The ocean moves – and so should marine conservation management
A casual glance at the ocean and you may just see a mass of blue. But take a closer look. There are waves, different colours, and different levels of water clarity. If you could peel back the layer of water, you would see environments that are not entirely alien - like mountains, canyons, forests, grass meadows, sand, mud, and volcanoes. The ocean is a mosaic of the most wondrous and splendid habitats, hosting a magnificent array of life.
Whilst the terrain itself may remain fairly stable, the ocean itself moves. It's not just the waves you can see breaking on the beach, nor the movement of the tides, or even those rip currents you really don't want to find yourself stuck in. Beneath the surface, you will also find movement- like currents flowing at different depths, upwellings that bring cold, nutrient-rich waters to the surface, and internal waves as tall as 244 meters. Sometimes you get two water masses moving either towards or away from each other, creating oceanic fronts.
Broadly speaking, there are two types of oceanic fronts. Convergent fronts occur when the masses move towards each other. Here, the water tends to be warmer than the surrounding area and accumulates all sorts of marine critters, algae, and even litter. In divergent fronts, where the water masses are moving away from each other, upwellings are created, bringing up nutrients from the deep. These nutrients support phytoplankton growth, which in turn supports zooplankton, which in turn supports other marine life – including species under threat such as the loggerhead sea turtle (Caretta caretta), and species we enjoy eating, like albacore tuna (Thunnus alalunga). As with many oceanographic features, fronts are not necessarily permanent features - they do not remain in exactly the same place, in perpetuity. The ocean is a highly dynamic environment, and as a result, the conditions that define the habitats for many marine species - particularly pelagic species that live in the water column - are also constantly on the move.
Being an apex predator (and a very efficient one at that), humans have had a significant impact on ocean ecosystems and the species that live in them through fishing. Not all of these impacts are on the species directly targeted by fishers either, with bycatch being implicated in the decline of many species, including the threatened wandering albatross (Diomedea exulans) and critically endangered vaquita (Phocoena sinus). There are a number of different methods being employed around the globe to reduce the negative impacts of fisheries on oceanic species, like gear restriction, or spatial management measures like seasonal area closures.
Traditional spatial management focuses on pre-designated areas, tending to be fixed in space and often in time. Even regulations such as gear restrictions may be subject to pre-determined spatial boundaries, limited to the jurisdiction of the management authority that implemented the measures. Pelagic species and their habitats pay no mind to human-delineated boundaries. Even though it may sound difficult, ocean dynamism isn’t necessarily a barrier to managing our activities in mobile hotspots of biodiversity, like oceanic fronts. Dynamic ocean management is, as Dr Sara Maxwell from Stanford University succinctly explains in a paper currently in review, “management that changes in space and time in response to the shifting nature of the ocean and its users based on the integration of new biological, oceanographic, social and/or economic data”.
Recently, Kylie Scales, a PhD student from Plymouth University and fellow researchers from around the UK have joined a growing number of people - including myself – who are suggesting that dynamic ocean management is potentially an essential part of ocean management. As for oceanic fronts… well, these, Kylie and the team of collaborators argue in their review paper recently published online in the Journal of Applied Ecology, are ideal candidates for dynamic ocean management.
Although oceanic frontal systems can be broadly categorised into either convergent or divergent frontal systems, there are in fact many different types of fronts. In the paper, Kylie et al. describe a host of these systems. For example, at the ocean-basin scale (1000s km) there are major frontal zones such as those in the Southern Ocean. These more permanent and extremely large-scale features support a host of species like penguins, seals, and whales. At the mesoscale (10s 100s km) to the sub-mesoscale (C. 1km), we have systems such as major currents, like the Kuroshio Current, which flows from Taiwan and past Japan, and its associated eddies. Scaling down even further, we find things like small-scale tidal fronts forming between stratified and well-mixed waters in shelf seas. With such systems retaining nutrients that increase seasonal phytoplankton production, these are key areas for species such as basking sharks (Cetorhinus maximus) and ocean sunfish (Mola mola). Even offshore banks can produce fine-scale tidal topographic fronts that result in a layer of phytoplankton (the subsurface chlorophyll maxima), creating foraging habitat for species like the black-legged kittiwake (Rissa tridactyla).
These biodiversity hotspots – especially the more predictable and more persistent frontal systems - are also the focus of human activities like fishing. Makes sense, right? If you want to fish, you go where the fish are. Unfortunately, fishing in frontal systems also results in a fair bit of bycatch – the accidental capture of species (or a subset of a species, such as individuals that have not yet reached maturity) during fishing operations. In fact, bycatch from fishing in these sorts of dynamic hotspots has been heavily implicated in the decline of many of the ocean’s top predators. But it’s not just fishing that’s an issue, Kylie notes. These convergent fronts accumulate debris, which in itself poses a threat to marine creatures, such as through ingestion of toxic particles or inhibiting efficient feeding. Developments like oil and gas platforms and renewable energy installations occurring in frontal system regions may potentially alter oceanic dynamics, having a knock-on effect for species that typically forage in these regions. Ocean management, if it is to be effective in reducing the human impact on the ocean ecosystem, cannot ignore these dynamic frontal systems.
But wait a minute - I said that these fronts don’t necessarily hang about. So if they are dynamic, how do we know where they are to manage our activities in them? The review paper also took a look at frontal mapping, noting that technologies such as remote sensing and autonomous marine vehicles allow us to uncover where fronts are. Sampling and biologging (tagging marine animals with devices that collect movement and environmental data) can reveal when and where species utilise different frontal systems. This data isn’t just limited to focusing on localised management areas either, with remote sensing systems such as Earth observation (EO) able to provide global front maps. Utilising such technology and then implementing management based on the data produced isn’t as far-fetched as it may sound.
In Australia, Dr Alistair Hobday and colleagues from CSIRO work with the Australian Fisheries Management Authority (AFMA) to produce a dynamic ocean management system to reduce bycatch of the critically endangered southern Bluefin tuna (Thunnus maccoyii)_as it undertakes its seasonal migration up the east coast of Australia into the grounds of the Eastern Tuna and Billfish Fishery. This system utilises biologging data to determine the tuna’s habitat preferences (derived from sea surface temperature readings) and Australia’s BLUELink oceanographic modelling system to determine the spatial location of the tuna’s preferred habitat in near real-time. As for more static front systems, some of these have already been included in static marine protected area design in the UK and the Mediterranean. As an added bonus utilizing dynamic ocean management for oceanic front systems creates a management system that also adapts to climate-induced changes in pelagic ecosystems.
We have the technology to implement dynamic ocean management on frontal systems, and we know that there is a need to do so. Of course, implementing such measures isn’t straightforward, not least because we still have a lot to learn about the ocean and the species that live in it. For nation-states that do not necessarily have the financial capacity to utilise the sorts of technology that are used to detect oceanic fronts, nor disseminate management regulations in near real-time, implementing this sort of dynamic ocean management may be more challenging. However, as Kylie notes, “We now have the opportunity to integrate satellite science, spatial ecology, oceanography and fisheries management to improve marine biodiversity conservation”. Front mapping isn’t just about ensuring the long-term persistence of marine species either, offering the “potential to help us balance the competing demands of human activity and biodiversity conservation in the oceans”. An idea worth exploring? I certainly think so.
This article was written for Biosphere Magazine.
