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In the context of maritime spatial planning and the implementation of spatialized Good Environmental Status indicators in the Marine Strategy Framework Directive (MSFD), the definition of a mosaic composed of coherent and standardised spatial units is necessary. We propose here a characterization of seascapes in time and space within the specific framework of the MSFD in the English Channel and the Bay of Biscay areas. A spatio-temporal classification of coastal-shelf water masses is carried out using twelve essential oceanographic and derived variables from operational coastal oceanography using the HYCOM model. Partitioning is computed using a multivariate hybrid two-step clustering process defining a time series of categorical maps representing hydrographical patch classes. Main patch occurrence is analyzed to understand their spatio-temporal dynamics and their oceanographic characteristics. Finally, patch classes are combined with MSFD marine sub-region delimitations to build seascapes, including ecosystem approach management and marine policy considerations.

Summary 1. Climate change and fishing impact marine ecosystems, potentially modifying the availability of small pelagic fish to marine top predators. Some seabirds that primarily rely upon these resources have switched to feeding on fishery waste. It has therefore been argued that seabirds might become dependent upon this artificial resource. 2. To test this hypothesis, we studied the foraging behaviour of Cape gannets Morus capensis breeding off the coast of South Africa using high-resolution Global Positioning Systemtracking in relation to the availability of pelagic fish assessed by acoustic at-sea surveys, and fishing effort by the two main south African fisheries (purse seiners that compete with seabirds for pelagic fish, and demersal trawlers that process fish at sea and discharge fish waste) tracked with vessel monitoring systems. Conjoint seabird, fish and fisheries information were analysed at mesoscale (c. 100 km) and sub-mesoscale (c. 10 km) in years of high (2002), medium (2009) and low (2005) pelagic fish biomass within gannets’ foraging range. 3. We found substantial inter-annual variability in spatial use by breeding gannets, which was driven primarily by pelagic fish availability. At the mesoscale, birds and purse seiners exploited similar marine areas, but no fine-scale dependence of birds on purse seiners was detected. Crucially, fine-scale dependence of gannets upon trawlers producing fishery waste was only detected in 2005, when pelagic fish biomass was lowest, indicating a direct effect of trawlers on gannet foraging behaviour in the absence of natural prey. 4. Further overlap analyses of gannet and trawler foraging areas during 2002–2010 confirmed that breeding birds only seek trawlers when pelagic fish availability is low, strongly suggesting reversible seabird dependency upon fishery waste. 5. Synthesis and applications. Our study demonstrates that seabirds such as Cape gannets depend on fishery waste when their natural prey is scarce, but revert to feeding on natural resources whenever available, showing highly flexible foraging behaviour. These results have important implications in the context of the anticipated legislation banning at-sea disposal of fishery waste in different regions, including European seas, highlighting the necessity to concomitantly promote sustainable fishing allowing the restoration of pelagic fish stocks.

Oceanic mesoscale circulation is a crucial structuring force in the marine environment. Dynamic processes associated with eddies, such as eddy-induced upwelling or eddy-eddy interac- tion, drive the transport and distribution of nutrients that support the whole food chain, presumably through bottom-up processes. Eddies can shape the distribution of organisms at both low (phyto- plankton, zooplankton and fish larvae) and high trophic levels (top fish predators, seabirds or turtles), but the impact of mesoscale features on intermediate trophic levels (micronekton) remains poorly understood. We analysed the influence of eddies on the distribution of micronekton aggregations in the Mozambique Channel by combining data from acoustic surveys and satellite sea topography. We demonstrated that large aggregations of micronekton occurred mainly in areas where the local horizontal gradient of sea level anomalies is strong, i.e. at the periphery of eddies. We observed that, in this region, eddies running along the coast advect coastal nutrient-rich waters at their edges, which support the base of the food chain. We propose that eddies can shape the distribution and the aggregation patterns of the prey of marine top predators through bottom-up processes.

Mesoscale physical features as fronts and eddies appear to play a key role in the dynamics of marine communities. In the Indian Ocean, the Mozambique Channel (MC) is a natural laboratory to investigate mesoscale eddies (100–300 km in diameter); indeed, four to seven eddies per year are know to transit through the Channel, from North to South. We studied the structuring role of the mesoscale eddies on spatial dynamics and foraging strategy of top predators using seabirds and tuna as examples. Emphasis was on the central part of the MC (16–24°S) where eddy activity is most developed. We integrated three main categories of information: (i) satellite altimetry for sea-level anomaly (SLA) and geostrophic current, remotely-sensed surface temperature (SST) and SeaWiFS data for chlorophyll concentration (CC); (ii) individual tracking of Great Frigatebirds ( Fregata minor ) to characterize foraging areas; and (iii) detailed catch statistics from purse-seine fisheries to describe distribution of tuna schools. Generalized Additive Models were applied to quantify the relative influence of mesoscale descriptors, SST and CC on foraging behaviour of Great Frigatebirds and location of purse-seine sets. Our results show that seabirds are more closely tied to mesoscale eddies compared to tuna. We underline the role of eddy boundaries on the response of frigatebirds and tuna. Good foraging conditions are promoted along the edge of eddies as a result of the interplay of the maturation process from cyclonic eddies and the concentration process by eddy interactions. A decrease in the number or intensity of eddies in the MC, as observed during strong El Nino events, could potentially affect the eddy-related ecosystem with putative negative repercussions on central-place foragers such as Great Frigatebirds. We discuss the importance of a better understanding of the “eddy system”, in marine conservation and tuna fisheries management in the Mozambique Channel.

