Timothy D. O’Hara, Museums Victoria, March 2023

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An assessment of the offshore marine natural values of Australia’s Indian Ocean Territories.

Executive Summary

Most of our knowledge of marine biodiversity of Australia’s Indian Ocean Territories (IOT) is restricted to coastal waters around Christmas and Cocos (Keeling) Islands. However, we do know that the vast majority of the marine realm across the two territories is deep-sea, with seafloor consisting of seamounts and ridges, abyssal and hadal plains. These deeper habitats around Christmas Island were explored by a 12-day 2021 expedition of the RV Investigator (IN2021_V04). A subsequent expedition (IN2022_V08) surveyed seafloor habitats in the Cocos (Keeling) Islands Territory in 2022. This bio-assessment draws upon knowledge of the IOT shallow water fauna, the IN2021_V04 and IN2022_V08 expeditions, other observations from infrequent scientific voyages to the area, and from better-surveyed deep-sea habitats elsewhere.

Depth (and its oceanographic correlates temperature, salinity, oxygen, nutrients and carbonate dissolution) and surface primary production are major determinants of seafloor community composition at large scales. At smaller spatial scales, substratum type (rock vs soft sediment) and current velocity will also be important.

Different communities are likely to occur in coastal (0-30 m), mesophotic (30-200 m), upper bathyal (200-1100 m), mid bathyal (1100-1900 m), lower bathyal (1900-3500 m), abyssal (3500-6000 m) and hadal (>6000 m) depths. Although, the boundaries between these layers are only approximate and individual species may inhabit only a part of one depth layer or parts of two or more layers.

Primary productivity varies in a NE to SW direction across the IOT, with the highest productivity occurring near Indonesia and the lowest in the mid Indian Ocean. This will alter the abundance and diversity of benthic communities.

There are four seamounts in the IOT that support rare mesophotic and upper bathyal habitats (30- 1100 m). These occur on the flanks of seamounts that underlie Christmas and Cocos Islands, and the upper flanks and plateau of the Muirfield Seamount. Upper bathyal habitats also occur on ‘Greeneye’ Seamount to the SW of Cocos that summits in ~440 m. Seamount biodiversity is generally structured by on-going or historical migration from surrounding regions that reflect predominant current patterns. As the currents at mesophotic and upper bathyal depths are similar in strength and direction to those at the sea-surface, it is probable that the biodiversity will reflect patterns seen in IOT coastal waters. Thus, the mesophotic and upper bathyal biological assemblages will be a mixture of Pacific and Indian Ocean species and populations characteristic of this depth, that arrive on the westward flowing South Equatorial Current from Indonesia and the South Java Current from the NE Indian Ocean. The biodiversity on these four seamounts are likely to be distinct, due to different primary productivity regimes and individual migration and environmental history.

Other seamounts, particularly in the Vening Meinesz and Cocos Keeling volcanic provinces, peak between 1100 and 1900 m (mid bathyal). These seamounts experience a mixture of intermediatedepth water originating from the south, north and east and so their community composition reflects this complex biogeography. The majority of other seamounts in the region, and the Investigator Ridge, peak between 1900-3500 m (lower bathyal) and experience deep-water masses originating in the south. The NE to SW gradient in primary productivity will alter benthic community composition and abundance. The Golden Bo’sunbird seamount chain (Christmas Island Marine Park) is in an area of relatively high productivity, while the Raitt seamount chain (Cocos (Keeling) Islands Marine Park) is in an area of relatively low productivity. The abyssal plain is very deep across the IOT, frequently below 5000 m and consists of siliceous sediments and manganese nodules. There is also some seafloor at hadal (> 6000 m) depths to the east and south of Christmas Island, and to the SE of Cocos 3 Islands. The fauna on and around manganese nodules has been found elsewhere to differ from that found on soft sediments.

A number of marine mammals, seabirds, turtles and fish feed in offshore waters. Species that breed on or around the islands typically forage in the surrounding waters, including the endemic Abbott’s booby and Christmas Island frigatebird. Migrating predators such as tuna, mackerel, dolphins, sharks, and seabirds tend to aggregate in the open ocean around schools of small fish that are in turn reliant on patches rich in plankton. The spatial and temporal distribution of these aggregations is unknown. Pelagic planktivores such as manta rays and whale sharks are known to occur around the two island groups. Whale sharks are known to target the abundant larvae of the Red land crab around Christmas Island during summer. The entire Christmas Island Marine Park is part of the only known spawning site for Southern bluefin tuna.

The oceanic seamount and island communities (30-1900 m) in the IOT are rare examples of these habitats in the eastern tropical Indian Ocean and are of national to international conservation significance. The presence of rocky substrata at lower bathyal depths, manganese nodules at abyssal depths, and troughs at hadal depths are significant in a regional context.