May 27, 2016

Improving Growth, Survival and Quality of Stocked Fish

The Freshwater Fisheries Society of BC is continually researching methods and techniques to improve the growth, survival, and quality of the freshwater fish stocked into British Columbia’s lakes and rivers. This work provides provincial fisheries managers and fish culturists tools for improving recreational fishing, and helps protect and conserve wild fish populations and biodiversity. The Freshwater Fisheries Society strives to improve fish performance by researching non-reproductive technologies and size at stocking.

Non-Reproductive Technologies

Triploid (3n) Technology

The use of non-reproductive technology in hatchery production has direct benefits to the fishery and protects wild fish populations. This technology involves a sterilization process that is unique to each species of fish. Such treatments increase the cost on a per-fish basis, but the fishing benefits can be substantial. Currently approximately 50% of the small lakes in BC are stocked with triploid fish. The Freshwater Fisheries Society produces triploid rainbow trout, eastern brook trout, kokanee, and cutthroat.

Benefits of Sterility:

  • triploid fish can potentially live longer than diploid fish
  • natural post-spawning mortality is reduced
  • can result in a higher number of larger “bright” fish being available to the fishery
  • has the advantage of reducing the risk of potential genetic interactions between hatchery and wild fish and/or preventing the establishment of hatchery populations in areas of conservation or biodiversity concerns.

One misconception about triploid fish is that they are Genetically Modified Organisms (GMOs). Triploid fish are not GMOs. Genetically modified animals are organisms that have had their DNA molecules altered by genetic engineering. Triploidy is a condition that can occur naturally in salmon and trout (albeit at very low rates) that makes the affected animals reproductively sterile. Currently the Freshwater Fisheries Society induces triploidy through the application of hydrostatic pressure to the eggs shortly after fertilization. This technique results in the retention of the second polar body normally extruded shortly after fertilization. This results in the retention of a third set of chromosomes instead of the usual two sets. This triploidy method achieves a 98-100% sterility rate in our facilities. The result is sterility in both males and females. Through this relatively simple pressure shocking process, no genetic material is introduced, moved or taken away. The fish are not “genetically modified” in any way.

All-Female (AF) Technology

The creation of an all-female population of fish is labour-intensive. The Freshwater Fisheries Society produces all-female populations of fish by crossing sex-reversed females with normal females. To create sex-reversed females normal fry are exposed to testosterone derivatives to halt ovary development and produce normal testes. These "masculinised" females remain genetically female (XX) but functionally male. The sperm must be extracted by hand as these fish generally do not develop a sperm duct. Once mature they are crossed with normal XX females to create all-female progeny for release.

An all-female stock has benefits over a mixed sex (male and female) stock in that it takes advantage of the slower maturation in females which means more time available to the fishery. Over 80 percent of males in some stocks exhibit precocious maturation (or “jacking”). These males mature during their first season in the fishery and have a high post-maturation mortality, lower flesh quality, and an overall smaller fish size. In addition, the stocking of all-female progeny in the absence of males effectively creates a functionally non-reproductive population.

All-female Non-Reproductive (AF3n) Technology

The production of AF3n fish for stocking derives the benefits of both the process of "feminization" and triploidization (AF and 3n). Sterilization manifests itself differently depending on the sex of the fish. Female triploids are hormonally and functionally sterile (i.e. no development of functional oocytes (eggs), and no behaviour changes). Male triploids are sterile, but still produce hormones that cause changes associated with maturation including deterioration of flesh quality, early mortality, and the development of testes and other secondary sex characteristics. In addition sterile males demonstrate "false" spawning behaviour including migration and interactions with other fish on the spawning beds regardless of their inability to reproduce. This behaviour can potentially displace wild functionally reproductive males possibly reducing the fitness of the wild population.

The double treatment (AF3n) for stocking programs further reduces the risk to wild fish since the females will not display “false spawning”. AF3n ensures that if there is a small percent of individuals for which the tripoidy process did not occur that the stocked population would be functionally unable to spawn by virtue of the fact that all progeny are females.

Eastern brook trout are not native to British Columbia in watersheds west of the Rockies. All eastern brook trout that are produced and stocked in the province of BC are all-female triploid (AF3n) to prevent any interbreeding with the closely related native char species (including bull trout, Dolly Varden and lake trout), or the establishment of naturalized populations. Although this is done mainly for conservation reasons, the quality of the fish in the fishery is also improved.

Stocking Sizes

Fisheries managers can improve small lake fisheries by matching specific lake characteristics with the appropriate strain and stock selection, stocking rates, regulations, and even stocking the appropriate size of fish. Although limited somewhat by geographic, economic, and individual hatchery facility constraints, the size of fish to be stocked into a lake is an important factor to consider depending on the lake. In monoculture lakes, survival of even small fish is very good, and recruitment into the fishery is likely. In mixed species lakes, lakes that contain non-salmonid competitors or predatory fish species size-at-release is more important. In a study comparing growth and survival of fish stocked at different sizes, survival of large fish (stocked at over 20 grams) was two to three times better than that of small fish (stocked at under 10 grams) in mixed species lakes.

Author: Staff, Freshwater Fisheries Society of BC
Photo Credit: Verna Cameron