The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 3. Fish

Diamond mining in southern Namibia creates seawalls that hold the ocean at bay, yet seawater overwashes or seeps through to fill landward excavated areas, forming numerous coastal ponds that reach 380 000 m2. These ponds span ages of 1–40 years, with the oldest ponds lying in the north, and the younger ponds in the central and southern areas. We investigated occupation of these mining ponds by marine fishes. The ponds offer sheltered, warm, nutrient-enriched environments, but become hypersaline after 10–12 years. The ponds contained predominantly west coast steenbras Lithognathus aureti and southern mullet Chelon richardsonii, with smaller numbers of seven other fish species, and all nine species being marine. The ichthyofauna was thus depauperate, but similar to that of periodically closed estuaries in the region. The size composition and gonadal development of both predominant species suggest they may breed in at least the younger ponds. However, in older, hypersaline, northern ponds, L. aureti transitions into an entirely female population via protandry, precluding breeding there. Body condition was low for both species in the northern ponds, and gut fullness greatest in the southern ponds. Growth of both species was faster in the ponds than in the sea, but after initially fast growth, L. aureti reached a plateau, suggesting stunting thereafter. Chelon richardsonii attained larger sizes in the ponds than in the ocean. For 10–12 years before hypersalinity sets in, the ponds constitute suitable habitat for fish. They are not estuaries as they have minimal freshwater input, but they do create conditions corresponding to closed estuaries that are otherwise scarce along this arid coastline. However, for a nursery function to be realised, the marine fish must return to the sea or else the ponds become an ‘ecological trap’.