General calling behavior and comparison with other species
The advent of animal-borne biologging devices and miniaturized technology including high-sample rate accelerometers has allowed us to compile this first description of fine-scale movement and calling behavior of tagged fin whales. Until now, our understanding of baleen whale calling behavior was based on broad interpretations from passive acoustic data, generalizations from tag data based on assumptions, or limited to behavior of single individuals followed by observers. These precautions were necessary to avoid ambiguity about which animal in a group was producing a call. Our analyses showed that fin whales exhibited shallow, relatively longer duration dive patterns during calling bouts, and statistical modeling additionally predicted little body movement, downward pitch, and a trend toward animals that showed directed travel at the surface. The confidence intervals on the model predictions were large, because our dataset was relatively small and our analysis probably did not include all the factors contributing to elevated calling rates. However, the variables that were identified were consistent with previous reports of calling fin whale behavior from passive acoustic data, as well as descriptions of calling from other baleen whale species.
In their detailed description of fin whale calls, Watkins et al.  reported that whales producing 20 Hz signals were located at approximately 50 m depth, and when tracked at the surface, swam slowly and with little overall body movement (such as vigorous fluking). Though the consistency in depth was based only on extrapolations from differences in sound arrival times on different hydrophones and phase reversals from reflections during propagation, our description of calling fin whale behavior based on tag data in Southern California is generally similar. Animals were shallower (approximately 15 m, although this could be an underestimate because depth measurements were a mean in a 1-min time bin rather than taken at the exact time of the call), but ODBA was low, indicating little movement. Traveling behavior is also consistent with PAM studies that have tracked individuals  (animals exhibiting slow, directed travel can still show low overall ODBA values).
Application of this behavioral profile of a calling animal may help surface-based visual observers identify callers, recognizing the limitations of the sample used to generate this study’s results. In addition, factors that impact sighting rates for systematic visual surveys include group size, surfacing rate, and surfacing behavior. Our results indicate that calling fin whales could be more difficult to see since they may surface less frequently, move away from their last surfacing position (and so may be more difficult to track), and maintain a low profile in the water without a very high arch since they are not diving deeply.
The behavior of calling fin whales also has similarities to other baleen whales. Tagged blue whales were found to produce calls at depths between 10 and 30 m [15, 21], and singing humpback whales are often found stationary at approximately 20 m, pitched slightly downward , similar to the fin whales in this study. It is possible that the similar depths, posture, and lack of large body movements among these large whales suggest either a physiological (based on the whales’ sound production mechanism) or physical (based on areas of best sound propagation) parameter that allows animals to optimize their ability to communicate.
Anatomical or physical/acoustic explanations for calling behavior
Aroyan et al.  proposed a model for blue whale calling that was based on anatomy. Given a monopole sound source with no air escaping during sound production, expansion of the laryngeal sac was the mechanism suggested for sound production. They predicted that calling should be shallow, at depths less than 220 m, and average depths as shallow as 30 m. They also predicted that calling whales should require a depth change during individual call production to supplement the muscular effort to drive the pneumatic source, or to maintain the inflation of the laryngeal air sac. Oleson et al.  did not observe depth changes during calls in their tagged blue whale dataset, but did record callers at depths consistent with the predictions of Aroyan et al. (20–30 m). We similarly did not observe changes in depth over the course of each call, although fin whale calls are much shorter in duration than blue whale calls, so such behavior would be difficult to achieve.
Oleson et al. also speculated that these are the depths where blue whales are close to neutrally buoyant, and, perhaps more important, that signal strength would be optimally increased (up to 6 dB) by surface reflection at these shallow depths. These postulated benefits gained from shallow water sound propagation in the case of blue whales could also be driving the sound production depths (10–15 m) of fin whales in this area. In our study area of Southern California, 20–30 m is a typical depth of the relatively warm surface layer, below which there is usually a sharp gradient in the pycnocline. This strong density difference is the location of significant changes in sound speed gradients as well as the right depth to get a Lloyd’s mirror surface reflection enhancement , and would be easily detectable from a buoyancy perspective. Though surface mixing could create a sonic layer that traps higher frequencies, making near-surface sound propagation more efficient, whether or how exactly low-frequency fin whales gain advantage in communication from these acoustic conditions is still unknown.
We can speculate that the downward-facing body orientation combined with sound production anatomy could help with horizontal propagation , as could particular propagation paths in the area, especially since tagged whales that received calls were often at similar depths to callers during sound production. In addition, if calling requires a large air volume in the lungs to drive pneumatic sound production, or a large air sac volume to maintain the appropriate frequency, then calling at deeper depths may be limited by reduction in air volume due to increasing ambient pressure at depth, and/or there may be a greater physiological effort to call at greater depths . These factors may also contribute to whales using shallower calling depths.
Accelerometer method vs. call acoustic parameters
Though differences we found in call levels and SNRs between calls produced by the tagged animal and other fin whales in the area were significantly different, the difference was not large in magnitude. This is likely due to several factors. In particular, any body movement or swimming by the tagged animal will increase accelerometer noise and flow noise, thereby masking recordings of less intense calls that are being received from more distant animals. Thus, it is possible that most of the non-tagged whale calls were produced by conspecifics at relatively short range (note, for example, the small sample size of non-tagged whale calls for bp13_257b, which may have increased variability in acoustic level measurements). In addition, overlap between the two datasets was substantial, and many calls identified to have been produced by conspecifics in the vicinity had levels and SNRs as high as or higher than some calls identified from tagged individuals. Readers should also keep in mind that accelerometer signals may be amplified by actual tissue vibration from sound production, making near-field particle velocity extrapolations less comparable with acoustic levels (refer to the appendix of  for further discussion on these issues). In general, the variability in acoustic properties of baleen whale sounds suggests that level-based discrimination should only be used with caution and with full understanding of its inherent assumptions, and research should continue to test methods for identifying individual callers in various situations.
Possible responses to disturbance
Regardless of the reasons for preferred calling depths and body orientation, these descriptions of fine-scale calling behavior give an important baseline for further studies of effects of disturbance from anthropogenic sources on calling animals. Watkins et al.  noted that “gaps” (quiet periods between 20 and 120 min during calling bouts) occurred at irregular times, sometimes at the approach of another fin whale, but also immediately corresponding to the close passage of a ship or the sound of a propeller cavitation starting up the area. Fin whales have also been shown to respond through changes in swimming behavior , but not in calling behavior , to the sound of earthquake noise in the ocean. It is clear that systematic experiments on fin whale responses to anthropogenic sound disturbance are needed. Our dataset was not large enough to address these questions, but such research is currently underway, and the baseline calling behavior reported here is a valuable standard against which calling behavior during and after potential disturbance can be compared in the future.