Project Goal
The goal of the Under Sea Ice and Pelagic Surveys (USIPS) project is to utilise Autosub-1, an Autonomous Underwater Vehicle (AUV), to investigate the physical and biological environment of the Antarctic Marginal Ice Zone (MIZ), and to assess the potential of AUVs for improving acoustic estimates of the biomass of fisheries resources.
General Objectives
The project has three main objectives:
- To investigate the distribution, abundance and behaviour of Antarctic krill in the marginal ice zone
- To investigate mesoscale variability in sea ice thickness
- To assess the potential for the use of AUVs in the acoustic assessment of fish stock biomass and to examine ship-avoidance behaviour of Atlantic herring.
Specific Objectives
Antarctic Marginal Ice Zone study objectives
- To quantify and compare the abundance and distribution of Antarctic krill within and without of the MIZ, and to investigate krill behaviour under ice relative to ice topography:
Although circumstantial evidence suggests that sea-ice is an important habitat for krill, little quantitative data exist to confirm or refute this contention. Interpretation of krill distribution and ice cover/thickness data together would allow both this, and hypotheses relating krill behaviour under ice with ice topography, to be tested. - To obtain measures of thickness of Antarctic sea-ice over a scale of tens of kilometres:
The study would yield an extensive series of under Antarctic sea-ice observations of thickness and distribution. Such data are essential for calculation of ice volume and water budget, and could lead to greater understanding of the likely response of the Antarctic sea-ice system to climate change. - To obtain measures of downwelling irradiance under Antarctic pack ice:
Underwater illumination in the MIZ is influenced by edge effects under floes, and the illumination varies in a non linear fashion as the proportion of ice cover changes. Understanding this is essential for modelling phytoplankton growth there (Mortan-Bertrand, 1988; Priddle et al., 1996).
Fisheries surveys objectives
- To increase the accuracy and precision of the North Sea herring acoustic survey:
Autosub-1 will be deployed on missions between vessel-survey transects, providing additional data that will enhance survey precision (Aglen, 1989). The intertransect distance in these surveys is currently restricted to 15 nautical miles, but there is evidence that herring may aggregate at scales smaller than this (Fernandes and Simmonds, 1997). The data obtained on the smaller scale by Autosub-1 will therefore increase knowledge of the spatial structure of herring, aiding the geostatistical evaluation of survey variance (Guiblin and Rivoirard, 1995). - To investigate the magnitude of vessel avoidance by herring during the North Sea acoustic survey:
Avoidance reactions to survey vessels by target species could contribute to an underestimate of biomass. There is however, conflicting evidence as to the magnitude of this reaction (Simmonds et al., 1992). This study would provide data on herring avoidance, valuable both for the survey and to the ICES FAST WG, which is currently assessing sources of uncertainty in acoustic surveys (Anon, 1997). - To observe surface schools during the North Sea herring acoustic survey:
Although there are considerable numbers of herring in the surface waters (upper 10 metres) of the eastern North Sea (Simmonds et al., 1997), this component is thought to be smaller in the west. We will use Autosub-1 to observe the surface from below and assess abundance there.
Project Outline
The necessary adaptation of acoustic instrumentation for autonomous deployment in Autosub-1 and the missions required to achieve project objectives will take place over four practical phases:
PHASE 1: Adaptation of the echo-sounder for use on Autosub-1
The Simrad EK500 is a scientific echo-sounder which uses echo-integration to provide quantitative data at high vertical and horizontal resolution (Bodholt et al., 1989). It is the standard acoustic research tool of both BAS and MLA. An existing MLA EK500 systems box will be modified to fit into a 500 metres depth-rated housing. The unit will be powered by an independent battery supply (also housed). Data will be logged autonomously to a local PC (contained in the EK500 housing) and accessed across a network facility at the end of each mission. The autonomous system will be tested in MLA's acoustic water-tank facility. Two transducers (38 and 120 kHz) will be fitted to Autosub-1, with the capability to be oriented in either upward- or downward-looking directions. Data from CTD and ADCP (logged to Autosub-1 bus) will be linked for analysis purposes to data from EK500 on the basis of time.
PHASE 2: Sea Trials
Field trials of the acoustic system on Autosub-1 will take place through a series of deployments from RV Calanus in a sea loch on the west coast of Scotland. The trials, during a five day period in May 1999, will consist of a variety of mission types concordant with the data collection objectives outlined in Phases 3 and 4. These would comprise five missions of up to 10 hours duration, with Autosub-1 at various depths (50, 100, 150 and 200 metres) with the echosounders orientated towards the surface, and missions at the surface, ahead of the vessel with the echosounders orientated towards the bottom. These will allow system performance to be evaluated, and should provide time for any required modifications to be carried out. In addition, deployment and recovery techniques will be practised.
PHASE 3: Fisheries applications of Autosub-1
Autosub-1 will be deployed from RV Scotia during the North Sea herring acoustic survey scheduled for a 24 day period in July 1999. During this cruise it is hoped to run a minimum of 10 Autosub-1 missions. The exact trajectory of these mission will be determined in situ (during re-charge time), such that the AUV can be recovered along the course of RV Scotia's cruise track: this allows for minimal disturbance of the acoustic survey, but would not restrict Autosub-1 missions. The survey has a four hour idle period per day (2300-0300) which provides ample time to deploy or recover Autosub-1. The normal survey time of 21 days has been extended to accommodate missions requiring Scotia to adopt a slower survey speed (four knots).
PHASE 4: Antarctic sea-ice study
Autosub-1 will be deployed from RRS James Clark Ross in the Marginal Ice Zone (MIZ) east of Signy Island, Antarctica, during a 14 day cruise in the period between October/January 2000/2001 (exact dates not yet set). The ice edge is a favourable location for deployment, since high frequency waves are damped in proximity to sea-ice, resulting in calm seas. A sea-ice and physical/biological oceanographic data-set will be collected around the planned deployment area as part of ongoing BAS research. This will include observations of ice characteristics/iceberg distribution, and will ensure that Autosub-1 is not deployed in places where icebergs may be encountered unintentionally. For the Antarctic Autosub-1 missions the EK500 and ADCP would be upward looking. A BAS Aquapack with PAR sensor will also be fitted.
Project Timetable