HADEEP / Life at extreme depth; Fishes and scavenging fauna of the Abyssal to Hadal boundary.

A team from the University of Aberdeen Oceanlab will join forces with the University of Tokyo, Japan and University of Tuebingen, Germany to undertake the first directed study on the distribution and behaviour of the deepest-living fishes in the world. Two unmanned autonomous lander vehicles will be built in Aberdeen; one equipped with a video camera and a second with a high resolution digital stills camera. They will be launched to depths of 5km to over 10km around the Mariana, Tonga and Kermadec trenches in the North and South Pacific Ocean from Japanese and German ships in 2007 and 2008. At the surface these ocean areas have clear blue water with low productivity and there is likely to be very little natural fall-out of food to the sea floor in the form of detritus and dead carcasses upon which the deep-sea animals living kilometres below are entirely dependent. Baits will be deployed on the sea floor in view of the lander cameras to mimic natural packets of food arriving from the surface and recordings will be made of fish, deep-sea shrimps and any other animals attracted into the observational area. The landers, equipped with depth sensors will provide accurate information on the depth of occurrence of different species, information that was difficult to obtain with equipment used to capture dead specimens that are found in different museums around the world. There is a chance that the team may capture the first living images of the world's deepest fish Abyssobrotula galatheae thought to occur down to 8370m. The deep sea is divided into three main depth zones, Bathyal down to 3000m, Abyssal between 3000 and 6000m and Hadal depths greater than 6000m. On the sea floor sharks are mainly limited to depths less than 3000m around the edges of the oceans, islands, sea mounts and on mid-ocean ridges. The abyssal zone accounts for 75% of ocean area and is populated by a surprising variety of life including active scavenging bony fishes such as grenadiers, cusk-eels, snail fishes and abyssal eels which have been filmed consuming fish and marine mammal carcasses at depths down to 5900m. It is not known what defines the boundary between the relatively rich life of the abyss and evidently sparse life of the hadal zone. The Hadal zone is mainly confined to trenches around the margins of the Pacific Ocean extending down to the deepest point on the planet, the Challenger Deep 10,896m in the Mariana trench. Two main explanations are likely: Firstly with increasing depth, food supply from the surface becomes sparser and a point is reached where there is insufficient energy to support active forms of life. Secondly pressure increases with depth and most animals have a maximum lethal pressure at which nerves, muscles and vital processes cease to function. Deep-sea animals have special adaptations to allow function at high pressures but there may be limits beyond which such adaptations are no longer possible. This project will not answer these questions directly but will make comparisons with information from shallower depths. If fish thrive down to 6000m with a constant cut-off at the start of the hadal region this would suggest a physiological pressure limit. However if the abyssal species peter out at different distances from the hadal boundary in different areas this may suggest limitation by local food availability. The time of arrival of animals at baits, their sizes, speeds, numbers and rate of consumption of bait will be measured. Comparisons will be made between species living at different depths. In particular fishes, which apparently do not penetrate the deepest parts of the trench systems, will be compared with amphipod shrimps that occur down to full ocean depth. Analysis will allow derivation of the first estimates of abundance and biomass of mobile life at these depths. This study will extend the depth range over which proposed fundamental laws of life in the deep sea can be tested.