Acoustic deep scattering layers as dynamic prey landscapes for air-breathing deep-diving Antarctic predators

Abstract

This PhD addresses the central hypothesis that acoustic Deep Scattering Layers (DSLs) are a prey landscape for deep-diving air-breathing Southern Ocean predators. In the open ocean, mesopelagic fish (including myctophids), zooplankton and other animals migrate down from the surface at dawn to the mesopelagic zone (200-1000 m) to avoid visual predators during daylight. There, they form layer-like aggregations known as Deep Scattering Layers that can be detected using echosounders. A large component of DSL biomass is comprised of myctophids, which are both a potential resource for fisheries and important in the diets of several iconic Antarctic predators such as King Penguins (Aptenodytes patagonicus) and Southern Elephant Seals (Mirounga leonina). Although these two predator species are amenable to bio-logging, there are very few simultaneous observations of DSLs and their foraging behaviour. Therefore, the importance of DSLs to Antarctic air-breathing diving predators is unknown. This is problematic given the predicted changes in DSLs in response to climate change and to the increasing interest shown in DSL harvest by commercial fishers. The 2017 Antarctic Circumnavigation Expedition (ACE), which is the first scientific expedition around the Antarctic continent stopping at most subantarctic islands to investigate a range of aspects of the Southern Ocean, provided a unique opportunity to simultaneously observe DSL characteristics acoustically from the ACE ship (at 12.5 kHz) and the foraging behaviour of predators using bio-logging. King Penguins and female Southern Elephant Seals appeared as good candidates to study the link with DSLs as they both mainly feed on myctophids, are both deep-diving predators potentially capable of reaching the depth of DSLs and are both known to dive deeper during the day compared to night time (like the Diel Vertical Migration (DVM) pattern of the components of DSLs), several clues that initially suggest that DSLs could be a prey landscape for them. I compiled a dataset of DSL depth and echo-intensity (proxy for biomass) along the circum-continental cruise track (~ 90,000 km, across 6 different frontal zones) and obtained dive data from 18 adult King Penguins breeding at South Georgia and from 8 adult female Southern Elephant Seals breeding at Kerguelen. This study aims to describe the distribution of DSLs in the Southern Ocean in order to build a DSL biogeography for this region and to investigate whether these Antarctic deep-diving predators rely on DSLs for food. In Chapter 2, it was found that DSL echo-intensity (proxy for biomass) was a function of Sea Surface Temperature (SST), and that DSL depth was significantly related to sub-surface temperature and salinity or surface density. These relationships were used to infer DSL properties throughout the Southern Ocean, and especially at predator dive locations. In addition, rather than being ubiquitous, the data from the present study suggest that DSLs disappear in places where SST values become lower than -0.4°C. Results from Chapter 3 showed that Southern Elephant Seals seemed to reach the bottom of the principal DSL (i.e. strongest DSL) or the top of the deepest DSL (i.e. most predictable DSL). In contrast, results from chapter 4 revealed that King Penguins preferentially selected habitats with dense and shallow DSLs, where the availability of DSL components was supposedly high. However, the dive depths of penguins were generally shallower than the DSL, suggesting that they did not feed on the layers themselves, but on prey patches that were observed acoustically above them. These patches may be associated with the layers. DSLs, as a prey landscape for these two species, also play an important role in the biological pump of the ocean (acting on climate regulation by sequestering carbon at depth) due to their DVMs. It is likely that DVMs have other implications, such as vertical mixing of nutrients or transport of contaminants through the water column. In this regard, it was found that King Penguin faeces contain relatively high concentrations of microfibers, which were likely indirectly ingested (i.e. from migrating prey consumed at depth) and might potentially be deleterious for them (Chapter 5). Chick-rearing penguins had lower levels of contamination compared to incubating birds, which are known to perform longer foraging trips and to reach lower latitudes, and are potentially more exposed to microfibre contamination. In that way, results suggest that microfibres provide a potential signature of foraging in King Penguins. The importance of DSLs for contamination should be further investigated (including the impact of DVMs and the quantities of microplastics that are brought on land). These findings resulting from a multidisciplinary approach using in-situ and remote sensing environmental data, acoustic surveys and bio-logging improve our understanding of predator-prey interactions in the Southern Ocean. Although Antarctic focused, the present study is relevant more broadly because several seal and whale species also feed on DSL components. Because the Southern Ocean is undergoing various threats such as climate change, overfishing and marine pollution, our findings regarding the biophysical relationships with DSLs and the link between DSLs and Antarctic predators serve to improve our understanding of mesopelagic dynamics. This study informs ecosystem-based management and conservation, which now adopt more holistic approaches when monitoring and assessing ecosystem health status, before any large-scale fishery exploitation of mesopelagic fish begins.