31 minute read
THE SECRET LIVES OF GTs
ARGUABLY FLY FISHING’S MOST LOVED SALTWATER BULLIES, THERE’S A LOT MORE TO GIANT TREVALLY (CARANX IGNOBILIS) THAN MEETS THE FLATS ANGLER’S EYE. TO UNDERSTAND THESE INCREDIBLE FISH A LITTLE BETTER, WE CHAT TO THREE LEADING FISHERIES BIOLOGISTS ABOUT THEIR WORK ON THE WORLD’S BIGGEST GIANT TREVALLY AGGREGATION OFF MOZAMBIQUE, WHY SOUTH AFRICA’S COASTAL GT FISHING HAS DETERIORATED SO MUCH AND JUST WHAT THE HELL GTS DO AT MTENTU.
Photos. Ryan Daly, JD Filmalter, Tessa Hempson, Mark Ziembicki
Think of “Giant Trevally” and what comes to mind is probably a massive, kingfish-destroying bonefish on a Seychelles flat, or maybe the plus-size specimens caught in the seas off Oman. Narrow down your brain’s search engine to “Giant Trevally from Sub-Saharan Africa” and your results thin out a bit. There are the small GTs caught occasionally in estuaries and off the rocks along the Wild Coast, but for adult GTs you would have to concentrate your attention on northern KwaZulu-Natal where fabled destinations like Kosi Bay mouth and Rocktail Bay were put on the map by this species. In the ‘80s and ‘90s, the GT fishing was incredible in these places but, despite the odd large fish being caught every now and then, additions to “The 100 Club” (fly anglers who have caught a 100cm+ GT on foot on the South African coast) are increasingly rare.
Someone who saw this area in its heyday is Andy Coetzee, one of the pioneers of GT fishing along the Northern KwaZulu-Natal/ Maputaland coast that borders onto southern Mozambique. Andy still works in the area, running diving and fat bike tours, but his interest in fly fishing for GTs has waned as the fishing has deteriorated. Andy says, “I started fishing Maputaland from about 1984 through to the early ‘90s with a fly rod and a heathen stick with poppers and live bait. I would go down to Kosi mouth and throw poppers and I would be guaranteed to hook at least two or three fish - anything from the range of 15-30kg. There were lots of Bigspot kingies and the Ignobilis, the GTs. The numbers that used to be there were absolutely phenomenal. I can remember going to Rocktail Bay on a spring high tide in the afternoon… eight casts, eight bustups, packed up and went home. That’s it.” “However, since then I’ve seen the decline. In 2014 and 2015 I went to Bhanga Nek with my brother and we fished the prime times, 3:30 am to 5:30am, prime light both in the morning and afternoon. We used a fly stick as well as a big stick, throwing poppers, surface lures and stick-baits. Nothing! It’s very disheartening because, in the 80s and early 90s, you’d catch big fish there. The kingies would smoke you, charge in right between your legs. I remember seeing shoals of big kingies chasing mullet and the mullet were jumping out of the water. You could pick them up, pin a 9/0 through a mullet’s arse and in three seconds you’d be tight. Those days are gone. You don’t even see mullet coming into Kosi mouth. I seldom pick up a fly rod anymore, because the effort to go and have one pull is hardly worth it.” While die-hards will continue to fish these areas and occasionally get a positive result, it’s easy to understand why Andy is so despondent. His experience is anecdotal, but it’s still an important baseline to remember because as humans we’re adaptable (just compare the idea of a national curfew today to the same idea two years ago). A good day out fly fishing for GTs in northern KwaZulu-Natal today looks a lot different to a good day out for Andy and his fellow pioneers back then.
Fortunately, it’s not quite time to slit your wrists when it comes to the future of GTs along our coastline. That frisson of optimism is thanks to a very special annual event that South African fisheries scientists have been studying for the last six years. Each year at a very specific time on a very specific moon in a very specific place off the Mozambican coastline, three scientists, Dr Ryan Daly, Dr Tess Hempson and Dr JD Filmalter come together for an orgy of Giant Trevally. ”Aggregation” is the scientific word for it, but to a layman that sounds a little too much like an Excel shortcut. Regardless, what makes this aggregation so special is that it’s the biggest aggregation of GTs on the planet. Where most GT aggregations from Hawaii to the Philippines and the Seychelles number in the hundreds, the southern Mozambican aggregation has been estimated at 5000 adult fish. That equates to 30 tons of adult Giant Trevally with some individuals estimated at 56kg.
