Pectoral herding: an innovative tactic for humpback whale foraging

Humpback whales (Megaptera novaeangliae) have exceptionally long pectorals (i.e. flippers) that aid in shallow water navigation, rapid acceleration and increased manoeuvrability. The use of pectorals to herd or manipulate prey has been hypothesized since the 1930s. We combined new technology and a unique viewing platform to document the additional use of pectorals to aggregate prey during foraging events. Here, we provide a description of ‘pectoral herding’ and explore the conditions that may promote this innovative foraging behaviour. Specifically, we analysed aerial videos and photographic sequences to assess the function of pectorals during feeding events near salmon hatchery release sites in Southeast Alaska (2016–2018). We observed the use of solo bubble-nets to initially corral prey, followed by calculated movements to establish a secondary boundary with the pectorals—further condensing prey and increasing foraging efficiency. We found three ways in which humpback whales use pectorals to herd prey: (i) create a physical barrier to prevent evasion, (ii) cause water motion to guide prey towards the mouth, and (iii) position the ventral side to reflect light and alter prey movement. Our findings suggest that behavioural plasticity may aid foraging in changing environments and shifts in prey availability. Further study would clarify if ‘pectoral herding’ is used as a principal foraging tool by the broader humpback whale population and the conditions that promote its use.


28-Aug-2019
Dear Ms Kosma On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-191104 entitled "Pectoral herding: an innovative tactic for humpback whale foraging" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Comments to the Author(s) The authors describe humpback whale behavior at a salmon hatchery with a focus on flipper movements. Humpback flippers are interesting structures that can produce significant hydrodynamic forces for a wide range of functions from social signaling to effecting both foraging and non-foraging maneuvers. The manuscript is well written but the data is very hard to understand and interpret as it is currently displayed and described in the manuscript. Some of the methods described for taking photo/video of whale behavior is incomplete, such as filming frequency, and the photos shown in figures 3, 4, 6, 8 do not show time stamps or time intervals. No video is provided for review. The schematics added in figure 2 and figure 7 do not add much given the lack of actual data presentation. Time scale of flipper movements should at least be a minimum for description of behavior given the focus of the manuscript. If drone data had RTK that should be used in coordination with altimeter data for kinematic analysis, if possible.
The title should reflect that the observed behavior is not natural. This study is an interesting experiment of how humpback whales exploit an artificial food source, but it is unclear how broadly relevant the present results are to our understanding of how whales use flippers in natural contexts on natural prey and in natural distributions/abundances.
The first paragraph of the discussion describes new technology used in this study. What technology are the authors referring to exactly? The last paragraph on page 8 states that tag deployed on whale dorsal surfaces only record movements of whole whale. There are papers recently published I suggest the authors refer to going back to 2017 and 2016 that describe the use of cameras within tags that show movements of jaws and appendages relative to the body.
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Review of Pectoral herding: an innovative tactic for humpback whale foraging
This study provides the first direct evidence for the 'pectoral herding' behavior of humpback whales. Bubble netting humpbacks wave their flippers in a non-hydrodynamically efficient way to scare schools of juvenile fish into their open mouths. Although this behavior has been previously hypothesized, it has not been otherwise confirmed. This study demonstrates video evidence of the flipper motion, how the prey responds to the flipper motion, and how it differs from other documented flipper uses. This paper is well written and very comprehensive. The authors sort through the different hypotheses for humpback flipper movements and convincingly demonstrate evidence for pectoral herding. Furthermore, the authors are careful to acknowledge the weaknesses of this study (n=2 individuals). I recommend this study for publication in RSOS and have only minor comments.

Observations:
These appear to be pretty slow lunges. Likewise, the flipper movements of whale B seem pretty slow compared to the thrust-generating flaps found in [25]. Particularly in your discussion pertaining to vertical herding I think the speed of the lunges and the flippers further supports your point of this not being a hydrodynamically active stroke. Since you're not measuring the speeds you can only say so much, but it may be worth mentioning the slower speeds involved.
The complex flipper movement of whale A is really interesting, and so is the head tilt towards the moving flipper. Furthermore, this whale is practically not moving forward at all. It would be really interesting to know what the back flipper is doing.
