The first is its environment - predominantly around the deep ocean seamounts, hills and plateaus 500 to 1,500 metres deep. The second is the age of the species. The third is its capacity to school in the millions around these seamounts, especially during feeding and spawning, when they are targeted by fishers. Knowing the species composition of fish aggregations that appear on echo sounders is essential to accurate assessment of the size of orange roughy stocks. The technique can be used to discriminate orange roughy aggregations from aggregations of other fish with a different pattern of reflectance across different frequencies, and is a significant advance towards improving the stock assessments used in fisheries management decision-making.
Seamounts are virtual oases on a comparatively barren sea floor.
Seamount communities appear to be sustained by a food chain that sweeps past in the currents. The food chain comprises small planktonic organisms fed upon by the corals and other suspension feeders, and the small fishes, squid and prawns comprising the diet of seamount-associated fish, such as orange roughy. Orange roughy travel considerable distances to spawn each year, aggregating at distinct deep ocean seamounts and plateaus. This aggregating behaviour makes them easier to catch but their migration between areas makes it difficult for scientists to distinguish stocks of fish from each other. Genetics, microchemistry and shape of otoliths are now being applied to distinguish the different stocks.