The data gleaned from their tagging efforts, is giving us more insight into the secret lives of Giant Trevally than ever before. We sat down with these scientists to discuss what they have since learnt about GTs ; why the once-famed GT fishing in South Africa is now so poor and just what the hell is going on at the fabled estuary of Mtentu on South Africa’s Wild Coast.
THE SCIENTISTS
JD Filmalter, Tess Hempson and Ryan Daly at work tagging adult GTs. Opposite Page: Tess Hempson and a colleague check an acoustic receiver on the drop-off at Vamizi island.
Somewhat like chess boxers, fisheries scientists are hardcore in that they are just as at home slugging it out with massive GTs and gargantuan Zambezi sharks (for research purposes of course), as they are sitting behind a computer analysing the data. As far as dream teams go, it does not get much better than these three. Outside of their PhDs and job descriptions, they have worked all over the world studying myriad species from coral reefs to bigeye tuna, Napoleon wrasse, grouper, Zambezi (bull) sharks and hammerheads.
Dr Tessa Hempson
Tess is the Programme Manager & Principal Scientist for Oceans Without Borders, which is a collaboration between conservation-focused luxury travel company &Beyond, and Africa Foundation – a non-profit organisation dedicated to community-led development and conservation of Africa’s natural environments. She spends a lot of time on location at &Beyond’s marine locations - from Mnemba island off Zanzibar in Tanzania, to Bazaruto in Mozambique and iSimangaliso in South Africa. But perhaps the best part of her job is her work on Vamizi, an island in the northern Quirimbas archipelago on the Mozambican channel, where the flats drop off into abyssal waters and amazing reefs. It was here that she found another aggregation of GTs, albeit much smaller than the southern Mozambican one.
Dr Ryan Daly
Ryan works for the Oceanographic Research Institute (ORI) where his focus is on Linefish & Marine Protected Areas. When he’s not on the road or on the open ocean tagging fish and sharks, Ryan is usually behind his computer in Durban. His bread and butter work is acoustic telemetry, tagging fish and sharks and then tracking their movements along our coastline. When it comes to GTs and sharks, few people can compare with Ryan’s deep knowledge.
Dr JD Filmalter
JD is a fisheries biologist and behavioural ecologist focused on shark and fish movements in Southern Africa. He lives in Cape Town, but still spends a lot of time on the Breede River where he set up the Acoustic Tracking Array Platform (ATAP), a tracking project for kob and he continues to do work there for SAIAB (South African Institute for Aquatic Biodiversity). He claims he taught Ryan how to catch fish.
WHY THERE?
A spawning aggregation is defined as an event that happens at the same time at the same place, year after year. That sounds simple enough but, finding an aggregation like the mammoth Mozambican one takes a lot of time, effort, data, local knowledge and a touch of luck. By going back to the area year after year, catching and tagging fish, noticing patterns linked to the time of year, the moon phases, water temperature, currents and so on, these scientists have gradually developed a very accurate idea of where and when these fish will be in big numbers.
Tess says, “There’s a lot we don’t know about aggregations, because they are so temporary. Unless you are there at the right time on the spot, or if you happen to have receivers in the area and see a whole bunch of tagged fish going to a specific area, you won’t know. You have to kind of tease out the data and start joining the dots.”
