Publication Abstract

Next Years TACs - How scientists estimate and forecast fish stocks

(this article was first published in Fishing News, 12 November 1999)

The system for managing fisheries in the North Atlantic, and in many other fisheries around the world, depends on setting total allowable catches (TACs). To do this an estimate needs to be made of the number of fish in the sea. Joe Horwood and John Casey explain how fisheries scientists make those estimates.

If we think about cod in the North Sea, the starting point for the number of fish is the extent and success of spawning each year. Each surviving female cod may produce several million eggs each year. The vast majority of these will not survive to become new young fish (recruits) because they will be eaten by other fish or perish in other ways. But a large number will survive to recruit to the fishery.

These recruits will, in turn, be reduced as a result of many being eaten by other fish. As they get bigger this hazard may decline but predation continues and numbers will also fall because of disease and age. In a natural state with no fishing activity the mature North Sea cod stock might have an age structure as in Figure 1.

The effect of fishing is to remove numbers of fish, which leaves the age structure looking quite different (see Figure 1).

Figure 1. The numbers of mature North Sea cod (in millions) by age in an unfished stock (vertical bars) and the average numbers at age with current levels of fishing (bold bars).

How are these numbers estimated each year? Fisheries scientists have several main ways of estimating the figure but the most common method uses information on: tonnes landed, commercial and scientific catch rates (catch-per-unit-effort) and scientific catches of pre-recruit fish.

For North Sea cod this data is put together and studied by scientists from seven countries each autumn. They meet in an ICES working group with a specific task of making assessments for North Sea cod and various other stocks.

For cod the catch data comes from Belgium, Denmark, Färoe Islands, France, Germany, Netherlands, Norway, Poland, Sweden and the UK. This data is broken down into the numbers of fish caught at each age. This is done using the results of widespread sample measurements of landings so that the spread of ages and size are known.

About 175,000 cod are measured for length and 35,000 have their ages checked from otolith (bone) measurements. The samples are used to estimate the spread of the total catch by age. Information on levels of discards is also available from periodic surveys. Data has been prepared in this way by ICES over a long period. More recent years are shown in Table 1 (The Working Group soon meets to update the information).

The trends in landings by age can also be checked against the trends in the catch-rates, by age, (e.g. kilogrammes of cod age two years caught per hours fishing) and this gives estimates of trends in the size of fish stocks of different ages.

For the North Sea cod fleet catch rates are used from: English trawlers; English seiners; two Scottish trawl fleets; Scottish seiners; French trawlers; the English research vessel survey; the Scottish research vessel survey; and a composite of other international surveys conducted at the same time of year by research vessels from seven nations.

Using all this data the Working Group can make comparisons and say if the stock is at a high or low level relative to its past. It can also calculate the current size of the stock. But more information and calculations are needed to estimate what level of the catches would be wise in the future and it is that figure which is of immediate interest to managers and fishermen.

Catches in 1999 will depend upon the numbers and size of fish left after fishing, and natural death, in 1998.

This information can be obtained from the above analysis. Catches will also depend upon the ‘recruitment’ of young fish entering the fishery for the first time in 1999.

Fishermen catch only a small proportion of one-year old cod, because they escape from nets, being too small to be caught, for most of the year. But many age-two cod are caught. These young cod typically comprise about 30% of the catch in weight and 50% of the numbers. It is therefore very important to know how many there are to accurately predict potential catches for the following year.

Research vessels sample these young and small fish, using nets smaller than those permitted for use by the commercial fleets. In the North Sea, estimates of the young fish are obtained from research surveys conducted by: Denmark, England, France, Germany, Netherlands, Norway, Scotland and Sweden. The results are put together by the ICES Working Group.

The results of each year’s survey of one-year-old cod by CEFAS’ research vessel Cirolana are shown in Figure 2. It shows the great year to year variability of the cod. In particular, it shows the large 1996 year class, which would have been noticed by fishermen in their own catches. It also shows the extremely low 1997 year class, which is causing a poor cod season this year.

Figure 2. Numbers of age one cod caught per 100 hours fishing from the CEFAS North Sea annual fishing survey. A large year to year variability is apparent. Of current interest is the relatively large number of cod from the 1996 year class (age 1 in 1997), followed by the scarcity of cod born in 1997. The next CEFAS survey is from 18 August to 20 September.

Bringing together all the data on catch and catch rates added to the estimates of the numbers of recruiting young fish, allows the Working Group to estimate what is available for the following year’s catch, and the effect of that catch on the size of the stock.

Advice of stock levels and catches.

The Working Group report goes directly to the ICES Advisory Committee on Fishery Management (ACFM). The ACFM usually meets in late October.

ACFM comments on the state of the stock, and whether it is within safe biological levels. The basis for the evaluation is now done using the ‘precautionary approach’. (The ‘limit’ and ‘precautionary’ levels of stock size and fishing rates are explained in a separate cefas handout entitled ‘The Precautionary Approach’).

The ACFM also advises on options for different catches that would be taken the following year with different levels of fishing. It advises whether such rates of fishing are within the precautionary approach to management. The most useful advice for fishermen and fisheries managers is the maximum TAC consistent with a precautionary approach, and ACFM usually will ensure it gives this option.

The advice is sent to the EU Commission and governments about mid-November. CEFAS briefs the fishing industry as soon as it can after that.

Credibility of advice

Good fisheries science is fundamental to good management, and so is the credibility of that science.

The credibility of the science has taken a bit of a battering in recent years. Perhaps the most important reason for this is the importance of, and lack of confidence in, the basic catch data.

Improvements in the accuracy of the data are still needed, since inaccurate estimates of the landings and the catches most directly affect the estimate of the TACs. We also need more and consistent information on the levels of discards.

However, the poor state of many of our stocks can be seen without detailed science.

For example, catch rates (as tonnes caught per 100 hours fished) of sole in the Bristol Channel have dropped by over 50% in 20 years (Figure 3). Another example is from our North Sea surveys.

Figure 3. Catch rates Bristol Channel sole by British vessels from 1925. The first series are from steam trawlers, the second from motor trawlers, and the third from otter trawlers corrected to a standardised effort. Modern beam trawlers are landing about 0.5 tonnes per 100 hours from the Bristol Channel.

Over the past five years more than 10,000 cod have been caught. Those aged four and over account for about 1% - this is the breeding population. The future of the stock would be much more certain if the numbers of these older fish were higher.

While we can argue about the detail of the precise size of next years TAC, scientists and fishermen do seem to be in general agreement that overall state of stocks need to be improved.

Dr Joe Horwood is MAFF’s Chief Fisheries Science Adviser.
Dr John Casey is a member of the ICES North Sea Assessment Working Group, and the Scientific, Technical and Economic Committee for Fisheries (STECF).