Marine Atlas: Maximizing Benefits for Tonga

HOW MUCH DO WE REALLY KNOW? COLD WATER CORAL HABITATS While quite a lot is known about Tonga’s inshore environment, some habitats are hard to explore and map, especial- ly those found deep below the surface where research is both expensive and complicated. To map Tonga’s important, cold-water coral communities, scientists use habitat suitability models that guide us to where they are likely to be found.

However, at this stage, because of the limited pub- lished data, habitat suitability modelling has been used to predict the likely occurrence of corals in the area. Habitat suitability was high in a continuous band along the main Tonga Ridge and island slopes. The distribution largely follows depth, with topography also a factor. The ridge is shallower than much of the abyssal plains offshore, with higher food avail- ability. The steep topography also provides hard rocky substrate which the corals need for attach- ment. Cold-water corals are sparse on the abyssal plains and are not predicted to occur at all in the hadal depths of the Tonga Trench. Although not presented, similar analyses have been carried out for five species of stony coral (order Scleractinia) (Davies and Guinotte, 2011). Depth, temperature, aragonite saturation state and salinity were the key environmental drivers for this taxonomic grouping. Their results indicate high suitability for some stony corals, such as Enal- lopsammia rostrata, especially in southern parts of the Tongan EEZ. Cold-water corals are widely regarded as being susceptible to damage from human activities, such as the direct effects of fishing and deep-sea min- ing and more indirect impacts from pollution and climate change. Many species of cold-water coral are structurally fragile and hence easily broken. They can also be long-lived and slow-growing, meaning that any recovery from damage or chang- ing environmental conditions is slow. This could have long-term effects on deep-sea ecosystems. Octocorals are one of the groups listed by the Food and Agriculture Organization of the United Nations (FAO) as potentially Vulnerable Marine Ecosystems (FAO, 2009), and which are required under United Nations resolutions to be protected from deep-sea fishing. The presence of cold-water corals can be an im- portant indicator for managing human activities to avoid or minimize impacts on deep-sea eco- systems. The habitat suitability map, although based on presence–absence, not on abundance, gives an indication of which areas may need protection from disturbance of the sea floor or climate change. Habitat suitability modelling examines the relation- ship between where the corals are known to occur and key environmental conditions at that location. This relationship enables extrapolation into areas that have not been sampled, based on the suitabil- ity of a range of globally recognized environmental factors. For octocorals, temperature, salinity, slope of the sea floor, ocean productivity, dissolved oxygen levels and calcite saturation state were important factors controlling habitat suitability (Yesson et al., 2012).

The Moon or the sea?

Corals are not restricted to shallow-water tropical seas. Deepwater or cold-water corals are regarded as occurring deeper than 50 metres and include five taxa and over 3,300 more species than their better known tropical coral reef counterparts: order Scleractinia (hard, stony corals), order Zoanthidea (zoanthids, gold corals), order Antipatharia (black corals), subclass Octocorallia (soft corals, gorgoni- ans, bamboo corals) and family Stylasteridae (lace corals) (Roberts et al., 2009). They are widespread throughout the Pacific Ocean. At present, cold-water corals have no economic importance for Tonga. However, many of them have been recognized as playing important eco- logical roles in the deep sea, since they can form large reef-like structures or have complex growth forms which in turn, provide habitat for many associated invertebrate and fish species. They are potentially common on seamounts in the southern There is a common misconception that we know more about the surface of the Moon than the ocean floor and that 95 per cent of the ocean is unexplored. The chapter “Voyage to the bottom of the sea” showed that we actually know a lot about the ocean floor. The entire ocean floor has been mapped to a maximum resolution of around 5 kilometres, unveiling most features larger than 5 kilometres across (Sandwell, 2014). However, only 0.05 per cent of the ocean floor has been mapped to a high level of detail, meaning Tonga’s waters un- doubtedly hold a lot of secrets, including deep- water or cold-water corals. These corals have a

depth range extending from around 50 metres to beyond 2,000 metres deep, where water temperatures may be as cold as 4°C (see also chapter “Still waters run deep”). While there are nearly as many species of cold-water corals as shallow-water corals, only a few cold-water species develop into traditional reefs. This is also why they are much harder to discover and map than their shallow-water counterparts. Nevertheless, scientists have created habitat suitability models that use information on the physical environment to predict their distri- bution and provide an understanding of their ecological requirements. The map shows the predicted suitability of habitat where octocoral species could occur. Octocorals are a highly diverse group, with soft corals, gorgo- nians, sea fans, sea whips, sea feathers, precious corals, pink coral, red coral, golden corals, bam- boo corals, leather corals, horny corals and sea pens among their estimated 2,000-plus species (Roberts et al., 2009). Globally accessible data for offshore corals are sparse in many Pacific Islands, including Tonga. In recent years, there have been deep-sea mineral exploration surveys, as well as research voyages (e.g. RV Falkor, 2017) that have generated images and samples of many species of cold-water coral from Remotely Operated Vehi- cle dives (Raineault et al., 2018). General species composition is likely to be similar to that known of the northern Kermadec Ridge off New Zealand. part of Tonga, which are of commercial interest for deepwater snapper and bluenose fisheries.

The bamboo coral Keratoisis grandiflora, which has been recorded in Tongan waters.

MAXIMIZING BENEFITS FOR TONGA

HABITAT VALUES

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