To understand why the GTs are there, it helps to understand how they behave as adults. Unlike a similarly large species like the Humphead (Napoleon) wrasse that Ryan studied when he was based in the Seychelles, on reaching maturity GTs begin to behave very differently. Ryan says, “Adult male Humphead Wrasses need caves of a particular size to hide in and sleep in at night. They also need to be close to a feeding site. The males defend their territory, which is a prime sleeping site next to a feeding site, which the females come to, and then they have their harem. The bigger they get, the less they move because they’re territorial. In contrast, the bigger GTs get, the more they move. As juveniles, nursery habitats are really important for them but, as soon as they hit maturity, they need to feed themselves and they need a broader range of habitats. As they get bigger, it costs them less energy to move. So they go further and further and further. We tagged GTs and Humpheads at the exact same site and they did completely opposite things because the drivers were completely different. For Humpheads it was mating. For GTs it was foraging.” Seychelles guides will tell you how intelligent GTs are based off observations of how they reject flies that they have seen too often (RIP Flashy Profile). When it comes to an aggregation like this, that attracts GTs from vast distances, just how these fish know where and when to go there is something that’s not 100% clear yet. There has to be some level of species intelligence and evolutionary sociability. Take the example Ryan gives of a GT that was caught as a juvenile in Durban harbour, grew up in an aquarium and was then released. “We tracked this fish and in the first year it swam around, kind of not knowing what it was doing but then, by the second year of liberty, it was at this aggregation site with all the other GTs, showing that it picked up what to do from other GTs out there. That was really, really interesting: how this captive fish, got talking to his mates and then knew how to become a wild fish and go to the spawning event. There’s also evidence from other spawning aggregations that if a spawning aggregation is decimated by fishing, the fish then won’t come back to the same place to spawn because they pick up social cues from other fish. So they’re essentially socialised, they share information and they
know what to do because they learn from each other.” Evolutionarily speaking, the question arises why these fish would come together in such a massive yet vulnerable aggregation as opposed to the smaller aggregations seen at Vamizi, or in the Seychelles, Hawaii and elsewhere. While GTs did not evolve to deal with the threat of a trawler’s nets, there are undoubtedly evolutionary reasons that draw them to mass spawning events.
Tess says, “Spawning sites are very often used by several species. During the annual spawning event in Mozambique we see mixed schools of kingies - GTs, big eyes and fulvies (yellow spotted trevally). The thing with mass spawning is that if you all breed at once, you pump it all into the ocean at once, the predators can’t eat all of your babies, so some of your offspring will survive. With GTs from all over, all spawning at the same time, you’re getting this mass mixing of genes, which means your genetic diversity is properly underwritten. At these sites there’s good water flow, so the larvae of the fish that spawn there will travel far. Evolution has dictated that the ones that breed there are the ones that generally survive and keep coming back to those spots.”
WHERE ARE THESE FISH FROM?
Thanks to the ATAP programme and the network of automated datalogging acoustic receivers that are moored to the ocean’s floor around the South African coastline, Ryan, Tess and JD are able to track the tagged fish from the southern Mozambique aggregation. Every time a tagged fish swims past a receiver, it’s logged. Perhaps the most fascinating take away so far from this aggregation is the fact that every GT that was caught, tagged and released by the team has been tracked going south from the aggregation site. None of them has moved north, not even to pop in at Tess’s smaller aggregation at Vamizi up in northern Mozambique. That means, you get to thump your chest with patriotic pride because these are Rainbow Nation GTs, as South African as biltong, Desmond Tutu and Rassie Erasmus’s dance moves.
Ryan says, “All of the GTs we’ve tagged in southern Mozambique have come south and none has gone north. So there’s a divide between our subtropical GTs on the coast, and the tropical GTs. They’re doing different things. The subtropical GTs can’t spawn in subtropical waters, because it’s too cold. Their larvae needs to develop in warmer water so they all have to travel very far north to get to warmer water to spawn, which is why this aggregation in Southern Mozambique is the biggest in the world because it draws all of the subtropical GTs up to spawn.” With an aggregation this big, I had assumed that the GTs attending it would be coming from all over the place like a UN summit - Mozambique, South Africa, Madagascar, hell maybe even some visitors from the Nubian Flats and Oman. A part of me likes the idea that a big oceangoing GT could frequent any island, atoll, estuary, flat or aggregation it likes. While in theory this sort of genetic mixing could happen and probably has over the centuries, it’s uncommon.
