Ocean-basin scale (1000s km)

Ocean-basin scale regions of intense mesoscale dynamics, such as those associated with the major water mass transitions discussed below, are ecologically significant features in the largely oligotrophic open oceans (Belkin, Cornillon & Sherman 2009). These regions are important foraging and migration habitats for pelagic marine vertebrates (Tittensor et al. 2010).

Mesoscale (10s–100s km) to SUB-MESOSCALE (c. 1 km)

Mesoscale and sub-mesoscale oceanographic processes drive front formation within large-scale transition zones and in regions associated with currents, upwellings and bathymetric features and appear to be of particular ecological importance. For example, hotspots of predatory fish diversity (tuna, billfish) are associated with mesoscale fronts within warm waters (c. 25 °C) across all the major ocean basins (Worm et al. 2005).

Major currents

Bioaggregating thermal, colour and density fronts frequently form along the boundaries of major current systems (Fig. 1). Seabirds and neonate sea turtles associate strongly with fronts and eddies formed along the Gulf Stream (Haney 1986; Witherington 2002; Thorne & Read 2013) and the Kuroshio Current (Polovina et al. 2006). The peripheries of frontal eddies formed along these currents are also of ecological significance (Haney 1986; Bailleul, Cotté & Guinet 2010; Godø et al. 2012).

Key biophysical characteristics of ecologically significant frontal zones

Current understanding indicates that accessibility, spatiotemporal predictability and relative productivity are central to the ecological importance of frontal zones (Hunt et al. 1999; Weimerskirch 2007). These insights are useful in predicting which taxa are likely to aggregate at frontal zones in different oceanographic regions, enhancing understanding of pelagic ecosystem function and identifying important at-sea habitats. For example, it is clear that large-scale frontal zones in the open oceans are often highly productive and persistent, and so predictable, yet are only really accessible to oceanic species and far-ranging central-place foragers (Bost et al. 2009; Tittensor et al. 2010). Predictable, productive mesoscale frontal zones associated with bathymetric features, currents and major upwellings attract marine vertebrates from diverse foraging guilds in contrasting oceanographic regions (Chavez & Messié 2009; Block et al. 2011). Persistent shelf-sea tidal mixing and tidal-topographic fronts create predictable foraging opportunities, accessible to coastal species such as colonial seabirds and some cetaceans. Recent work in the Celtic Sea highlights temporal persistence as a key component of frontal zones used as foraging features for a piscivorous seabird (Scales et al. 2014), presumably as persistence enhances both productivity and predictability.

The literature documenting associations between marine vertebrates and fronts has yielded valuable insights, yet many questions remain. For example, despite the implicit assumption that fronts generate suitable foraging conditions, the mechanisms linking physical processes and prey dynamics are not well understood (but see Cox, Scott & Camphuysen 2013). In many cases, it remains unclear how habitat utilization changes through the annual cycle, through ontogenetic development and through life cycle stages (i.e. breeding, migration; but see e.g. Votier et al. 2011). In addition, little is known about the ways in which many species perceive and respond to environmental cues (but see Nevitt & Bonadonna 2005; Tew Kai et al. 2009; Votier et al. 2013; Tremblay et al. 2014). Moreover, it is important to determine whether fronts are significant foraging features at the population level. This has not yet been achieved, to our knowledge, but is possible through estimation of the proportion of a population using a frontal zone, or the spatial range over which animals are attracted. Future work should address these questions, improving capacity to locate ecologically significant features.

HOTSPOTS of anthropogenic threat

Frontal zones appear to be hot spots of overlap between potentially critical at-sea habitats and spatially explicit anthropogenic threats (e.g. fisheries), particularly in the coastal zone (Halpern et al. 2008). The major fisheries threats to marine vertebrates are bycatch (Gilman et al. 2008; Anderson et al. 2011; Žydelis, Small & French 2013; Lewison et al. 2014) and competition for resources (e.g. Bertrand et al. 2012). Comprehensive data are difficult to obtain, but industrialized fisheries, particularly pelagic long-lining fleets, target persistent frontal zones (Podestá, Browder & Hoey 1993; Hartog et al. 2011), generating significant risk of conflict with other apex consumers. Spatial overlap is particularly pronounced within the coastal zone, along shelf breaks and in upwelling regions (Halpern et al. 2008; Lewison et al. 2014), especially those around Africa and South America (Zeeberg, Corten & de Graaf 2006; Pichegru et al. 2009). Within these regions, frontal zones are logical areas in which to target measures for mitigation of fisheries threats. In addition, convergent fronts can concentrate pollutants and floating debris such as oil and plastics, potentially increasing exposure of marine vertebrates aggregating to forage (Bourne & Clark 1984; González Carman et al. 2014).

