Temporal and spatial dietary dynamics of the longspine glassy (<i>Ambassis ambassis</i>) in the St Lucia estuarine system, iSimangaliso Wetland Park

Abstract

Among the 155 recorded fish species in the St Lucia estuarine system, Ambassis ambassis is one of the most prominent. After a decade dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically with the onset of a new wet phase in 2011. In response, A. ambassis has expanded its distribution throughout the system. Stable δ 15 N and δ 13 C isotope analysis was used in conjunction with gut content analysis to elucidate the diet of this species at 5 representative sampling localities. Zooplankton as well as terrestrial and aquatic insects were prevalent in the diet. Non-metric multidimensional scaling analysis revealed a considerable dietary overlap for the fish collected at the different localities. Seasonally, trophic position differed significantly, with the dry season showing consistently higher isotopic signatures at all sites. A significant relationship was identified between trophic position, salinity and temperature, indicating the potential effect of these variables on the diet. The ability of A. ambassis to link lower trophic level organisms, such as zooplankton, with larger piscivorous predators is thus indicative of the vital role this species occupies in the food webs of the St Lucia system.

Insect meals in fish nutrition

“…Some insects in the orders Hymenoptera, Diptera and Coleoptera have also been found in the guts of marine or brackish fish (Darnell ; Elliot ; Dyer et al . ). Insects are a well‐documented part of fish diets, particularly in the early stages for carnivorous and omnivorous species, and are components of the diets of continental fish (Spataru et al .…”

“…; Ulrich ) as well as marine fish and fish that tolerate a wide range of salinity that can be found in brackish water (Darnell ; Dyer et al . ). Some fish species opportunistically feed on insects when food is abundant (Carolsfeld et al .…”

“…) and terrestrial insects that primarily belong to the families Vespidae and Formicidae (Dyer et al . ; Ferrareze et al . ).…”

Turbidity effects on the feeding, respiration and mortality of the copepod Pseudodiaptomus stuhlmanni in the St Lucia Estuary, South Africa

“…As such, a full Mfolozi-St Lucia reconnection would likely lead to a decline in the secondary production and abundance of all dominant St Lucia zooplankton species. P. stuhlmanni has been particularly singled out as a key prey item of higher trophic-level species (Dyer et al, 2015;Jerling et al, 2010;Nhleko et al,, 2012;Peer et al, 2013). A population decline of this species might result in cascading effects on higher trophic levels (Cyrus et al, 2011).…”

“…At St Lucia, studies have been conducted to determine the effect of turbidity on the feeding and mortality of two dominant zooplankton species, the mysid Mesopodopsis africana and the calanoid copepod Acartiella natalensis (Carrasco et al, 2007. Pseudodiaptomus stuhlmanni is another abundant calanoid, which has been singled out as a key prey item for several fish species at St Lucia (Dyer et al, 2015;Nhleko et al, 2012;Peer et al, 2013). The present study aims to expand on the past studies, by investigating not only how turbidity affects mortality and feeding, but also the respiration rate of P. stuhlmanni.…”