Underwater TV surveys are used as a fishery-independent survey method to assess the size of Norway Lobster (Nephrops norvegicus) stocks.
Nephrop s live in burrows in muddy seabed, and are fished with trawls in most areas. While in their burrows, Nephrops are protected from trawling, and as burrow emergence varies in relation to a number of factors (including light level, tide, season and sex), trawl catch rates at the same location can vary greatly with time of day or year. Thus trawl surveys are not considered to be a reliable way of estimating the numbers of Nephrops.
The Nephrops' burrows lend themselves to visual underwater surveys because they are present in the seabed irrespective of the time of day or year. These burrow systems have been studied in Scotland (by Marine Scotland Science (MSS)/Fisheries Research Services (FRS) and by the University Marine Biological Station Millport UMBSM) since the 1970s, using both SCUBA diving and underwater television, and are the basis for the underwater TV surveys currently performed for Nephrops stock assessment.
How Does an Underwater TV Survey Work?
An underwater TV camera and lights are mounted on a sledge towed over the seabed. From video recordings made during these surveys, counts of the burrows and information on the area covered on each tow are used to estimate the density of Nephrops on the ground. Nephrops densities are known to vary with the type of mud in which they live, so TV surveys are designed to sample each type of ground, to give a good estimate of the density in each. When information on the average size and weight of the animals is available, these densities can then be converted into stock biomass.
Where are the Surveys Conducted?
Underwater TV surveys have been carried out on Scottish Nephrops grounds by Marine Scotland Science, as the FRS, since 1992. In the first year, only the Fladen Ground was examined, but since then the number of grounds examined each year has been increased, and in recent years all the main Scottish Nephrops stocks have been surveyed annually, using both MRV Scotia and MRV Alba na Mara. The Farn Deeps ground is mainly fished by English vessels, and this is surveyed annually by the Centre for Environment Fisheries and Aquaculture Sciences (CEFAS). The surveys are expanding beyond these main stocks, and since 2000, underwater TV surveys have also investigated Nephrops densities on the continental shelf edge to the west of the Hebrides (to a depth of 630 metres) at the Stanton Banks, and in a number of west coast sea lochs.
What do the Surveys Show?
The underwater TV surveys show burrows of animals smaller than those usually caught in commercial trawl gear, and therefore provide a useful indication of future recruitment to the stock which is very valuable to scientists (it can be used as an indication of future catches). As well as providing valuable information on the overall size of the stock, TV surveys indicate how the stock is distributed. This is particularly useful for large grounds such as the Fladen Ground, or where densities vary considerably within a stock.