By JD Filmalter

If you fish at sea in South Africa – in the surf, from the rocks or in an estuary  – you know what a dusky kob is. They are iconic. They are prized. They are magnificent.

They are disappearing…

THE FACTS

  • Juvenile dusky kob live in their home estuary for the first 3-5 years of their life and almost never leave
  • Juveniles from the coastal environment seldom enter estuaries
  • Adult dusky kob live mostly at sea and only visit estuaries for short periods
  • Coastal movements of adults are limited
  • The national stock consists of sub-populations that very seldom mix
  • Currently very few juveniles are making it out of estuaries to join the adult population.

 

THE CONTEXT

Back in the mid 90s Dr Mark Griffiths officially described the species Argyrosomus japonicus in South Africa while conducting his PhD research. This separated them out from the silver kob A. inodorus, with which they had been lumped as Johnius hololepidotus since people started recording such things. Since then the scientific endeavour to understand more about them has been relentless.

Griffiths did extensive work on their biology himself, establishing that this was a long-lived, slow growing and late maturing species. Almost the diametric opposite of its silver sister-species. Dusky kob typically live in excess of 30 years, with the oldest recorded at 42 years. They reach maturity after 5-7 years, with males maturing slightly earlier than females. At this age they are 90 cm – 1.1 m in length. They also get big and inhabit estuaries and shallow coastal waters, which make them highly attractive targets for recreational anglers.

dusky kob

A shoal of silver kob in suspension (Photo: Steve Benjamin/Animal Ocean)

During his work, Griffiths also conducted extensive research on catch records and, using these in conjunction with biological traits, determined that the dusky kob population had collapsed, largely as a result of overexploitation of juveniles in estuaries. He estimated that the biomass of adults was between 1 and 4.5% of pristine levels – when no one was fishing. At the same time dart-tagging and catch trends were used to infer movement patterns which led to the belief that adults generally migrated to Kwa-Zulu Natal during the winter months to spawn.

There was some evidence of sub-populations (separate groups with almost no mixing) along the coast but this was difficult to verify at the time. With technological advances in fish tracking methods, these beliefs are now being redefined.

Over the past two decades Dr Paul Cowley and his students at the South African Institute for Aquatic Biodiversity (SAIAB) have led the charge on fish tracking research in South Africa. From the humble beginnings of following a single dusky kob around a small Eastern Cape estuary this research has led to the development of a nationwide monitoring program called the Acoustic Tracking Array Platform (ATAP) with a network of over 100 automated acoustic listening stations deployed at sea and in estuaries from the Berg Estuary to Ponta do Ouro in Mozambique. Today over a thousand fishes, sharks and rays, tagged with acoustic transmitters, are being constantly tracked by the ATAP network. Many researchers have climbed onboard, each focusing on a specific stretch of the coast or a particular species. For Paul and his team however, the dusky kob has always held a special focus.

 

EARLY DAYS IN THE EC

From 2008 to 2012, 96 dusky kob, between 24 cm and 1.2 m in length, were tagged and tracked in estuaries and the coastal environment of the Eastern Cape with Dr Amber Childs leading this work during her PhD.

The findings were remarkable. Almost all juveniles remained resident within the estuary in which they were tagged, with only two fish venturing into other estuaries. As they grew and approached sexual maturity they used the marine environment more and more. Juvenile dusky kob tagged on the coast, on the other hand, very seldom entered estuaries, mostly visiting them to avoid sudden cold-water events at sea. This behaviour of staying put, either in the sea or in the estuary during the juvenile stage suggests that there are probably two groups in the juvenile population which likely reflects where the individual grows up: Either they live in the surf zone, or an estuary.

“When we gave larval dusky kob, about 1cm in length, a choice between sea water and estuarine water they chose turbid estuary water 100% of the time.”

When recently speaking with Amber, who is now at the SAFER lab at Rhodes University, she explained that some of the work she and Dr Nikki James of SAIAB have been conducting is showing just how important turbid (muddy) estuaries are as nursery habitats for dusky kob. “When we gave larval dusky kob, about 1cm in length, a choice between sea water and estuarine water they chose turbid estuary water 100% of the time.” This implies that settling in estuaries with high fresh water input is the favored strategy. This also suggests that those that grow up at sea may do so simply because they were unable to locate a suitable estuary during their early development phase (1-15cm).

Larger fish, most of which were tagged in Algoa Bay, initially moved very little, but over several years some individuals were recorded as far north as Port St Johns, a distance of roughly 400km form their tagging site. Amazingly, after 8 years there are still some tagged dusky kob that have remained resident in Algoa Bay, suggesting only a portion of the population actually undertake significant longshore movements.

