The management of the fish stocks requires knowledge of their size and other characteristics, but also knowledge of the ecosystems of which the stocks are a part.
Each year, the abundance of several marine resources is mapped in scientific surveys, using echo sounders, trawling, abundance estimations of eggs and larvae, tagging, or just counting along transects when whales are estimated. The surveys are often joint investigations with participation from many countries, such as for instance the International Bottom Fish Surveys (IBTS) in the North Sea. The data from each vessel participating in the survey are combined to make a total estimate of a particular species in a specific area.
In Norway, the Institute of Marine Research is responsible for monitoring the fish stocks and the other living marine resources in Norwegian and adjacent waters. From about 80 species exploited in Norwegian fisheries, scientific advice on approximately 20 species based on systematic stock monitoring, while advice on further 20 species is based on catch data. Many of the species have little commercial significance, but may none the less be important in the food chain and the ecosystem.
In fish stock assessments, scientists combine data from these scientific surveys with available information from fisheries, catch statistics, and information on the state of the ecosystem, including food supplies for the individual species.
For most stocks of interest to Norway, assessments are made jointly with scientists from several countries under the aegis of the International Council for the Exploration of the Sea (ICES). Scientists from the ICES member countries work together on the collected stock data in annual working groups. The calculation tools are mathematical models, the choice of model depending on the characteristics of the stock and what data are available. The Norwegian input to these models is generally based on both catch and cruise data.
Most of the mathematical models that have been used up to now are based on individual stocks, because this has proven to be an operational way of creating a basis for advice on harvesting quotas. However, many factors affect fish stocks, one being the availability and quantity of food on the various links of the food chains. Several species appear to be more closely linked than others. For example, there is a close interplay in the Barents Sea between cod, capelin and herring. To be able to take into account as many as possible of the factors that may affect the individual stock, multi-stock models are being developed. Such models have now been adopted for the assessment of individual stocks where the link to and dependence on other stocks seems clear.
Advice on fishing quotas
The ICES Advisory Committee for Fishery Management (ACOM) use the results from the stock assessment working groups to formulate advice for the management of marine resources. An important element in the advice is the notion of Total Allowable Catch (TAC) of a given stock.
The report from ACOM is sent to member states, EU and fishery commissions, which are regional bodies for the management of marine resources in specific areas. Even if total quotas are subject to political influence, they are based on data from the national research institutions. Through ICES, this information is used as a basis for impartial and non-political advice.
Development of ICES stock advices takes place in several stages
1. Scientists in the various countries obtain basic data from fish landings, discards and scientific surveys.
2. This information is used in the ICES working groups to assess the status of the stocks .
3. The results from the working groups are reviewed by ACOM, which decides what the ICES official advice on the management of the stocks is to be.
Biological reference-points – the precautionary principle
Sustainability is the basic principle of any management strategy. Optimal resource utilisation and predictability for the fishing fleet are other criteria that it is reasonable to include. In order to evaluate the degree of exploitation and also the condition of the stocks in relation to such criteria, biological reference-points have been developed.
Biological reference-points represent either a level of fish mortality or a level of the spawning stock. Since 1998, ICES has defined precautionary-principle reference points and attempted to quantify these for most stocks. The reference-points embrace both degree of exploitation (fishing mortality) and the size of the stock:
On the basis of historical stock data and assumptions about the spawning stock and recruitment, attempts have been made to define a bottom limit for the spawning stock of each species (Blim). If the spawning stock falls under that limit, there is a great chance of poor recruitment.
In the same way, an upper limit for fishing mortality has been defined (Flim). If this limit is exceeded over a long period, there is a high probability that the stock will fall below Blim, to a level where poor recruitment can be expected.