Acoustic telemetry reveals the drivers behind estuary–sea connectivity of an important estuarine-dependent fishery species, Pomadasys commersonnii, in the Breede Estuary, South Africa

Abstract

The spotted grunter Pomadasys commersonnii is an important estuarine-dependent fishery species known to spawn along the east coast (KwaZulu-Natal Province) of South Africa. However, recent evidence suggests that this species also spawns along the southwest coast in the Western Cape Province (WC), which has necessitated the investigation of estuary–sea movements of adult P. commersonnii in the WC, and their link with spawning season and changing environmental conditions. Seasonal gonad maturation of adult P. commersonnii (n = 112) was investigated using histology and the gonadosomatic index. Seven adult P. commersonnii were acoustically tagged and monitored in the Breede Estuary (WC) between November 2016 and March 2020. Tagged individuals spent more time in the estuary (83.5%) than in the adjacent marine environment. Sea trips (n = 90) lasted on average 3.23 (SD 4.55) days (range 1–37 days), with most sea trips (82.6%) having a duration of less than 5 days. The majority of sea trips (>50%) occurred in austral summer, which coincided with the peak spawning season. Increases in estuary water temperatures in summer and river inflow in winter also influenced the presence of tagged fish in the sea. The high degree of residency of P. commersonnii in the Breede Estuary and short sea trips, along with increased frequency of reproductively ripe fish during summer months, supports the conclusion that regional spawning is occurring along the southwest coast. We suggest that management strategies should be put in place to protect the P. commersonnii population along the South African southwest coast, especially during summer, for the recovery of this population.