Characteristics of stone crayfish populations along a disturbance gradient – a case study of Kustošak Stream, Croatia

The Kustošak Stream is partially located within the Medvednica Nature Park, which is known for abundant stone crayfish (Austropotamobius torrentium) populations. This species is sensitive to changes in habitat so any anthropogenic impact puts stone crayfish populations at risk. This study aimed to assess how modifications of the natural features of a freshwater habitat and subsequent water quality changes affect populations of the stone crayfish in the Kustošak Stream in Zagreb, Croatia. For the study we selected three spatially close locations but under differing levels of anthropogenic disturbance: (1) no disturbance, (2) recently modified part of the stream during construction and (3) long time modified part of the stream through channelization and artificial modification of streambed. During a week-long period in September 2021, at each site, we measured physicochemical parameters of the water and collected crayfish with baited handmade traps. Caught individuals were used to estimate population size by mark-recapture method, and to analyse population structure by recording sex ratio, animal size and condition (Fulton’s condition factor). We then compared all the measured population and water parameters between sites. Results showed that population density of the site without disturbance (1) was significantly higher in comparison to the two sites that are anthropogenically impacted. Also, the crayfish size structure of this site differed significantly as opposed to the two hydromorphologically altered sites. Furthermore, PLS analyses has shown that crayfish abundance (expressed as CPUE) is strongly affected by physicochemical parameters of water, including among others: oxygen, calcium ions, nitrates, nitrites, COD, pH and water temperature. We discuss our findings in the context of effects of anthropogenic disturbance on viability and persistence of this EU priority species.