Please do ensure that the important videos are attached in the supplementary material or a repository (currently they are available on dropbox), since they are very important to this paper. Not all the videos need to be presented, just the ones that you show in the figures.

Minor comments:
The introduction could use some additional copy editing. A few examples: 1) a comma after 'sizable', 2) the ";" after "prey with lunge feeding" should probably be a ":". There are a few more instances as well. In this respect, the rest of the paper is pretty good.
Pg7 "Our current understanding about lunge feeding revolves around the theory that whales must either actively increase lift with their pectorals and/or use them to stabilize their body during a lunge." I think this sentence is a bit misleading. During lunge feeding, it has been shown that humpback flippers can be used to generate additional thrust and it has been hypothesized (although it is almost certain) that they can be used for stability and maneuvering. Additionally, they could also play a hydrodynamically passive role during some lunging events and either be held in a neutral position or their motion could be a reaction to the accelerative and decelerative forces of the body. I think 'must either' is too simplistic and I would rephrase the sentence.
Pg8 "It is well known that humpback whales aggregate their prey with a bubble-net." I think this paragraph is confusing. Has it actually been shown that "bubble-nets are rendered more inefficient if the prey does not naturally aggregate into dense patches"? Isn't this the point of the bubble netting? I get the gist, but I would rephrase the first few sentences of the paragraph.
Pg8: "However, our study reveals a flaw with the current tagging technology. Our findings illustrate that prey aggregation and capture are not limited to movements of the head, caudal peduncle, and tail flukes, but current tagging protocol deploys the tag on the back of the whale, recording only movements of the whole whale." A slight caution with this sentence: camera tags do allow for examination of the flippers and have been around for several years (see The role of flippers, flukes, and body flexibility in blue whale maneuvering performance).  Abstract 10 11 Humpback whales (Megaptera novaeangliae) have exceptionally long pectorals (i.e., flippers) that aid in 12 shallow water navigation, rapid acceleration, and increased manoeuvrability. The use of pectorals to herd or 13 manipulate prey has been hypothesized since the 1930s. We combined new technology and a unique viewing 14 platform to document the additional use of pectorals to aggregate prey during foraging events. Here, we 15 provide a description of "pectoral herding" and explore the conditions that may promote this innovative 16 foraging behaviour. Specifically, we analysed aerial videos and photographic sequences to assess the function 17 of pectorals during feeding events near salmon hatchery release sites in Southeast Alaska (2016 to 2018). We 18 observed the use of solo bubble-nets to initially corral prey, followed by calculated movements to establish a 19 secondary boundary with the pectorals -further condensing prey and increasing foraging efficiency. We found 20 three ways in which humpback whales use pectorals to herd prey: 1) create a physical barrier to prevent 21 evasion, 2) cause water motion to guide prey toward the mouth, and 3) position the ventral side to reflect 22 light and alter prey movement. Our findings suggest that behavioural plasticity may aid foraging in changing 23 environments and shifts in prey availability. Further study would clarify if "pectoral herding" is used as a 24 principal foraging tool by the broader humpback whale population and the conditions that promote its use. 25 26 Background 27 28 The metabolic demand of a large body size forces baleen whales to require sizable dense prey patchesLarge 29 body sizes of baleen whales generate high metabolic demands that require the consumption of sizable, dense 30 patches of prey [1][2][3]. However, filter feeding on these patches is energetically demanding for these animals 31 and requires inventive effective methods for prey aggregation [2]. BIn rorquals, behavioural plasticity and 32 foraging innovations are common among rorquals [4,5]. H and humpback whales (Megaptera novaeangliae) 33 provide an excellent example of how individual advances changes in behaviour can lead to diverse foraging 34 tactics that maximize feeding efficiency [6-9]. They are known for a wide variety of foraging strategies 35 including:Such foraging includes lunge feeding [6,10], bubble-net feeding [6, [11][12][13][14] Humpback whales are one of the world's largest filter-feeders and regularly use lunge feeding to capture prey. 39 with lunge