Saturday, October 04, 2014

Fish Counts - better on Rebreather!

John's geek on rebreather, surveying a deep reef at Johnston Atoll. Source.

Cool stuff, and I cite,
Inside MPAs, fish surveys conducted with close-circuit rebreathers (CCR) recorded similar community metrics to fish surveys conducted with conventional open-circuit SCUBA. 
In contrast, outside the MPAs, the bubble-free diving system recorded 48% more species and up to 260% greater fish abundance. These differences reflected the ability of a diver wearing the silent CCR unit to sample the larger, most heavily targeted species that are shy of divers in fished areas. This difference was also large enough to change some results from ‘reject’ to ‘accept’ the null hypothesis of ‘no significant differences exist between fished and protected areas’. 

The use of CCR for fish surveys clearly minimizes behavioural biases associated with fish avoiding open-circuit SCUBA divers. 
We recommend the use of this bubble-free diving system for surveys assessing reef fish populations, especially in areas where fish are heavily targeted by spearfishing. If fish behaviour is not accounted for, surveys using SCUBA could result in erroneous conclusions when comparing fished and protected areas. While the behaviour of fish towards divers is rarely mentioned in conclusions from studies using Underwater Visual Census, it is an important source of bias that should be acknowledged and minimized where possible.
Now that I read it, it's pretty obvious.
With one caveat: only in places where spearfishing on SCUBA is legal!
Where it is not = where spearfishing is only being conducted on breath-hold, any such recorded difference between the counts of SCUBA divers vs those of rebreather divers would be perplexing.

Anyway, good to keep this in mind.
Paper here.


Unknown said...

Does this mean PA will have to use rebreathers from now on? haha

DaShark said...

Well that's the question isn't it.

Spearfishing on SCUBA is illegal in Fiji = rather uncommon.
With that in mind, like I say, I'd be highly surprised if your observations were affected by the same sampling bias.

Why don't you test that in a shallower site by comparing data collected via free diving vs SCUBA.