Striped & Black Marlin Movements in the Indian Ocean with Dr. Chris Rohner

MMF Principal Scientist Dr. Chris Rohner has been working on tracking marlin recently. This was the first time that MMF researchers have worked on these amazing, highly-migratory oceanic predators. Chris has now published two of the first scientific papers on black and striped marlin movements in the Indian Ocean, so I chatted with him to discuss the results.

Fishing for Marlin information

A few years ago now, we were approached by sport fishers for marlin in Kenya who were concerned that little was known about these fishes. Sport fishing for marlin is typically catch-and-release and, following capture, they were able to deploy satellite-linked tags on the fish to track their movements. Analyzing the data is quite technical though, so they asked us for scientific support.

The sport fishers focused on tagging two species, black and striped marlin. The black marlin is one of the biggest of all bony fishes, with a trophy fish being a 'grander' weighing over 1000 lb. Striped marlin are oceanic speedsters whose stripes light up when they're hunting. They’re both amazing fish.

It's ended up being the first satellite-tagging project for marlin in the Indian Ocean, and one of the largest studies conducted on these species so far. It's been great to work with the incoming data and help the fishers get the information they were hoping for.

Swimming far and fast

It's hard to keep a tag on a marlin – they're among the fastest of all fish, and they jump too – but, generally, the longer the tag stayed on, the further that individual marlin dispersed from Kenyan waters. We found a lot of similarities between the species; they both swim far and fast, covering 40–50 km per day. Most moved northwards from Kenya towards Somalia and Oman, but there was plenty of individual variation too. One of the striped marlin swam all the way over to the Maldives, and a black marlin swam south into the Mozambique Channel. The longest track was 11,944 km over 167 days.

Marlin hunt oceanic fish and squid. We don’t know the distribution of these prey in real-time, but we can retrospectively compare the marlin tracks to oceanographic features, like fronts. Fronts are where different water masses meet, and we can monitor their location through remote sensing satellites. Fronts concentrate plankton at a high density, so planktivorous fish and everything else congregate in these areas, including top predators like marlin. That’s not all we can see with the satellite data: we can look at ocean temperature and chlorophyll (phytoplankton) density as well, which is helpful too.

Striped marlin followed ocean productivity through the year, with high productivity off Kenya during the Dec-Mar period when they were caught, before moving north towards the Horn of Africa around the island of Socotra. There was lots of cool-water upwelling there, which means productive surface waters and probably dense prey. We looked at four years of satellite data for that area and there's a consistent front present that is clearly very attractive for oceanic predators.

Black marlin were similar during the first half of the year, moving north to the same general area, but they moved back down the coast to Kenya in the second half of the year. That return movement is difficult to explain because there's no particular front, upwelling, or obvious productivity we could detect through those months. Generally, though, both species are going to be moving around to find food, so there's probably a prey source present that we can't identify remotely.

There's also the possibility of spawning activity, which we haven't gotten to the bottom of yet. Striped marlin larvae have been found up north, off Oman. No Indian Ocean spawning areas for black marlin have been identified. We think they might be spawning off Mozambique, so we'll be looking into that for the next phase of the project.

Model Marlin

We had some helpful external reviewer comments for the black marlin paper that encouraged us to explore advanced modeling work to understand marlin habitat preferences. With the help of some mathematician friends at The University of Queensland, we randomized the real marlin tracks, then set computer-generated 'model marlin' to (digitally) swim across the ocean. By comparing random model fish movements to where the real fish went, and the oceanographic conditions both swam through, we could test whether the real fish were actively choosing particular areas and conditions.

Real marlin preferred more productive waters than the digital marlin, which makes sense of course: they want to be in places where they can find prey. Even though it's a simple result, it was tough to calculate. For each real track we generated 100 model tracks, then sampled the environmental conditions along all of them, so there were about half a million data points to extract for each variable that we were interested in. That would have taken weeks of solid processing on my trusty computer, so we borrowed some time on a big supercomputer. Even that took a couple of days. It was worth it in the end though, the results were pretty cool.

