The Status and Potential of Europe’s Atlantic Fisheries – Part 1

The Status and Potential of Europe’s Atlantic Fisheries

Part 1

 

This is the first of a 3 part series on Europe’s Atlantic Fisheries. In part 1 we explore the status and potential of stocks that have been subjected to scientific analysis by the ICES Science Process.

In part 1 we explore the status and potential of stocks that have been subjected to scientific analysis by the International Council for the Exploration of the Sea (ICES). The ICES process of stock assessment has teams of scientists use all available data to estimate the abundance and potential of a stock, this is the best indicator we have for overall stock health. The stocks presented in Part 1 constitute almost all (>95%) of the catch in the European Atlantic; the numerous, very small stocks that compose the rest of the catch are not evaluated by ICES. Part II will explore the status and potential using a statistical method that also estimates the status of the many very small stocks.

The Northeast Atlantic is one of the most productive fisheries regions in the world and the place where most modern industrial fishing originated. Statistics on catch in this region are available from FAO as region 27, which almost totally overlaps with the ICES statistical areas shown in the Figure 1 below.

Figure 1. ICES Statistical Areas in the NE Atlantic

Figure 1. ICES Statistical Areas in the NE Atlantic

The total catch reported to FAO is shown in Figure 2 and, since the 1970s, has fluctuated between 9 and 13 million metric tonnes. There has been a consistent decline since about 2000, so the catch is now about 8 MMT.

Figure 2. Total catch for FAO region 27 from the FAO catch data base.

Figure 2. Total catch for FAO region 27 from the FAO catch data base.

174 individual stocks are assessed through the science process of ICES and these assessments generally provide the history of catch, total abundance, spawning stock size, fishing mortality rate and annual recruitment. These data are available in the RAM Legacy Stock Assessment Database.

The total catch represented in the assessments in the RAM Legacy Database for NE Atlantic is shown in Figure 3. From 1950 to 1980 the ICES assessed stocks in that period covered roughly ½ of the total catch reported to FAO since few ICES assessments begin as early as 1950. However, since 1980 the assessments have covered 90% of the total catch reported to the FAO from the NE Atlantic.

Figure 3. Total catch from assessments in the RAM Legacy Database for NE Atlantic fisheries.

Figure 3. Total catch from assessments in the RAM Legacy Database for NE Atlantic fisheries.

Thus, through the scientific process of ICES we have a good idea of the trends in the status of the fish stocks that make up most of the food production from the NE Arctic with almost total coverage of the abundance of stocks since 1980.

A key feature of fisheries stocks and catch in the NE Atlantic is the great diversity in the size and potential catch of the fish stocks. Although there are landings data for 465 species or taxa of fish and invertebrates in the FAO catch data base for the NE Atlantic, 4 species (Herring, cod, mackerel and capelin) constitute over 50% of the catch (in weight) since 1950, 14 species constitute 80% of historical catch, and 50 species constitute 95% of the historical catch. The 174 stocks assessed by ICES (many of the same species from different regions) constitute 90% of the potential catch.

This is illustrated below where the species are ranked in order of catch and the cumulative percentage of catch up to and included that species is shown. The largest species, Atlantic herring, constitutes 20% of the total catch; adding Atlantic cod brings us to about 30% of total catch and the first 20 species constitute 85% of total catch.

Figure 4. The taxa are sorted in order of their total catch from 1950-2013 and the fraction of the total catch represented by each taxon and all taxa with larger catch are shown.

Figure 4. The taxa are sorted in order of their total catch from 1950-2013 and the fraction of the total catch represented by each taxon and all taxa with larger catch are shown.

So in any analysis dealing with the potential yield of these fisheries we must be clear whether we are giving all stocks equal weight or considering the importance of the larger stocks. If our concern is yield and food production, we must consider the size of the stock.

Trends in abundance

For the assessed stocks, trends in abundance are shown in figure 5.

Figure 5. Estimated total biomass of stocks assessed by ICES, corrected for stocks where a biomass estimate was not available.

Figure 5. Estimated total biomass of stocks assessed by ICES, corrected for stocks where a biomass estimate was not available.

The total abundance estimate declined from 110 MMT in the early 1950s to a low of 46 MMT in 1990 but has grown to 62 MMT as of 2014.

