Sonobuoys are expendable temporary hydrophones typically used by the military, but they can and are used for real-time tracking and localization of vocalizing marine animals. There are obvious benefits to using sonobuoys (e.g., McDonald & Moore, 2002; McDonald, 2004; Wade et al., 2006; Oleson et al., 2007; Soldevilla et al., 2022), and specialized software has even been developed for analyzing the data (e.g., PAMGuard and Ishmael). But they last only up to 8 hours and ultimately contribute to additional trash and plastic to the ocean. Some cruises regularly deploy them by the dozens. Is the data worth the environmental cost? Are there sustainable alternatives to sonobuoys for identifying and localizing vocalizing marine animals?
Sonobuoys drift with current, transmit data as long as battery last or deployment time is exceeded, then they sink to bottom.
The reason of this approach is that originally sonobuoys were deployed from aircrafts are date were also recorded by aircrafts. So there is no opportunity to recover. But this is not the case when used from ships and boats for civilian research.
Alternatives are surface drifting buoys with suspended hydrophones. Such systems are indeed used by the bioacoustic community. One issue I see is the tendency of developers to add functionality, so system can become more expensive.
If you have access to sonobuoys it would even be possible to take them apart and add the receiver and transmit system to a buoy system. IMO, this would constitute the cheapest maritime PAM system possible.
Edit: IIRC, when setting up sonobuys there are three operations: setting up radio frequency, depth and duration. This means they will sink after the pre-programmed duration. When deployed, sonobuoy first sink to programmed depth, release then a small radio antenna that is in a small air filled Portuguese man o' wars (Physalia physalis) type of cover returning back to surface. So by construction you cannot recover the buoys (at least easily). Therefore ne need for a different type of construction: Surface float with submerged hydrophone. And yes, you must plan your survey to come back to the buoys to recover them.
There have been numerous attempts to rebuild sonobuoys such that they can be reused, and I have been involved in at least 3 of these, each of these were unsuccessful to varying degrees. I've also spent a bit of time trying to decide if sonobuoys would be useful for population-level studies on our surveys.
For what it is worth... Overall, my assessment was that for our surveys, they did not bring enough value to serve as a tool for population level monitoring (or, not enough to justify the negative environmental impact). For our purposes (NOAA surveys in offshore waters of the US West Coast), they still remain valuable for collecting recordings in the presence of sighted baleen whales (and especially rarely encountered whales such as sei whales).
First, regarding recovery of sonobuoys at sea: This is not advised. The batteries are meant to be in seawater and you do not want these on your boat if you retrieve them. If you want to retrieve the parts, then you would be wise to extract the battery and dispose of that in the water (see this video or this publication for more info on batteries). Beyond the batteries, all the materials are highly engineered to be (relatively) good at one job, and to do it for a maximum of 8 hours. The wires, bungee, drogue-- none of these will last for more than a few deployments.
Previous Versions of Rebuilt Sonobuoys: Most attempts to rebuild try to reuse the electronics and the 'DIFAR Ball', but in a different housing, and with rechargeable or replaceable batteries.
The 'Donobuoy' was a reusable sonobuoy developed by Don Ljungblad (very briefly explained in this overview of IWC-SOWER Surveys in Antarctica). Reception range was VERY short, so these would be tethered to the ship for overnight recording and were therefore dominated by ship noise. The batteries could be recharged, but as I recall, ship noise dominated the DIFAR signal. If target species were detected, sonobuoys would be deployed. In 2013 I worked with Don to build several-- but these never worked out (they are still in my lab, waiting for their next life?).
I have also worked with others to try to rebuild buoys-- but they were also unsuccessful. I also have a large PVC version built by Jay Barlow (never worked). I have heard rumors that an 'RMS' (Real Smart Cookie) will be making a directed effort in the upcoming years, and I eagerly await this effort (as well as offload more of my aging sonobuoy parts).
Using sonobuoys successfully: As the OP has mentioned, there has been some great improvements in use of sonobuoys & DIFAR for marine mammals. PAMGuard makes DIFAR analysis very easy (yay!).
I put together this report that outlines some considerations that should be taken when using sonobuoys for marine mammal research. An important lesson learned (from a Navy Sonobuoy Technician) is that the common method of removing 'extra' plastic bits may be problematic. These highly engineered devices include a pin that is held in a precarious position and can be easily knocked out of place if the parachute & plastic holding the parachute are removed. This pin plays a critical role in depth of hydrophone deployment. Basically, if you remove the plastic, you cannot be guaranteed that you will get the intended depth of hydrophone deployment. The way I see it is: If you are going to throw this garbage in the ocean, you surely want to be sure you are getting the best data possible!
I would highly recommend consideration of recent work by Aaron Thode using 'azigrams' for directional information from DIFAR buoys (see link here) and Brian Millers' work on Estimating Drift of Sonobuoys from Acoustic Bearings (see here) to get the most out of your sonobuoys. Keep an eye on these two for new & exciting advances in software (and potentially hardware).
Alternatives to Sonobuoys: Vector sensors have great potential, but have limited use in marine mammal surveys to date. To date, the main vector sensor used is the DASAR by Greeneridge Sciences (see here), which is a moored seafloor vector sensor using DIFAR capabilities. These have been in use in the Arctic for decades. I expect the increased need in recent years, along with alternative monitoring platforms and companies working with marine acoustics, may increase the options for future research.