Ichthyofaunal Community of the Anthropogenically Altered Seekoei Estuary in Warm Temperate, South Africa

The temporarily open-closed (TOCE) Seekoei Estuary, on the warm temperate coast of South Africa, has been subject to various anthropogenic modifications such as impoundments, water abstraction, causeway construction, and artificial breaches that have impacted its estuarine functioning. To determine the impact of these combined factors on the fish community, the Seekoei Estuary was sampled twice per season over 2 years, which included a hypersaline phase. A total of 92,026 fishes, comprising 12 families and 25 species, were caught from five different habitat types using a 30 m x 1.7 m seine net (10-mm mesh aperture). The catches were dominated by the exclusively estuarine species Gilchristella aestuaria (81.0%) (Clupeidae), followed by the estuarine and marine Atherina breviceps (11.4%) (Atherinidae) and the marine estuarine-dependent Rhabdosargus holubi (5.1%) (Sparidae). Fish catch-per-unit effort (CPUE) were similar among seasons and sites; however, species richness was highest (d = 1.24) at the site directly below an obstructing causeway hindering fish migration. Additionally, reoccurring hypersaline conditions due to low freshwater inflow and artificial breaching have resulted in frequent fish kills of mainly marine origin species, creating an anthropogenically mediated ecological trap. The current anthropogenic alterations require amelioration for the conservation of the fish and the ecological functioning of this estuary. Rehabilitation to restore functioning is important for estuaries globally, especially estuaries with low inflow.

Automated summary

The Seekoei Estuary, located on the coast of South Africa, has been extensively modified by human activities such as impoundments, water abstraction, causeway construction, and artificial breaches. This has had a significant impact on the fish community, which was studied over a period of 2 years. The study found that the estuary was dominated by specific estuarine and marine species, but frequent hypersaline conditions and habitat alterations created an ecological trap.