The fish-based assessment tool (FAT) for transitional waters was adapted to the Ems estuary within the framework of the Dutch-German cooperation for the implementation of the Water Framework Directive (WFD) in the Ems-Dollard region. Application of the procedure requires appropriate monitoring. Bio-Consult was contracted by Wageningen Marine Research to fish at three sampling stations in the Ems transitional waters by means of stow nets according to the requirements of WFD/FAT in spring and autumn 2021.
Overall, the study resulted in a spectrum of 39 fish species. In spring 25 species occurred, of which 21 were detected at station Spijk, 19 at station Oterdum and only 9 at station Terborg.In autumn 32 species were identified, of which 22 species were documented at Spijk, 24 at Oterdum and 16 at Terborg.
The species belonged to 6 ecological guilds. Along with 11 estuarine resident species, 9 marine-juvenile, 5 diadromous, 5 marine-seasonal, 5 limnic and 4 marine (adventitious visitors) species, were recorded.
In spring 2021, the mean total abundance at Spijk was approx. 761 ind./h/80 m², at Oterdum 801 ind./h/80 m² and only 137 ind./h/80 m² at Terborg. In autumn the result was on a much higher level at all stations, especially at Spijk station with an average of 8.088 ind./h/80 m². At Oterdum station the mean value was 7.101 ind./h/80 m² and 720 ind./h/80 m² at Terborg. The higher abundance (ind./h/80 m²) occurring in autumn in relation to spring is also reflected in the total biomass recorded at the single stations.
In the meso- and polyhaline zones Herring (Clupea harengus) represented a dominant species in spring and autumn, with shares of up to around 78% in both seasons. In the oligohaline zone, the dominance structure differed from the other two stations. At Terborg station in 2021 herring was only abundant in very low numbers, therefore smelt and gobies (P. minutus & P. microps) where clearly dominant in both seasons.
The stow net fishing operations in the Ems in 2021 showed that the fish fauna was characterized by a pronounced seasonal and also spatial variability. This was indicated by all parameters described (abundance, biomass, dominance structure). Similar to the findings in recent years, the findings for 2021 reflect qualitative and quantitative deficits of the fish community, especially in the oligohaline zone, due to unfavourable habitat conditions (oxygen deficiency, fluid mud), mainly upstream Terborg.
Looking at the mean abundance and the mean biomass on an annual basis, the numbers for 2021 are on a remarkable lower level compared to the period 2018 – 2020. The high level for mean abundance and mean biomass in 2018 and 2019 were strongly related to an overall increasing abundance of herring in autumn. In 2020 lower number of herring were recorded, but still higher, compared to 2021. The decline of total abundance and biomass in 2020 and 2021 strongly reflects the lower catch numbers of herring.