Assessing the state of fish stocks
Introduction
Fisheries management involves a number of political and scientific processes that aim to continue the business of providing a sustainable supply of fish. Each year Fisheries Research Services (FRS) carries out the research required by the Scottish Government Marine Directorate to support fisheries management. Scientific work is undertaken at FRS to understand the biology and the state of commercially important fish stocks. FRS investigates the life history of each stock, for example when the fish spawn, how long they live, and what they eat. Staff also research how changes in the marine environment may affect fish stocks. The state of the stock refers to the numbers of fish in the population or their total weight, the stock biomass. The aim is to provide fishery managers - the Government and the European Commission - with information on the state of the stocks. They can then make decisions on exploitation rates to sustain the stocks and the management actions necessary to achieve them.
FRS scientists look at four main characteristics of an exploited fish population to determine its 'health':
• Fishing Mortality (F) - 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;
• Recruitment - The number of young fish produced each year which survive from spawning to enter the adult stock and the fishery;
• Landings - The total annual tonnage of fish taken from the stock and landed by the fishing fleet.
Every year, scientists at FRS, as well as other members of the International Council for the Exploration of the Sea (ICES), look at the changes in these characteristics and forecast what is likely to happen in the future. In general the predictions are based on a pool of information provided by several European fishing nations. The process of estimating the past and current state of the stock is known as stock assessment.
Information Required for Stock Assessment
To carry out a stock assessment it is necessary to build up a picture of how the fish populations change over time, and examine the historical trends. The information required comes from market sampling, discard sampling and research vessel surveys.
Market Sampling
Throughout the year, landings at Scottish fish markets are sampled regularly by staff from FRS. Information on length and age are collected along with details of the catches of the fishing fleets. The most common method used to age fish is to examine the ear stone or otolith which is found in the head of the fish. These bony structures give an accurate picture of
the life history of the individual fish. They consist of layers that are built up like the rings on a tree. Each year can be identified by a dark and light band corresponding to winter and summer growth. The age of the fish can be determined by counting the number of bands on the otolith. By counting the ages of a large number of fish the age structure of the whole population can be estimated. This gives a good indication of how the stock is standing up to the effects of fishing. If there are not many young fish, then recruitment has been poor and this could mean problems for the future. If there are no old fish present in the stock, this could be an indicator of over-fishing. If the stock is in a healthy state, fish of a wide range of ages are present. Each year FRS staff sample around 1,000 catches from the landings of 18 different species. Approximately 800,000 fish are measured and around 90,000 are aged.
Discard Sampling
Sampling the landings is very important but it gives no information about those fish which are caught that never reach the market. Many of the fish caught at sea are thrown away, either to comply with the quota limits or because they have no commercial value, or because they are below the legal minimum landing size. With the co-operation of the fishing industry, FRS staff carry out around 80 voyages a year to collect information on the amount of fish discarded, as well as details of their length and age composition. Each year around 280,000 discarded fish are measured and 14,000 are aged. This information is then combined with the landings' data to give a complete picture of the effects of fishing on the stocks.
Research Vessel Surveys
Research vessel surveys give 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. Bottom trawling surveys are used for cod, haddock and whiting, whereas acoustic surveys are used for herring. Shoals of herring show up on echosounders as 'marks'. Analysis of the strength of the echosoundings gives an estimate of the total biomass.
For mackerel, surveys are carried out to estimate the total number of eggs produced during the spawning season. This is used in combination with information on the numbers of eggs produced by female mackerel of different lengths to estimate the size of the population.
Summary
The Main Characteristics
By collecting information each year from the fish landings, discard trips and research vessel surveys, scientists build up a picture of how the stocks change over a long period of time. Each year when they carry out stock assessments, scientists plot out the important characteristics of the population. They look at the trends over time and decide on the 'state of the stock'. they also try to predict what will happen to the stock in the future. These can be used to answer questions such as:
• Are landings increasing or decreasing?
• Is recruitment high or low in recent years?
• Is spawning stock biomass at a high or low level?
• Is fishing mortality rising or falling?
These trends show that this stock maybe outside safe biological limits.