Finding mesopelagic prey in a changing Southern Ocean

Abstract

Mesopelagic fish and squid occupy ocean depths extending below the photic zone and their vertical migrations represent a massive pathway moving energy and carbon through the water column. Their spatio-temporal distribution is however, difficult to map across remote regions particularly the vast Southern Ocean. This represents a key gap in understanding biogeochemical processes, marine ecosystem structure, and how changing ocean conditions will affect marine predators, which depend upon mesopelagic prey. We infer mesopelagic prey vertical distribution and relative abundance in the Indian sector of the Southern Ocean (20° to 130°E) with a novel approach using predator-derived indices. Fourteen years of southern elephant seal tracking and dive data, from the open ocean between the Antarctic Polar Front and the southern Antarctic Circumpolar Current front, clearly show that the vertical distribution of mesopelagic prey is influenced by the physical hydrographic processes that structure their habitat. Mesopelagic prey have a more restricted vertical migration and higher relative abundance closer to the surface where Circumpolar Deep Water rises to shallower depths. Combining these observations with a future projection of Southern Ocean conditions we show that changes in the coupling of surface and deep waters will potentially redistribute mesopelagic prey. These changes are small overall, but show important spatial variability: prey will increase in relative abundance to the east of the Kerguelen Plateau but decrease to the west. The consequences for deep-diving specialists such as elephant seals and whales over this time scale will likely be minor, but the changes in mesoscale vertical energy flow have implications for predators that forage within the mesopelagic zone as well as the broader pelagic ecosystem.

Figure 1

Spatial and vertical representations of the study domain. (a) Tracks from 98 adult females instrumented between 2004 and 2016. Tracks in the focal region of this study (the open ocean region of the Antarctic Zone) are highlighted in black and the grey are locations outside the study area. The solid black lines represent the climatological position of the major Southern Ocean fronts. STF = subtropical Front, SAF = SubAntarctic Front, APF = Antarctic Poloar Front, sACC = southern Antarctic Circumpolar Current Front and sbACC = southern boundary of the Antarctic Circumpolar Current. The red box indicates the transect illustrated in (b). (b) A meridional transect centred along 80°E (5° degree width) showing the salinity of the upper water column from the surface to 1000 m. The relationship between S_diff (calculated as salinity at 600 m minus salinity at 200 m, dashed lines) and CDW is illustrated. Where CDW is shoaling towards Antarctica S_diff is small (shoaling CDW(i)). In comparison, farther north where CDW lies deeper in the water column S_diff is large (deep CDW(ii)). The shaded regions are outside our study domain. (c) A climatology of the salinity difference between 600 and 200 m (S_diff) compiled from 14 years of seal and Argo float CTD data within the study domain between the Antarctic Polar Front and the Southern ACC Front.

Figure 2

Aspects of female elephant seal diving behaviour at a range of temporal scales that indicate that they forage on mesopelagic prey. (a) The proportion of daily dives at each depth for each day during the post-moult period (February–October), (b) mean dive depth relative to the time of day and day of the year and (c) the distribution of dives depth during the day (light blue) and night (dark blue), with solid black lines indicting the mean depths.

Figure 3

Conceptual representation of the relationship between seal  dive depth (m) and hunting time in relation to the time of day and the vertical proximity of Circumpolar Deep Water (CDW). The x-axis represents the difference in salinity from 600 m to 200 m (S_diff), used as an index of the relative vertical position of saline CDW (S1c). Where S_diff is small, CDW is relatively close to the surface whereas larger values indicate CDW remains at depth. The y-axis indicates seal dive depth. The dashed blue lines are the fitted night time dive depths (left panel) from the model: depth ~ S_diff *day/night (S3), and the fitted day dive depths (right panel). The dashed white dive profiles are stylised seal dives. The coloured boxes  present a heatmap for the time seals actively hunted for prey, where red represents the longest hunting times (at night when CDW is shoaling), followed by day dives under shoaling CDW conditions (orange), day dives when CDW is deep (green) and night dives when CDW is deep (blue). The number of fish and squid in each dive pictorally represents a relative measure of feeding success following the above heatmap, showing that seal dives were most successful during night dives when CDW was shoaling, followed by day dives under shoaling CDW, day dives when CDW was deep, and least successful when diving at night when CDW was deep.

Figure 4

Projected change in vertical distribution and relative abundance of mesopelagic prey in the Antarctic Zone from the present to 2100. (a) hunting time changes (seconds) during the day and (b) hunting time changes during the night, showing that in the eastern part of the domain mesopelagic prey will be relatively more abundant (i.e. a positive change of up to 30 s hunting time), (c) dive depth changes (metres) during the day, showing that in the western part of the domain prey will be somewhat deeper (i.e. a negative change of up to 15 m), (d) dive depth changes during the night.