Non-lunge feeding behaviour of humpback whales associated with fishing boats in Norway

Top marine predators, such as odontocetes, pinnipeds, and seabirds, are known to forage around fishing boats as fishermen aggregate and/or discard their prey. Recently, incidents of humpback whales interacting with fishing boats have been reported. However, whether humpback whales utilise discard fish as a food source and how they forage around fishing boats is unknown. This study reports, for the first time, the foraging behaviour of a humpback whale around fishing boats. Three whales were tagged using a suction-cup tag containing a video camera, and a behavioural data log-ger in the coastal area of Tromsø, Norway. Video data from one tagged whale showed that the whale remained in close vicinity of fishing boats for 43 min, and revealed the presence of large numbers of dead fish, fish-eating killer whales, fishing boats, and fishing gear. In waters with large numbers of dead fish, the whale raised its upper jaw, a motion associated with engulfing discard fish from fishing boats, and this feeding behaviour differed markedly from lunge-feeding observed in two other whales in the same area. This behaviour was defined as “pick-up feeding”. No lunge feeding was seen on the data logger when the whale foraged around fishing boats. This study highlights a novel humpback whale foraging strategy: low energy gain from scattered prey but also low energy costs as high-energy lunge feeding is not required.

before engulfing a large volume of prey-laden water (Goldbogen et al., 2017).However, it is thought that lunge feeding would be an inappropriate strategy around fishing vessels, since it is inefficient for scattered immobile prey and could increase the risk of collision with vessels.
In the coastal area of Tromsø, north Norway, local fishermen operate set net fishing for herring Clupea harengus which attracts killer whales Orcinus orca around fishing boats for opportunistic feeding events (Mul et al., 2020).Humpback whales Megaptera novaeangliae, a rorqual baleen whale species, also feed on herring in this region (Jourdain and Vongraven 2017) and have recently been reported to conduct similar interactions with these fishing boats (Basran & Rasmussen, 2021).Discard fish are a potential low-cost food source for humpback whales.However, it is unknown whether humpback whales forage on discard fish during these interactions.Here, we report the first case of a humpback whale foraging on discard fish from herring fisheries, and the low-energy cost-feeding strategy used to obtain this prey.
Tags included a behavioural data logger (W1000-3MPD3GT; diameter × length: 26 × 175 mm; 140 g mass in air; Little Leonardo, Tokyo, Japan), a video camera (DVL400M130-2R; length × width × height: 68 × 21 × 22 mm; 47 g mass in air; Little Leonardo Ltd.), a Very High Frequency (VHF) transmitter (Advanced Telemetry Systems, USA), and a satellite transmitter (SPOT6; Wildlife Computers), and a single suction cup for attachment.The behavioural data logger records diving depth, temperature, conductivity, and swim speed and 3-axis magnetic field strength at 1 Hz as well as dorso-ventral, lateral, and longitudinal accelerations at 32 Hz.The video camera recorded 30 frames per second with a 43° field-of view on land and 31° field-ofview underwater.Humpback whales were slowly approached from a 6 m boat and tags were attached using a 6 m carbon fibre pole (Aoki et al., 2015;Johnson & Tyack, 2003).Tags did not have programmable pop-off mechanisms but naturally detached after several hours.The data loggers were retrieved using ARGOS satellite telemetry to locate the general area of the floating tag along with VHF transmitter signals to determine the precise location for tag recovery.
The study was carried out under a permit issued by the Norwegian Animal Research Authority (FOTS ID 8165).

