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• W4384202258 abstract "The fin whale(Balaenoptera physalus) is a large cosmopolitan cetacean whose range covers most of the world's oceans from temperate to polar latitudes (Aguayo et al., 1998; Aguilar & García-Vernet, 2018; Edwards et al., 2015; Reeves et al., 2002). The fin whale has been classified as a Vulnerable species by the IUCN (International Union for the Conservation of Nature) and as Endangered by Chile's Ministry of the Environment (http://www.mma.gob.cl), based on the sharp reduction of its populations due to intensive commercial whaling off Chile from 1964 to 1966, and off the Antarctica Peninsula from 1911 to 1929 (Clarke et al., 1978). Today, fin whales in Chile are distributed from Antofagasta (23°29′S) to Cape Horn (56°48′S), including the Juan Fernández Archipelago (Aguayo et al., 1998; Buchan et al., 2019, 2020; Pacheco et al., 2015; Pérez et al., 2006; Sepúlveda et al., 2018; Toro et al., 2016). The fin whale populations off Chile could be part of populations migrating to the Southern Ocean, but this has not yet been determined (Buchan et al., 2019; Clarke et al., 1978). Passive acoustic monitoring (PAM) has proven to be an effective tool for the study of whales, allowing the acoustic presence of whales to be determined over long periods and in remote areas (Buchan et al., 2015, 2019; Leroy et al., 2016; Mellinger & Barlow, 2003; Širović et al., 2017; Stafford et al., 2005). Particularly, fin whales emit low frequency song and nonsong calls. Song calls are low-frequency (~20 Hz) short duration (~1 s) pulsed signals whose source level can reach up to 190 dB re: 1 μPa (Širović et al., 2007; Thompson et al. 1992). These are produced in regular sequences as triplets, pairs, or individual pulses (Archer et al., 2019; Delarue et al., 2009; Hatch & Clark, 2004; Watkins et al., 1987). These songs have been reported worldwide (e.g., Buchan et al., 2019; Castellote et al., 2012; Edds, 1988; Nieukirk et al., 2004; Širović et al., 2013; Thompson et al., 1992; Watkins, 1981; Watkins et al., 1987) and appear to be produced by males only, suggesting a reproductive purpose (Croll et al., 2002). There are indications that the intervals between song pulses and the higher frequency components of song pulses could be used to identify acoustic populations (Širović et al., 2017). Nonsong calls are often called 40 Hz calls or downsweep calls (Širović et al. 2013). Fin whale downsweeps are high intensity (>180 dB re 1 μPa) and short duration signals of higher frequency than the 20 Hz song calls (e.g., Castellote et al., 2012; Garcia et al., 2019; Miller et al., 2017, 2021; Širović et al., 2007; Weirathmueller et al., 2017; Wiggins & Hildebrand, 2020). There appears to be seasonal variation in the occurrence of downsweep calls, which are more common during the late spring and summer feeding season, by comparison to 20 Hz songs being more common during the winter (Buchan et al., 2019; Oleson et al., 2014; Širović et al., 2013; Watkins, 1981). A variety of downsweep signals have been reported in areas of high productivity, suggesting that they may be associated with foraging behavior (Boisseau et al., 2008; Castellote et al., 2012; Cummings et al., 1986; Garcia et al., 2019; Romagosa et al., 2021; Schevill, 1962; Širović et al., 2013; Watkins, 1981; Wiggins & Hildebrand, 2020). Other authors have suggested that these signals are associated with social interactions (Boisseau et al., 2008). For fin whales, Širović et al. (2013) found that the number of downsweep calls were higher during the day compared to dusk and dawn in the Bering Sea and the Gulf of California but found downsweep calls to be more common during the day and at dusk in Southern California; these authors propose that diel calling changes depending on behavior and prey preference. Fin whales aggregate to feed on euphausiids (Buchan et al., 2021) in waters surrounding Isla Chañaral off northern Chile during the austral spring. Around Isla Chañaral (29°02′S, 71°36′W; Figure 1), groups of fin whales have been observed feeding in large numbers during the austral spring, summer, and autumn (Buchan et al., 2021; Pérez et al., 2006; Sepúlveda et al., 2018; Toro et al., 2016). This study focuses on characterizing the summertime acoustic repertoire of fin whales in the