Improving British Columbia’s Freshwater Fishery through stocking optimization, one gram at a time
British Columbia is home to many native rainbow trout strains and some of the best recreational fishing opportunities in the world. The Freshwater Fisheries Society of BC (FFSBC) has become a leader in developing and stocking sterile forms of trout, char and kokanee, which improves angling while protecting wild fish populations. The key components of a successful stocking program involve determining the appropriate strain, size, density and timing of release. Each of these factors is complex and varies depending on the physical characteristics of the lake, predators, weather and many other factors. As part of continued efforts to provide great fishing opportunities, FFSBC is evaluating the post-stocking survival of our rainbow trout to see if we could improve survival by optimizing the stocking size. The key questions included: Does stocking size affect survival and what is the optimal survival size? In order to determine this we looked at the relative survival between different fry size classes and yearlings for the Blackwater and Pennask rainbow trout strains.
Blackwater and Pennask rainbow trout vary considerably in their life history traits and habitat use so it was useful to compare both strains in the stocking size experiment. The Blackwater River rainbow tend to thrive with competitor species: they spend most of their time on shoals, and they can grow to very large sizes. The Blackwater strain is more heavily spotted than the Pennask strain. They tend to have body spots from head to tail, with a heavier concentration of spots above the lateral line. This body-spotting pattern appears to be typical of coastal rainbow trout. These fish also tend to be fairly aggressive, opportunistic feeders and are caught readily by all gear types. In stark contrast to the Blackwater, the Pennask Lake strain rainbow trout do not thrive in lakes with other fish species. Pennask Lake is naturally devoid of other fish species and the natural prey items are mainly aquatic and terrestrial insects. A key life-history trait of Pennask Lake rainbow is that they are able to survive long winters and have an exceptional ability to conserve lipids over the winter. This makes them ideal for stocking cold, high-elevation lakes.
Having a sufficient supply of stored energy is critical if fry are to survive their first overwintering period, regardless of strain type. While it is assumed that larger juveniles may survive the winter with greater success, they too may starve if they do not maximize their energy stores or expend too much energy searching for food over winter. The experiment was conducted among eight small lakes in the interior of BC to assess the performance of the two wild brood stocks (Pennask and Blackwater) at three different sizes of fry and yearling life history stages. The lakes chosen for the experiment are productive monoculture lakes and are representative of the lakes in BC where fry are currently stocked.
Each group of fish in this study was reared to the target size through feed management, and then hand graded prior to stocking the fish in mid-September. The following fall when these fish were 1.5 years of age each of the lakes was assessed using standardized gillnets to look at the relative survival of each of the pre-determined size classes.
The data collected from this study illustrates that larger size fish do survive better for both the Pennask and Blackwater rainbow trout strains. However, it is also apparent, at least in the Blackwater strain, that there is a point in which survival will plateau despite the increase in size.
Another key component to this study was an examination of fry to yearling survival. Stocking fry would be more cost effective for the fisheries program when compared to stocking yearlings if survivals were comparable. At present, rainbow trout are most often stocked as yearlings. Fry were stocked in equal densities to yearlings from the same brood year in four small lakes, and assessed in the fall at 1.5 years of age to determine the relative survival of fry to yearling for both Pennask and Blackwater rainbow trout stocks. Survival differences between the strains were noted in large fry compared to yearling stockings. Blackwater fall fry had equal survival to yearlings while survival of Pennask fry was half that of yearlings.
The results of this project will greatly improve how we stock rainbow trout in the province. Optimal stocking sizes and age class requirements (fry vs. yearling) will result in lower mortality for stocked rainbow and allow FFSBC to more cost-effectively improve freshwater angling in stocked lakes.