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Demersal Trawl Surveys

Demersal trawl Survey areasMarine Scotland Science (formerly Fisheries Research Services (FRS)) has a long history of undertaking demersal trawl surveys. Data collected from these surveys forms a time series that dates from around 1920. Initially these surveys were self-contained and as a result data was only gathered from sea areas of paramount interest to Scottish fishermen. It became clear that more useful data could be obtained by integrating the FRS surveys with those of other interested countries, as most fish stocks are mobile and do not recognise international boundaries. Since 1960 FRS has been one of the leading proponents of closer international co-operation and has participated in a number of international co-ordinated surveys. The longest time series of co-ordinated surveys relates to the North Sea and can trace its origins back to 1960. The original objectives of the survey were:

  • To identify the main areas of abundance of pre-recruit herring
  • To determine whether North Sea herring comprised one or more unit stocks

Between 1960 and 1967 these annual surveys were used to estimate abundance of juvenile herring. However, it became clear that these surveys could also be used to estimate the abundance of juvenile gadoids, especially cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus) and Norway pout, as well as pelagic fish.

How the North Sea Survey Works

Currently eight nations (Scotland, England, Norway, Sweden, Denmark, Germany, France and the Netherlands) participate in the North Sea surveys, and wherever possible all nets and protocols are standardised. These surveys form the International Bottom Trawl Survey (IBTS) series and are conducted in the spring and autumn.

  • The trawl net used was designed specifically by the French Research Institute for Exploitation of the Sea (IFREMER) for this task and is called a Grande Overture Verticale trawl (GOV trawl)
  • The cod-end and extension are fitted with an internal liner with a mesh of 30 mm so that the majority of young fish caught are retained for examination on board the research vessel
  • The individual national surveys take place within agreed segments of the North Sea and within a strict time frame. The data are pooled to provide an international relative measure of abundance for the entire area in which the stocks are distributed

Scope of the Surveys

These surveys provide scientists with valuable biological information on age, weight, sex and maturity of the stocks. All these data are essential parameters used in the stock assessment process. In addition, environmental information such as surface and sea bottom temperatures and salinity are collected. These environmental data can be linked to fishery data via the International Council for the Exploration of the Sea ( ICES) to enable understanding of links between fisheries and the environment.

Building on Success

The success of the co-ordinated North Sea surveys has led to the expansion of co-operation to other sea areas of interest to Scottish fishermen. Since 1997 Scotland, England, Ireland, Northern Ireland, France, Spain and Portugal have been co-operating in a late autumn survey from the North of Scotland to Gibraltar. Whilst this survey has not reached the level of integration enjoyed in the North Sea, it has already provided a more cohesive set of data for fisheries on the western seaboard of Europe.