waters off Isla Chañaral. Passive acoustic monitoring data were collected during two periods: November 26, 2017, to August 16, 2018 (with instrument turnover on February 14 and 15), and November 28, 2018, to April 23, 2019. The subsurface mooring was deployed at a site (29°1.305′S, 71°32.336′W) with a water column depth of 104 m, with the hydrophone at a depth of 80 m (Figure 1). An Ocean Instruments SoundTrap STD300 sound recorder with an integrated hydrophone was set to a 24 kHz sampling rate and a duty cycle of either 10 min/hr (2018) or 20 min/hr (2019). Only the austral summer months (January to April) were analyzed due to this being the season with the greatest presence of whales in the area (Sepúlveda et al., 2017). Spectrograms were created with Raven Pro 1.5 software (Center for Conservation Bioacoustics, 2014) to visually identify, manually annotate, and count the number of song and nonsong fin whale calls (Figure 2). To identify the downsweep calls of fin whales, prior descriptions of these signals in the literature were used (e.g., Boisseau et al., 2008; Garcia et al., 2019; Širović et al., 2013; Watkins, 1981). All the spectrograms were made with Hann window; FFT 16,384; 50% overlap. Annotated fin whale calls were compiled using the selection table function of Raven Pro 1.5 where the minimum frequency, maximum frequency, peak frequency, duration, and signal to noise ratio (SNR) were measured (Figure 2). The measured SNR values were analyzed using the SNR NIST Quick measurement tool of Raven 1.6. Because fin whale downsweeps are similar to other calls produced by other species, such as blue whale D-calls and sei whales downsweeps (Baumgartner et al., 2008; Ou et al. 2015), the spectrogram window length was adjusted to 60 s because there are differences that allow them to be distinguished when visualized, such as the duration of the signals. Fin whale downsweeps have a duration of about 1 s while blue whale D-calls last between 1 and 4 s and sei whale calls are around 1.4 s (Baumgartner et al., 2008; Berchok et al., 2006; Madhusudhana et al., 2009; Ou et al., 2015; Watkins, 1981). To characterize fin whale downsweeps, average frequency and temporal characteristics from the selection tables were calculated and compiled over both study periods. The peak frequency distribution and duration characteristics of the recorded signals were examined by means of histograms in MATLAB. A MATLAB function suncycle (Pawlowicz, 2009) was used to determine the sunrise and sunset based on latitude and longitude. Twilight was considered to be dawn and dusk. Dawn was defined as 30 min before and 30 min after sunrise; day was defined as the period of time between the end of dawn and the beginning of dusk; dusk was defined as 30 min before and after sunset; and night was defined as the period between the end of dusk and the beginning of dawn. To determine whether there was diel variation in calls, mean adjusted call rates (calls/hr) for the three light periods (twilight, day, and night) during 2018 and 2019 were calculated using only days with at least one detection. Because the durations of each daily period are different, the calling rates were divided by the duration of the period in hours, to obtain normalized calling rates. The data were also adjusted by subtracting the average hourly detections on a day from the hourly detections for that day, thus removing the effect of variations in daily detection rates across periods (as per Wiggins et al., 2005). The null hypothesis that the calling rate is the same for the three periods of the day was evaluated with a Kruskal-Wallis and additional pairwise Wilcoxon Rank Sum test with a Bonferroni correction (as per Leroy et al., 2016; Wiggins et al., 2005). A comparison with the literature from other areas of the world was done, specifically looking at the minimum frequency, maximum frequency, peak frequency, duration, and SNR; standard deviation was also considered (if reported) or calculated using the information provided if possible. Four hundred and eighty hours of recordings were analyzed for the 2018 period and a total of 1,103 downsweep calls were annotated, and 890 hr for the 2019 period were analyzed with 355 downsweep calls annotated. Fin whale 20 Hz song calls were not found during this analysis. All signals recorded (n = 1,458) were low frequency (<140 Hz), highly irregular calls with frequencies ranging between average minimum frequency of 48.9 ± 8.1 and average maximum frequency of 81.3 ± 11.0 Hz. The mean peak frequency was 62.1 ± 9.9 Hz and the average pulse duration was 1.0 ± 0.1 s. The mean SNR was 22.2 ± 5.5 with a range of 7.3 to 41.1 (Table 1). The peak frequencies of downsweep calls were normally distributed between 36 and 108 Hz, with maximum values between 50 and 70 Hz (Figure 3). 