| Analysis of video and behavioural data
Video footage was first analysed to determine whether foraging events had occurred.Data of behavioural data logger was then assessed to identify the foraging strategy, and whether humpback whales used lunge feeding around fishing boats.
We analysed depth, speed, and acceleration data using IGOR Pro version 8.0 (WaveMetrics).As measured by power spectral density, the fluke stroke of the tagged whale was detected as the dominant cycle frequency of the longitudinal acceleration data (for method details, see Iwata et al., 2021).Swim speed was recorded as the number of rotations per second of an external propeller and was converted to swim speed (m s −1 ) as previously described (Sato et al., 2003).Swim speeds slower than the stall speed of 0.2 m s −1 that tag cannot measure speed due to too slow were not included in the analysis.Diving was defined as spending longer than 10 s continuously at a depth greater than 3 m.Feeding events in humpback whales are commonly detected by identifying lunge events from biologging tags (Goldbogen et al., 2008;Simon et al., 2012).In previous studies, lunge feeding of humpback whales has been detected by triaxis accelerations and swim speed calculated by flow noise (Simon et al., 2012).In this study, lunge feeding events were detected by the following protocol.( 1) "Jerk", the rate of change in acceleration, was calculated from tri-axis accelerations (Ydesen et al., 2014), (2) coefficient of variation which is one of variation and a standard deviation divided by mean, of jerk and swim speed were calculated from moving average values of every 5 s, (3) one peak of maximum value of the coefficient of variation were detected within a time window of 30 s, (4) when the peak of the coefficient of variation in jerk and swim speed were detected simultaneously within a time window of 5 s, the events were defined as lunge feeding.The histograms of the peak coefficient of variation in jerk and swim speed demonstrate an unimodal distribution, and values exceeding the right edge of the peak were attributed to intense or high-speed movements.Therefore, the value at the rightmost edge of the peak was set as the threshold for lunge event detection.Fluke stroke rate, maximum swim speed, and events of lunge feeding during diving were calculated.Statistical analyses were performed in R (R Development Core Team, 2022).
Fluke stroke rate and maximum swim speed during feeding of each whale were compared using Dunnett's multiple comparison test (Rolland et al., 2005) which is used for making multiple comparison between a single control and several treatments using the multcomp package in R.

| RE SULTS
Three humpback whales were tagged, IDs mn17_022LLa, mn17_022LLb, and mn17_026LLa, recording a total of 32 hours of behavioural data and 17 hours of video data.Several humpback whales were observed visually around fishing boats and whale mn17_026LLa was tagged around fishing boats.Video data from animal mn17_026LLa around fishing boats showed that the whale remained in close vicinity of fishing boats for around 43 minutes after tagging, and revealed the presence of large numbers of dead/almost dead fish (herring and cod species), fish-eating killer whales, fishing boats, and fishing gear such as ropes (Figure 1, Video S1).In waters with large numbers of dead/almost dead fish, the whale raised its upper jaw in 10 times, a motion associated with engulfing the fish (Figure 1, Video S1).We defined this behaviour as "pick-up feeding" since whales caught discarded fish from fishermen.There was no lunge feeding signal in behavioural data of whale mn17_026LLa throughout tag period.However, several lunge feeding signals in behavioural data of whales mn17 _022LLa and mn17_022LLb were detected with values of jerk and swim speed changing abruptly and largely (Figure 2).Fluke stroke rates during a diving of whale mn17_026LLa during pick-up feeding and whales mn17 _022LLa and mn17_022LLb during lunge feeding were 8.4 ± 2.5 (SD) (n = 22 dives), 13.3 ± 1.6 (n = 5 dives), and 10.4 ± 1.7 (n = 9 dives) per minutes respectively.Fluke stroke rates during pick-up feeding were significantly lower than fluke stroke rates during lunge feeding (mn17_026LLa < mn17_022LLa (p < .001),mn17_026LLa < mn17_022LLb (p < .05),Dunnett Contrasts).
Maximum swim speed during a diving of whale mn17_026LLa during pick-up feeding and whale mn17 _022LLa and mn17_022LLb during lunge feeding were 2.4 ± 0.7 ms −1 , 4.8 ± 0.1 ms −1 , and 4.7 ± 0.4 ms −1 respectively.Maximum swim speed during pick-up feeding were ) which performed pick-up feeding.Video data showed that pick-up feeding of the whale for 43 min immediately after tagging then changed behaviour to not related feeding until tag detached.Video cameras worked for this period.(b) a part of the record of a whale (ID mn17_022LLb) which performed lunge feeding.Values of jerk and swim speed changed abruptly and largely during the feeding phase.Video camera worked for this period but all the footages were too dark for analysing.Video data for 8 hours from whale mn17_026LLa following pick-up feeding and video data from whales mn17_022LLa and mn17_022LLb did not show any scenes related to feeding behaviour.

