Stow net fishery Ems 2023 : fish fauna monitoring in accordance with the WFD
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 moni toring. BioConsult 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 2023.
Overall, the study resulted in a spectrum of 37 fish species. In spring 31 species occurred, of which 18 were detected at station Spijk, 20 at station Oterdum and 19 at station Terborg. In autumn 26 species were identified, of which 22 species were documented at Spijk, 25 at Oterdum and 22 at Terborg.
The species belonged to 6 ecological guilds. Along with 9 estuarine resident species, 9 marine-juve nile, 7 diadromous, 2 marine-seasonal, 8 limnic and 2 marine (adventitious visitors) species, were recorded.
In spring 2023, the mean total abundance at Spijk was approx. 515 ind./h/80 m², at Oterdum 503 ind./h/80 m² and 498 ind./h/80 m² at Terborg. In autumn the result was on a higher level at all stations, especially at Spijk station with an average of 2.581 ind./h/80 m². At Oterdum station the mean value was 1.094 ind./h/80 m² and only 227 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. Compared to other years the abundance was low in 2023.
In the meso- and polyhaline zones Herring (Clupea harengus) represented a dominant species in spring and autumn, with shares of up to around 85% in both seasons. In the oligohaline zone, the dominance structure differed from the other two stations. At Terborg station in 2023 herring was only abundant in very low numbers, therefore gobies (P. minutus & P. microps) where clearly dom inant ins spring, in autumn smelt (Osmerus eperlanus) was the most frequent species.
The stow net fishing operations in the Ems in 2023 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 2023 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 2023 are on a remarkable lower level compared to the period 2018 - 2020, and even lower than 2021 and 2022. 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. The numbers show an overall decreasing trend from 2020 - 2023. The decline of total abundance and biomass from 2020 to 2023 strongly reflects the lower catch numbers of herring.