Density‐dependent growth in brown trout: effects of introducing wild and hatchery fish

Abstract

Summary 1.Although it is not clear to what extent density dependence acts on the survival, emigration or growth of organisms, experiments testing alternative explanations are rare. A field experiment on 1-year-old brown trout (Salmo trutta L.) was undertaken to address the following questions: are the mortality, movement and growth of wild stream-living trout affected by population density? If so, are the density-dependent effects of released hatchery trout different from the effects of wild fish? 2. In each of two small streams, two replicate treatment blocks were used, each with four treatments assigned to stream sections 50-70 m in length: (1) control, no fish was introduced and population density was kept at its original level. (2) Trout biomass was doubled by introducing additional wild fish. (3) Trout biomass was doubled by introducing additional hatchery fish. (4) Hatchery trout were introduced, but biomass was kept at its original level by the removal of some resident wild fish. 3. We found no treatment effects on the recapture rates of resident trout, which suggests that survival was not strongly affected by competition. They were also remarkably stationary, regardless of treatment. However, trout growth rate was reduced to the same extent in both treatments with increased density, suggesting that growth was negatively density-dependent, and that the density-dependent effects of hatchery trout and introduced wild fish were similar. 4. Wild resident fish grew faster than introduced wild trout, which in turn grew faster than hatchery trout. Hatchery fish and introduced wild fish moved more than wild resident fish. 5. The results show that population density affected growth in trout parr. We conclude that competition is not limited to the underyearlings, as has previously been suggested, and that density-dependent growth is the main density-dependent response in yearling trout. Furthermore, this effect was the same for wild and hatchery-reared competitors, suggesting that stocking of hatchery fish may affect natural populations negatively through density dependence.

Brown trout growth in Minnesota streams as related to landscape and local factors

“…In contrast, brown trout relative abundance, grassland and cultivated riparian area, and thermal sensitivity had minor or nonexistent effects on growth. This supports conclusions reached by Dieterman et al (2012), who noted that limestone bedrock geology and agricultural watersheds promote high productivity in southeast Minnesota streams such that typical brown trout growth factors like prey availability, intraspecific density, social dominance are less important than in other regions (Bohlin et al 2002;Kaspersson & Hojeso 2009). Although thermal sensitivity did not explain spatial variation in an annual index of growth (MBLAA), previous research indicated late-winter brown trout condition (relative weight; Neumann et al 2013) was positively associated with groundwater input and negatively associated with thermal sensitivity in southeast Minnesota streams (French 2014;French et al 2014).…”

Coping with divided attention: the advantage of familiarity

“…Group members were defined as familiar neighbouringterritory holders. Fishes from different sites (more than 100 m apart) were defined as unfamiliar with one another because trout very rarely move more than 100 m (Bohlin et al 2002). Groups of captured trout were transported back to the laboratory in separate containers of water.…”

Distribution and growth of brown trout in pristine headwaters of Central Europe

“…The observed populations in the main river basins (Vydra and Křemelná Rivers) are essentially dependent on spawning migrants from downstream areas of the Otava River. Therefore, it can be suggested that wise management of hatchery fish stocking and the fisheries themselves [49,50] and the restoration of longitudinal river continuity [51] in downstream river stretches are necessary to achieve viability of the brown trout populations in Šumava National Park.…”