feeding; an This particular technique is energetically costly [19] and sequential two-step 40 processrequires a two-step process. The whale first uses a high-velocity lunge to engulf large volumes of prey-41 laden water. The whale then closes its mouth and the baleen acts as a sieve to filter prey [14,20].First, during a 42 high-velocity lunge the whale engulfs a large volume of prey-laden water and second the prey is filtered from 43 the water with specialized feeding anatomy [14,20]. Appendix B 2 [20,22]. T; this acceleration maximizes the amount of water engulfed and aids in the capture of active prey 48 [26]. Humpback whales feeding near the surface exhibit an array of lunge types [6,12,15) and some are in 49 association with the creation of bubbles. A bubble-net is denoted by the formation of a ring of bubbles in a 50 clockwise fashion to enclose prey [6,7,12,13,27] and this strategy can be employed by an individual or a group 51 of whales. Bubble nets serve as a physical barrier to increase lunge efficiencies and are most commonly used 52 on naturally schooling fish (i.e., Pacific herring). 53 This bubble barrier makes the whale's lunge more efficient and is most commonly used on naturally schooling 54 fish (i.e., Pacific herring). 55 56 Humpback whales have a distinctive body morphology that allows for the successful and efficient capture of 57 prey [28,29]. Notably, they have the longest pectorals (i.e., flippers) of any cetacean, measuring from one-58 quarter As the buccal cavity expands during a lunge, a hydrodynamically optimal position for the pectorals is for one or 69 both would generate lift and increase propulsive thrust during an engulfment event: 1) both pectorals must move 79 symmetrically, 2) pectorals are angled into the path of the stroke, 3) the stroke is oriented perpendicular to 80 the whale's body, and 4) the stroke is aligned with the direction of travel [25]. Lift is generated as pectorals are 81 rotated at an angle to the water flow (angle of attack or  photographic sequences with a Nikon D7000 camera whenever whales were observed feeding at the surface. 130 In 2017, we also used a GoPro Hero5 Black video camera affixed to the end of a 3.5-meter pole to provide an 131 aerial perspective while standing on walkway platforms attached to hatchery net pens. These platforms 132 provided a unique and close-up perspective without disturbing whale behaviour that enabled camera views 133 directly above or within bubble-nets created by the feeding whales. In 2018, we used an unoccupied aerial 134 vehicle (UAV; DJI Mavic Pro with 4k video at 24fps) to capture footage of whales surface lunge feeding near 135 the facility. In addition to visual prey identification, we used a cast net and herring jig to sample prey in 136 foraging areas. We removed juvenile salmon otoliths to differentiate hatchery-reared and wild origin fish 137 according to methods described by Volk et al. to make the background section more concise and easier to read. Additionally, we took out "novel" as the editor requested and added the third potential use of pectorals as hydrodynamically passive (or decreasing drag with there placement) requested by reviewer 1 We used Adobe Premiere Pro to analyse video footage and Adobe Lightroom to assess photographic 142 sequences. Kinematic assessments of whale foraging behaviour were made, with particular focus on the use of 143 pectorals. We recorded pectoral positions, movements, and prey locations (when possible) using real-time and 144 frame-by-frame processing. Whale foraging movements were then 3D modelled using Blender, with post-145 processing in Adobe Photoshop to accurately illustrate foraging behaviours seen in footage and photographs. 146 Lunge durations were calculated from videos, when possible. All footage and photographic sequences were 147 viewed and categorized based on surface foraging behaviour. Bubble-net feeding was denoted by the 148 formation of a ring of bubbles followed by a lunge through the centre.  pectoral 51.9% of all documented feeding events. In these cases, the lower jaw was tilted at an angle that 185 exposed prey to the largest circumference of the buccal cavity (Fig. 4) events, all of which were in the vicinity of newly released hatchery-reared juvenile coho salmon (Fig. 5). We 214 observed two well-documented types of kinematic feeding behaviours for Whale B: vertical lunge and lateral 215 lunge. We also documented vertical pectoral herding, which has not been previously documented in the 216 scientific literature. Video footage depicting all three lunge types is provided in supplemental material (S4). 