishing, catch-size trout can be stocked in fall or when temperatures have dropped to below 21˚C (70˚F). Avoid stocking more fish than the amount that can realistically be caught before late spring of the subsequent year.
Stocking grow-out ponds with trout fry involves more complex scenarios. As these fish will presumably be raised to harvest size before the succeeding summer, you will have to account for their growth rate and potential losses due to mortality. Grow-out ponds, which are usually single-species ponds, are more equipped to deal with high stocking rates.
Stocking larger-sized fingerlings, which may measure anywhere from 3 – 8 inches (7.6 – 20 cm) long, would lead to a shorter rearing time for obtaining catchable sizes. This option often results in lower mortality rates, but it would undoubtedly cost more. Some farmers opt to stock their ponds with smaller fry (1 – 2 inches or 2.5 – 5 cm) despite the dangers of higher mortality rates and a more lengthy grow-out period. Make sure to keep in mind that, in winter, outdoor ponds will not be able to sustain a rapid rate of growth due to decreased temperatures.
Management of Rainbow Trout Ponds
In recreational trout ponds, responding to growth or disease-related issues usually involves a “trial-and-error” method. For example, if the harvested sample of trout appears to have grown at a slow pace or shows signs of stunted growth, the farmer may opt to either increase feeding rates or remove a portion of the population.
In aquaculture ponds, where larger densities are produced to meet commercial demand, poor growth is often related to exceeding the carrying capacity of the pond. This is why stocking rates are fine-tuned beforehand and why several parameters (DO, suspended solids, hardness, pH, and nitrites) are tested each day. Water conditions need to be optimized, and farmers may conduct either a partial or full harvest in response to poor growth.
One of the trickiest problems for trout farmers, regardless of whether their ponds have been fully optimized prior to stocking, is disease. Poorly managed ponds subject rainbow trout to many stressors, increasing their susceptibility to both environmental toxins and infectious diseases.
While testing for certain pathogens before the stocking period may help alleviate the chances of disease-related losses, it cannot ensure a successful harvest. Even sourcing your trout fry from sterile and certifiably disease-free hatcheries does not guarantee immunity. Be prepared to repeatedly assess pond conditions, respond to problems in a time-sensitive manner, provide medication if necessary, and have the manpower for emergency harvests.
