Evaluation of visual encounter surveys as a method for the rapid assessment of fish presence and relative density in high mountain lakes

Abstract

1. Introduced fish are a widespread ecological threat in originally fishless high mountain lakes. However, basic distribution data are largely missing for most high mountain regions.2. Using time-consuming standard methods (e.g. Nordic standard fishing nets) to assess fish distribution and relative densities at a relevant spatial scale can be impracticable, because of the large number of high mountain lakes. To overcome this problem, alternative rapid monitoring methods would be helpful.3. Visual encounter survey (VES) is a candidate method that enables observing fish from the shoreline. It takes only minutes to implement and is already widely used for amphibian monitoring in high mountain lakes and ponds. VES was evaluated as a method for monitoring introduced salmonids and cyprinids (the most widespread fish families) in 52 high mountain lakes.4. The probability of detecting both families by VES rapidly approaches 100% as the relative densities of fish increase, and false absences are restricted to populations living at low relative densities. VES also provides simple indications about fish relative densities, distinguishing between high-density and low-density populations.5. As VES usually does not enable fish species identifications, we propose VES as a useful method to describe large fish distribution inventories, not needing high taxonomic detail, but necessary for planning large-scale conservation measures.

Persisting in extreme environments: What are the drivers of body conditions of introduced fish in high mountain lakes?

“…In addition, summer water temperature indirectly affected salmonid body condition in a negative way owing to its positive effect on density of deep benthic invertebrates and salmonids, and forage fish presence. Despite that salmonid densities in these mountain lakes can be 50% lower than in other similar ecosystems (Pastorino et al., 2020; Tiberti et al., 2022), this result suggests that negative effect of density‐dependence – here competition – could be at play for altering Kn . In fact, density‐dependence may be exacerbated when resources are scarce (Deyle et al., 2016) as often identified in mountain lakes.…”

Environmental DNA Characterization of the Fish Species Composition in the Mukawa River and Adjacent Habitats

“…The rapid assessment concept of obtaining the maximum amount of information for a minimum of effort and cost [67] to determine species presence or compositions of a particular location or type of habitat [68] has been applied using various sampling methods such as visual census [69,70], remotely operated vehicles (ROV) [71], or more recently using eDNA samples, so as to evaluate the distributions of invasive species (e.g., [72,73]). An expanded version of the low-effort present study that would include a few more water samples taken in freshwater reaches where there is easy access, could for example, be used to compare the fish species present in a series of nearby river systems over a few days, such as those on either side of the Mukawa River, which includes rivers flowing out of the Hidaka Mountains, as seen in Figure 1b.…”