Regarding the possibility of GTs from Southern Africa intermingling with those from Seychelles, the Red Sea or even Oman, Ryan says, “I think there’s some connectedness in terms of the genetic flow over time. But even in the Seychelles, we tagged GTs on the Amirantes bank, which is separated from Desroches by a channel. Those GTs would go throughout the Amirantes bank, which is roughly 220 kilometres north to south, but they would not cross the channel to Desroches which was only 40 kilometres away. So, much like Hawaii, island-based GTs stick to their patch of island and they don’t go oceanic. In a way, island populations are more isolated populations of GTs.” JD says, “It would be incredible to see some mixing, but I suspect it’s probably not likely to happen. We don’t have this data yet, but my assumption is that the further you move into the tropics, the more stable the environments become, the less they’re going to travel. That said, it just takes a few individuals to move from one population group to another and spawn successfully with the other group for their genes to quickly spread through the population. Even over timespans of fifty to a hundred years, if you have one fish that decides to swim to Madagascar, it can impact the connectivity. Each species needs those outliers -that’s their survival strategy. You need the ones that are going to go out and explore.” A recent example of a GT that went off piste is that of a 14.7kg specimen caught in the Breede River in April 2021. That catch was particularly bizarre for two reasons. Firstly because, in reference to the impacts of climate change, in issue 26 of The Mission (March/April 2021) we jokingly predicted we’d be “fishing for GTs in the Breede River in about ten years’ time”. Two weeks after that issue came out, that fish was caught, leading us to head to Grand West Casino for a flutter that did not end well. The second, more-pertinent reason that fish was so special, is because it’s basically unheard of for GTs to be caught in the Breede River as it’s way south of their usual range. Tess says what happened with that Breede GT could be down to a couple of things.
“It could be that with global warming you’re getting warmer temperatures further south. For example, in Sydney on the east coast of Australia, they’re getting butterfly fish in the Harbour. Butterfly fish are a coral reef species and Sydney’s eco-system is similar to Cape Town. Or, it could be that there was an eddy off the Agulhas current. The current comes down the east coast and then it hits the ITCZ (Intertropical Convergence Zone), which is the next layer of weather systems and ocean. It then retroflects back on itself around Agulhas Banks where that coastal flat gets really wide. As that bend retroflects back on itself it can move in and out from the coast. It can vary from day to day. That’s why some days the water temperature will feel like you’re on the West Coast, and the next day you’ve got warm Durban water. That’s the bend in the Agulhas current moving in and out from the shore. But then the other cool thing that can happen is that the bend that comes down the coast can create an eddy of warm water that pinches off like a blob from a lava lamp and then spirals down the coast. So potentially, a GT in an exploratory mood might have been swimming in one of those warm eddies and got stuck way further south than usual, ending up in the Breede River.”
As the number of receivers stretching from Mozambique up the east coast of Africa increases and more data comes in, scientists will get an even better understanding of how these fish behave, where they go and who they mingle with. Advances in genetic sequencing are also moving at lightning speed to the point where tagging won’t be necessary any more as they will be able to monitor a fish population just by taking genetic samples. For now, the long and the short of it is that not all GTs are the same. They are very adaptable when they need to be. As the tropical GTs of the Seychelles or Vamizi have shown, they can stay super local if everything they need from food to spawning is close by. Or, in the case of subtropical South African GTs, they can travel incredibly long distances following warm water currents in search of food, spawning and perhaps just a little bit of adventure.
WHAT DOES THE SPAWNING LOOK LIKE?
What I wanted to know, was what GT sex looks like. Is it like a feeding frenzy similar to those videos of the “pets” at Farquhar, a gang of human-habituated GTs that hang around like street dogs looking for scraps? Or, is it more a case of The Best of Sade and a bottle of Chardonnay? Ryan explained that GTs are gonochoristic broadcast spawners, the ’gonochoristic’ bit meaning they are either male or female as opposed to some fish species which are hermaphrodites. The ’broadcast spawning’ bit means the females release their eggs and the males release their sperm into the ocean at the same time. Some of the eggs will be fertilised by the sperm and will float and drift on the current, the larvae developing and eventually hatching into mini GTs, which will hopefully find a healthy estuary like Mtentu to grow up in. What that means is that the adult GTs have to all come together in the same place at the same time for a mass release of spawn and eggs. The location is probably dictated by habit/previous success and passed on from adults to juveniles (and even, after a rigorous vetting process, to that Durban fish released from an aquarium). The timing of the event, which is dictated by the season and the moon is a formula. Once the team worked out the formula, the aggregation has occurred like clockwork. It is that predictable.