TAGGING IN THE WC

More recently, I took up the acoustic tagging of dusky kob in the Western Cape, which was focused in and around the Breede Estuary and the De Hoop Marine Protected Area. This work formed the basis of my postdoc research at SAIAB over the past four years, falling under Paul’s national project investigating the movements of estuarine fishery species. Since 2016, 61 adult (101 – 172c m) and 20 juvenile (64 – 97 cm) dusky kob were tagged.

To date, a little more than 2.5 years since the first fish was tagged, 80% of the juvenile dusky kob tagged in the Breede have been caught and kept.

To date coastal movements of these fish have been even more restricted than those tagged in the Eastern Cape. No fish have been recorded more than 150 km from where they were tagged. Adult dusky kob have spent the majority of their time at sea, but have visited all of the monitored estuaries in this area (Breede, Duiwenhoks, Goukou and Gouritz) for brief periods. Those tagged in the Breede have returned to the estuary every year in alarming numbers.

Each year the vast majority of adult dusky kob tagged in the Breede Estuary have returned.

 

Graeme Forrer with a beautiful reward after many miles of walking. Photo: Steve Benjamin/Animal Ocean

Similar to the findings from the Eastern Cape, juveniles dusky kob tagged in Breede have shown extremely high levels of residency. These fish were all above the minimum size limit (60 cm), but were several years shy of adulthood. The majority of them never left the confines of the estuary. During the heaviest flooding events they would move right down to the mouth, but within a day or two would venture back up into the estuary.

A 65 cm Breede juvenile getting ready for a tag (photo: JD Filmalter)

These fish were tagged with the specific purpose of understanding how they transition from estuarine residents as juveniles to marine users as adults. Unfortunately, the details of this habitat shift will remain a mystery. Not because it hasn’t yet happened, but because almost all the tagged juveniles are no longer with us…

To date, a little more than 2.5 years since the first fish was tagged, 80% of the juvenile dusky kob tagged in the Breede have been caught and kept.

And as many die-hard anglers will be amazed to hear, they were not caught by mysterious illegal foreign trawling fleets, or netted by poachers, or even harvested by the commercial sector. All of them were caught by recreational anglers in estuaries. None of these fish had reached maturity.

dusky kob

Silver prize after a long day on the sand (Photo: Steve Benjamin/Animal Ocean)

While this mortality rate is staggering, it is not all that surprising. During Paul’s earlier tracking in the Great Fish Estuary, some 16 years ago, 49% of the juvenile dusky kob tagged were captured within 4 years. Again, when the focus shifted to the Sundays Estuary during Ambers’ studies, 35% of the tagged kob were caught and kept within 4 years, of which nearly 90% were still under the minimum size limit. Together these statistics paint a dire picture for the dusky kob population.

 

THE DECLINE

Looking at the bigger picture, taking growth rates and movement patterns into consideration, population declines are inevitable, even in the absence of rigorous catch statistics. Juveniles are struggling to join the adult population, while the long-lived adult stock is slowly being eroded.

However, for many fishermen, the population decline has gone unnoticed. This is not because they don’t know how, or refuse to see it, but rather because its simply not reflected in their catches and so doesn’t seem to be happening.

 

THE BASIN EFFECT

Anglers go to their favourite estuary (the Breede for example) and catch the big kob they came for. Where’s the problem? – They ask. The explanation is simple, and the tracking data is clearly showing it. Estuaries are magnets for these fish.  When adult dusky kob enter an estuary, they become concentrated in a confined space. The number of fish per unit area of habitat becomes astronomically higher than when they are at sea. Similarly, the fishing effort per unit area is far higher than when fishing at sea.

So, it’s not entirely surprising that catches (and vulnerability) are much higher in estuaries than at sea. Furthermore, we are seeing more and more that estuaries and the adjacent marine environment are aggregation zones for adult dusky kob. So effectively most of the fishing effort for this species around our coastline occurs within aggregation zones. This means that you will continue to be successful, with little noticeable change in catch rates, when fishing in these areas despite population declines at a national level. This is a well known concept in fisheries, termed the basin effect. If your measure of water in the basin is taken directly above the drain hole, there will always be more water there than on the edges, right until it is completely empty.

 

CAN THEY BE SAVED?

The future of dusky kob hinges on what happens in and around our estuaries. This is where the biggest source of the problem lies. What the tracking data is showing is that this species could be managed at a more local level than a “one size fits all” for almost all of its distribution. Provincial, municipal or even estuary specific regulations, could go a long way to helping rebuild local populations and the stock at large. But the success of any new regulations would require vast improvements in enforcement and compliance across all fishing sectors – a tough ask in SA.

dusky kob

Releasing a Breede breeder (Photo: JD Filmalter)

A growing culture of catch and release is an immensely encouraging sign that many anglers are taking responsibility for their impacts on this species. And the bottom line is if you release your fish in your local estuary or stretch of coast, it is the local population that will benefit first, ultimately resulting in better fishing in the future.

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