Bioenergetics of fishes in a high-energy surf-zone

Abstract

Energy budgets are proposed for four teleost and two elasmobranch species as well as for the main icthyofauna groups for a surf-zone ecosystem. The ecology of surf-zone fishes of eastern Cape beaches, Algoa Bay South Africa, is reviewed. Using the equationC=F+U+R d +R R +ΔB, the following general energy budgets were derived for fishes: teleosts − 100=10+4+21+23+42; elasmobranchs − 100=11+2+16+24+48; whereC: food consumption;F: faeces;U: nonfaecal excretion;R d : apparent specific dynamic action;R R : routine metabolism;ΔB: growth. These show that most of the energy consumed is used in metabolism (R d +R R ) and growth (ΔB) whereas excretion (U) only accounts for a small portion. The energy budgets developed are within ranges recorded for other species. The main feeding groups of surf-zone icthyofauna are the southern mulletLiza richardsonii, the sandsharkRhinobatos annulatus, benthic feeders, zooplankton feeders, omnivorous and piscivorous fish with biomass values of 1000, 1000, 3000, 2400, 400 and 400 kJ m−1, respectively; and annual consumption budgets of 22107, 13725, 65710, 65476, 9758 and 8517 kJ m−1 yr−1, respectively.L. richardsonii feeds mainly on surf diatoms, consuming 0.5% of total diatom production. Zooplankton production supplies 91%, and macrobenthic production 9%, of the energy needs of other non-piscivorous carnivorous fishes. Piscivorous fishes consume 30% of the available fish production. Nonfaecal-energy production (8229 kJ m−1 yr−1) is utilised by surf diatoms, and faecal-energy production (30 341 kJ m−1 yr−1), is returned to the detritus pool to be utilised by the microbial loop in surf-waters. Our current knowledge of surf-zone energetics indicates that fishes are important predators. This study confirms the concept that the ecosystem generates adequate food for the fish assemblage. Fishes recycle energy, as excretory products, via the detritus pool and surf-diatoms, while fishes moving across the outer boundary of the surf-zone export energy from the system. Data presented, therefore, also support the general concept of a self-sustaining beach/surf-zone ecosystem.