217 218 Vertical pectoral herding was used in 30.8% of all feeding events. We identified vertical pectoral herding when 219 Whale B moved its pectorals from a neutral state (as in vertical lunge and lateral lunge) to a protraction-220 abduction posture (Fig. 6). After establishing this posture, the whale simultaneously moved both pectorals 221 forward and into a V-shaped position on either side of its mouth, with pectorals curved ventrally (Fig. 7)). A 222 vertical lunge was used during 23.1% of all feeding events. When employing this technique, the whale's 223 pectorals first abducted with the tips curved up. Prior to closing its mouth, the pectorals adducted to a vertical 224 lunge position, tight against the side of the body. Finally, the pectorals retracted and angled posteriorly as the 225 whale lunged to the surface (Fig. 7). The distinguishing feature between vertical lunge and pectoral herding 226 was a slight upward dorsal oriented curve to the pectorals and less visibility of the pectorals as they were 227 abducted with a swept-back configuration. A lateral lunge was used in 46.2% of the feeding events (Fig. 6). 228 When using this technique, the whale pivoted on its left pectoral and rolled approximately 90 degrees while 229 lunging. The left pectoral was exposed and occasionally broke the surface of the water as the whale used it to 230 manoeuvre. 231 232 When documenting Whale B's feeding events, we observed notable differences in light conditions. Both 233 vertical lunge (3 of 3) and lateral lunge (5 of 6) occurred in shaded waters. All vertical pectoral herding events 234 (4 of 4) occurred in sunlit water, which was easily identified from photographs due to a sun-induced green tint 235 of the water (Fig. 6). Whale B employed different tactics in the same location only when light conditions 236 varied. In general, Whale B appeared to use vertical pectoral herding in sunlit areas but switched to vertical 237 6 lunge or lateral lunge when the same area became shaded. The single lateral lunge event in sunlight waters 238 was located near a surface obstacle in the centre the bubble-net. Possible avoidance behaviour was 239 documented as the whale lunged near the buoy. Prey movement in the direction opposite of vertical pectoral 240 positioning was visible in 2 of 13 engulfment events (Fig. 8). In "before" snapshots (i.e., images taken prior to 241 vertical pectoral positioning), we observed a dense aggregation of prey between the mouth and pectoral. In 242 "after" snapshots (i.e., images taken once pectorals were placed in V-shaped position) we observed less dense 243 prey patches in the area between the mouth and pectoral. We also identified a greater relative density of prey 244 that had moved toward the whale's mouth. We could not calculate lunge duration for Whale B because the 245 whale started to lunge in water too deep to visually see the whole process using aerial footage. The variation 246 in light conditions also prevented the identification of consistent cues for the start of a lunge. 247 248 Discussion

250
It is well known that humpback whale pectorals aid in acceleration and manoeuvrability during feeding events 251 [28,29] conjunction with a bubble-net (as a secondary barrier) had never been documented. Using direct video 257 footage and photographic sequences, we described this foraging technique as "pectoral herding", with two 258 methods of execution: horizontal pectoral herding and vertical pectoral herding. We observed two humpback 259 whales using bubble-nets as a primary barrier to corral prey, proceeded by deliberate movements of the 260 pectorals to establish a secondary barrier before the lunge. These observations suggest that pectorals are 261 used to further condense prey inside the bubble-net, thereby increasing feeding efficiency for each event. 262 From our results, we found three ways in which humpback whales use pectorals to herd prey: 1) create a 263 physical barrier to prevent evasion by prey, 2) cause water motion to direct prey movement, and 3) position 264 the white coloration on the ventral side to reflect light, causing prey to move in the opposite direction [12,39]. 265 These three methods of pectoral herding are not mutually exclusive and can be used in conjunction with one 266 another. 267 268 Horizontal Pectoral Herding rates with and without horizontal pectoral herding supports our hypothesis that any additional movement 281 must substantially aid in prey capture. We conclude that Whale A used its pectorals in two of the three ways 282 to herd prey: 1) create a physical barrier to prevent evasion by prey and 2) cause water motion to direct prey 283 movement. In addition, pectoral movements could create eddies and/or drag that increases the whale's 284 capacity to alter prey movement. We note that our descriptions of horizontal pectoral herding rely upon 285 7 observations from a single whale. However, we documented the use of this particular foraging technique by 286 one additional whale, suggesting potential for cultural transmission of this foraging behavior. 