Ryan says, “To see so many GTs is unbelievable. The scale of it is constantly phenomenal. You just want to be in the water as much as possible as they give you a short window into their lives. This year, we were particularly fortunate because the conditions were so good that we got to spend a lot of time in the water with them, observing the Zambezi sharks swimming through them, and observing how they socialised which involves colour dimorphism. The males and females will start to change colours, going dark and light, dark and light as they start to signal to each other that they are ready for spawning. It’s the same for many king fish species. Bludgers, fulvies, black tips, and bigeyes all do this. Interestingly, bigeye kingfish will pair off much more, so there’ll be one dark and one light, but the giant trevally
seem to be more mixed. The grand finale of them actually releasing spawn still eludes us. I think it’s because it happens when we are never around, very late in the afternoon on a very specific day when the weather has to be correct. So I don’t exactly know what the final event looks like.” Though they may feel like disappointed GT pornographers, we predict with the many hours the team spends fishing, tagging and diving at the GT mega-aggregation, it’s just a matter of time until they witness the money shot.
THE MTENTU MYSTERY
It’s important to understand that there are spawning aggregations of GTs and then there are non-spawning aggregations that no one really understands just yet. The southern Mozambican GT aggregation is the only local spawning aggregation they have found (the jury is out on what the 1000-fish strong aggregation at Vamizi was about). While those two are relatively new, perhaps the most well-known aggregation is the one that occurs at Mtentu estuary on the Wild Coast of South Africa’s Eastern Cape province. Part of the Pondoland MPA (Marine Protected Area), Mtentu is a vital stronghold for South Africa’s GT population.
For sub-tropical GTs, estuaries are nurseries, the equivalent of an atoll for tropical GTs. For juveniles, they are safe spaces where youngsters can grow and feed before reaching sexual maturity. The GTs of Mtentu are not spawning so what they are doing there is still up for debate. The answer is probably multi-factorial, but for Ryan the main reason is that Mtentu is, to put it simply, their home.
Ryan says, “Mtentu is one of South Africa’s best estuaries. Whereas most other estuaries are temporarily open, or are closed or aren’t as tidal, Mtentu is really unique because it’s tidal, there’s sea water for four and a half kilometres up river, and the mouth never closes. You can’t fish in there. There’s no pollution. It’s absolutely pristine. It is just the most perfect place for GTs to live in and it remains one of the most pristine nursery habitats for them on our coast. I think that they’re doing this thing called natal philopatry where adults come back to the place that they were living in as a nursery. Animals seem to have this really strong draw to come back to the places of their birth or where they grew up.” JD says, “Mtentu has got some really deep parts but its catchment is quite short, so it doesn’t often get super dirty, unless there’s massive rain. It has no agriculture upstream. Because it’s so short it’s pristine. I have no idea what it is about that estuary that makes it so different and so attractive for the GTs because there are GTs in almost all the Transkei estuaries, but they are not coming in in those big numbers you get at Mtentu. Maybe it’s because Mtentu’s mouth is big, deep, and permanently open whereas other estuaries along the Transkei coast are less predictable and open and close.” Another theory Tess has is that the fish go to Mtentu to rid themselves of unwanted passengers. She says, “What the GTs are doing at Mtentu is the big question, isn’t it? One of the things I did in my PhD was collect chocolate grouper. I was catching them, bringing them up to the surface alive in cages, then putting them in aquariums for a while to get their diets kind of consistent before putting them on to these reefs that I built. I needed to make sure that they were all starting from an equal footing, which meant ensuring some fish weren’t carrying a heavy parasite load while others didn’t. The way you do that is you put them in fresh water. If you put a saltwater fish into fresh water, parasites come out of everywhere…literally through their skin, out of their gills, out of their mouths. It’s unreal. You don’t do it for too long, generally less than a minute
Daisy-chaining GTs mill around in the pristine Wild Coast estuary of Mtentu.
