Southern African temporarily open/closed estuaries are often characterized by sand dominated mouth regions, a feature in contrast to many permanently open estuaries of the region which regularly have substantial rocky areas. Here we explore how habitat characteristics may reduce nesting opportunities and potentially explain population trends for a common estuarine fish species, the speckled sand goby Psammogobius knysnaensis, in many temporarily open/closed estuaries in South Africa. The present study is the first to document that P. knysnaensis is a speleophilic nester, utilizing the underside of rocks and other hard substrata for egg attachment. Furthermore, our results suggest that ideal nesting habitat for the speckled sand goby is potentially a limiting resource in sand-dominated temporarily open/closed estuaries. We postulate that this is a factor contributing to the relatively low numbers of adult P. knysnaensis found in sandy temporarily open/closed estuaries of the region, although the role of marine connectivity, food availability, predation pressure and other environmental factors in this regard still remain largely unassessed.
