Intertidal seagrass beds (Zostera noltii) as spawning grounds for transient fishes in the Wadden Sea

Since the major seagrass loss in the 1930s, the former extensive seagrass beds (Zostera marina) in the Wadden Sea (North Sea) are now exclusively limited to the intertidal area and are dominated by Z. noltii. We hypothesised that intertidal seagrass beds are important spawning areas for common North Sea fishes. To study the role of Z. noltii beds as a spawning ground in general, different investigative approaches had to be used with respect to the biology of fish species and the timing of field investigation within the spawning processes. In situ spawning experiments combined with comparisons of fish activity and abundance on vegetated and non-vegetated tidal flats indicated the importance of intertidal seagrass beds as spawning grounds for the garfish Belone belone. The tidal migration of B. belone was found to be specifically directed to intertidal Z. noltii beds for spawning; furthermore, the seagrass plants were found to be significantly more effective in adhering eggs than alternative plant substrates such as the brown algae Fucus vesiculosus. Besides being found in Z. noltii beds, eggs of herring Clupea harengus were also found attached to F vesiculosus associated with man-made constructions for coastal protection located in the upper intertidal zone. However, comparisons of egg numbers in relation to plant biomass m(-2) revealed that densities of herring spawn were about 20 times higher in seagrass beds than in brown algal patches. Disguised nests of the three-spined stickleback Gasterosteus aculeatus could only sporadically be observed within the intertidal seagrasses canopies. Since over 50% of the female individuals of G. aculeatus captured within Z. noltii beds carried ripe ovaries, we proposed that intertidal seagrasses are suitable spawning grounds for small species such as G. aculeatus, which have distinct parental care behaviour.