So why have stocks declined and then increased? The answer is quite simple: fishing pressure was too high. With improved management, fishing pressure has been reduced and abundance is increasing. The fraction of the total biomass of fish that was caught increased from about 10% in 1950 to an average of 25% from 1970-1995 but has since declined to a little less than 15% (Figure 6).

Figure 6. Fraction of total biomass caught.

Figure 6. Fraction of total biomass caught.

A central feature in most national and international legislation is the concept of Maximum Sustainable Yield (MSY) as the goal of MSY management is to maximize long-term catch and stock health. For any given fish stock, there are three associated quantities: MSY, Umsy, and Bmsy. MSY is the average tonnage of fish that can be caught each year in the long term on a sustainable basis. Associated with MSY is a rate of harvesting, commonly called Umsy. Umsy is the fraction of the population harvested each year that will produce MSY. Finally, there is Bmsy, the average biomass the stock would be at if it was harvested at Umsy. For most stocks we can estimate MSY, Bmsy and Umsy. Plotting Umsy against Bmsy gives what is known as a Kobi plot seen in Figure 7.

Figure 7. Stock abundance and fishing pressure relative to the levels that would achieve MSY.

Figure 7. Stock abundance and fishing pressure relative to the levels that would achieve MSY.

In this plot, each stock is a circle. The size of the circle is proportional to the MSY for the stock, so big stocks are big circles and small stocks are small circles. To maximize long term sustainable yield, the stocks should be near 1 on both axes, the place where the two orange lines cross, which divide the space into four quadrants.

Quadrant A (green) is where abundance is high and fishing pressure low; stocks in this quadrant are underfished, meaning there is potential for more food and money to be collected from these stocks. Quadrant B (orange) is low fishing pressure but also low abundance; stocks in this quadrant were probably overfished and on the road to recover since fishing pressure is low. Quadrant C (red) is the area of most concern, where abundance is low and fishing pressure high. Finally, Quadrant D (purple) is stocks that are still above Bmsy but have fishing pressure higher than MSY; stocks in this quadrant are fine for now, but may warrant management interventions to get them closer to the intersection of Umsy and Bmsy. Generally, stocks above the horizontal orange line will need to have fishing pressure reduced to maximize long term yield, while stocks below the horizontal orange line would need to have fishing pressure increased to maximize long term yield. Stocks in Quadrants B and C would increase in abundance (move to the right) if fishing pressure is set at Umsy (1 on the Y axis). The black + mark denotes the median B/Bmsy and U/Umsy. The median values denoted by the large + mark shows things are good on average, but lots of stocks need lower fishing pressure, and if we want to maximize yield lots of stocks need more fishing pressure. Stocks in quadrant B need to rebuild to achieve their potential, but would rebuild even if fishing pressure is increased to Umsy. They will rebuild faster if fishing pressure is kept below Bmsy.

Fisheries management is largely about controlling the rate of harvest; what managers generally seek to do is keep harvest rates at or near Umsy. Figure 8 below shows the median U divided by Umsy and is quite close to MSY target exploitation rate and biomass, but many individual stocks are well above or below that level. In this graph, as in the next few, each stock is given equal weight, whereas in earlier figures dealing with total biomass, the large stocks had much more influence.

Figure 8. Median U/Umsy by year for stocks that were assessed in that year.

Figure 8. Median U/Umsy by year for stocks that were assessed in that year.

Here we see that from about 1970 to 2000 the median fishing mortality rate was about 1.4 times Umsy (too high to produce maximum yield), but, beginning about 2000, the fishing mortality rate declined so that it is now at or near Umsy. The decline in fishing pressure we see in this graph is quite a bit stronger than the decline seen in Figure 6. This is because the really large stocks in the NE Atlantic, which dominate Figure 6, were not as overfished and have seen less reduction in fishing pressure. But, on average, across assessed stocks, fishing pressure has declined about 33% since the mid 1990s.

Using the Bmsy reference point, we see stocks declined to below Bmsy until about 2000 then began to recover and are now, on average, just below Bmsy.

Figure 9. The trend in stock biomass relative to Bmsy.

