Why the health of Breede River Estuary is important for Spotted Grunters

Spotted Grunters (Pomadasys commersonnii) are a common game fish in the Breede Estuary that is often seen in the shallows with its tail waving above the water as it blows small creatures out of holes in mud and sand. These fish get their name from their appearance and the grunting noise they produce when grinding their strong jaws together.

Spotted Grunters utilise both sea and estuaries during their life cycle, and like most estuary-dependent fish, they move to the sea for spawning – the reproductive process whereby females release eggs for males to fertilise. Spawning is triggered by several environmental factors such as seasonal shifts in temperature or weather events. Lunar phases and day length also play a role. In South Africa, P. commersonnii is known to spawn along the east coast (KwaZulu-Natal Province), however, recent evidence suggests that this species also spawns along the southwest coast in the Western Cape Province.

As a result of Spotted Grunters being heavily targeted by recreational anglers and small-scale fishers, the species has been listed as Vulnerable in the South African National Biodiversity Assessment (2018) and is therefore in need of improved management. However, to improve conservation practises it is essential to understand the ecology of the species, including what drives their movements and tendency to return to the same place.

A recent study conducted by local fish scientist JD Filmalter et al. (2023) investigated this using acoustic telemetry to track Spotted Grunters

across the Southern Cape. Seven adults, all measuring over 40 cm in length, were tagged with acoustic transmitters in the Breede Estuary and monitored between November 2016 and March 2020. Fish were captured using rod and line, and once captured were placed in a water-filled sling with the ventral side of the fish facing upwards, ensuring the head remained submerged to facilitate respiration. An acoustic transmitter was surgically implanted into the body cavity of each fish through an incision posterior to the pelvic girdle. A network of receivers were placed throughout the Breede Estuary and adjacent marine environment outside the mouth. Over the course of the study period (2016-2020) fish movements were monitored using the data generated by each receiver every time a tagged fish would pass by.

The results of this study showed that all acoustically tagged fish spent more time in the Breede Estuary (82.6%) than at sea. However, all fish undertook trips to sea at some point during the study period, with the majority of their trips to sea lasting less than five days. These trips mostly took place during the summer season, which coincides with the peak spawning season. It was also confirmed that increases in estuary water temperatures in summer and river inflow in winter influence the movements of these fish between estuary and sea.

These findings supported that regional spawning is taking place on the southwest coast due to the Breede Estuary’s population exhibiting a high degree of residency. Thus, due to east and southwest coast populations being genetically isolated, management strategies are needed along both coasts to protect Spotted Grunters, especially during summer months for the recovery of this species. This study also highlighted the importance of the Breede River Estuary and the crucial role that it plays in supporting biodiversity.

Map showing the location of the Breede Estuary and the positions of the acoustic receivers in the estuary and adjacent marine environment used to monitor movements of tagged Pomadasys commersonnii between November 2016 and March 2020.

[BA Ziko, TS Murray, TF Næsje, JD Filmalter & PD Cowley (2023): Acoustic telemetry reveals the drivers behind estuary–sea connectivity

of an important estuarine-dependent fishery species, Pomadasys commersonnii, in the Breede Estuary, South Africa, African Journal of Marine Science, 45:3, 201-213.]

Share this newsletter!