Fish are more abundant in salt marsh creeks with a salinity gradient and low drainage capacity

Salt marsh conditions and morphology influence the ecological function of marsh habitats. However, little is known about how the interplay between these factors impacts fish in the highly modified marshes typical of north-western Europe. We conducted a two-year survey at two salt marsh areas in the Dutch Wadden Sea to test the influence of marsh creek morphology on in-creek abiotic conditions and the abundance of four numerically dominating fish species representing the different life-history strategies of fish in the marshes. We measured fish abundance and several abiotic factors seasonally, and quantified the morphological attributes of each creek (such as volume), and of the creek basin (such as drainage density and bathymetry). Modelling results revealed a salinity gradient and/or a greater capacity for water retention (creek volume or drainage density) were associated with higher abundances of Atlantic herring ( Clupea harengus ), European eel ( Anguilla anguilla ), gobies ( Pomatoschistus sp.) and the three-spined stickleback ( Gasterosteus aculeatus ) across most seasons. During recruitment months sticklebacks and gobies were more abundant in brackish water conditions with a higher capacity for water retention. Eels were more abundant in creeks with larger volumes and lower salinities. Larval and juvenile herring were more numerous in brackish water than in water with higher salinity, but an effect of creek morphology on herring abundance was not detected. While recruitment processes were important determinants in the seasonal abundance of most species, creek morphology and abiotic conditions affected the use of salt marsh habitat by the fish species studied. Intertidal marsh creeks that retain water during low tide and have a salinity gradient hold more fish than marshes lacking these characteristics. These findings highlight the effects of local salt marsh conditions on fish assemblages. Taking the specific marsh conditions and their effects on fish into account in salt marsh management can improve outcomes for fish. • Salt marsh morphological and abiotic conditions affect habitat use by fish • Low drainage density and a salinity gradient are favourable for salt marsh fish • Modified marshes are missing a salinity gradient and capacity for water retention • Incorporating these values back into modified marshes would be beneficial for fish