287 288 In over half of the documented events, Whale A rotated its head in the direction of the left pectoral before 289 closing its mouth (during all other fulling documented events, the head remained centred and never rotated in 290 the opposite direction). This suggests that the left pectoral was herding prey and that the whale turned its 291 mouth into the path of swimming prey, further increasing the amount of fish consumed per lunge. The lower 292 jaw turned at an angle that exposed prey to the largest circumference of the buccal cavity, which likely 293 prevented escape between the lower jaw and the surface of the water. The rostrum was also above the 294 surface of the water to avoid blocking prey from entering the buccal cavity when the whale turned its head. 295 When the whale's head remained centre, the lower jaw surfaced to meet the upper jaw. showed that fish avoid lights and seek out dark refugia when lights were activated and/or flashing. Because 344 juvenile salmon were the target prey during Whale B's feeding events, we believe that light reflected off the 345 ventral surface of the pectorals served as a stimulus to scare fish in the direction of the dark "refuge" of the 346 whale's mouth. We were able to directly observe prey movement toward the mouth in response to Whale B's 347 pectoral placement in some of the videos. Pectoral movement and flashing may directly stun or disorient prey 348 [7]. 349 350 It is well known that humpback whales use bubble-nets to aggregate prey [12,27]; however, bubble-nets may 351 not be as efficient are rendered more inefficient when prey do not naturally aggregate into dense patches. 352 This is because schooling fish would aggregate within a single area of the bubble-net, enabling the 353 consumption of most contained fish in a single lunge. Non-schooling fish may very well distribute 354 themselves throughout the bubble-net, which is larger than the size of the whale's mouth, resulting in fewer 355 fish consumed per lunge. Acoustic prey surveys at our study site showed that groups of juvenile coho 356 (Oncorhynchus kisutch) and chum (Oncorhynchus keta) salmon were small, patchy, and short-lived compared 357 to those formed by herring and krill [59]. Whales tend to moderate their behaviour to efficiently exploit 358 different prey types and respond to dynamic prey conditions [14,60]. It is possible that the two whales we 359 observed have independently altered their foraging strategies to accommodate non-schooling fish and more 360 effectively incorporate hatchery-released juvenile salmon into their diets. Because aerial documentation of 361 solo bubble-netting whales has been limited, we cannot conclude whether or not pectoral herding is restricted 362 to these whales and the unique prey resource of hatchery-reared juvenile salmon. Pectorals are an efficient 363 secondary barrier and may be used by other whales lunging on different prey. For Whales A and B, 93.9% of 364 pectoral herding events exclusively targeted juvenile salmon. The remaining events may have also targeted 365 herring as prey. Additionally, a bubble-net may be substantially larger than the size of a whale's open mouth, 366 restricting engulfment to only a portion of the prey enclosed within the net. A secondary barrier further 367 condenses prey, conceivably enhancing the energy gained per lunge. 368 369 McMillan et al.
[18] documented humpback whales using a feeding strategy called "trap-feeding". The authors 370 inferred that whales use pectorals to manipulate prey by flicking fish into their mouth. The available footage 371 of the pectoral movement in this study relies on a lateral perspective with poor visibility below the water's 372 surface and no view of prey. This makes it difficult to connect pectoral movements to a specific behaviour or 373 make inferences about prey responses. Additionally, lateral footage makes it difficult to differentiate between 374 the use of pectorals as a stabilizing force during a lunge and pectoral movements to manipulate prey. In 375 general, most whale observations are obtained from land or boat, yielding lateral views that limit the 376 perspective and skew our perception of individual behaviours. With innovative technology (e.g., unoccupied 377 aerial vehicles, small video cameras), we can now gain the perspectives necessary for more accurate 378 interpretations of marine mammal foraging tactics. Our observations, which relied on an aerial perspective, 379 provide insight into the position of humpback whales in relation to prey (above and below the water) as well 380 as a more detailed depiction of the whale's movements and position during feeding events. Based on lateral-381