Figure 9. The trend in stock biomass relative to Bmsy.

Potential yield

So what is the potential of these fisheries to produce more fish? The answer is quite simple, stocks that are fished too hard need to be fished less and stocks that are lightly exploited need to be fished harder. We can calculate how much yield would be produced at current fishing pressure for each stock, and how much it would increase for stocks that need to have fishing pressure reduced, and how much potential increase from stocks that would need to be fished harder.

We can get an idea of this by comparing the size of the stocks above and below the horizontal orange line in Figure 7. We see far more large stocks below the line (fishing pressure too low) compared to large stocks above the line. Figure 10 shows how much yield would be obtained at current fishing pressure, almost exactly 10 million tons, and how much could be increased by fishing those stocks above the line less (0.7 MMT), and how much catch could be increased by fishing harder on stocks below the line (2.9 MMT). Though management can and should reduce fishing pressure in stocks above the line, to maximize yield we would need to increase fishing pressure in underexploited stocks which will create more jobs, provide more high quality nutrition, and increase revenue.”

Looking back at Figure 2, the current catch is only about 8 MMT although we estimate that at current fishing pressure, yield will rise to 10 MMT. Under current fishing pressure almost all of the stocks that are now below Bmsy will increase in abundance, and thus, catch will increase.

Figure 11 shows the Kobi plot shaded by what fraction of the potential yield is obtained at the current fishing mortality rate. Points shaded dark green achieve >90% of the potential yield, and different shades indicate decreasing proportion of potential yield being achieved.

To achieve maximum yield, stocks above 1.0 on the Y axis need to have fishing pressure reduced, and those below 1.0 should have fishing pressure increased. Clearly, both should be done and we estimate catch could rise to a total of 13.6 MMT.

Figure 10. Long term yield to be obtained under current fishing pressure, and how much could be obtained by fishing overexploited stocks less, and underexploited stocks more.

Figure 10. Long term yield to be obtained under current fishing pressure, and how much could be obtained by fishing overexploited stocks less, and underexploited stocks more.

Figure 11: Kobi plot with points shaded by fraction of potential yield obtained at current fishing mortality rate.

Figure 11: Kobi plot with points shaded by fraction of potential yield obtained at current fishing mortality rate.

Figure 12. The relative importance of different management actions in maximizing long term yield from NE Atlantic fisheries.

Figure 12. The relative importance of different management actions in maximizing long term yield from NE Atlantic fisheries.

Final Cautions

The calculations shown in Figure 10 and 12 are predicated on two key assumptions; (1) that the objective is to maximize the tonnage landed of each stock, and (2) that each stock can be managed individually.

Neither of these is likely true. Management goals are often a mix of maximization of yield, reduction of costs of fishing, decreased ecosystem impact, and a range of social goals. Society and fishery managers may choose to forgo some potential yield for other purposes, so the calculations in Figures 10 and 12 need to be regarded as “what if” questions. If the goal was only to maximize yield, Figures 10 and 12 show what would need to be done.

Many fish stocks are harvested in mixed stock fisheries where it is impossible to harvest each stock at its targeted exploitation rate. This is particularly true for demersal species caught by trawling (cod and haddock for example) where many species are caught in the same location with the same gear. Thus, it is generally not possible to apply a specified exploitation rate that is distinct for each stock.

 

This post was turned into a lecture by Ray Hilborn. Watch it below:

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2 Comments

  1. Could you provide a list of the species that are above and below target biomass and fishing levels? In other words, we can see generally how many stocks are above and below target rates and population sizes from these charts, but it would be helpful to add a list showing which stocks are which. Thanks for considering it.

  2. The calculations of potential yield are predicated on at least three key assumptions, not two as stated here. We can define a technical interaction between species as the catch of a particular species taken when fishing for other species and a biological interaction between species as the mututal effects they have on each other due to predation, competition etc. The estimates of potential yield shown here are predicated on a third assumption: that they are not affected by biological interactions. Some of the consequences of biological interactions on potential yield are explored in Andersen et al. Ecological Applications, 25(5), 2015, pp. 1390–1396 Biological interactions, which can have very large effects on potential yield, make it difficult to define what we mean by terms such as MSY, which do not include biological interactions.

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