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Assessing Fish Stocks

Every year, scientists undertake a programme of work to assess the state of fish stocks in European waters. Marine Scotland scientists at the Marine Laboratory in Aberdeen collect data on stocks in the North Sea and the west of Scotland. This information is combined with data from other European nations who fish in these waters, and is then considered by the International Council for the Exploration of the Sea (ICES).

ICES estimates the current state of the stock, and forecasts what is likely to happen in the future. This process, known as stock assessment, aims to provide fishery managers in the Scottish Government and European Commission with the necessary information on which to base decisions on how much fishing should take place.

Stock Assessment

To determine the 'health' of a fish stock, scientists estimate the following levels:

  • Fishing Mortality - a measure of the proportion of a fish stock taken each year by fishing
  • Spawning Stock Biomass - the total weight of mature fish (capable of spawning) in the population, and the minimum weight of mature fish required to ensure a sustainable fishery
  • Recruitment - the number of young fish produced each year which survive from spawning to enter the adult stock and the fishery, and
  • Landings - the total annual tonnage of fish taken from the stock and landed by the fishing fleet

Additional information is collected relating to when fish spawn, how long they live, what they eat, and changes in the marine environment that may affect fish stocks. The information required to carry out a stock assessment comes from a number of sources including:

  • Market Sampling - information on the length and age of fish is collected at fish markets, along with details of catches of the fishing fleets
  • Discard Sampling -  to provide length and age information on fish that are caught that never reach the market. Discard sampling, in addition to market sampling, provides a more complete picture of the effects of fishing on the stocks, and
  • Research Vessel Surveys - these provide information on the numbers of young fish (recruits) which are too small to be caught and landed by commercial vessels. They also provide information on changes in the distribution and abundance of the adult stock.

ICES Management Advice

ICES provides fisheries advice that is consistent with the broad international policy norms of maximum sustainable yield (MSY), the precautionary approach, and the ecosystem approach. ICES recognises that the fisheries for which it provides advice have not, in general, been managed with MSY as an objective, and that current European Commission policy does not call for fisheries to be managed in accordance with MSY until 2015. Therefore, the nature of ICES advice is evolving, including options for a 'transition process' to attain full implementation of the MSY approach by 2015.

Within the Precautionary Approach framework, ICES continues to define reference points in terms of spawning stock biomass (the total weight of mature fish in the sea capable of spawning) and fishing mortality (a measure of the rate of removal of fish by fishing). Stocks are classified in terms of reproductive capacity in relation to spawning stock biomass and sustainable harvest in relation to fishing mortality.

Spawning stock biomass and fishing mortality reference points are defined individually for each stock, and consist of limit reference points which signify stock conditions to be avoided, and precautionary approach reference points which give a high probability of avoiding the limit reference points.

Further information and detailed advice on stocks is available on the ICES website.

Developments in the Management Approach

Total Allowable Catches (TACs) have been a principal management tool for many years. Technical measures to help control the size composition of catches are also employed. While TACs and quotas based on pre-agreed allocation keys can be helpful in establishing upper limits on landings and stability in the sharing out of fish resources, they have proved less satisfactory in controlling fishing mortality. Part of the problem is that TACs refer to the landed component of a catch which means that once a TAC is exhausted, additional fish taken during a fishing trip have to be disposed of by, for example, discarding over the side.

In recent years, management of effort through a European Days at Sea scheme has also been in operation in an attempt to control mortality more directly by limiting fishing activity itself. Days fishing to be applied across different types of fleet (large meshed trawlers, beam trawlers, etc.) is agreed at the Council of Ministers and operated within each Member State. There have been some signs that this approach, along with significant decommissioning schemes (such as in Scotland), have had some effect in reducing fishing mortality for some species, including Cod. With continued reduction in time at sea, however, there is an increased possibility of more targeted effort through changed fishing behaviour.

Since 2009, Scottish fishermen have been taking part in a Catch Quota Scheme aimed at avoiding Cod. This allows fishermen to land all the Cod they catch, provided they carry cameras on board and stop fishing altogether in the North Sea when they reach their Cod quota. Catch Quota is only one management tool available but this, along with the use of selective gears and Juvenile Real Time Closures, can make a real difference. It is hoped that measures to encourage and reward avoidance behaviour will help reduce Cod fishing mortality further.