Spotted grunter spawning aggregations observed in Knysna Estuary
It has been generally accepted that many marine fish species, including spotted grunter, make annual migrations from the southern Cape up into KwaZulu-Natal (KZN) waters to reproduce. This tactic has advantages in that the eggs and larvae can develop in warmer waters whilst the Agulhas current transports them back down the coastline and into more nutrient rich cooler waters along the Cape coast.
Spotted Grunter aggregation site in Knysna Estuary. © Arland Reed
Spotted grunter, Pomadasys commersonnii, is found in tropical to warm-temperate waters of the Western Indian Ocean from India, along the east African coastline to the Western Cape. Genetic studies have indicated no differentiation between fish from across this range, suggesting that the population is well mixed and highly connected. It is a popular angling species and important within the recreational and subsistence linefisheries. It comprises between 25-50% of east coast estuarine linefish catches and up to 60% of angler catches in the Breede Estuary. Population abundance assessments indicate over-exploitation. The species is in a state of collapse, with large spawning adults now comprising less than 25% of pre-exploitation levels.
Spotted Grunter aggregation site in Knysna Estuary. © Arland Reed
Two large fish were caught, fortuitously a female and male, and dissected. Macroscopic staging of the gonads indicated that the fish were likely to be spawning as the gonads were large, well vascularised and eggs were noted within the ovary. Gonads were removed and sent to the Ichthyology and Fisheries Science Department at Rhodes University where microscopic histological evaluations were undertaken to verify the macroscopic staging. The combination of in-field observations, macroscopic and histological examination of gonads provided empirical evidence for the spawning of spotted grunter in an estuarine system outside of KZN. This contributes towards an accumulation of evidence which suggests that South African spotted grunter likely employ alternative spawning strategies.
Macroscopic staging of the female gonads. Photo © Kyle Smith.
An anglers catch of spotted grunter in Knysna Estuary. Photo © Kyle Smith.
Using diverse life-history tactics and strategies likely contributes towards population diversity and buffers against environmental changes and poor recruitment years. Basically, using more than one reproductive strategy may promote resilience of the species to environmental change. However, to truly understand the role that non-migratory spawning plays in population abundance requires more work to understand what drives these alternative strategies, the frequency and timing of the events, where they occur and whether they significantly enhance rates of survival and recruitment.
Reference: https://www.tandfonline.com/doi/abs/10.2989/1814232X.2022.2042383
A version of this story was also published in the 2021/2022 Research Report.
