Aerial view of Kanamai, Kilifi County, Kenya. Image © CORDIO East Africa.
New research led by the University of Oxford has found that that oceanographic connectivity (the movement and exchange of water between different parts of the ocean) is a key influence for fish abundance across the Western Indian Ocean (WIO). ScienceVega spoke to lead author Laura Warmuth to get her take on the potential implications of the research.
ScienceVega: To what extent can we quantify oceanographic connectivity?
LW: On a large scale, there are quite a lot of good models and in my case I used an Indian Ocean-wide model. It had a sub model for the Western Indian Ocean region and we can model big dynamics fairly well. The main thing we can’t do is really small-scale things, so smaller circulations and edges around coral reefs that we can maybe do at an individual site, but not really at a bigger range or scale. We looked at different coral reefs across several countries and ignored all of the smaller scale data to really look at the big dynamics. The main way in which we looked at this was to see just how many particles, how much current and how much connectivity there was between two different sites. It is broken down from everything you could do, but it gives us a hopefully good applicable indicator.
Munje, Kenya. Image © CORDIO East Africa.
ScienceVega: Do you see similar patterns in all oceans or are they distinctly different based on their own geographies and currents?
LW: The oceans are vastly different. I’m a biologist, not an oceanographer or a physicist, but to my knowledge every ocean, even every region within an ocean, is vastly different. In the Western Indian Ocean, it is seasonal; we have four seasons, which are the monsoon, transitional, and dry seasons, all of which are about three months long. That means there are four different patterns and depending on when you look at your sites they show different connectivity patterns.
That’s quite unique to the monsoon region and the Western Indian Ocean has monsoon-driven patterns that really affect how different sites are connected. Other oceans have different dynamics and there are bigger scale processes that connect oceans like the gulf stream for example. But you don’t find the same patterns across oceans.
We looked in great detail into the Western Indian Ocean with a model that was just for this ocean. It was a sub model that allowed us to only look at mid- to large-scale patterns and tried to break them down. We did look into seasonality, but we didn’t include it for the current modelling. We’re planning on doing that in the future because that’s obviously a big part of connectivity dynamics.
If these vastly change, you can’t break them all down together into getting just an average, so in our case we did match the monthly connectivity with the monthly ecological data based on time and location. Depending on when fish were surveyed, we used the oceanographic data from these months so it would match to the season’s connectivity pattern. But if you want to see concrete future trends, you would really need to find out how oceanography is driving different ecological processes. For that you need to do a model for each season and then look at them separately.
Gillnet fishers from Mkunguni, Kenya. Image © CORDIO East Africa
ScienceVega: Are you seeing any changes to oceanographic connectivity due to climate change?
LW: We do. We have our climate change predictions, the Shared Socioeconomic Pathways (SSP) scenarios, and there are different paths we can take; the newer ones now include socioeconomic data as well. If you look at the best- and worst-case scenarios, you see differences. We see them in the Indian Ocean as well, not necessarily the strongest ones, but we are seeing predicted changes between the most drastic climate change scenarios.
If you look at the more realistic scenarios, those that are between perhaps 1°C and 3°C warming then oceanography is less predicted to change than for example sea surface temperatures or ocean acidification and salinity.
ScienceVega: To what extent have the reefs you have studied been affected by a warming of the oceans?
LW: Quite a lot, and that’s another reason why we’re looking at the Western Indian Ocean as well. The local communities depend strongly on the coral reefs and the reefs are particularly drastically affected by climate change. The Indian Ocean is also one of the large oceans that is warming at a faster rate than other oceans, and that affects the coral reefs. There are reefs where, even under the more optimistic and best-case climate scenarios, the majority of corals are predicted to bleach. They potentially won’t recover as well since bleaching becomes more and more frequent.
So I would say the Indian Ocean, and particularly its coral reefs, are more threatened than a lot of other marine ecosystems across the world.
Camouflage grouper. Image © Paul Asman and Jill Lenoble, CC BY 2.0 < https://creativecommons.org/licenses/by/2.0 >, via Wikimedia Commons.
