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Fully recoverable mooring systems can be important in oceanic environments when we don’t want to leave anything behind. What are the pros and cons of different recoverable mooring designs like landers, tripod frames (e.g. https://doi.org/10.1111/2041-210X.13404), and acoustic release rope canisters? What are the maximum deployment depths for the different systems?

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There should be no depth limitation for fully recoverable mooring system. IMO, it is only a question of budget.

A 'normal' deep mooring system (say bottom depth 1000 m, recorder 150 m over bottom) with no surface expression has typically a dead weight and a chain that are left on bottom.

To recover also dead weight and chain you need not only a surface expression, or a reliable and proper constructed release rope cannister, but also a vessel that has a winch that can pull in the weight and hold 1 km of rope, in addition to a A-frame/Crane to lift the weight on board.

AFAIK, most research vessels (that are usually expensive) and deep-see trawlers (not sure what they would charge) would be suited to recover full moorings. That is why I say, it is a question of budget.

Concerning Pro and Cons of different mooring systems, it depends on application and deployment depth. IOW, how close to the bottom do you wanted the hydrophone to be. For this to determine, you should inspect the sound speed profile. In deep water where you have upward refracting propagation, you wanted the hydrophone well above the bottom, otherwise you will miss the direct path of even close-by bottom feeding cetaceans and will detect only surface reflections. So, ranging is pretty tough. In the application referred to by the cited Goossens paper, bottom mounted hydrophones maybe OK.

When designing bottom moored recorder you must consider also hostile interaction. A system that is moored in 800 m with the hydrophone 150 m over the bottom can easier be picked up by a deep-sea trawler than a trawl-save construction where the fishing net simply slides of the system.

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Instead of making the mooring fully recoverable I know some research groups have used natural/degradable materials that are left on the sea bottom. I think a coffee beans bag filled with gravel/rocks was used for the dead weight along with a sturdy hemp rope.

I have not tested this myself and would probably ask around for details/lessons learned before doing it this way.

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Coastal processes and sand movement can thwart weight recovery devices if they become buried!

I deployed a handful of homemade rope canisters in depths <100 m (see underater video of one in action here), and suffered a few losses in an area along some sand dunes. Fortunately, my acoustic release and instrument were recovered, but the buckets were either partially or completely filled with sand and in one case, I suspect the entire weight retrieval system was buried in sand after a 3 month deployment. We used a heavy-duty winch to try to recover the weights, but eventually the line snapped under tension (dangerous!) and the weights were never recovered. Had the sand accumulated enough to cover the acoustic release, I suspect we would have lost everything. Using multiple releases can help to prevent total losses, but they come at a considerable expense.

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Other things to consider include:

  • Colonization rates in your deployment environment (in other words, when do you expect encrusting organisms to attach to your gear/hydrophones? Best be down to clean or exchange them before this happens as this can affect your acoustic recordings)
  • Consider any noise (chains clanking, etc.) that might contaminate your acoustic recordings
  • Acoustic releases useful but expensive, and not always reliable.
  • Consider the flow of your deployment environment. If you're in a tidal rapid, for example, your gear may drift as it surfaces and not 'pop up' (via acoustic release) quite where you expect it to. Also, if you're in a high-flow environment, you should expect to use more weight on your tripod/lander.
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