Dr John Filmalter is currently tagging important estuarine fishes in the Breede River estuary as part of a national project examining their migration patterns along our coastline which is being run by Dr Paul Cowley from SAIAB. The focal species are dusky kob, spotted grunter, leervis and white Steenbras. Fish are tracked using acoustic tags, which are implanted inside the fish. An array of acoustic receivers, along our coastline and in many of our estuaries, then detects and records the tag’s information as the fish swim past.
Some of the highlights presented at the talk were:
- To date, grunters appear to be highly resident and seldom venture out to sea
- Preliminary data suggests that individual grunter may utilize limited portions of the estuary, a few kilometers in length, for prolonged periods before moving to another area. One tagged grunter in the Breede River moved 20 km up river from where it was tagged and stayed there for several months, only returning to the lower reaches briefly on two occasions.
- A grunter tagged in the Goukou estuary in Stilbaai swam over 190 km’s in 4 months to the Knysna lagoon where it was recaptured.
- So far leervis display the largest movements of all fish tagged.
- Leervis appear to move between estuaries and along our coast during both summer and winter
- A leervis tagged in the Breede estuary was recaptured in Balito on the KZN north coast 3 months after is left the Breede. Another leervis, also tagged in the Breede has been recorded in the Kowie estuary in Port Alfred.
- Juvenile estuarine-dependent fishes move into estuaries to avoid predation after they are spawned at sea.
- Several factors are likely to have contributed to the declining abundance of white Steenbras and the increased occurrence of spotted grunter in the Breede Estuary over the past 30 years. White Steenbras numbers are very low along our entire coastline at present and have declined dramatically in recent years. They grow slower and reproduce at a greater age than spotted grunter, and as such are less likely to adapt to rapidly changing environmental conditions. Sevier estuarine degradation along the East Coast has reduced the potential habitat for the spotted grunter population in SA waters. Simultaneously, climate change has possibly led to slightly warmer conditions on average in our coastal environment. These factors, along with the abundance of food resources within the Breede River estuary have all likely contributed to their increased abundance.
- For the first time ever a tagged seven gill cowshark was recorded in an estuary. The shark was tagged by researcher Dr. Alison Kock and her team, at Robben Island in Table Bay in October 2016 and briefly visited the Breede estuary in April 2017.
- The outcomes to date highlighted the importance of the Breede River estuary in the region. The Breede River is one of the few estuaries where adult Kob is still regularly caught. The reason for this could be the fact that the De Hoop Marine Protected Area, where fishing is entirely prohibited, is located so close to the estuary. This reserve could provide the fish a safe refuge when they go to sea.