Seasonal and diurnal patterns in the microhabitat use of fish in the littoral of gravel pit lakes, with special reference to the use of supplemented dead woody habitat

The habitat quality of the littoral zone of lakes is known to play an important role for most lake dwelling fish species. Different species and life-stages vary in their use of certain habitat types on a diurnal and seasonal scale. Artificial lakes, such as gravel pit lakes, often possess structurally homogenized littoral zones, regularly offering low habitat quantities especially for structurally orientated fish. The introduction of deadwood increases the availability of littoral structures and might act as a suitable habitat for fish. In the present work the diurnally and seasonally varying use of different microhabitats by six dominantly occurring fish species (eel (Anguilla anguilla), perch (Perca fluviatilis), pike (Esox lucius), roach (Rutilus rutilus), rudd (Scardinius erythropthalmus), and tench (Tinca tinca)) was studied in eight gravel pit lakes with added woody structures in the littoral zone. The tested hypotheses were that these six fish species use the new littoral microhabitats, and that littoral microhabitat use intensities differ by size. Further, it was hypothesized that fish increasingly use long lasting deadwood structures during winter when macrophytes die off and that littoral variables (microhabitat and littoral depth) are of high importance in explaining fish abundances in the deep, steeply shored gravel pit lakes. The use of the littoral habitats was assessed in each of the four seasons using random point abundance sampling by electrofishing. Patterns of fish abundances in two dominant natural habitats (unstructured littoral and emerged macrophytes) and supplemented deadwood habitats were analyzed using generalized linear mixed modelling (GLMM). Additionally, boosted regression trees (BRT) were run to identify species- and size-class specific patterns of microhabitat use and to compare the relative importance of lake-level (e.g., trophic state) and littoral-level variables on fish abundance. The usage of the available physically complex shoreline structures by all assessed species was detected, especially during daytime. Additionally, increased abundances of primarily roach and perch were detected during nighttime in the unstructured open littoral habitats. Fish used the deadwood structures immediately after the supplementation in all seasons, and larger piscivores (perch, pike and eel) in particular were positively associated with the artificial habitats. During winter, roach, perch and pike intensively used deadwood structures and were rarely found in the unstructured habitat. Besides the structural properties, shallow water was also found to be an important determinant of littoral fish abundance. Generally, littoral variables were similarly or even more important in explaining fish abundances compared to lake-environmental variables in the sampled gravel pit lakes. The present work suggests that the implementation of deadwood structures can increase the habitat quality for structurally orientated fish in gravel pit lakes with limited littoral habitats and may thus support fish populations.