Dropping in for a fish census

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Special to WCS by science writer, Amy West

Ready to deploy the BRUV off the support boat

Ready to deploy the BRUV off the support boat

A curious visit by a human or motored robotic vehicle likely sends deep-water fish fleeing. It’s difficult to accurately tally fish if we appear as an ungainly and potential predator. In addition, the robot’s electrical field and narrow camera view may not reflect the actual fish composition. So dropping video festooned with bait that sits patiently on the bottom can harmlessly capture which fish are out and about.

 

An Australian duo from Western Australia University aboard the Waitt Institute research vessel has dropped nearly 60 of these camera systems, fondly called  “BRUVs” (baited remote underwater video). Todd Bond and Jordan Goetze have perfected the art of heaving these awkward rebar frames into and out of the water with sheer brute strength, and, well, the help of a cleverly rigged PVC and wooden winch thanks to the Waitt Institute crew. Angling the pair of cameras also allows length measurements of the fish. Needless to say, these boy’s fish ID skills are top notch.

Winching made easier thanks to the Waitt Institute crew

Winching made easier thanks to the Waitt Institute crew

“Hey what is that fish?” I ask trying to describe the coloration.

Coris aygula,” says Jordan without missing one beat.

“And that one on the…,” I ask.

Heniochus  acuminatus,” he says before I can finish my sentence.  Later a fish shoots across the pixelated television screen, totally unrecognizable to all of us. “That’s an jobfish!” hollers out Todd.

 

 

Dropping these frames along Fiji’s reefs or seamounts gives us a peek into the fish assemblages at deeper depths; which are essentially unexplored. Luckily for us the reef system provides natural ledges. Reefs worldwide will typically level out at roughly 30 meters and 60 meters, and some at 90 meters etc… These natural platforms form from erosion thousands of years ago when sea level stabilized, allowing centuries of wave action to eat away at the coral. When sea level rises or falls, a reef is normally dynamic: either building up or dying off.

 

Finding a ledge after 60 meters was challenging-- it just drops off.

Finding a ledge after 60 meters was challenging– it just drops off.

When the team could locate these ledges on the sonar, they were ideal for their camera frames since most of the reef is so sheer. But the systems can go missing or slip off the line. Their recent loss of two cameras to the deep blue was hard to swallow, but par for the course when tossing expensive equipment overboard beyond our reach. Knots may come undone, and gear can be ripped away by currents or snagged on the way up.

 

 

 

Reviewing the footage back on board reveal fish fighting for bait, groupers stopping in for a mouth cleaning at cleaner stations, and a black blotch emperor at ~50 m losing its colors when near the bait. The sea bream at 70 meters changed its colors and striped pattern, too. If they didn’t capture this metamorphosis on video, it could be challenging to identify these fish from a still image without their markings. On this trip the duo have seen more cod and emperor fish at 70 to 80 meters, which have been largely fished from the shallower depths. They’ve also seen fish that extend well beyond their range as noted in fish ID guides.

 

Checking to make sure the cameras stayed on and what fish they captured

Checking to make sure the cameras stayed on and what fish they captured

Recently Jordan and WCS used this video technique to conclude whether the marine protected area had an effect on sharks. These baited cameras inside and outside of Fiji’s largest protected area, Namena Marine Reserve, showed twice as many sharks in the shallow waters of the reserve. Their numbers are thought to be higher inside the safe zone because of the larger number of prey fish available to them.

 

 

Surprisingly it’s the selective practice of spearfishing that’s taken a toll on Fiji’s reefs. It’s too craggy and steep to cast nets on the offshore barrier reefs, but with the expanding market for fish, more poaching, more spearfishing at night, and more boats carrying spearfishers occurs. There’s hope though: fish may be more intelligent than we think. The idea of “deepwater refuges” is a relatively new concept, but an old simple observation made by many divers: more fish are seen below the limits of where they are fished. The BRUVs finally quantified that observation. By dropping cameras both inside and outside an established and newer marine reserve at five meters and 30 meters, scientists found that fish targeted by fishing were missing in the shallower areas, yet seen deeper. The types of fish found deeper were similar within and outside the two marine reserves, indicating depth can be a natural refuge.

 

Though these one-hour videos give just a glimpse into a tiny cross section of the ocean, over time the data can show trends to help manage marine life that are beyond our view.  It keeps the fish counters safe and dry, which the boys aren’t ecstatic about, but they know sacrificing underwater time means collecting more data. All they need to do is grab a cup of coffee and start shouting out fish names.

 

Amy West has traveled worldwide as a marine scientist, specializing in fisheries and deep-sea ecology. Now as a science communicator she brings stories about ocean realms to the public through radio, video, photography, and writing. She’s usually diving into adventurous stories that take her on or below the water.

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