Seamount Environment Disappearing

Author:  Halliday Elizabeth
Institution:  Marine Biology
Date:  April 2005

The first time I saw orange roughy, it was marinated in Teriyaki sauce. I didn't know that the fish was probably older then my grandmother, or that it came from one of the most surprising and mysterious environments on our planet. At ocean depths generally perceived as abyssal and void of life, there are thriving coral reefs and conglomerations of diverse marine life.

The deep sea is loosely defined as waters below 200 meters, and is characterized by the hardly hospitable physical conditions of cold temperatures, darkness, and increased pressure. Dotting this dark expanse are geological formations similar to those found on land – rifts, peaks and seamounts, which are underwater mountains formed by volcanic activity.

Seamounts are particularly unique because they block the path of cold, oxygen-rich, deep-water ocean currents. This causes nutrient-rich currents to converge on the seamount's peak, concentrating organic matter that runs down its slopes. This provides a food source for benthic (bottom-dwelling) organisms that in turn sustain small fish (such as squids and prawns) that are preyed upon by seamount-associated fish like orange roughy.

"Seamounts are disjunct, isolated oases in the deep sea," explains Tim Shank, an assistant scientist at Woods Hole Oceanographic Institute who researches seamounts. "They tend to host isolated populations, where communities take advantage of nutrient-rich currents." Planktonic larvae, which can usually travel with the currents to new habitats, are stopped from dispersing to other seamounts by the closed circulatory patterns in the surrounding water. "Because larvae can't break free of the vortex to leave the seamount, you tend to find species highly endemic to the individual seamount," says Shank. Additionally, the lack of nutrients and distance between seamounts makes migration nearly impossible for most organisms.

Orange roughy fisheries went unknown until 1975 when the depletion of other fisheries forced fishermen to turn from coastal waters to more mysterious and previously inaccessible depths. Aided by advances in technology, more powerful boats and better fishing gear, New Zealand fishermen were happily surprised when they discovered huge aggregations of fish, bright orange in color and rough in scale, around seamounts in deep waters. Its flesh was easily processed into freezable fillets and tasted delicious. A market instantly emerged and orange roughy quickly became one of New Zealand's most profitable exports. The rush for roughy was on – and in the early years, the orange roughy "hotspots" yielded a tremendous volume of fish. However, this bounty did not persist and it was soon apparent this was not a renewable boom-and-bust cycle.

Deep-sea fishes are long-lived and slow developing; orange roughy generally mature between 23 and 40 years of age, and are one of the longest-living marine organisms with a lifespan exceeding 100 years. Females produce few eggs in comparison to other fishes and reproduction is irregular. All these factors hinder a population rebound after over fishing.

Orange roughy numbers are difficult for scientists to monitor. In an evolutionary concession to the high pressures of deep water, the fish has lost its swim bladder. This air-filled sac found in most fish is used to control buoyancy. However, it also provides a contrasting physical matter to the aquatic environment and thus reflects acoustic energy, which is one method scientists use to measure numbers of fish in schooling populations.

Orange Roughy,is it on the menu tonight? Source: World Wildlife Fund.

Orange Roughy,is it on the menu tonight? Source: World Wildlife Fund.

Roughy aren't the only denizens of the deep that are rapidly disappearing. "My big worry in the Pacific," says Dr. Les Watling, of the University of Maine in Walpole, "is that these big fishing boats are literally going from seamount to seamount, we don't know what they're hitting, and the south Pacific is so big that monitoring them is nearly impossible." Further, "Orange roughy gear is absolutely massive...a lot of coral beds are being scraped clean. It's a very significant problem." Cold-water corals are extremely slow growing, at a rate of about 1 – 2.5 cm per year. Coral dating methods have placed living deep-water corals in the North Atlantic at about 10,000 years old.

The same thing happens in the North Atlantic. Shank recalled a submersible dive in Alvin in 2003 to the seamounts he studies, which stretch in geographic range from Cape Cod to the Mid-Atlantic ridge. At a depth of 2000 meters, he saw a beautiful yellow coral forest stretching out, with a path of broken coral through it and a ripped net. "It was clear to me there had been trawling, which was previously unknown, and there had been a tremendous impact."

Dark, cold waters across the globe are host to thriving coral communities. Although fishing expeditions revealed deep-sea coral structures over two hundred years ago, these corals remained relatively unknown for decades, and their biology is still mysterious. "The bottom line is – if there's rock, there's coral," says Watling, "All these seamounts are loaded with corals, gardens so thick and dense you wouldn't believe it."

The corals are filter feeders, dependent on the currents and the sinking organic matter for energy. "There are six months of primary phytoplankton production," Watling explains, and after these surface blooms, "A lot of it sinks to the bottom. This is a great food source for the corals."

Ecologically, these deep-sea reefs are extremely important because they increase habitat complexity in the deep. Deep-water corals support and shelter an amazing array of organisms including sponges, polychaete worms, bryozoans, and echinoderms such as starfish, brittle stars and urchins. The diversity of the reefs in Norway is estimated to be three times more diverse then the surrounding soft sediment, and a study of the reefs on seamounts near Tasmania found a total of 300 species of fish and invertebrates, of which at least a quarter of were unknown to science and one-sixth to one-third were restricted to the seamount environment.

Deep sea fishing activity results not only in species depletion, but also in habitat destruction. Source: Australian Fisheries Management Authority.

Deep sea fishing activity results not only in species depletion, but also in habitat destruction. Source: Australian Fisheries Management Authority.

The practices used in coastal fisheries cannot be applied in a sustainable way to the deep-sea environment. Trawling is leaving a swath of destruction through ancient reefs all over the world. Shank warns that, "There's fundamental data we don't have yet about reproduction and the degree of endemism of these seamounts. If you knock out a population on a seamount, we don't know how fast or if it'll ever come back."

It is astounding that we may so quickly lose an environment that has only been recently discovered. Conservation is especially difficult because of the many scientific unknowns. The question now is whether the environment will last long enough to do the research?

References and Suggested Reading

Milius, Susan. Corals Without Borders:

New Zealand Fisheries Dept. Orange Roughy: delicacy of the deep.

Clark, Malcolm and Rowden, Ashley. Uncovering secrets of our seamounts.