Przegląd Geograficzny (2025) vol. 97, iss. 1
Natural riffles – biodiversity havens: analysis of spawning grounds for fish and habitat functions for invertebrates
Przegląd Geograficzny (2025) vol. 97, iss. 1, pp. 11-25 | 
Full text
doi: https://doi.org/10.7163/PrzG.2025.1.1
Abstract
This paper presents the results of implementation activities aimed at reconstructing a natural riffle along the bed of the River Mierzawa at Przyłęczek, Poland, with a view to spawning grounds for brown trout Salmo trutta being restored and created.
Each species of lithophilous fish requires specific spawning conditions due to differences in reproductive biology. Thus, for instance, salmonids including brown trout bury their eggs in gravel. However, the choice of a spawning site depends on factors such as water velocity, depth, riverbed slope and sediment grain-size. These parameters are crucial for the proper incubation of eggs by the trout, which takes approximately 400 degree-days in the species in question. The appropriate grain-size allows eggs to be placed properly between gravel particles, and ensures adequate rinsing and gaseous exchange during incubation. The sizes and shapes of grains are important, as those with sharp edges can injure female fish during nest building.
High water velocity in spawning areas influences the transport of sediment, preventing the deposition of fine-sand fractions that could block spaces between gravel, and hinder water flow through the nest. Water depth, slope, flow velocity, and bottom-sediment permeability are all key to water infiltration into the spawning substrate. Deeper water and steeper slopes increase hydraulic pressure and gradients, promoting groundwater flow. Higher flow velocities can clear sediment pores, encouraging infiltration, whereas slower flows promote particle deposition, reducing infiltration intensity. Substrate permeability determines the rate and extent of water infiltration, which is vital for maintaining oxygen-rich conditions for egg incubation. Fish also avoid the shallowest depths in order to ensure that nests are protected from exposure during periods of low water.
The studied riffle’s hydrodynamic and granulometric parameters were studied so as to allow for an evaluation of its potential as a reconstructed spawning ground for brown trout in an upland stream, and verify utilisation by fish of the reconstructed spawning site. Implementation work involved enrichment of the selected section of riverbed with gravel and stones in the following quantities:
• the 8-20 mm fraction: approximately 10 tonnes
• the 20-31.5 mm fraction: approximately 50 tonnes
• the 31.5-100 mm fraction: approximately 40 tonnes
These restoration efforts were prompted by year-on-year reductions in the areas of riverbed covered by gravel, as well as an observed increase in the proportion of sand in the studied riffle’s bottom sediments, which can be considered as degrading spawning-ground parameters.
The introduced gravel was distributed to cover a larger area than before (with an existing hole in the bed below the bridge filled). The area of the active spawning ground was increased in this way. Granulometric analysis of bottom sediment, water-velocity measurements, geodetic surveys, and habitat studies were all conducted before and after the intervention.
The Sundborg diagram (Fig. 8) makes it clear that, following the riffle modification work, sediment grains in the spawning section remained stable and were not mobilised, thereby ensuring that the spawning grounds were not washed away. As a sand layer remained in place beneath the layer of gravel and stones 15-30 cm thick, the riverbed was supplemented with coarser stones to increase flow roughness and stabilise the new sediment. Sand is expected to settle upstream of the spawning grounds, where a sediment trap was created by raising the riffle crest. This trap should prove effective at retaining sand – as is confirmed by the observed reduction in water velocity. The impacts of the revitalisation work (Fig. 8) were for hydrodynamic conditions at the riffle to improve, with a stop therefore put to the unfavourable accumulation of sand in the spawning area.
Before restoration, a maximum of three nests of brown trout were observed in the middle section of the riffle, with these also characterised by low spawning efficiency – to the extent that only about 50 summer brown-trout fry were recorded annually (Fig. 10). Following restoration, five nests were observed in the first year, and eight in the second year. The number of summer brown-trout fry obtained during the first year was 287.
The restoration work yielded the following positive effects where conditions for reproduction among brown trout were concerned:
1. The improved grain size of the bottom sediment reflected elimination of a sand fraction undesirable in spawning nests. The increased grain size and reduction in amounts of sand resulted in a more homogeneous sediment composition.
2. The gravel spawning area increased in size, with the active surface expanding from 60 to 160 m². The specially-designed shape of gravel bars also introduced varied hydrodynamic conditions, protecting riverbanks from erosion and creating diverse potential spawning sites.
3. Enhanced sediment stopping was a consequence, as water velocity decreased significantly upstream of the spawning ground, encouraging sand deposition. This makes it unlikely that sand will be transported into the spawning grounds, as the upstream section now functions as a natural sedimentation trap. Hydrodynamic parameters were thus optimised in the spawning grounds as compared with the conditions pre-restoration.
4. A positive ecological impact can be said to have been exerted by the restoration efforts along the section of river, with the population of brown trout increased as a result, and biodiversity enhanced.
Keywords: spawning grounds, spawning nest, brown trout, Salmo trutta, hydrodynamic conditions, gravel-stone sediment
[michalmaciejbien@gmail.com], Okręg Polskiego Związku Wędkarskiego w Kielcach; Uniwersytet Rolniczy w Krakowie, Wydział Inżynierii Środowiska i Geodezji
[karol.plesinski@urk.edu.pl], Uniwersytet Rolniczy w Krakowie, Wydział Inżynierii Środowiska i Geodezji
Citation
APA: Bień, M., & Plesiński, K. (2025). Naturalne bystrza – ostoje bioróżnorodności: analiza funkcji tarliskowych dla ryb i siedliskowych dla bezkręgowców. Przegląd Geograficzny, 97(1), 11-25. https://doi.org/10.7163/PrzG.2025.1.1
MLA: Bień, Michał and Plesiński, Karol. "Naturalne bystrza – ostoje bioróżnorodności: analiza funkcji tarliskowych dla ryb i siedliskowych dla bezkręgowców". Przegląd Geograficzny, vol. 97, no. 1, 2025, pp. 11-25. https://doi.org/10.7163/PrzG.2025.1.1
Chicago: Bień, Michał and Plesiński, Karol. "Naturalne bystrza – ostoje bioróżnorodności: analiza funkcji tarliskowych dla ryb i siedliskowych dla bezkręgowców". Przegląd Geograficzny 97, no. 1 (2025): 11-25. https://doi.org/10.7163/PrzG.2025.1.1
Harvard: Bień, M., & Plesiński, K. 2025. "Naturalne bystrza – ostoje bioróżnorodności: analiza funkcji tarliskowych dla ryb i siedliskowych dla bezkręgowców". Przegląd Geograficzny, vol. 97, no. 1, pp. 11-25. https://doi.org/10.7163/PrzG.2025.1.1