The orange roughy Hoplostethus atlanticus has been first described from off the Azores, North Atlantic, by a Danish fish expert some 125 years ago. This usually brightly brick-red coloured fish occurs however in many areas of the world’s oceans north and south of the tropics between New Zealand in the south Pacific to Greenland in the north Atlantic. This species became suddenly interesting for fish industry when discovered in large aggregations around seamounts (undersea mountains) off New Zealand and Australia and caught in high numbers with use of modern echolocation and fishing techniques from depths of often more than 1000 m.
Orange Roughy seen through the lens of the ROV on top of the MAR at 1000m depth.
The English name “orange roughy” is derived from its colouration and the special scales (scutes) formed along the belly thus making this part of the body resemble like an “armoured chest”. Its size is up to about 60 cm in length and it has a white flesh with a mild taste. It became very popular on the American frozen fish dinner market because it retains high quality even if it is frozen and thawed several times. In the 1980s and 90s up to 60 000 thousand tonnes and more were caught per year especially around New Zealand until many of the local seamount stocks were depleted.
Can be 150 years old
Biological and ecological studies on orange roughy, carried out in parallel to fisheries exploitation, showed that this deep-sea fish is a particularly long-lived species. Based on ageing with radionuclide chemistry estimates of age up to 150 years are obtained. Consequently, growth is rather slow and the age of sexual maturity is not reached before about 33 years (!). This has a major consequence also for fisheries, as overexploitation may result in a large time gap for the recruitment of the respective stock. Although this species is widely distributed, immigration rate is rather low and hence the threat of local extinction through excessive fisheries is quite high. Therefore strict fisheries management based on a “precautionary” approach is required to allow sustainable harvesting.
Orange roughy. Photo: Thomas de Lange Wenneck
That much more research is needed to fully understand the biology of this interesting fish species was revealed by submersible investigations in the Bay of Biscay off southwestern France. In this area an aggregation of orange roughy was found on the bottom of a deep-sea canyon at about 1400 m depth. Surprisingly, these fishes could be approached as close as one meter and did not avoid the submersible. This is in contrast to earlier observations with underwater cameras in the Pacific, where aggregations dispersed when approached with this apparatus. This was also demonstrated by accompanying echolocation analysis.
White orange roughy
But what was even more exciting was that a great number of the orange roughy encountered in the depths of the Biscay bay were not reddish but completely white. When the submersible came closer, some individuals showed a colour change towards red and at the same time slowly became active and moved away. During the observation period this part of the canyon was without any currents, although just a few hundred metres away strong water movements had been encountered during the transect crossed with the sub. Are these fish able to use such remarkable differences among deep-sea habitats for adopting a lifestyle intermediate between hectic search for fresh prey accumulated through currents in the open water, and relaxation phases in adjacent quiet bottom zones? White colouration would then probably contribute to energy saving. And the red body colour derives from carotinoid-containing substances that have to be extracted from the diet consisting mainly of live crustaceans. Further evidence for these findings was now encountered during our cruise in both investigation areas along the Northern Mid-Atlantic Ridge with the ROV`s Bathysaurus and Aglantha.
An orange roghy caught on the MAR-ECO cruise with RV G.O. Sars, July 2004. Photo: Thomas de Lange Wenneck
Is orange roughy really so “clever” that it selects alternative deep-sea habitats which differ in hydrological conditions with good conditions for relaxation and making feeding excursions? And do they indeed “switch off” their red colour to save energy? For the moment these observations still raise more questions than they can give answers. And they clearly demonstrate that we are just at the beginning of disentangling the mysteries of the deep sea!
By Franz Uiblein, Institute of Marine Research, Bergen, Norway