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Surveys

FRV ScotiaMarine Scotland Science conducts a number of surveys to determine the abundance, age structure and geographical distribution of the principal commercial fish stocks in Scottish waters. All surveys are designed to accomplish two objectives:

  • The survey must provide an estimate of average fish density over the entire spatial range of the stock. This can then be used as a relative measure of abundance (for example, average number of fish per hour towed) or converted to an absolute measure (for example, total fish biomass) depending on the specific data available and the method of analysis
  • The spatial distribution of the stock must be properly mapped i.e. the survey area must cover the entire area of distribution for the stock, to ensure that it is sampled properly

Different Fish Need Different Surveys

There are many different types of survey as each type is suited to a particular species or assemblage of fish:

  • Acoustic surveys are used for pelagic species such as herring and mackerel. These sample most of the water column continuously, over the entire area occupied by the stock, using vertical scientific echosounders. The echosounder output is converted to fish numbers and biomass once the detected echoes have been assigned to a species (and size) using a pelagic trawl
  • Demersal trawl surveys are used for demersal species such as cod, haddock and whiting. These sample fish using a number of bottom trawl hauls over the entire area occupied by the stock using a planned sampling design
  • Deep water surveys are used for deep water fish such as grenadiers and orange roughy. These are similar to bottom trawl surveys but are taken over much greater depths
  • Egg Surveys (and larvae surveys) are used for species which have pelagic eggs (and pelagic larvae) such as mackerel and horse mackerel (and herring, in the case of larvae). These sample the water column for fish eggs (and larvae) during the spawning season. The number of adults required to produce the abundance of eggs (and larvae) is then estimated
  • TV Surveys are used for Nephrops. These take video samples to count the number of burrows in a sampled area. The number of adults is estimated based on occupancy rates of burrows