Mark Johnson’s research while affiliated with Aarhus University and other places

The EU-funded SATURN project contributes to improve our understanding of the effects of ship noise on aquatic animals through several studies. Assessing the effects of underwater noise on aquatic animals is complex due to the diversity of taxa involved, each with their own sound sensitivity in terms of spectral and temporal aspects, as well as behavioural and physiological sensitivity to acoustic disturbances. To conduct exposure experiments on fish and invertebrates that are sensitive to the particle motion component of sound, we have developed innovative laboratory setups. Invertebrates are studied in an AQUAVIB exposure chamber, while migratory fish species are studied in a MIGRADOME swimming tunnel. The project also focuses on three marine mammal species: porpoises, pilot whales, and harbour seals. These species provide broad biological coverage of acoustic specialisations and relevant European habitats. Field data were collected using tags deployed on wild animals and dose-response relationships of marine mammal behaviour and energetics were established as a function of real-world exposure to ship noise. The effects on porpoise population are assessed using the updated DEPONS model. The results of SATURN will be transferred to stakeholders to inform management actions for the mitigation of the effects of acoustic pollution in marine habitats.

Suction cups are commonly used to attach biologging tags to cetaceans, and interact mechanically with compliant integument, an organ primarily composed of skin and blubber. However, the impact of compliance on suction cup performance is difficult to predict because knowledge about in vivo integument mechanics is lacking. Here, an experimental approach is used to investigate the mechanical properties of common bottlenose dolphin ( Tursiops truncatus ) integument using a custom instrument, the static suction cup (SSCup), to collect data from both trained dolphins and wild individuals ( n = 17) during a static pose. Three loading profiles were applied at three sites to quantify nonlinear stiffness, hysteresis, and creep. The site at the dorsal fin insertion exhibited the highest stiffness, while sites posterior to the blowhole and above the pectoral fin showed greater energy dissipation during cyclic loading. Viscoelastic behavior was observed across all sites. Suction cup performance on a surrogate material with broadly similar compliance showed reduced performance compared to cups on rigid acrylic: the maximum applied force at detachment on acrylic (50 N) was twice as large as the compliant substrate (25 N). Site‐dependent compliance of integument may lead to varying performance of suction cups as an attachment method for tags.

Many large terrestrial mammalian predators use energy-intensive, high-risk, high-gain strategies to pursue large, high-quality prey. However, similar-sized marine mammal predators with even higher field metabolic rates (FMRs) consistently target prey three to six orders of magnitude smaller than themselves. Here, we address the question of how these active and expensive marine mammal predators can gain sufficient energy from consistently targeting small prey during breath-hold dives. Using harbor porpoises as model organisms, we show that hunting small aquatic prey is energetically cheap (<20% increase in FMR) for these marine predators, but it requires them to spend a large proportion (>60%) of time foraging. We conclude that this grazing foraging strategy on small prey is viable for marine mammal predators despite their high FMR because they can hunt near continuously at low marginal expense. Consequently, cessation of foraging due to human disturbance comes at a high cost, as porpoises must maintain their high thermoregulation costs with a reduced energy intake.

The giant rorqual whales are believed to have a massive food turnover driven by a high-intake lunge feeding style aptly described as the world's largest biomechanical action. This high-drag feeding behavior is thought to limit dive times and constrain rorquals to target only the densest prey patches, making them vulnerable to disturbance and habitat change. Using biologging tags to estimate energy expenditure as a function of feeding rates on 23 humpback whales, we show that lunge feeding is energetically cheap. Such inexpensive foraging means that rorquals are flexible in the quality of prey patches they exploit and therefore more resilient to environmental fluctuations and disturbance. As a consequence, the food turnover and hence the ecological role of these marine giants have likely been overestimated.

Sperm whales are known for producing powerful clicks that allow them to echolocate mesopelagic prey at long ranges in the darkness of the deep ocean, during long and deep dives. However, sperm whales sometimes switch from pelagic to benthic or benthopelagic foraging at or near the highly reflective seafloor, raising the question of how the most powerful biosonar system in the world is operated in such a highly reverberant environment. We hypothesized that sperm whales foraging near the seafloor produce faster but weaker echolocation clicks. Using data from DTAGs deployed on seven sperm whales from populations in the Mediterranean Sea and the Gulf of Mexico, we compared echolocation sound production in the descent and bottom phase of benthic (n=23) and pelagic (n=19) dives. During descents, whales employed around 3 dB weaker apparent output levels (AOL) and 0.2 s shorter inter-click-intervals (ICIs) in benthic dives compared to pelagic dives, consistent with a shorter acoustic scene ending at the seafloor. Furthermore, whales gradually reduced ICI and AOL during benthic descents, likely to reduce backward masking from the seafloor. In comparison, AOL adjustment showed fewer stepped slopes in pelagic dives. Surprisingly, we observed no biosonar adjustments in the bottom phase of benthic dives. Instead, the whales spent a 12.3% longer time swimming horizontally upside-down during the benthic dives. As the hearing of sperm whales is located in their lower jaw, the large body of the whale likely blocks returning echoes from the seafloor, thereby allowing for a long-range biosonar strategy while swimming upside-down. Hence, sperm whales employ contextual adjustments of their biosonar sampling scheme, acoustic gaze, and body orientation to facilitate prey search and capture in different foraging habitats.

