Oceanic fronts and marine megafauna

It has long been recognised that larger marine animals, such as cetaceans and basking sharks, forage along ocean thermal fronts where phytoplankton production is enhanced and zooplankton is concentrated, thus where their food resources accumulate. Dr Peter Miller, NEODAAS Technical Lead, has developed techniques that enable front maps detected through remote sensing to be used for modelling the distribution or various marine animals that feed near the ocean surface, and for the designation of marine protected areas (MPAs). Oceanic front products are now available for any global region through NEODAAS (Miller, 2009; see Fig. 1a). Contact Dr Miller to discuss collaboration on more advanced analyses.

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In a project for the UK Department for Environment, Food and Rural Affairs (Defra), maps derived from 30,000 satellite images collected between 1999 and 2008 were generated to show the most frequent front locations around the UK and were used as a proxy for biodiversity ‘hotspots’ to assist in the designation of possible MPAs (Miller, et al., 2010; see Fig. 1b). ‘Ground truthing’ of these front maps is gained from comparison with known areas of pelagic ecological importance. This quantitative front mapping is likely to reveal new insights into the dynamic distribution and behaviour of marine megafauna (Oppel, et al., 2012), and enable marine management and protection strategies that might vary inter-annually or seasonally (Miller & Christodoulou, 2014). These techniques are also being considered for reducing the impact on biodiversity of offshore renewable energy installations (Witt, et al., 2012).

Composite front map, indicating the location, strength and 
            persistence of all fronts observed during May 2008 around UK shelf Frequent front map, indicating the percentage of time a strong 
            front was observed at each location during summer 1999-2008

Figure 1 (a) Composite front map, indicating the location, strength and persistence of all fronts observed during May 2008 around UK shelf; (b) Frequent front map, indicating the percentage of time a strong front was observed at each location during summer 1999-2008.

By combining front maps with the recorded tracks of basking sharks, insights have been gained into the foraging strategy of the species. Further work with colleagues from the seabird community, is looking at how fronts influence foraging and productivity in gannets and kittiwakes to identify remote offshore feeding areas, which are crucial to their survival.

Earlier work

NEODAAS data were previously used to compare locations of migrating animals with boundaries or contours of surface temperature or colour properties (e.g. Hays et al. 2006; see Fig. 2). SST data have also been used in a novel manner to fix latitude along the longitude on tracked basking sharks by comparison with tag recorded water temperature (Sims et al., 2005; 2006).

The position of the 15 ºC surface isotherm in different 
            years

Figure 2: The position of the 15ºC surface isotherm in different years. a) and b) AVHRR satellite images comparing years when the position of this isotherm in August was at its most southerly extent (1985) and most northerly extent (2000). (c) The increasing northerly extent of the surface 15ºC surface isotherm in August showing a northerly shift of 330 km (McMahon & Hays, 2006).

Hays, G.C., Hobson, V.J., Metcalfe, J.D., Righton, D. & Sims, D.W. (2006) Flexible foraging movements of leatherback turtles across the north Atlantic Ocean. Ecology, 87(10), 2647-2656.

McMahon, C.R. & Hays, G.C. (2006) Thermal niche, large scale movements and implications of climate change for a critically endangered marine vertebrate. Global Change Biology, doi: 10.1111/j.1365-2486.2006.01174.x.

Miller, P.I. (2009) Composite front maps for improved visibility of dynamic sea-surface features on cloudy SeaWiFS and AVHRR data. Journal of Marine Systems, 78(3), 327-336. doi:10.1016/j.jmarsys.2008.11.019. Animations

Miller, P.I., Christodoulou, S. & Saux-Picart, S. (2010) Oceanic thermal fronts from Earth observation data – a potential surrogate for pelagic diversity. Report to the Department of Environment, Food and Rural Affairs. Defra Contract No. MB102. Plymouth Marine Laboratory, subcontracted by ABPmer, Task 2F, pp. 24.

Miller, P.I. & Christodoulou, S. (2014) Frequent locations of ocean fronts as an indicator of pelagic diversity: application to marine protected areas and renewables. Marine Policy 54, 318-329. doi: 10.1016/j.marpol.2013.09.009

Oppel, S., Meirinho, A., Ramírez, I., Gardner, B., O’Connell, A., Miller, P.I. & Louzao, M. (2012) Comparison of five modelling techniques to predict the spatial distribution and abundance of seabirds. Biological Conservation, 156, 94-104, doi: 10.1016/j.biocon.2011.11.013.

Sims, D.W., Witt, M.J., Richardson, A.J., Southall, E.J. & Metcalfe, J.D. (2006) Encounter success of free-ranging marine predator movements across a dynamic prey landscape. Proceedings of the Royal Society B-Biological Sciences, 273(1591), 1195-1201.

Sims, D.W., E.J. Southall, G.A. Tarling, & Metcalfe, J.D. (2005) Habitat-specific normal and reverse diel vertical migration in the plankton-feeding basking shark, Journal of Animal Ecology, 74(4), 755-761.

Witt, M.J., Sheehan, E.V., Bearhop, and others including Miller, P.I. (2012) Assessing wave energy effects on biodiversity: the Wave Hub experience. Philosophical Transactions of the Royal Society A, 370, 502-529, doi:10.1098/rsta.2011.0265.