1,458 48.1 ± 7.4 (30.1–76.0) 51.4 ± 9.0 (27.3–76.1) 48.9 ± 8.1 (27.3–76.1) 80.6 ± 10.6 (55.2–120.2) 83.3 ± 12.1 (40.2–119.8) 81.3 ± 11.0 (40.2–120.2) 61.5 ± 9.5 (36.6–104.0) 63.9 ± 10.6 (30.7–106.9) 62.1 ± 9.9 (33.7–106.9) 1.0 ± 0.1 (0.6–1.3) 1.0 ± 0.1 (0.7–1.5) 1.0 ± 0.1 (0.6–1.5) 21.9 ± 5.5 (7.3–41.1) 22.9 ± 5.2 (8.3–37.4) 22.2 ± 5.5 (7.3–41.1) The daily calls rates ranged from 0 to 14 calls/hr for 2018 and from 0 to 9 calls/hr for 2019, with the largest calls/hr in 2018 (Figure 4). For the 2018 period, the total number of calls per month was lower at the beginning of the summer (229 calls in January) and increased by the end of that season (484 calls in April). For the 2019 period, the number of calls was higher at the beginning of the summer (273 calls in January) and decreased towards the end of the summer (19 calls in April), when few calls were detected. No clear diel calling pattern was observed. The analysis of the daily cycles during the 2018 and 2019 periods showed significant differences between day, night, and twilight periods (Kruskal Wallis test for 2018 and 2019, respectively: χ2 = 23.33 and 23.84, p = 8.59e−6 and 6.66e−6). A Wilcoxon pairwise comparison test showed significantly (p = .019) higher calling rates at night compared with during the day in 2018 (Figure 5, top panel) and in contrast, 2019 showed significantly (p = .025) higher calling rates during the day compared to at night (Figure 5, bottom panel). In both years, twilight had significantly lower calling rates than either day or twilight (p = 0.002, p = 3.3−5 for 2018) and (p = 2.9−6, p = .025 for 2019). Fin whale calls detected around Isla Chañaral had a maximum frequency below 140 Hz, with the majority of calls (n = 1,222, 83.8%) having a frequency range of 80–50 Hz; these values fall within the ranges reported by other studies (Table 2). Upon comparison with the available literature (Table 2), the frequencies of downsweeps in this study were most similar to reports from the North Atlantic (Boisseau et al., 2008; Delarue 2008), but were higher than downsweeps reported for other regions (Table 2). The duration of the downsweeps in this study, i.e., 1 s, matched descriptions by Boisseau et al. (2008) for the North Atlantic, and Širović et al. (2013) and Wiggins & Hildebrand (2020) for the North Pacific (Table 2). The downsweep frequency ranges of fin whales recorded near the Reserva Marina Isla Chañaral were similar to those found near Antarctica with a frequency range between (Miller et al., 2021). — ~0.59 ~0.73 1,283 76.2 ± 16.8 (41.3–127.2) 49.9 ± 15.1 (15.8–96.4) 0.7 ± 0.2 (0.3–1.8) 81.3 ± 11.0 (40.2–120.2) 48.9 ± 8.1 (27.3–76.1) 62.1 ± 9.9 (33.7–106.9) (Peak) 1.0 ± 0.1 (0.6–1.5) A complication in comparing these data with the literature is that frequency metrics are reported differently in different studies, some studies reported characteristics based on a very small sample size (e.g., De Vreese et al., 2017) and most authors did not report peak frequency. Considering the variation in maximum and minimum frequency measurements that may arise due to variation in the SNR of signals, we propose that peak frequency may be better conserved under SNR variations and therefore is a better parameter to describe and compare the vocal repertoire of fin whales across geographic locations. Manual detection of whale vocalizations represents a challenge for researchers in the field of bioacoustics because the selection of each signal is a slow process and influenced by analyst subjectivity. Širović (2016) found that the final result of detections from the same data set can be affected by the analyst's subjectivity in identifying vocalizations due to each analyst selecting signals according to their personal criteria and experience. The presence of blue (B. musculus) and sei (B. borealis) whales (which have occasionally been seen in the north of