6 pages, 4 figures, 1 table.-- PDF ArXiv pre-print: http://arxiv.org/abs/0904.1959, Meso- and submesoscales (fronts, eddies, filaments) in surface ocean flow have a crucial influence on marine ecosystems. Their dynamics partly control the foraging behaviour and the displacement of marine top predators (tuna, birds, turtles, and cetaceans). In this work we focus on the role of submesoscale structures in the Mozambique Channel on the distribution of a marine predator, the Great Frigatebird. Using a newly developed dynamical concept, namely the Finite-Size Lyapunov Exponent (FSLE), we have identified Lagrangian coherent structures (LCSs) present in the surface flow in the Channel over a 2-month observation period (August and September 2003). By comparing seabird satellite positions with LCSs locations, we demonstrate that frigatebirds track precisely these structures in the Mozambique Channel, providing the first evidence that a top predator is able to track these FSLE ridges to locate food patches. After comparing bird positions during long and short trips, and different parts of these trips, we propose several hypotheses to understand how frigatebirds can follow these LCSs. The birds might use visual and/or olfactory cues and/or atmospheric current changes over the structures to move along these biological corridors. The birds being often associated to tuna schools around foraging areas, a thorough comprehension of their foraging behaviour and movement during the breeding season is crucial not only to seabird ecology but also to an appropriate ecosystemic approach of fisheries in the channel., A Ph.D. fellowship for E.T.K was provided by the InstitutdeRecherche pour le Développement and the University Pierre and Marie Curie. Ph.D. financial support for V.R was provided by the Direction Ge´ne´ rale de l’Armement. The Laboratoire d’Etudes en Géophysique et Océanographie Spatiale contribution is supported by Centre National d’Etudes Spatiales funding. The Instituto de Física Interdisciplinar y Sistemas Complejos contribution is supported by Ministerio de Ciencia e Innovación and le Fonds Européen de Développement Régional through project FISICOS (FIS2007–60327), and by Consejo Superior de Investigaciones Cientificas through the Intramural Frontier Projects OCEANTECH. H.W.’s contribution was supported by the REMIGE project funded by Agence Nationale de la Recherche (ANR 2005 Biodiv-011).

We analyse the coupling between sea surface chlorophyll concentration (CC) and the physical environment in the Mozambique Channel (MZC) using statistical models. Seasonal and interannual patterns are studied along with the role of mesoscale dynamics on enhancement and concentration processes for phytoplankton. We use SeaWifs data for CC and two other remotely sensed data sets, TMMI for sea surface temperature (SST) and merged altimetry products for sea level anomaly and geostrophic current. Empirical Orthogonal Functions (EOF) on SSC and SST show strong seasonality and partition the MZC into three distinct sub-areas. The chlorophyll variability is mostly driven by seasonality, but more in the North (10 degrees S-16 degrees S) and South (24 degrees S-30 degrees S), and explains respectively 64% and 82% of the CC variance. In the Central part (16 degrees S-24 degrees S), the seasonal signal has less influence (60% variance). There, complex EOFs on Sea Level Anomaly (SLA) highlight the role of mesoscale activity (i.e. eddies and filament structures) in the spatial distribution of chlorophyll. Five mesoscale descriptors (shear, stretch, vorticity, deformation and eddy kinetic energy) are derived from the altimetry data to quantify the eddies-related physical patterns in the central region of the MZC. We use generalized Additive Models to explain the effect of those features on phytoplankton enhancement. The best model fit (r(2) = 0.73) includes shear, stretch, vorticity and the latitude-longitude interaction as eddies are well structured in space. Cyclonic eddies associated with negative vorticity are conductive to phytoplankton enhancement by the effect of upwelling in the core notably during the spin-up phase. The interaction between eddies generate strong frontal mixing favourable to the production and aggregation of organic matter. The mesoscale activity is also affected by interannual variability with consequences on CC. We highlight a substantial reduction of the SLA pattern in 2000-2001 when the SOI positive phase is peaking (Nina-type pattern). The strong relationship between mesoscale eddies and SOI suggests that primary productivity in the MZC is also under the influence of distant forcing at a basin scale.