Tuesday, 14 April 2009 11:35

The Oceanic Fisheries Programme of the Secretariat of the Pacific Community has recently completed a study on tuna fisheries around seamounts in the WCPO. This research is part of the Pacific Islands Oceanic Fisheries Management Project which is supported by Global Environment Facility (GEF).  Biodiversity on seamounts is high for benthic communities and they are considered important in the conservation of marine ecosystems.  Little however is known about their importance for pelagic species such as tuna. The study was undertaken in 3 parts:  the first used remote sensing data and existing literature to identify and validate the location of all seamounts in the western and central Pacific Ocean; the second examined whether tuna catch was higher on seamounts as opposed to coastal or other oceanic habitats and the third analysis examine whether pelagic biodiversity was higher on seamounts than coastal or other oceanic habitats.

icon Tuna fisheries and pelagic biodiversity around seamounts in the Western Central Pacific Ocean (WCPO)


Seamount location

Seamounts provide both commercial resources and, in some cases, unique biodiversity; they are clearly of particular interest and an accurate inventory of seamounts appeared necessary at both national and regional levels for fisheries management and conservation strategies. Several studies have been recently conducted to locate and quantify the extent of these features at the global scale, however the number of detected seamounts varies widely between the different data sets and databases have been to date largely not validated. Twenty datasets on seamounts and bathymetry from different sources and scales (from individual cruise to worldwide satellite data) have been gathered to compile a detailed list of underwater features for the Western and Central Pacific Ocean with some further extensions in the South Pacific RFMO area. The Kitchingman and Lai (2004- Fisheries Centre Research Reports 12(5):7-12 dataset (KL04) from satellite altimetry data provided the baseline of this study because it covered the entire region of interest and includes depth information Dr Allain said. All KL04 potential seamounts were cross-checked with other datasets to remove any atolls and islands incorrectly classified as seamounts, to add seamounts previously undetected by KL04, to update the overall database (geolocation, depth) and provide a 12-classes typology of the different types of underwater features. Dr Allain explained that of the 4627 potential seamounts identified in KL04 in the area of interest, 822 (18%) were actually emerged banks, atolls and islands and 272 were multiple identifications of the same underwater features (e.g. multiple peak seamounts), leaving 3533 actual underwater features. Conversely, 490 underwater features documented in other datasets but not registered by KL04 were added. The screening of all the potential WCPO seamounts produced a final list of 4023 underwater features with accurate position and information.

Allain V., Kerandel J.-A., Andréfouët S., Magron F., Clark M., Kirby D. S. & Muller-Karger F.E. 2008. Enhanced seamount location database for the western and central Pacific Ocean: screening and cross-checking of 20 existing datasets. - Annex Deep-Sea Research I. 55(8): 1035-1047. 

Allain V., Kerandel J.-A. & Clark M. 2008. Potential seamount location in the South Pacific RFMO area: prerequisite for fisheries management and conservation in the high seas. Fifth International Meeting on the Establishment of the South Pacific RFMO. 10-14 March 2008. Guayaquil, Ecuador. SPRFMO-V –SWG 05.

Allain V., Kerandel J.-A., Andréfouët S., Magron F., Clark M. & Muller-Karger F.E. 2007. Enhanced seamount location database for the Western and Central Pacific Ocean: screening and cross-checking of 20 existing datasets. Third regular session of the Scientific Committee of the Western and Central Pacific Fisheries Commission. 13-24 Aug. 2007. Honolulu, USA. WCPFC-SC3 – EB SWG/IP-9: 1-17.

Allain V., Kirby D & Kerandel J-A. 2006. Seamount Research Planning Workshop Report 20-21 March 2006. Second regular session of the Scientific Committee of the Western and Central Pacific Fisheries Commission. 7-18 Aug. 2006. Manila, Philippines. WCPFC-SC2 – EB IP-5: 1-56.

Area of interest bounded by the 45ºS-32ºN / 130ºE-120ºW domain where 4627 potential seamounts were identified by KL04. It includes Exclusive Economical Zones of most Pacific Ocean countries, and several high-seas international areas.

The 4,023 underwater features in the WCPO

Illustration of problems identified in the Kitchingman and Lai (2004) dataset. Top panel: regional view of the patterns in Tuamotu Archipelago (French Polynesia) highlighting confusion between KL04 seamounts (triangles) and atolls (black outlines show the rim of the atolls). This example also illustrates how large single features, here atolls, are identified as several potential seamounts. Bottom panel: enlargement and illustration of the same issues around Tahanea Atoll in Tuamotu Archipelago.

Example of the seamount databases before and after cross-checking for the Wallis and Futuna area. Top panel: all datasets are presented, using different colors and markers. Bottom panel: only the final validated underwater features are shown. Duplicates and false-positives have been removed.


Fishing around seamounts

This study also undertook a Pacific Ocean scale analysis of oceanic tuna longline catch and found higher catch rates of tuna species and other pelagic species close to some seamounts. Dr Morato explained that “there is considerable statistical uncertainty associated with these study but definitely some of them aggregate tuna around their summits”. This study estimated that about 17 thousand tons for yellowfin, bigeye and albacore are being caught annually on seamounts. The results have important implication for tuna fisheries management, particularly for yellowfin and bigeye Dr Morato said. “When overall population abundances decline, such as the present situation of yellowfin and bigeye, fishing vessels may concentrate on areas where fish remain. Such aggregation areas may promote what fisheries scientists call hyperstability of catch rates.” Whilst this is generally a positive for the viability of the tuna industry it is important that such hyperstability does not hide real trends in the data that might be indicating a decline in the status and viability of tuna stocks. The results from this study will allow OFP to include the effect of seamounts when preparing data for the regular assessment of stock status in western and central Pacific Ocean.
Cumulative longline tuna catch in Western Central Pacific Ocean (WCPO) seamounts in thousands of metric tons


 Morato T., Allain V., Hoyle S., Nicol S. 2009. Tuna longline fishing around west and central Pacific seamounts. Fifth regular session of the Scientific Committee of the Western and Central Pacific Fisheries Commission. 10-21 Aug. 2009. Port Vila, Vanuatu. WCPFC-SC5 – EB IP-4: 1-19.


Biodiversity around seamounts

This study also used observer data to investigate the role of seamounts in aggregating large pelagic biodiversity and to identify those pelagic species that are associated with seamounts. Dr Morato said that their “analyses suggest that seamounts, mainly within 30-40 km from its summits, are hotspots of pelagic biodiversity since they showed consistent higher species richness than coastal or oceanic areas”. Many species were observed to aggregate close seamount features such as blue shark, oceanic whitetip shark, swordfish, moonfish and sunfish, but also albatross and dolphins. These results suggest that seamounts are potentially areas of “special interest for conservation” Dr Morato said, particularly since many occur within the EEZ’s of SPC member countries. Management of oceanic ecosystems is considered easier with the boundaries of EEZs than in the high seas. Observer data is insufficient for identifying which seamounts aggregate biodiversity more than others but with continual improvement and expansion in observer programs of SPC members such analysis are possible in the near future.
Mean species diversity rarefied from 40 individuals as a function of distance to seamount summit. The fitted logarithmic regression is also shown (orange line).


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