ScienceVega: How can decision-makers incorporate connectivity when considering conservation projects?
LW: That is what we were trying to do in really breaking down the connectivity dynamics. We’re focusing on a very simple pattern to count the modelled particles going between sites and find one parameter which gets assigned to all the reef sites we have. This gives us a proportional index between zero and one, or 0% and 100%. Then we can see whether a reef is highly connected, medium well- connected or whether it is very little connected and isolated.
Based on that, climate risk can then be classified between very well-connected reefs and not so well-connected reefs. That in turn can be incorporated into planning marine protected areas. If you have a budget to design a network between either different reef sites or different countries, you can manage it based on connectivity.
What we are envisioning is that you have a really simple indicator that can classify your reefs and that way you can include them in any sort of conservation or marine protected area management by knowing the number of well-, medium-well and not so well-connected reefs. We need something that is easily understandable and widely applicable. That is why we try to really break down a lot of complex oceanographic models and dynamics into something where you can have a range of values on a scale of how connected the reef sites are. Based on that we can assess them, and we have done that for around 50 sites in the Western Indian Ocean. You can use the same modelling process to create this indicator for different sites or areas and include it in conservation management plans.
Sights from the Mange Reef, Mafia Island, Tanzania. Image © CORDIO East Africa.
ScienceVega: How is climate change impacting coastal communities within the WIO?
LW: That is a major reason as to why we’re working there, because local communities really depend on coral reefs. There is a lot of research that has been done already where ecology moves a bit into social science and looks at how local populations depend on coral reefs. But the focus and the knowledge is much more present, for example, in the Indo-Pacific region, where there is a lot more NGO work and a lot more historical research.
In the Western Indian Ocean and rest of the Indian Ocean, we know that the warming is accelerating quite a lot faster and that it is one of the warmest oceans. The conservation and research efforts are more limited than in other regions, and we know that communities depend on small-scale fishing a lot, but very little of that is measured. Our knowledge is limited on how these fish populations are impacted by different climate scenarios. How will they develop? How do different functional groups affect each other? How does the food chain alter if groups higher or lower in the food chain decrease in numbers quite a lot? How might that affect the whole ecosystem? And then, in turn, how will that affect small-scale fisheries and the local income or protein source for coastal communities?
This is a tight connection, which is why we’re also focusing on reef fish groups that are relevant to small-scale fishing and local communities. All of the reef fish groups we’re looking at are important ecologically, but also important to local communities and fishing. They provide an income source for people in these coastal and island regions.
ScienceVega: How do you plan to build on the research you have done so far?
LW: We are planning to create separate models where we look into seasonality so we can make predictions based on which quarter of the year you’re looking at. We are currently including social data as well; given that the development of abundance or biomass of different reef fish groups is so crucially important not just for the coral reef, but also for local communities, we have included socioeconomic data. This encompasses variables such as how close is the nearest market centre, the nearest town, or the nearest local population of people.
We are also looking at, for example, how much is the local population predicted to increase over the next couple of decades and how that would develop in parallel to what fish populations we are predicting. We’re trying to find out whether, in certain areas, there is higher stress because there are increased fishing and population impacts, so more demand from the sea, and whether that can be integrated into conservation measures. If there are certain areas where we know there is going to be human pressure, how can we counteract that? How can we create a marine protected area design that could buffer for that? We are looking at whether we can at least try to predict and model areas for the next couple of decades. So we are looking to expand out to the social human side as well.
The main goal is that this is a process that breaks down complex oceanography connected with ecology and really gives a tool that others can either recreate or use what we have already published and translate that into conservation. We are already working with a local NGO (CORDIO East Africa, Mombasa, Kenya), so they are right in the field and that was the main motivation – to really have research that can be translated or applied by stakeholders, by local NGOs.
The focus now is to also include the human pressure perspective to find out not only whether we can prepare for climate change, but whether we can also prepare for population increase, for fishing demand and how we can protect coral reefs, or at least buffer their resilience for the future.
