JD Filmalter is doing invaluable research on one of SA’s most iconic fish. The man is a mine of information on everything Argyrosomus japonicus. Here he shares some insights:
As fisheries biologist, JD’s day-to-day entails conducting research on fish and fisheries with the aim to ensuring the sustainable use of marine ecosystems. This, of course, means fishing too, but he didn’t get to ‘fish all day’ overnight, he crunched the books for a good decade before received his PhD in 2015. (Dr JD is it?) Currently he’s monitoring the movements of dusky kob in and around the Breede Estuary on a 3-year Post-doctoral research grant for SAIAB (South African Institute for Aquatic Biodiversity).
“The story of the dusky kob is actually quite sad,” JD explains. “The species occurs along our entire east coast, and just a few generations back it was extremely abundant. Today catching a size dusky kob is a rarity for most rock and surf and estuary anglers. Almost 20 years ago a population estimate based on declining catch rates put the number of breeding adults at less than 5% of their historic unfished levels.”
Research that leads to angler education and ultimately catch-and-release philosophies throughout (i.e conservation of the species by sports fishers) is crucially important.
Right now we don’t really know if adult dusky kob swim from the Western Cape to KZN or never leave the proximity of the estuary in which they grew up.
JD highlights three things most don’t necessarily know about the species:
- They are dependent on estuaries in their juvenile years
They enter estuaries from about 1 cm in length (between 25 days to a few months after
they are spawned) and they will hardly ever leave the system for the next 2-3 years.
- They get VERY old!
A big kob of 60kg could be anywhere from 35 – 50 years old. Once they reach maturity at about 1 m or 10kg, after 6-7 years, their growth slows right down as they put more energy into reproduction.
- Egg production increases exponentially with size
So, for example, if a 10kg female spawns 100 eggs per season a 20kg
female will spawn 10 thousand and a 30kg female can produce several
million. That’s why protecting the big old females is so critical.
– Learn more about JD Filmalter and his research on the dusky kob (Argyrosomus japonicus) here.
– Follow JD (@the_fishing_scientist) on Instagram. The oke catches (and releases more fish than you care to know about. Don’t say you haven’t been warned.
Thanks for the post.
If only more of the fishing fraternity would get it in their heads the impact they are having when they remove a 15-20kg kob from the ecosystem. Not to mention the pollution of these essential estuaries. On the flip side, how proud we can be of our impact when letting one swim free.