Fishing at its limit
The coming and going of fishFish stocks increase and decrease, with or without fishing activity. We have been aware of this natural phenomenon for hundreds of years. In the past it has spelled disaster for many people when fish stocks have suddenly declined. For instance, in 1714 and 1715 the cod inexplicably failed to appear along the barren west coast of Norway. In the poor region of Søndmør the fishermen, to avoid starvation, were forced to sell their most important possession – their boats. For a long time it was unclear what triggered such fluctuations in fish stocks. Many fishermen and scientists believed that in some years the fish simply migrated to other maritime regions. Finally in 1914 the Norwegian fisheries biologist, Johan Hjort, produced a comprehensive statistical analysis of data he had gathered over numerous research expeditions. One of his most important findings was that variability in the number of fish and offspring is largely dependent on environmental factors – including the salinity and the temperature of the water. Hjort’s work dates back almost 100 years. Since then, our knowledge about the growth and decline of many fish stocks has increased tremendously. Today we know that many factors impact on the natural development of stocks. We still do not fully understand how everything interacts, however. The natural factors with the greatest impact include the biotic environment with its species interactions, and also the abiotic environment, particularly the salt and oxygen content, temperature and quality of the water. The latter are also changed by long-term climate fluctuations – a further complicating factor in reaching an understanding of stock development. Of course, the size of fish stocks is not affected only by nature but also by human fishing activity. The condition of an exploited stock can be described by the following three factors:
- STOCK BIOMASS (B) is the total weight of all large and small, juvenile and adult fish in a stock. This figure is estimated with the aid of mathematical models using fisheries’ catch data and scientific samples and is quoted in tonnes. But even these mathematical estimates are riddled with uncertainty. Biomass can also fluctuate greatly from year to year. Of particular significance is the number of adult, sexually mature fish – the spawners – because they are responsible for producing offspring. This section of the stock is known as the “spawning biomass” which is also stated in tonnes. The spawning biomass level is crucial for fisheries scientists because they use it to derive vital benchmarks, known as reference points, used in fishery management. The total biomass of a stock is made up of the spawning biomass and the biomass of the juveniles, which have not yet reached sexual maturity.
- THE FISHING MORTALITY RATE (F) is a somewhat abstract measure of fishery pressure. It can be converted to a relative value which indicates the proportion of the stock biomass which is removed by the fisheries.
- THE PRODUCTIVITY of a stock is calculated by subtracting the number of fish which have died of natural causes from the increase in mass resulting from offspring and natural growth of the fish. This correlation makes it clear that the productivity of a stock is highly dependent on the spawning biomass. It follows that the stock declines when the natural mortality rate and the fishing mortality rate together are greater than the productivity.
- 5.2 > Barren land, poor fishermen: In the Søndmør region of western Norway people’s livelihoods used to depend almost exclusively on fishing, and particularly on the development of fish stocks.
- The offspring production of a fish stock is limited. If the spawning biomass is large, the habitat at some stage reaches its maximum carrying capacity. Even if the spawning biomass then continues to grow, the number of juvenile fish remains at a certain level. At this stage the amount of offspring depends entirely on the environmental conditions. Various factors come into play here: eggs and larvae may be eaten by predators, for example, or starve because insufficient food is available. In addition there can be competition for suitable spawning sites to deposit eggs. The Baltic Sea herring, for example, deposits its adhesive eggs on aquatic plants. When there are too many spawners, they deposit the eggs on top of each other, and those underneath die from a lack of oxygen. As these conditions can fluctuate from year to year, so too does the number of offspring when spawning stocks are high. There can be strong but also very weak years for offspring. If a stock is exploited too intensively the following can occur. The spawning mass is at some stage so small that few offspring can be produced. In such a case the number of offspring depends directly on the number of spawners. It is no longer capable of reaching its carrying capacity, even when good environmental conditions prevail. The value at which the spawning biomass is so small is called limit biomass (BLIM). The corresponding fishing mortality rate is described as FLIM.
The failure of the precautionary approachThe massive overfishing of many stocks by the industrial fishing industry in the 1970s, 1980s and 1990s made the importance of limiting catch volumes abundantly clear. In 1995, the international community adopted a more cautious approach to fishing with the United Nations Straddling Fish Stocks Agreement (UNFSA). In the same year the Food and Agriculture Organization of the United Nations (FAO) published its Code of Conduct for Responsible Fisheries. The overriding aim of this precautionary approach (PA) is to prevent a stock from being reduced to such an extent that it can no longer produce sufficient offspring and becomes overexploited. It also stipulates that fisheries should err on the side of caution: the less that is known about a stock and its development the more carefully that stock should be managed, and the less it should be exploited. In principle, therefore, the precautionary approach aims to avert the risk of harm to fish resources. Limits were accordingly set for many commercially exploited fish species, in order to minimize fishing mortality and prevent severe depletion of stock biomass. For example, each year the EU Council of Ministers sets the total allowable catch (TAC) for stocks in the waters of the European Union, thus stipulating how many tonnes of a fish species may be caught in a specific area. The precautionary approach also takes the dynamics of the stocks into account, because environmental conditions can change the size of a stock. If there is little food available, for instance, the productivity of the stock declines accordingly. The biomass shrinks. If there is plenty of food, the productivity rises and the stock grows. The fishing industry must take these stock fluctuations into account and adjust catch volumes accordingly, not con-tinue to catch the same amount of fish. Such adjustment should be achieved using several benchmarks and limit reference points, terms which are still used for fishing according to the precautionary approach:
- BIOMASS PRECAUTIONARY APPROACH (BPA): It is difficult to predict the status of a fish stock, for several reasons. One is that the current fishery and research data used to calculate fish abundance is unreliable. Another is that all mathematical analysis programs are to a certain extent inexact. There is no 100 per cent certainty. The Limit Biomass (BLIM) is therefore too risky as a reference point. The probability is too great that the biomass does actually fall below this limit in any given year, threatening population growth. In line with the precautionary approach, therefore, it was decided to stipulate a limit reference point which takes such uncertainties into account. This limit is known as the Biomass Precautionary Approach (BPA). It is designed to guarantee that the biomass does not inadvertently fall below the BLIM-threshold. The area between BLIM and BPA is therefore a buffer zone, as it were. Today it is still the most important benchmark used to ascertain the health of many stocks.
- PRECAUTIONARY FISHING MORTALITY RATE (FPA): As the biomass is a fundamentally unreliable and changeable variable which cannot be directly influenced by human activity, it is not practical to stipulate a limit reference point for the fisheries which takes only the biomass as its parameter. Therefore there is an additional limit reference point which is derived from the BPA. This is known as the FPA. This point specifies the maximum fishing mortality rate permissible to stay below the BPA. Scientists use the FPA to calculate the maximum annual catch tonnages for the next season. However, this is only possible when the current status of the stock is known. For this purpose the researchers use the catch data of past years, which provides information on the long-term development of the stock. They then add the catch data from the current season along with the data gathered by research vessels. Finally, they must make assumptions for the current year for which no fisheries data is yet available. From these figures they use mathematical models to estimate the status of a stock for the next season, which forms the basis of their catch recommendations for the fisheries. Adhering to these maximum catch tonnages ensures that fishing remains within the FPA. >