On the continental shelf, the expansion of marine renewable energy installations (MREIs) has the potential for direct and indirect effects on marine vertebrates (Inger et al. 2009; Grecian et al. 2010; Scott et al. 2014). MREIs that rely on tidal flow are likely to be concentrated in the vicinity of hydrographically dynamic tidal mixing fronts (Miller & Christodoulou 2014), altering habitat dynamics and displacing foraging effort. These impacts may be particularly pronounced for coastal central-place foragers (Scott et al. 2014). While more research is needed to determine whether MREIs have population-level effects, marine spatial planning can be improved by identification of vulnerability hotspots.

Front mapping to identify priority conservation areas

Technological innovations in remote sensing, biologging, autonomous marine vehicles and vessel monitoring hold promise for identification of priority conservation areas (Palacios et al. 2006; Grantham et al. 2011; Miller & Christodoulou 2014) and spatially dynamic, near-real-time threat management (Hobday et al. 2014). Front mapping via EO remote sensing (Fig. 1; Miller 2009) enables high-resolution, automated detection of frontal zones anywhere in the global ocean. Seasonal or climatological products are potentially useful for marine spatial planning, identifying priority areas for threat mitigation both on-shelf (Miller & Christodoulou 2014) and in areas beyond national jurisdiction (ABNJ; the ‘high seas’). Moreover, near-real-time front mapping augments the suite of tools with potential to inform spatially dynamic ocean management (Hobday et al. 2014), enabling identification and monitoring of critical ephemeral habitats (Fig. 2).

Remotely sensed oceanographic data have been used to inform spatially dynamic fisheries management in several cases. For example, historical and near-real-time SST imagery, coupled with satellite telemetry and spatially explicit fisheries data, has been successfully used to reduce bycatch of loggerhead turtles along the TZCF north of Hawaii (Howell et al. 2008). The Australian Fisheries Management Authority has used a comparable approach using in situ sensors to regulate exploitation of southern bluefin tuna Thunnus maccoyii (Hobday & Hartmann 2006). Although there are few examples of such innovatively managed fisheries (Dunn, Boustany & Halpin 2011), similar methods are applicable to other species of conservation concern (Hobday & Hartmann 2006) and may be critical in mitigating future marine biodiversity loss.

Marine protected areas (MPAs) can regulate overlap between spatially explicit threats and critical at-sea habitats. MPAs are most tractable on-shelf, within Exclusive Economic Zones (EEZs), where anthropogenic threats to marine vertebrate populations, such as fisheries pressure, MREI development, noise and habitat degradation, are also concentrated (Maxwell et al. 2013). Spatially predictable biophysical hotspots, such as those associated with persistent tidal mixing, tidal-topographic and upwelling shadow fronts, are logical candidates for within-EEZ MPAs and easily identifiable. Indeed, hot spots associated with quasi-stationary frontal zones have been explicitly included in MPA design in the UK (Miller & Christodoulou 2014) and the Mediterranean (Panigada et al. 2008).

In the open oceans beyond EEZs, persistent frontal zones, such as that associated with the Charlie Gibbs Fracture Zone in the North Atlantic (Fig. 1), are also amenable to site-based management. However, effective conservation of pelagic biodiversity in ABNJ rests not only upon the identification of vulnerability hotspots but also upon the capacity to track how these hotspots shift with changing oceanographic conditions (Hooker et al. 2011; Lascelles et al. 2012; Fig. 2). Spatially dynamic ocean management (Hobday et al. 2014) may be more effective in managing threats to marine vertebrate populations in some highly dynamic regions, and for increasing adaptability as pelagic ecosystems undergo changes related to climate variability. High-resolution front frequency maps, both near-real-time and seasonal/climatological (e.g. Fig. 1), coupled with real-time monitoring of anthropogenic activity and marine vertebrate habitat use (Fig. 2), present managers with data of value for more effective management of pelagic ecosystems.

Conclusions

Associations between marine vertebrates and oceanographic fronts vary spatially, temporally and between taxa, influenced by both the biophysical properties of fronts and taxon-specific foraging ecology (Hunt et al. 1999). Despite this variability, there now exists a considerable body of evidence indicating that persistent mesoscale frontal zones are ecologically significant across the oceans (e.g. Polovina et al. 2001; Bost et al. 2009). As areas of existing and potential overlap between critical habitats and anthropogenic threat, persistent frontal zones represent tractable conservation areas, in which to target threat mitigation measures. Continued integration between remote sensing science, spatial ecology, oceanography and fisheries management has potential to improve marine biodiversity conservation by (i) bridging the gaps in our understanding of the oceanographic drivers of marine vertebrate space use and (ii) feeding into systematic conservation planning through mapping and real-time monitoring of threat hot spots (Grantham et al. 2011; Hobday et al. 2014). Such integration is vital if we are to balance the competing demands of anthropogenic activities and biodiversity conservation in the vast and dynamic oceans.