Seamounts: rest stops and filling stations for tuna and swordfish

Date: September 11, 2006

Seamounts (underwater mountains) serve as oceanic filling stations or rest stops for commercially important fish species such as tuna and swordfish, scientists have concluded. At a workshop held in New Caledonia in March this year, scientists examined the pivotal role of seamounts in relation to commercially important pelagic species such as albacore (Thunnus alalunga), bluefin tuna (Thunnus orientalis), bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) and concluded that there is strong evidence of a link between these species and seamounts. (1) Pelagic fish are attracted to seamounts by high concentrations of zooplankton and mircronekton found around these deep-sea features.

Seamounts trap zooplankton around them because layers of these animals (known as the deep-scattering layer or DSL) migrate towards the sea surface at night, moving with the currents over seamount summits. When the zooplankton attempts to migrate into deeper water at dawn, it is trapped by the seamount’s physical presence. Evidence from acoustic studies, the analysis of zooplankton and its distribution and of fish gut contents, has pointed to the fish intercepting the migrating layers of DSL, and to a complex interaction between many levels of the ecosystem. Pelagic fish primarily use seamounts as feeding grounds, but may also use the areas for spawning and nursery grounds. Other scientists believe they could potentially be used as navigational markers.

Recreational and commercial fishers, including deep-sea bottom trawling vessels, target seamounts because they know that is where deep-sea fish tend to concentrate. Fished seamounts have half the biomass of un-fished seamounts. (2) Most seamounts are located in international waters where there is no international law to protect them. In the light of the threats faced by seamounts from destructive fishing practices such as high seas bottom trawling and in consideration of strong evidence linking seamounts to valuable pelagic fisheries it appears highly prudent that international bodies act now to protect these vulnerable resources.

Notes: (1) The workshop was held in Noumea, New Caledonia, in March 2006.
DSCC briefing (pdf)
Report of the Seamount Research Planning Workshop held at the Secretariat of the Pacific Community, Noumea, New Caledonia, 20-21 March 2006, Allain et al. (pdf) (2) Stone et al. (2003)

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