## 4 Discussion
These are not optimal for the detection and quantification of fish, due to the acoustic frequencies and pulse repetition frequencies employed. However, to expand glider capabilities and perform fish acoustic assessments in the future, a 200 kHz transducer can be adapted. Another potential approach is operating the echo sounder in CW mode (narrowband) and utilizing transducers with a wider beamwidth. The results presented here corroborate the effectiveness of gliders equipped with passive acoustic monitoring devices in collecting long-term marine mammal data and anthropogenic noise across a wide geographical area, with minimal disturbance to animals [11; 37]. The novel Seaglider(r)-integrated hydrophone used in this study was able to record cetacean vocalizations. The production of time series datasets during periods of unknown animal distribution contributes to the growing body of knowledge on marine mammal distribution in the Norwegian sea outside traditional summer survey periods. Although it is generally known that sperm whales are often found along the continental slope, this fact had not yet been documented in the Norwegian Sea and outside summer months. Furthermore, recordings of humpback whale occurrence beyond the months previously reported in the literature for low-latitude migration shows that not only can gliders equipped with passive acoustic equipment document species distribution, but they can also address gaps in areas and periods of the year for which there is a severe lack of information [44; 73]. Advantages of using gliders for recording cetaceans include cost efficiency, the capability of working under adverse weather conditions, and the collection of passive acoustic data over periods of weeks to months (e.g., [11]). Gliders are quiet monitoring platforms with a much lower acoustic impact on the ecosystem compared to traditional ship-based survey systems. As for zooplankton data collection, this is not achievable with traditional research platforms such as research vessels, but it is essential to detect ecosystem changes brought about by anthropogenic or natural disturbances (i.e., storms). Although both the Seaglider(r) and Wave Glider were equipped with hydrophones, only the data recorded by the Seaglider(r) were processed due to the large amount of data collected by each platform and time/budget constraints associated with data processing. Furthermore, considerations would have to be made for sampling that occurred at a single depth (Wave Glider) and sampling across multiple depths of the water column (Seaglider(r)), which would likely not be comparable due to the latter recording data at a different spatial plane. This study also showed the potential of gliders to detect and record anthropogenic noise, and when overlapping with cetacean vocal activity, how noise may represent a potential stressor for marine mammals. Underwater noise radiated by ships was previously recorded in LoVe and is one of the main sources of anthropogenic marine noise [74; 75]. On the technical side, the placement of acoustic sensors onboard autonomous vehicles will influence the quality of the data obtained. In the first Wave Glider deployment, the hydrophone was installed on the keel of the surface float, resulting in the sound of the wave breaking on the keel saturating the spectrogram for the duration of the survey. Yet,we observed that installing the hydrophone on a towed cable attached to the subsurface glider decreased drastically the recording of the self-noise produced by the float. Regarding ocean modeling, our results showed the importance and complementary role of gliders in increasing the accuracy of the ocean circulation models. We noted that by assimilating a few sporadic glider observations, our model's performance was greatly improved. A systematic glider-based observing system can be an important complement to better constrain small-scale circulation features [53].