Aquatic highways: understanding the importance of habitat connectivity for migratory fish species, especially anguillid eels.

Abstract

Connectivity among freshwater, estuarine, and marine ecosystems is crucial, facilitating the transfer of biomass among them and providing pathways for migratory aquatic species. This thesis focused on the importance of connectivity in these ecosystems and the impacts that impaired connectivity had on migratory species and fish communities residing within them. The first objective of this study was to evaluate the impact of four instream barriers (weirs) on fish species movement and migration in the uMkhomazi River, KwaZulu-Natal, South Africa. The second was to determine which factors influenced anguillid eel recruitment into the uThukela, uMkhomazi and uMtamvuna estuaries in KwaZulu-Natal, South Africa. The third and final objective was to assess the factors shaping fish community structures within the uThukela, uMkhomazi, and uMtamvuna estuaries in KwaZulu-Natal, South Africa. To achieve the first objective, sampling was conducted during the rainy season (March– April) and the dry season (June) in the Mkhomazi River. Sampling was conducted above and below four weirs along the river using electronarcosis to catch the individual fish. The KwaZulu-Natal yellowfish Labeobarbus natalensis was the most abundant fish species caught, and species abundance and diversity decreased with increasing altitude. Significant differences in species unevenness were present between communities in the respective river fragments, separated by the existing weirs, which potentially restricted the longitudinal movement of fish. Furthermore, eels were found present in all river fragments except the most upstream river fragment. It is suggested that all existing weirs be retrofitted with suitable and effective fishways, and that the proposed weirs and dams for the river include fishways to maintain a strong level of connectivity and better protect and conserve migratory fish within the system. For the second objective, sampling was conducted from October 2024 to May 2025 in the three estuaries, namely uThukela, uMkhomazi, and uMtamvuna estuaries, along the KwaZulu-Natal Coast. Eight glass eel nets were placed in each estuary for a duration of three nights per month. Nets were checked every 12 hours. Anguilla spp. in the glass eel life cycle stage were only recorded in the uThukela Estuary during the study. In the uThukela Estuary, connectivity between the marine and estuarine environments, high tides, moon phases with lower light intensity and increases in certain water quality variables, namely dissolved oxygen, oxygen percentage and electrical conductivity (when related to nutrient concentration), were found to positively influence anguillid glass eel recruitment. The outcomes of this study provided insights into the cues that trigger anguillid eel recruitment from the ocean into estuaries along the east coast of South Africa, which improved our understanding of the recruitment of four globally important eels. It is suggested that conservation efforts aimed at maintaining the natural degree of connectivity between marine and estuarine systems would allow the continued recruitment of these eels into these estuaries. Additionally, management practices tailored to preserve naturally occurring water quality would help protect and conserve these ecologically and economically important eel species. For the third objective, sampling was conducted from October 2024 to May 2025, during which eight glass eel nets were deployed in each estuary for three nights per month. The nets were checked every 12 hours. The degree of connectivity present in the uThukela was found to be greater than that of the uMkhomazi and uMtamvuna, as it contained the most diverse and abundant migratory species. The fish communities residing within the three selected estuaries were found to be shaped by numerous abiotic variables, including connectivity, water temperature, salinity, and dissolved oxygen. A combination of these factors and suitable habitat is required to maintain the fish community structures in the selected estuaries. Factors shaping estuarine fish community structures along the east coast of South Africa are complex, but increased temperatures and longer estuary closure times have an impact and thus affect the conservation of these fish communities as the climate changes. This thesis highlights the importance of connectivity for four globally important freshwater eel species and other migratory fish species that form part of the estuarine fish communities. It is evident from these findings that conservation interventions aimed at restoring connectivity are necessary to protect and preserve migratory fish species, including economically and ecologically important species such as anguillids.