Bjarte Bogstad and Harald Gjosaeter
Institute of Marine Research, P.O. Box 1870 Nordnes,
N0-5817 Bergen, Norway

Central to all advice for fish stock management and exploitation is the scientific stock assessment.

The stock assessment typically consists of an updated stock history, an estimate of the present stock situation, and a prognosis of how the stock will develop in the short/medium term, for different management options.

In most cases, and where data is available, an age-based analysis of the stock is at the core of such an assessment. The tools used differ from stock to stock, but in most cases a model of the type "catch-at-age-analysis" is applied for the Barents Sea fish stocks. In short, the assessment consists of an analysis of catch in numbers in each age group back in history, fitted to various indices of stock abundance. Assumptions about the proportion of fish dying from other reasons than fishing (the 'natural' mortality) are also needed.

The catch in numbers at age data is derived from information on total commercial landings (weight) by the countries fishing on the stock in question, combined with age and length distributions of samples from commercial fisheries. Such samples may be taken either onboard the fishing vessels or in the port where the fish is landed. It is important to have a good coverage of samples in space and time, as well as collecting samples from the various fleets/fishing gear types.

The indices of stock abundance used in assessments are both data derived from scientific surveys and from catch-per-unit-effort (CPUE, e.g. kg caught per hour) information from the commercial fishery. The data from scientific surveys can be either indices of abundance calculated directly from trawl catches, or indices from acoustics (echosounder) measurements combined with age/length compositions from trawl samples.

For some stocks, also data from mark-recapture experiments as well as mortality estimates derived from stomach samples of the predators on the stock in question are utilised in the assessment.

FISHERY IN NORTH ATLANTIC: REALITY AND PROSPECTS