as that’s all it takes for the parasites to freak out. Then you stick the fish back in salt water. That opportunity to get rid of parasites is part of what I think draws the GTs to Mtentu.” Whether it’s natal philopatry, parasite management or something else that draws GTs to Mtentu and other Wild Coast estuaries, these fish are clearly not deterred by how far they have to swim when it comes time to spawn. Ryan says, “What we see is that juvenile GTs are in Mtentu year round and over the summer period the adults come back into the estuary to hang out. But then those same adult fish go to southern Mozambique to spawn and then go back to Mtentu the same month. Last summer we had GTs arriving in Southern Mozambique to spawn and when the moon changed those same GTs were back at Mtentu. So the same individuals are traveling massive distances up and down, spawning in Southern Mozambique, but not spawning in Mtentu, which makes sense. It’s too cold to them to spawn in Mtentu, but there’s good feeding, good habitat and it’s the site of their birth.”
If the spawning aggregation is Matric Rage / Spring Break for randy GTs, Mtentu is somewhere between your mum’s house for Sunday roast, a spa day and a sensory deprivation chamber free from boats, anglers and nets. That’s why it is so vital to keep it off-limits.
HOW DO WE PROTECT THEM?
When you picture an aggregation of this size, the immediate thing that comes to mind is how vulnerable these fish are. It’s necessary to divide the threats to these GTs into immediate and long-term threats. Immediate threats would be something like one idiot of a trawler (or 30 small boats with the coordinates) coming along and wiping out the population in no time. Long-term threats are trickier to grapple with, ranging from overfishing, to climate change and the destruction of their nursery habitat via development and the impact of centuries-old agriculture. In the immediate threat column, it helps that this aggregation falls into the greater Maputo National Park, which incorporates both a terrestrial park and a huge stretch of coastline. Not only would you really struggle to find it without the working knowledge of the GT-SexFormula™ that these scientists have developed, but as far as Marine Protected Areas (MPAs) go, this one is well policed and if you tried you would hopefully get shot out of the water for your efforts. Regardless, Ryan, Tess and JD are intentionally vague about both where and when the aggregation takes place. That’s not to say that you, dear reader, are an unscrupulous bastard with fishcakes on the brain, a cousin with a midsized trawler or a bar stool with your name on it at the ski boat club. But you could be… Tess says, “The one super important thing with these aggregations, and the reason that it’s so important to learn about them and focus conservation efforts on them, is that if one fishing vessel with a big seine net comes through, it can smash a population in an instant. So figuring out where these spots are and then not letting on where they are, is the one major thing. But making sure that if they are known that they are properly protected is also super critical.” Tess has firsthand experience of what happens if you properly protect a marine environment because a large part of her work with Oceans Without Borders involves working with the local community on Vamizi to ensure they have a sustainable fishery for years to come. “Vamizi is an amazing success story. In Nampula province, the next province down in Mozambique, there’s been a huge amount of fishing pressure by commercial vessels offshore, but also subsistence fishing from growing coastal populations where people are very poor and directly reliant on the reefs. But on Vamizi Island, it’s different. In the fisheries laws of Mozambique, they have a thing called a CCP (Conselhos Comunitarios de Pesca or Community Fishers’ Councils), which is a volunteer fishers’ council. Through a CCP, fishermen that live in a particular community have the right to decide how they want to manage their local marine areas within three nautical miles of the shore. They can lock it down completely and say, ‘Sorry, sanctuary area, no one goes in here.’ Or they can say ’Only we fish here and we only use this gear,’ or ’We close for this time of year,’ or whatever it is they choose. “On Vamizi, the various stakeholders that have been working in tourism there for a long time have worked with the local CCP to establish a marine sanctuary area off the eastern side of the island. That’s just about to be declared a nationally gazetted MPA which will be the first gazetted MPA that’s been community driven and not from national legislation. So it’s not top down, it’s bottom up. The dividends of that sanctuary are already paying off because all the way down the coastline for hundreds of kilometres, everyone knows that Vamizi has fish because it’s got the sanctuary area. Through war and everything else that’s happened in Mozambique, that CCP has kept on policing that space. Over the 20 plus years that it’s been in existence, they’re now seeing the impact and the message is that if you conserve an area, this is how it pays off. Because of Vamizi’s reputation for having fish, the CCPs on other islands close by have realised Vamizi is onto something. They’re designing MPAs for their local fish stocks, dictating which gear, nets etc can be used and they are managing it themselves. So it can work.” Turning our attention to the long-term threats, things get a little more complicated. If estuaries are the nurseries for coastal GTs, the future of these kids is not bright. To the north, with the exception of places like Vamizi, fish stocks in Mozambique have been hammered by rampant netting and the estuaries are particularly bad. In South Africa our estuaries are also in a horrendous state, but JD points out that trawlers and netting are not the problem.
