Unique videos of orange roughy give scientists new insights
NIWA scientists have captured never seen before footage of schools of orange roughy swimming above a seamount nearly a kilometre below the ocean surface.
Moored video cameras were used to film the orange roughy swimming above a seamount called “Morgue”, located on the northern Chatham Rise, about 500 kilometres east of New Zealand. The seamount rises from a depth of 1250 metres to a peak of 890 metres.
“Morgue” is one of 19 seamounts around New Zealand which were closed to all fishing in May 2001 under a Seamounts Management Strategy.
Using state-of-the-art scientific echo-sounders and underwater cameras deployed from the research vessel Tangaroa, scientists made the first ever visual observations of schools of orange roughy swimming in the water column away from the seabed.
Dr Richard O’Driscoll, one of the NIWA scientists on the voyage, says capturing the footage represents a significant breakthrough for future information gathering.
“Technology like these video observations will help us make better measurements of orange roughy abundance in both fished and unfished areas,” Dr O’Driscoll said.
Ministry of Fisheries Chief Scientist Dr Pamela Mace said the footage provides new insights into orange roughy behaviour and will improve the accuracy of stock assessments.
“Assessing fish numbers at such great depths is very challenging and there is a lot of uncertainty in interpreting echo-sounder signals from schools of fish that are often a kilometre or more underwater,” Dr Mace explains.
“This footage and the other observations from this voyage will help us calibrate how we interpret echo-sounder readings and give a more accurate picture of fish numbers.”
Commercial fishers have long suspected that orange roughy make up a large proportion of the "plumes" of fish observed on echo-sounders swimming above seamounts, but orange roughy are only caught by trawling on or near the seabed.
Previous attempts to verify what type of fish are forming these seamount plumes have failed because the fish are very wary and avoid nets and cameras towed through the water towards them.
“Our solution was to use stationary video cameras anchored at several heights above the seabed,” says Dr O’Driscoll.
These fish live at great depths in perpetual darkness and shun the bright lights needed to capture video footage. To solve this, cameras and lights were timed to come on for only two minutes every two hours. This tactic was very effective, capturing footage of schools of orange roughy as well as smooth oreo, deepwater sharks, and squid.
The experiment was conducted as part of a voyage funded by the Ministry of Fisheries and Land Information New Zealand. Other voyage highlights included taking specimens of rare fish and invertebrates from depths down to 1850 metres, as well as scientific measurements and video observations of fish being herded into trawls.
Home video camera saves the day!
The cameras being used by NIWA scientists were off-the-shelf handycams mounted with a custom-built timing circuit in an underwater housing. During their first deployment early in the three-week voyage, three of the cameras were destroyed due to housing leaks.
Running low on spares, the team resorted to using a scientist’s personal video camera for the final mooring operation, which successfully captured some of the best pictures in the experiment.
Orange roughy are known to spawn on seamounts and it is spawning time now (late June to early July). However, some fish live on seamounts all year round, which may be because the current flowing past the seamount brings a “conveyor” of food (small fish and crustaceans) for the fish to eat.
The “Morgue” seamount is part of the “Graveyard” complex together with seamounts called “Crypt”, “Zombie”, “Mount Doom”, “Voodoo” and “Ghoul”. The area was named by the crew of fishing boats who explored and mapped the undersea terrain using depth-sounders. Some of the names were based on the theme of movies being watched by the crew when they were off duty; on this voyage evidently horror movies.