2.2. Data analysis
Potential trends in the crayfish catches (e.g. increase or decrease of crayfish abundance) were determined for the period
between 1995 and 2015. Quantum GIS Wien 2.8.1 software (www.qgis.org) was used to map the distribution of the crayfish
species. To obtain insight in changes in crayfish distributions over time, the observations were divided into four time periods:
from 1995 to 2000, from 2000 to 2005, from 2005 to 2010 and after 2010. Comparing the distributions between these periods
showed either a decreasing, stable or increasing range per species.
We counted the number of crayfish observations in the different types of waterbody categories. Additionally, the average
abundance was calculated per species per water body type. Finally, crayfish occurrence in relation to surface water velocity
was also calculated.