By Claire Asher, on 4 June 2015
a guest blog by David Curnick, written for the 2015 Write About Research Competition.
With a global human population of over 7 billion it is becoming ever more important to manage our natural resources effectively. For centuries, the oceans have been seen as an endless bounty, ripe for harvesting. However, this simply isn’t the case and concerns are growing over the status of many fish populations, particularly those of predatory fish, and this has led to concerns over potential impacts on food webs and ocean health. A number of management initiatives have been adopted in an attempt to address these perceived declines such as restrictions on fishing gear, catch limits, closed seasons and the establishment of marine protected areas (MPAs).
Recently, there has been a growing trend for the establishment of large MPAs, such as that around the Chagos archipelago in the Indian Ocean, as countries attempt to meet global conservation targets (e.g. the Convention on Biological Diversity states that 10% of the ocean should be under protection by 2020). However, whilst the benefits of smaller coastal MPAs have been widely documented, there is uncertainty about their effects on wide-ranging oceanic species, such as tunas and sharks. This is because oceanic species tend to utilise the open ocean more than their coral reef counterparts and therefore have the potential to cross MPA boundaries and render such reserves obsolete. Therefore, for spatial management to work for these wide-ranging species, the MPAs would need to be large enough to either encapsulate the entirety or a significant proportion of their home range, or protect them during crucial life stages such as during breeding or to provide nursery grounds.
Given society’s fondness of tuna, you would think that we would know everything there could possibly be to know about our easily tinnable friends. Unfortunately, unlike the well-documented migration of wildebeest, migratory patterns in tuna are not that well understood, and nor are many of their life histories. In fact, the range size of tunas, a key consideration for spatial management, is hotly debated in the literature. Sibert and Hampton (2003) suggested that the average lifetime distance travelled for skipjack tunas in the Pacific Ocean ranged from 420 to 470 nautical miles (nm) and that for yellowfin tuna it was about 20% less. This led Sheppard (2010) to propose that, should this also hold true for populations in the Indian Ocean, then potentially there is a large resident tuna population within the Chagos MPA year-round. The fact that a longline fishery used to operate in Chagos throughout the year seems to support that there are at least tunas in Chagos year round, although whether they are the same individuals is impossible to say at the moment. In contrast to Sibert and Hampton’s findings, Hallier and Million (2012) found that tunas tagged off East Africa, the Seychelles and the Maldives travelled significantly further, with an average of 800nm for yellowfin, and 600nm for bigeye and skipjack tuna. Such a large discrepancy in range estimates existing between sub-populations of species only serves to highlight our lack of understanding, and could be the difference between a marine reserve proving effective or not. An additional issue with these tagging programmes is that they are based on simple mark-recapture studies and therefore we have no idea of the route that the tuna took between point A and B (being tagged and being re-caught). Maybe they actually travelled much further than the estimates, or maybe they spent 90% of their time in one location. We simply do not know from these data. It is also apparent that both of these studies assume that all individuals within a population follow the same migratory pattern and that behavioural traits of species are similar across regions and even oceans. In reality, neither of these assumptions is likely to hold true and therefore we need to investigate the tuna populations on a case by case basis to understand the specific ecology of these animals better.
So how are we addressing this knowledge gap in Chagos? Well, until recently, our knowledge of oceanic predators in Chagos was limited to historical fisheries records and a handful of observations on coral reef focused scientific expeditions. Together with colleagues from Stanford University, the University of Western Australia, the Bertarelli Foundation and the Zoological Society of London, we have been studying sharks and other ocean giants within the Chagos MPA to try and find out exactly what role the reserve has within the context of the wider Indian Ocean, and if it truly can protect wide-ranging species. Covering 640,000km2, Chagos is certainly large, in fact it’s about the same size of mainland France. Oceanic sharks, like tunas, are often branded as ‘highly migratory’, but a number of studies have shown these species to exhibit site-fidelity around nutrient-rich seamounts and upwellings which are known to be abundant in Chagos. If high site-fidelity is observed in sharks around Chagos, this would indicate that the reserve may be of huge benefit to sharks, tuna and other migratory species in the Indian Ocean. Alternatively, sharks may range far beyond the boundaries of the reserve and this may therefore raise questions of its efficacy for these species. Through satellite tagging sharks and tunas, we will get high resolution data on their movements which will help us to understand how much time they spend within this mega reserve, whether there are any hotspots of activity or aggregations that could suggest feeding or breeding grounds, and how connected populations are with others across the Indian Ocean. So far, 152 sharks and 25 manta rays have been tagged in Chagos in order to find out their movements inside and outside the reserve. Only through understanding these key baseline data can we then start to assess the potential impact of the Chagos MPA.
A final point to note is that in terms of marine resource management, we are currently operating with one hand tied behind our back. Why? Well, in the current legislative environment, marine reserves and other initiatives can only really be established within the 200nm national jurisdiction (Exclusive Economic Zone – EEZ) of willing countries. This means that within the ocean beyond, which amounts to 64% of the ocean and 50% of our planet, management options are limited. Previous studies have suggested that, particularly within the Indian Ocean, important breeding grounds for tunas may exist within this legal black hole. Therefore it is quite possible that our best chance for ocean sustainability is currently just beyond our 200nm reach.
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