Hi Sol,

I think your concern is well placed.  The pros typically tow an array of hydrophones, in its simpler configuration it looks like a long fat rubber hose containing maybe a dozen transducers feeding their electrical signals to a recording unit back on the ship.  All this is done to reduce noise from the ship, from waves crashing, and flow noise.  The multiple transducers can also be electronically tuned to be directional so that it can be "pointed" away from a noise source (like the ship).

In your position, I would just try the simplest thing that could work, then fix the problems as they arise.  It could be you may need to be dead in the water while recording.  To address surface noise (slapping waves, wind), you could mount the hydromoth low down on a spar buoy, which you tow into position.

Best of luck, it sounds like an interesting project (c:

Hydromoths are great for the price but they do not have the most streamlined housing and audio quality won't be as good as something like a SoundTrap or really any recorder with a proper hydrophone and 16-bit +DAQ system.

If you can afford it, this is an excellent SoundTrap based towed autonomous system NOAA have been using. It might work towed behind an autonomous vehicle

Alternatively, if you can have something inside the vehicle, a simple tape recorder (e.g. Tascam DR40X) and hydrophone on cable  will provide excellent sound quality. You could also use something like a Raspberry Pi with audio focussed ADC hat to record but that would require a bit more programming. Even consider a standard AudioMoth and plug a proper hydrophone into the audio jack - this would still have a 12-bit ADC but would provide better sound quality than a hydromoth (hydrophones are more omnidirectional and there's no air filled causing reflections and attenuation)

Happy to help Samuel, will send a message

I'd love to!

Hi Joan,

Sounds like a great project! I would recommend having a look at some of the nice review literature and guidelines that are out there, like - 

• https://www.researchgate.net/publication/368683386_Good_practice_guidelines_for_long-term_ecoacoustic_monitoring_in_the_UK 
• https://academic.oup.com/bioscience/article/69/1/15/5193506 (Terrestrial Passive Acoustic Monitoring: Review and Perspectives)
• https://zslpublications.onlinelibrary.wiley.com/doi/full/10.1002/rse2.227 (Optimizing tropical forest bird surveys using passive acoustic monitoring and high temporal resolution sampling)
• https://onlinelibrary.wiley.com/doi/full/10.1111/brv.12890 (Acoustic indices as proxies for biodiversity: a meta-analysis)
• https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/2041-210X.14194 (Using acoustic indices in ecology: Guidance on study design, analyses and interpretation)

And some specific to nocturnal birds:

• https://pdfs.semanticscholar.org/f720/4b65248c2d9335dc0b70d1ac3b748145398e.pdf (In the still of the night: revisiting Eastern Whip-poor-will surveys with passive acoustic monitoring)
• https://link.springer.com/article/10.1007/s10531-023-02642-7 (Passive acoustic monitoring in difficult terrains: the case of the Principe Scops-Owl)
• https://www.tandfonline.com/doi/abs/10.1080/01650521.2021.1933699 (Passive acoustic monitoring of the Ferruginous Pygmy-Owl (Glaucidium brasilianum) over a complete annual cycle: seasonality and monitoring recommendations)
• https://www.biorxiv.org/content/10.1101/2023.05.22.541336v1.abstract (Nighthawk: acoustic monitoring of nocturnal bird migration in the Americas)

And a study on bird acoustic monitoring in Nigeria:

• https://www.sciencedirect.com/science/article/pii/S1470160X20312115 (Passive acoustic monitoring gives new insight into year-round duetting behaviour of a tropical songbird)

In terms of processing and analyzing the data, I work for Rainforest Connection which maintains Arbimon - a free, no-code ecoacoustic analysis platform to help automate species detection and classification within soundscapes. If you're interested, you can get started with our support docs!

There are also a number of stats packages for analyzing soundscape data (seewave, monitoR, warbleR in R; and OpenSoundscape, scikit-maad in Python).  

The 18650s should be packed in discrete holders to separate them and prevent them from shorting. I've taken up to 50 on a plane before. 100 might be a tall ask since most planes have limitations on total Amp Hours you can bring. 

These are the holders I use.

Good luck.

I have an AudioMoth that they are welcome to.  I'm down in Moss Beach (just north of Half Moon Bay).