Marine Atlas: Maximizing Benefits for Tonga

UNDER WATER MOUNTAINS: SEAMOUNT MORPHOLOGY Tonga has 44 submarine mountains or seamounts (including guyots). These enhance productivity and act as biodi- versity hotspots attracting pelagic predators and migratory species such as whales, sharks and tuna. Vulnerable to the impacts of fishing and mineral resource extraction, seamounts are becoming increasingly threatened.

Underwater mountains

Seamounts are important features of the ocean landscape, providing a range of resources and benefits to Tonga. Many have elevated biodiversity compared to surrounding deep-sea areas. They can therefore function as stepping stones, allowing hard substrate organisms to disperse from one under- water mountain to another, thereby expanding their range across ocean basins. Seamounts are also key locations for many fisheries (see also chapter “Fish- ing in the dark”) and are known to contain valuable mineral resources (see also chapter “Underwater Wild West”). As demand for these resources con- tinues to grow, the need for focused management is increasing. The adverse impacts of mismanaged mineral resources extraction have the potential to severely impact seamount ecosystems. Just like mountains above the sea, seamounts dif- fer in size, height, slope, depth and proximity, with different combinations of these factors recognized as different morphotypes likely to have different biodiversity characteristics (Macmillan-Lawler and Harris, 2015). The map presents a classification of seamounts identified by Harris et al. (2014) into morphotypes within Tonga’s waters. Physical variations such as depth, slope and proximity are Imagine the shock of the captain who, in 2005, ran his submarine, the USS San Francisco, at full speed (35 knots) into an unknown solid ob- ject at a depth of 160 metres (Doehring, 2014). It was neither a whale nor a hostile submarine. The mysterious object in fact turned out to be an underwater island, or seamount. Vessels on the surface can easily look out for islands, either visually or using bathymetric maps (see chapter “Still waters run deep”), and the same applies for submarines. Unfortunately, at the time, the charts did not show the seamount near Guam that the submarine ran into. The fact that this feature was not on the charts is due to the nature of seamounts—mountains rising from the ocean floor that do not quite reach the water’s surface.

But how quickly things can change! By 16 January 2015, after a large eruption and ash plumes reach- ing 10 kilometres high, a former seamount became a new Tongan island, Hunga Ha’apai, now 2 kilo- metres long and 100 metres high (NASA, 2015). While some islands are newly born and others disappear amid rising sea levels (see chapter “Hot- ter and higher”), there is a third kind that seems to come and go. Home Reef, created by another Tongan seamount, surfaced in 2006, sending vast rafts of floating pumice drifting over to Fiji. And yet, by 2008, Home Reef was already gone. A sub- sequent eruption in 2015 did not bring Home Reef back, but the seamount may yet have another chance to metamorphose into an island (Smithso- nian Institution, 2017). known to be important factors for determining the structure of biological communities. For example, many species are confined to a specific depth range (Rex et al., 1999; Clark et al., 2010). There- fore, both the minimum depth (peak depth) and the depth range (height) are likely to be strongly linked to the biodiversity of a given seamount. Slope is also an important control in the structure of seamount communities, with steep slopes, which are current-swept, likely to support different communities to flat areas, which may be sedi- ment-dominated (Clark et al., 2010). Seamounts in close proximity commonly share similar suites of species with one another and also with nearby areas of the continental margin. The 43 seamounts and one guyot in Tonga’s water represent eight of the 11 global morphotypes. Un- derstanding this distribution of the different mor- photypes is important for prioritizing management actions. For example, seamounts with shallow peak depths that fall within the Epipelagic (photic) zone are hotspots for biodiversity. In Tonga’s case, this includes the large, tall and shallow peaked seamounts (morphotypes 9 and 10), the majority of

which are found north of the main islands, with the exception of the Capricorn Guyot (morphotype 9) to the east of the islands. Over half the seamounts are part of the intermediate seamount group (mor- photypes 3, 5 and 11). These are small to medium in size, with medium heights and a gradation in peak depths from moderately shallow through to moderately deep. Of the remaining seamounts, nearly one quarter are small with deep peaks (mor- photype 1). Those with shallow or moderately shallow peak depths are more likely to be exposed to fishing impacts than deeper-peaked ones. The remaining seamount morphotypes are characterized by deep to very deep peak depths, so are less likely to be targeted directly by fishing. However, with the push to explore seabed mineral resources, sea- mounts—with their associated cobalt-rich crusts— are likely to come under increasing pressure.

Seamount morphotypes found in Tonga waters

Large and tall seamounts with a shallow peak – Morphotypes 9 and 10 .

Peak depth

Peak depth

Medium-height seamounts with moderately deep peak depths – Morphotype 3, 5, and 11 .

Proximity

Proximity

Height

Height

Percent escarpment

Percent escarpment

Small seamounts with a deep peak – Morpho- types 1, 2, and 4 .

Basal area

Basal area

Small and short seamounts with a very deep peak – Morphotypes 7 and 8 .

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MAXIMIZING BENEFITS FOR TONGA

SUPPORTING VALUES

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