Seagrass (Zostera capensis) bed development as a predictor of size structured abundance for a ubiquitous estuary-dependent marine fish species

Abstract

Episodic river flooding has the capacity to ‘reset’ estuaries and redirect these systems on a new path towards some sort of dynamic physical and biological equilibrium. A major river flood through the Kariega Estuary, South Africa, during 2012, provided an ideal opportunity to monitor the occupation of seagrass (Zostera capensis) beds by juveniles of the marine sparid Rhabdosargus holubi, post flood recovery. The estuary contained extensive dense submerged Z. capensis beds throughout the system prior to the flood, almost all of which was removed from the system as a result of the flood. Over the following four years there was a gradual recovery in the extent of cover of submerged Z. capensis beds, which by the end of 2015 had recovered to near the same levels of cover as the pre-flood conditions. In addition to the positive relationship between seagrass development and R. holubi abundance that was recorded, there was also a greater utilisation of this habitat type by the smaller (10–40 mm SL) compared to the larger (41–60 mm SL) juvenile size classes. This new finding emphasizes that occupation of particular estuarine nursery habitats by the juveniles of estuary-associated fish species may not necessarily be uniform and may change with age. The study also reinforces the changing nature of estuarine environments over time and highlights, once again, the importance of this habitat type for the most abundant estuary-dependent marine fish species on the African subcontinent.