JD says, “There is no massive targeted commercial fishery for GTs anywhere on our coastline. Even in southern Mozambique, there’s no dedicated commercial fishery for them. There are so many factors involved when looking at declining fish stocks, but it wouldn’t surprise me to find that habitat degradation plays a massive part in declining populations. It’s the juvenile, estuarine nursery habitats that are getting hurt on our coastline and that’s a key part. We don’t know exactly how critical that is for the entire population, but we do know that you find baby GTs in estuaries from the Wild Coast/Transkei all the way up the coast. We don’t have many sheltered bays on our coastline, it’s all very exposed so, for these species, it’s the estuaries that serve as nurseries.
Tess Hempson working with the local CCP on Vamizi (left) and downloading tagging data (right).
“Our subtropical estuaries are broken, whereas our temperate estuaries further south in the Transkei and Eastern Cape are a bit better. In KwaZulu-Natal the estuaries don’t work; they’re pretty much all stuffed. My guess is that’s due to the plantations of sugar cane, pines and blue gums, which remove so much water out of the soil compared to natural vegetation, that the water table disappears. When it rains, the runoff hardly gets into the rivers and the rivers struggle to feed the estuary. The estuaries become too shallow for adult fish to come in and too silted and turbid for the plant life (like eel grasses) to grow that need to be there to make the nursery habitats. The diversity just disappears as the estuary stops working. “Take St. Lucia, which makes up something like 40% of South Africa’s estuarine surface area. It doesn’t function. It’s pretty much permanently closed and is now just a shallow silted-up bog. So that’s almost half their habitat gone straight away. I’ve seen photos from the ’50s, ’60s and ‘70s of people shoulder to shoulder, fighting and climbing over each other catching mountains of kob and shad and they just kept it all. There used to be the mullet run at St Lucia and that was when everything came, because the mullet would migrate to spawn and it was this big event. None of that happens anymore. Nothing. So all the fish populations that used to rely on the mullet have gone or have had to move to less favorable environments and are less successful in their spawning which means there’s less recruitment the next year. The knock on impacts are massive.”
So where does all this leave us?
We know that our GTs probably travel further than most other GTs in order to keep their populations going.
We know that strong estuaries equal strong fisheries, not just for GTs but the many other species from kob, to grunter and leervis that are as important to fly anglers (and our niche needs) as they are to the incredible biodiversity and food webs of our marine ecosystems. We know that despite plenty of threats, GTs are relatively fast-growing and adaptable… to a point. We know that the fabled fishing grounds of northern KwaZulu-Natal are not what they used to be and we need to remember that. Because, while it’s unlikely that we can reverse the damage done to our estuaries and bring those fisheries back to life, things can easily get worse if we do not protect the species and places that we have left.
Tess sums up the state of GTs when she says, “Compared to some other marine species, I think GTs are doing okay, but the pressure on marine ecosystems is insanely heavy and the problem of shifting baselines is severe. What we knew the oceans to be when we were children versus what our grandparents knew is vastly different. That’s why Andy’s experiences are so important. If, as a scientific exercise sampling the ecosystem, you could spend a day at sea fishing back then with what was probably very inferior kit and then look at what you bring home compared to today, the difference scientifically would be insane. We’ve got to remember those baselines, because we might think GTs are doing pretty well, but it’s relative because everything else is so trashed. How were GTs doing fifty years or a hundred years ago? There might still be a good few around today, but they’re in decline. We can’t be complacent. Properly working Marine Protected Areas are really critical. There’s a drive to declare thirty percent of the planet’s coastal waters protected by 2030. It’s something we definitely need to be striving for, but declaring is not enough. We need to be enforcing it, whether enforcing means ‘keep everybody out’, or establishing what the key species are and explaining that we don’t touch and this is why.”
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