Preference, avoidance or coincidence? How fish and crustaceans use intertidal salt-marsh creeks in the German Wadden Sea

Worldwide, salt marshes have proven important or even essential for a wide variety of marine nekton. But in the world's largest intertidal wetland, the Wadden Sea, their role for nekton is surprisingly unexplored. Building on extensive field sampling, this study presents an inventory of fish and crustacean species using German Wadden Sea salt marshes and investigates their habitat use and affinity. Ten species dominated the nekton assemblage of the salt-marsh creeks both in frequency of occurrence and in relative abundance. Repetitive samplings resulted in highly variable catches, probably caused by the abundance in the subtidal. However, common goby (Pomatoschistus microps) and brown shrimp (Crangon crangon) were depleted with the 9 consecutive hauls when occurring in high numbers, indicating a high habitat affinity. Seasonal and diurnal quantitative sampling revealed seasonal shifts within the nekton community and significant higher densities during night than day for seven out of the nine dominant salt-marsh nekton species. Nekton densities tended to be lower than elsewhere in the Wadden Sea and thus animals using the salt-marsh creeks may benefit from lower resource competition. Compared to the subtidal not only the species composition differed significantly but also the size composition. The majority of fish in the marsh creeks were small (97% < 4.5 cm), consisting of larvae, juveniles and adults of small species. Thus, the use of salt-marsh creeks seems predominantly be triggered by size than by life stage. The absence of aquatic and avian piscivores indicates that salt-marsh creeks function for small nekton as refuge from predators. However, the habitat value depends also on the connected low tide refuges and the access pathways in between. The difference of the nekton community of the marsh creeks from the subtidal raises the demand for habitat-specific monitoring programmes and further research on the connectivity and interactions between different habitat types, both considering seasonal and diurnal abundance variabilities.