Holding their breath?

Squid are an important part of the diet for both species, so they dive a lot. One individual striped marlin moved almost 15 km vertically over a single day. There are a couple of factors that come into play on deeper dives. The first one is temperature. It gets colder when they dive past the surface mixed layer, and both marlin species dived down to 450–500 m on occasion, with the coolest temperature around 10ºC. Marlin are cold-blooded, or ectothermic, so the water temperature dictates their body temperature. However, they have a special blood circulation adaptation to route warm blood from their swimming muscles to move past their eyes and brain, so their sensory system maintains a more constant temperature and remains fully functional for hunting at depth. It’s very clever. Still, they will want to come up into warmer surface water to heat up their body after a long period at depth.

Their other consideration is oxygen. Because billfish swim so fast to catch their prey, they have a high metabolic demand. That means the oxygen content in water can actually be a limiting factor for them. There are areas with low oxygen close to the surface in the northern Indian Ocean area, where these marlin migrated to, forcing the marlin to dive into what we call the oxygen minimum zone (OMZ). That will limit their movements after a short time exposed to those low oxygen conditions, as the muscles can’t get enough oxygen to function properly. That's another reason why they'd have to swim up and down; to reoxygenate their gills near the surface before swimming down again.

With the changing climate, the OMZ is becoming shallower over time in some areas. In the Atlantic, the OMZ has expanded so dramatically that marlin habitat has shrunk. They have less vertical space to hunt in, because they have to spend most of their time in this increasingly narrow band where they can get enough oxygen. 

One of the other areas where deoxygenated water is likely to be a problem for marlin is up around Oman, where some of our marlin swam. In some areas up there, the OMZ is only about 20 m deep. It's harder for them to hunt in such a narrow range, and it also affects their vulnerability to fisheries – there's no depth refuge from longlines and nets. Some of the gillnets up there are 30 km or more in length, and there are many thousands of fishing boats. Marlin need more help from people if we’re going to improve their outlook for the future.

Open-ocean conservation

When we started this project I was surprised at how little was known about marlin, considering how popular they are with sport fishers. What we quickly learned was that there are real conservation concerns around marlin. Commercial catches of marlin have increased quite dramatically, tenfold since just the 1990s for some species in the Indian Ocean. These are big fish that can clearly swim long distances, so nobody is sure how many are actually out there. Fisheries observers often struggle to identify the species; there's a shortage of even the most basic information. That makes it difficult to manage these fisheries, obviously, because so little is known about their biology and ecology. At the moment there aren't any legal limits on how many can be caught.

There are two areas of high commercial catches for striped marlin in the region: up north of the Horn of Africa, and off Kenya. Those were the areas our tagged marlin used too. Prior to this work, we didn’t know that it was the same fish moving between both. Now we have shown that this whole region needs to be considered as a single management unit. The entire Indian Ocean might, in practice, be a single population for both species – our tags were set to pop-off the marlin after six months, so they're only capturing a snapshot of their movements, and one swam over to the Maldives in that time alone.

People tend to think of the open ocean as a fairly homogenous environment, but there are definitely some areas that are in much higher use by open ocean predators like marlin, tuna, and sharks. Commercial fishers know that too, and they use the same remote sensing data we analyzed here to identify where the best catches will be. The challenge for conservation in this open ocean habitat, which covers a good portion of the planet, is that it is international waters where no particular government has jurisdiction. That means there needs to be international cooperation to do obvious things, like institute more protection for critical habitats of overfished species, but these habitats like frontal zones aren't necessarily at the same location throughout the year. And then, these areas are far offshore and it is hard to enforce any protections that are put in place.

There are broader social considerations to this too. Marlin swim from coastal areas to the open ocean and back. At the coast, they’re a target for artisanal fishers and an important source of protein. In some areas, including Kenya, there’s a big economic benefit from sportfishing for marlin too. It’s all linked. When open ocean commercial fisheries deplete the stock, coastal communities suffer the consequences. At least our new results allow us to inform these conversations with management authorities. The open ocean is the new frontier for marine conservation.