Chile) around Isla Chañaral makes it difficult to identify fin whale calls, but an adjustment in window size and careful checking of signals allows for better identification (Acevedo et al., 2017; Ou et al., 2015; Redaelli et al., 2022; Širović et al., 2016). Fin whale downsweeps have been reported at feeding grounds, such as in Southern California and the Bering Sea, where they peak during the northern summer, based on PAM of complete annual cycles. In the North Atlantic, fin whale downsweep signals were detected in summer in the Gulf of Maine and the Gulf of Saint Lawrence, but based only on PAM during summer months (Delarue, 2008). In the Southeast Pacific, downsweeps have been reported in the Antarctic whose characteristics are similar to those found around Isla Chañaral (Miller et al., 2017, 2021). Although there are other studies on fin whale calls in Chile, these describe only 20 Hz calls and are therefore not suitable for direct comparison (Buchan et al., 2019, 2020). In this study, we analyzed 4-month periods in two consecutive summers, so we cannot comment on the seasonal occurrence of these signals, but downsweeps were present during all months analyzed. Interestingly, downsweep occurrence increased during the summer in 2018 and decreased during the summer in 2019, possibly due to changes in prey biomass between the two summer seasons. In effect, Romagosa et al. (2021) reported a significant effect of prey biomass on fin whale downsweep production. A recent study carried out with an Acoustic Zooplankton and Fish Profiler during February 2018 and 2019 revealed a higher acoustic backscatter of zooplankton in February 2018 compared to 2019 around Isla Chañaral (Oyanadel, 2020). Romagosa et al. (2021) hypothesizes that downsweep calls could provide information to other individuals about food sources where prey is abundant. In particular around Isla Chañaral, feeding activity of fin whales on Euphasia mucronata has been reported (Buchan et al., 2021; Pérez et al., 2006; Toro et al., 2016). Širović et al. (2013) found different diel patterns for fin whale downsweeps at different areas (Gulf of California, Southern California, and the Bering Sea). Here, we found a significant difference in call rates between day and night for both study periods, but the patterns were opposite for each summer seasons. In the literature on blue whale songs, some authors have proposed that diel patterns in song are linked to changes in feeding behavior but this has not been shown for fin whale calls (Leroy et al., 2016; Stafford et al., 2005; Wiggins et al., 2005). In blue and sei whales, diel calling behavior has been identified, possibly related to prey vertical migration patterns (Baumgartner & Fratantoni, 2008; Leroy et al., 2016; Stafford et al., 2005; Wiggins et al., 2005). More studies are needed to determine the behavioral context of fin whale downsweep calls and understand variations in call rates in response to environmental changes and social interactions. This is one of the first studies to describe the downsweep calls of fin whales in the Southeast Pacific. This characterization can serve as a basis for long-term PAM studies of these signals with automated detection techniques. We hope that future research will focus on achieving better characterizations of these signals globally and in the Southern Hemisphere, and a better understanding of their function. Maximiliano Vega Cortés: Formal analysis; investigation; methodology; software; validation; visualization; writing – original draft; writing – review and editing. Susannah J Buchan: Conceptualization; data curation; funding acquisition; investigation; methodology; resources; supervision; validation; writing – original draft; writing – review and editing. Carlos Olavarria: Conceptualization; funding acquisition; investigation; methodology; project administration; resources; supervision; writing – review and editing. Marcel Ramos: Methodology; supervision; writing – review and editing. Maria Valladares: Methodology; writing – review and editing." @default.
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• W4384202258 title "Preliminary characterization and diel variation of fin whale (<i>Balaenoptera physalus)</i> downsweep calls off Isla Chañaral, northern Chile" @default.
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