Dusky kob have dominated Dr JD Filmalter’s work life for the past few years and few in South Africa know the species (and its plight) better than him.

“The story of the dusky kob is actually quite sad,” says JD, who has been studying the movement patterns of the species using acoustic tracking technology for the past four years.

“The species occurs along our entire east coast, and just a few generations back it was highly abundant. Today catching a size dusky kob is a rarity for most rock & surf, and estuary anglers,” he says, explaining that 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 un-fished levels.

JD Filmalter

Logically you need adults to produce juveniles and replenish the stock, and understanding the movements of adults is essential for effectively protecting them. Also, you need an understanding of the species’ breeding timelines and, exactly ‘why’ it is they need to live so long.

“The maximum lifespan at the moment is 42 years, of a fish that was aged,” says JD. “That specimen was in the high 40s in terms of kilos. So, those 50- and 60-kilo kob are in all likelihood well over 50 years old. That is old for a fish.”

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“From an evolutionary perspective it obviously makes sense to reproduce as early, and as often as possible if you’re looking at replacing yourself (to at least keep the population stable) or to increase the population – which ultimately every species is trying to do,” JD explains.
Dusky kob

This puts the dusky kob at a distinct disadvantage. “They only start breeding (there is a difference between males and females, but on average) at about 10 kilos, or around 1.1metres. They reach that size anywhere from five- to eight-years of age. In the animal world, quite a long time that you are existing but not reproducing,” he says.

“From an evolutionary perspective it doesn’t seem to make a lot of sense, but…”

A large female will produce well in excess of a million eggs in a single spawning season, so one would expect (in a pristine state) a huge population. Over its lifespan that equates to many many millions of potential offspring. “In the ‘stable population’ scenario, where humans are out of the equation, for me means that there is a massive mortality rate in the larvae before they recruit into the estuary. Plus, there is a massive mortality all the way up until they can reproduce,” says JD.

It’s a law-of-averages kind of thing then: If your offspring are going to die in massive numbers, you need to spawn enough to balance it out.

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Two species of kob commonly found in SA. Can you tell them apart? Top is a silver kob (Argyrosomus inodorous) and below is the dusky kob (A. japonicus) which has more common names than you can shake your rod at (daga, silver salmon, kabeljou, jewfish, mulloway- and in Japan giant black drum, to name a few). Silver kob occur in Southern Africa from the Kei River to Namibia. They start breeding between 1 and 3 years at size of about 35cm. Their latin name refers to the lack of the distinct metallic smell associated with dusky kob. They grow quite fast and attain a max size of about 1.2m after roughly 12 yrs. Max reported age is 25 yrs Dusky kob have a wide distribution from South Africa through the Arabian Gulf to Australia and Japan. In SA females mature at roughly 1 m after 7-9 years. From then their growth slows drastically. The max reported age is 42 yrs with a max size of 1.8m (roughy 70kg). Only dusky kob enter estuaries. They are hard to tell apart. In the hand silver kob are much slimier. Their eye is larger and their tail is far more translucent. While tail shape may appear to be diagnostic in these pics, both species display a wide range of tail shapes. . . . #fishscience #knowyourfish #kob #duskykob #mulloway #jewfish #silverkob #feathersandfluoro #saltfishingsouthafrica #fishing #fish #southafrica

A post shared by JD Filmalter (@the_fishing_scientist) on

What we do know is that larval dusky kob will enter into an estuary (‘recruit’ into it) and then use the estuary as a nursery area for those seven-odd years it takes to mature, while some will recruit into the surf zone and live most of their sub-adult live there.

According to JD, from larval stage to 80 – 90cm fish are dying. “it’s a classic population pyramid, with a wide base – billions of larval fish – that slopes sharply inwards until the adult size class is reached”. This has always been the case, historically due to many natural factors (being eaten by predators), but now, lots of fish are dying because of fishing pressure and the lack of recruitment in estuarine systems.

Says JD: “The success of the recruitment each year is highly variable and related to the rainfall in the catchment of the estuaries where they are going to recruit. The larval fish need fresh water from the estuaries, which carries chemical signals, to be pushed out to sea so that they can ‘smell’ their way into the estuaries.”

In simple terms then, recruitment doesn’t happen in years when you don’t have sufficient rain coinciding with the time that the fish are in the recruitment stage (which is probably just a few months a year).

“You can potentially have massive failure in recruitment for an entire spawning season in a particular area because the larvae fail to find the estuaries before they are eaten at sea. That is one thing, the other is that weather and oceanic conditions can be such that the spawn gets swept offshore and there is also no recruitment.”

Herein lies the crux of why dusky kob grow so old.

To overcome all of these massive hurdles, dusky kob have to live really long. “So that they can outlast drought periods – which could last for 10s of years – and all sort of other environmental and oceanic factors. Basically they live long so that they will at least see good recruitment at some point in their lifespan.”

The trick here is that dusky kob have enough natural challenges to spawning without human interference.

The usual suspects such as human population growth and the resultant increase in agriculture, results in removal of massive amounts of water from our estuaries. “Add to that something like climate change where we see massive shifts in ocean currents and rainfall patterns and, in all likelihood, it is all going to result in further negative effects on the success of recruitment,” says JD.

So to give them the opportune chance to replenish their stocks, we need to return all caught dusky kob. “For the stock to be healthy in the long run you have to let them live long. If we catch them soon after they have spawned once, that doesn’t mean that they have replaced themselves. They need to spawn for twenty/thirty/forty years to make sure they replace themselves,” JD says.


*Side note – silver kob
Silver kob grows much faster and spawns much earlier. They spawn at a much smaller size which means they are far more resilient to fishing pressure. They also live in a far more stable environment (in the ocean) and don’t rely on recruit into estuaries for their growth phase to be successful – meaning they have a much better chance to survive to maturity.

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