Fish fauna structure, feeding ecology and growth of keystone fish species in the Elbe and Odra estuaries

Abstract

In the Elbe and Odra estuaries essential feeding grounds, nursery areas, spawning grounds and transitional habitats for several fish species are found. With climate change and human activities (e. g. channel management, waste water sewage and nutrient inputs) both estuaries have faced several anthropogenic stressors in the past and in the present. These stressors are likely to have an influence on estuarine ecosystems and their food webs. Keystone fish species play an important role in estuarine food webs. With this regard, in this thesis the feeding ecology and growth of keystone fish species in the Elbe and Odra estuaries were analysed. Additionally, in the Elbe estuary, the structure and stock development of the fish fauna, as well as fish health, were evaluated based on stow net catch data and tissue-specific stress responses of the holobiont. Analyses on feeding and growth of the four keystone fish species zander Sander lucioperca (Linnaeus, 1758), smelt Osmerus eperlanus (Linnaeus, 1758), ruffe Gymnocephalus cernua (Linnaeus, 1758) and flounder Platichthys flesus (Linnaeus, 1758) were carried out, as these species occur in high numbers throughout both estuaries and play key roles in the respective aquatic food webs. The food webs in estuarine ecosystems are important biological indicators, therefore the feeding ecology of the four keystone fish species was studied by stomach content analyses (chapter 2). The most important prey of zander in both estuaries were fish and mysids. For smelt amphipods were the most important prey in the Elbe estuary, whereas in the Odra estuary smelt mainly consumed mysids. The most important prey of ruffe in the Elbe estuary were also amphipods, while annelids (lower estuary) and insect larvae (upper estuary) were the dominant prey in the Odra estuary. In the Elbe estuary, flounder fed mainly on copepods, whereas in the Odra estuary they preferred bivalves. In the Elbe estuary higher dietary overlaps were found, particularly between smelt and ruffe, zander and ruffe, as well as zander and smelt. By comparing the present diets of zander, smelt and ruffe from the Elbe estuary with previous studies (1990s), it was noticeable that the consumption of amphipods increased, while mysids and copepods decreased in their diets. These shifts indicated a restructuring of the food web, likely driven by environmental changes, which highlights the vulnerability of estuarine ecosystems. In chapter 3 we aimed to analyse the growth patterns of the four keystone fish species to identify regional differences from both estuaries and to compare present findings with those from past studies. Age determination on hard structures (otoliths, scales, opercula) and length measurements were carried out. This allowed the determination of size ranges of the individual age groups, which were used to calculate the parameters of the von Bertalanffy growth function. The asymptotic lengths of smelt, ruffe, and flounder were larger in the Elbe estuary compared to the Odra estuary, whereas their growth coefficients were smaller in the Elbe. The growth of zander, smelt and ruffe from the Elbe. estuary were inferior compared to previous studies. The growth of juvenile zander in the Elbe estuary showed a noticeable reduction in the maximum turbidity zone located in the middle estuary. Slower growth at these sites appears to be linked to reduced fish prey intake, as juvenile zander relied more on invertebrates like amphipods, mysids, and decapods. Flounder from the Odra estuary showed smaller asymptotic length compared to other studies from the Baltic, but had a higher growth coefficient in contrast to studies from the Pomeranian Bay and Gulf of Gdańsk. Our findings indicate a connection between environmental changes and fish growth via changes in feeding behaviour, especially in the Elbe estuary. We created a periodic time series over the last four decades (1984-2022), combining fish species densities data from stow net catches alongside with environmental data in the Elbe estuary. In chapter 4, we showed that the fish fauna of the Elbe estuary has become more similar in guild structure to that of macrotidal estuaries in Europe, with a relative increase in marine-estuarine opportunists and a decrease in diadromous species. Due to an improvement of the water quality in the 1990s, fish densities, especially of smelt, increased until 2010. However, anthropogenic hydromorphological interventions, have led to an increase of suspended particular matter until 2022. Together with a reduced river runoff and poor oxygen condition, these factors acted as stressors for fishes in the estuary. In 2021-2022 mean fish densities dropped by over 91 % compared to 2009-2010. This decline was mainly driven by a reduction of the key species smelt, alongside decreases in twaite shad Alosa fallax (Lacepede, 1803), flounder, ruffe, common bream Abramis brama (Linnaeus, 1758), and other species. In contrast, densities of marine-estuarine opportunist species such as herring Clupea harengus Linnaeus, 1758 and whiting Merlangius merlangus (Linnaeus, 1758) increased. Overall, this time-series provided insights into the strong impact of human intervention in estuarine ecosystems. In chapter 5, we integrated tissue specific gene expression data from fish host (gill and liver RNAseq) and 16S rRNA metabarcoding of gill mucus microbiota with physiological and abiotic parameters to assess the health of juvenile zander along the Elbe estuary. Liver specific gene expression patterns in fish from highly turbid areas in the middle estuary indicated starvation, which aligned with the compromised body condition. The gill microbiome derived from the freshwater area with local occuring oxygen minimum zones was dominated by potentially pathogenic taxa, including Shewanella, Acinetobacter, Aeromonas and Chryseobacterium. Their presence was found alongside with strong immune responses in the host gill tissue and increased energy demand in the liver tissue, supporting a potentially pathogenic role. Together, these results demonstrate how physiological and microbiome responses in estuarine fish can reflect the cumulative impact of abiotic stressors, which are likely to intensify under ongoing climate change.