NORTH SEA RAC – DEMERSAL WORKING GROUP
GLASGOW, 17 TH & 18 TH MAY 2005
DISCUSSION PAPER
LONG-TERM MANAGEMENT OBJECTIVES FOR FISHERIES: THE
WSSD TARGETS
OBJECTIVE OF THE PAPER
To facilitate a discussion on the implications of the WSSD target of bringing stocks to maximum sustainable yield levels by 2015 and the anticipated Commission paper on this subject.
To determine whether further work needs to be undertaken or commissioned.
WSSD TARGET
The World Summit on Sustainable Development (WSSD) agreed inter alia :
30. To achieve sustainable fisheries, the following actions are required at all levels:
(a) Maintain or restore stocks to levels that can produce the maximum sustainable yield with the aim of achieving these goals for depleted stocks on an urgent basis and where possible not later than 2015;
The WSSD objective is one of a package of measures to ensure sustainable fisheries, building on previously agreements such as those below.
RELATED OBJECTIVES
The FAO Code of Conduct FOR Responsible Fisheries has :
7.2 Management objectives
7.2.1 Recognizing that long-term sustainable use of fisheries resources is the overriding objective of conservation and management, States and sub-regional or regional fisheries management organizations and arrangements should, inter alia, adopt appropriate measures, based on the best scientific evidence available, which are designed to maintain or restore stocks at levels capable of producing maximum sustainable yield, as qualified by relevant environmental and economic factors, including the special requirements of developing countries.
The United Nations Agreement on Straddling Fish Stocks and Highly
Migratory Fish Stocks seeks measures to ensure sustainable fisheries, including :
[Article 5] ensure that such measures are based on the best scientific evidence available and are designed to maintain or restore stocks at levels capable of p. 1/5

producing maximum sustainable yield, as qualified by relevant environmental and economic factors,
The Agreement gives Guidelines for Precautionary Reference Points :
[ANNEX 2] For stocks which are not overfished, fishery management strategies shall ensure that fishing mortality does not exceed that which corresponds to maximum sustainable yield, and that the biomass does not fall below a predefined threshold.
GENERAL DEFINITION
There are a variety of different definitions, this is a relatively straight forward one :
The maximum sustainable yield (MSY) is the largest average catch or yield that can continuously be taken from a stock under existing environmental conditions.
The size of the stock at which MSY is achieved is termed the maximum sustainable yield level (MSYL).
The fishing mortality rate which would give MSY and MSYL is F(max) .
BENEFITS OF WSSD TARGETS
The objective is to ensure sustainable fisheries. In so doing this would manage fisheries at lower fishing mortalities and lower fishing effort resulting in greater security of the stock and the fishery, and greater stability of both.
CALCULATION OF MSYL FOR SINGLE STOCKS
There are two different approaches to calculating MSY and MSYL for any one stock.
Yield per Recruit
Over a large range of stock size, recruitment is often constant on average. The Yield and SSB can be calculated for a single recruit entering the fishery for a range of values of fishing mortality (F). Plots such as that below can be constructed and the value of F giving the maximum Yield per Recruit, Fmax, can be seen. The example is for
North Sea cod, and F(max) is 0.2. ICES presents this statistic for most stocks. p. 2/5

Figure 1 : Example of MSY calculated as Yield per Recruit (kg) and
SSB per recruit (kg) for North Sea cod (ICES 2004). F(msy) is at 0.2
Models combining a stock-recruit relationship
The above "model" assumes a constant recruitment at all stock sizes.
At low stock size this is unrealistic, and at very high stock size recruitment can also be depressed. If a stock-recruit relationship is build into the Yield per Recruit model then a different relationship between MSY and F may arise.
An example based on North Sea cod is given below. In this case the
F(msy) has moved considerably.
Figure 2: Illustration of a Yield v F relationship (yield on the left axis, and F on the bottom axis) if a general production model or a stock-recruitment relationship is used. This example is based on one developed for North Sea cod. Here F(msy) is at F = 0.8 p. 3/5

OTHER ISSUES STILL WITH A SINGLE STOCK APPROACH
Other factors can affect the calculation of F(msy) and the management towards MSYL.
The above relationships will change depending on whether or not discards are included in the calculation of "yield".
The above relationships also change with different mesh sizes.
Some stocks do not have a value for F(msy) at any reasonable level due to their mesh size and exploitation pattern. Often F(0.1) is used instead; F(0.1) is always a lower value than F(msy).
Variable recruitments need to be factored into an understanding of MSY and MSYL, including implications of potential ecological changes in the
North Sea.
FISHERY AND ECOLOGICAL IMPLICATIONS
Most North Sea demersal fisheries catch more than one species of fish.
It is unlikely that stock targets of F(msy) will be simultaneous achieved in such mixed fisheries. Does one manage to the lowest F(msy)?
However, the MSY from the mixed fishery as a whole may be different from the individual targets. Should one consider, in some sense, a mixed-fishery MSY, "over-exploiting" some and "under-exploiting" others.
This then leads to questions of how would one combine fish – by weight
– by value ?
Fish eat many other fish. Whiting are predators, and earlier scientific advice, based on multispecies models, indicated that single stock analyses of benefits of increased mesh size would be reduced if predation was taken into account. Some studies have indicated that greater productivity could be achieved by a degree of over-exploitation of top predators.
WHERE ARE THE FISHERIES NOW ?
ICES regularly reports upon the current status of stocks in terms of
F(current) and F(max) based upon the Yield per Recruit model. Most of the EU stocks are fished well above Fmax but not all. Based upon the
October 2004 advice some examples from the North Sea are: p. 4/5

F(current) F(max) cod 0.75-1.00 0.2 haddock 0.25 0.32 whiting saithe plaice sole unknown not given
0.29
0.71
0.44
0.25
0.17
0.34
It is assumed that fishing mortality and fishing effort are approximately proportional for towed gears, and so the difference between F(current) and F(max) represents the reductions in effort required to meet the
F(max) target.
CONCLUSIONS
WSSD targets should give greater security to the stocks and the fisheries.
They generally imply lower fishing mortality rates and lower effort on stocks.
However, there are a variety of ways in which the target fishing mortality can be calculated.
There are further unresolved issues associated with management of mixed fisheries. p. 5/5