Deep diving toothed whales evolved to predate on the biggest biomass on Earth in the Deep-Scattering-Layer (DSL) and Benthic-Boundary-Layer (BBL). The behavioural ecology of these more than 20 species of air-breathing predators might include interspecific competition leading to spatial segregation or coexistence. Here we used DTAGs to investigate the vertical foraging niches of 10 Cuvier's (Zc) and 16 Blainville's (Md) beaked whales, and 27 short-finned pilot (Gm) and 4 sperm (Pm) whales. Pm and Zc were tagged in the Ligurian Sea and Md and Gm were tagged off El Hierro and Tenerife. Depth and altitude above the seafloor of the whales while emitting echolocation clicks and buzzes indicate that: i) The four species mainly hunt in the mesopelagic realm; ii) Zc is the only species routinely targeting the bathypelagic while iii) Gm exploits the migration of the DSL to epipelagic waters to feed at night; iv) all prey on benthopelagic resources although Gm does so rarely. We show that all the studied species have niche overlap, and this is higher during the day than at night. Niche overlap and social structure might explain observations of interspecific agonistic behaviours in species with large body size (Pm) or large group sizes (Gm) that allow them to defend territories, in contrast with the spatial coexistence of species with small group sizes (Md and Zc) in spite of high niche overlap. The reliance of deep diving apex predators on resources of the DSL and BBL means that they would be negatively affected by emergent human activities such as fishing of the DSL or deep-sea mining.

The North Sea faces intense ship traffic owing to increasing human activities at sea. As harbour seals (Phoca vitulina) are abundant top predators in the North Sea, it is hypothesised that they experience repeated, high-amplitude vessel exposures. Here, we test this hypothesis by quantifying vessel noise exposures from deployments of long-term sound and movement tags (DTAGs) on nine harbour seals from the Wadden Sea. An automated tool was developed to detect intervals of elevated noise in the sound recordings. An assessment by multiple raters was performed to classify the source as either vessels or other sounds. A total of 133 vessel passes were identified with received levels > 97 dB re 1µPa RMS in the 2 kHz decidecade band and with ambient noise > 6 dB below this detection threshold. Tagged seals spent most of their time within Marine Protected Areas (89 ± 13%, mean ± SD) and were exposed to high-amplitude vessel passes 4.3 ± 1.6 times per day. Only 32% of vessel passes were plausibly associated with an AIS-registered vessel. We conclude that seals in industrialized waters are exposed repeatedly to vessel noise, even in areas designated as protected, and that exposures are poorly predicted by AIS data.

Predators that target multiple prey types are predicted to switch foraging modes according to prey profitability to increase energy returns in dynamic environments. Here, we use bat-borne tags and DNA metabarcoding of feces to test the hypothesis that greater mouse-eared bats make immediate foraging decisions based on prey profitability and changes in the environment. We show that these bats use two foraging strategies with similar average nightly captures of 25 small, aerial insects and 29 large, ground-dwelling insects per bat, but with much higher capture success in the air (76%) vs ground (30%). However, owing to the 3–20 times larger ground prey, 85% of the nightly food acquisition comes from ground prey despite the 2.5 times higher failure rates. We find that most bats use the same foraging strategy on a given night suggesting that bats adapt their hunting behavior to weather and ground conditions. We conclude that these bats use high risk-high gain gleaning of ground prey as a primary foraging tactic, but switch to aerial hunting when environmental changes reduce the profitability of ground prey, showing that prey switching matched to environmental dynamics plays a key role in covering the energy intake even in specialized predators.

Recent fieldwork strategies and early results on tagged Short-Finned Pilot Whales during 2021 and 2022 in South Tenerife, Canary Islands, Spain. Published in the SATURN's Project Newsletter. https://www.saturnh2020.eu/post/newsletter-issue-no-2