| DISCUSS ION
This study reveals that humpback whales utilise discard fish near fishing boats for foraging.Whale mn17_026LLa did not perform lunge feeding around fishing boat but instead used a potentially lower energetic cost 'pick-up feeding' where the whale opened its upper to engulf fish.Number of strokes and swim speed in marine mammals is used an index of energy consumption (Davis et al., 1985;Williams et al., 2000).Whale mn17_026LLa around fishing boats did not conduct lunge behaviours, fluke stroke actively, and swimming without high speed indicating that pick-up feeding may be foraging strategy associated with low energy costs.Additional underwater, high-resolution documentary footage of this behaviour from this region, supports our findings, indicating that whales caught dead fish while travelling at low almost drifting speeds (Figure 1, Video S2 and S3).Although footages from animal-borne video did not give an accurate number of fish eaten, pick-up feeding was only conducted 10 times for 43 minutes, and, as such, relatively few fish, compared to lunge feeding events, would have been eaten (Goldbogen & Madsen, 2018).Despite, this pick-up feeding was still likely of net benefit to the whale due to the low energy costs of this behaviour.
Moreover, it is plausible that whales may benefit from the ready detectability of fishing boats, potentially leading to a reduction in prey searching duration (Mul et al., 2020).
Rorqual whales primarily employ lunge feeding as a foraging strategy but several other strategies have been reported.They include trap-feeding (McMillan et al., 2019) and bottom feeding (Parks et al., 2014) in humpback whales, tread-water feeding (Iwata et al., 2017) and head slap feeding (Izadi et al., 2022) of Bryde's whales Balaenoptera edeni edeni and B. e. brydei, and skim feeding of sei whales B. borealis (Segre et al., 2021).This study documents an alternate, novel, humpback whale foraging strategy, called pick-up feeding, that is low energy gain from scattered prey but also low energy costs.A previous study reviewing foraging behaviour in rorqual whales has suggested that recent recovery of whale populations may driving the increase in novel foraging strategies (McCarthy et al., 2023).The new pick-up feeding strategy of humpback whales seen here could be a result of competition between whales for prey.
Pick-up feeding includes risks of entanglement with net, which is a common and significant threat to baleen whales (Berrow & Whooley, 2022;Clapham et al., 1999).In addition, bycatch of large animals, such as baleen whales, also pose an issue for fishermen's safety and may cause costly damage to equipment.As such this behaviour could pose a threat to humpback whales and, should it become more common, avoidance sounds, such as pingers (Guidino et al., 2022) to deter whales from feeding close to fishing boats should be utilised to potentially minimise this behaviour.This study showed a novel feeding behaviour in humpback whales, which provides information on the ecology and behaviour of this species which could help inform conservation and management of humpback whales around fisheries in this region.

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Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/eth.13419by University Of St Andrews University, Wiley Online Library on [10/11/2023].See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions)on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License E 1 Underwater video footage from a whale mn17_026LLa.Images of (a), (b), (c), and (d) were recorded by animal-borne video camera and an image of (e) was taken by an underwater camera of documentary program industry staff (taken by Andreas B. Heide from the sailing vessel Barba).(a) killer whales and fishing ropes.(b) dead cod.(c) dead herring.(d) a tagged whale raised its upper jaw, presumably a motion associated with engulfing the fish.(e) a humpback whale and killer whales feed fish together around fishing boats (photo by Andreas B. Heide).F I G U R E 2 Time series records of behavioural data which are swim speed, diving depth, dynamic acceleration, and jerk, of humpback whales.The orange dots on the dynamic acceleration and the orange squares on the diving depth indicate fluke strokes and lunge feedings respectively.(a) a part of record of a whale (ID mn17_026LLa