1 120 120 100 100 80 80 recruitment recruitment 60 60 40 - - PDF document

▶

Apr 09, 2023 354 likes •416 views

120 2.5 ICES Workshop on Limit and Target Reference 100 2 recruitment as O-ringers in bllions Points (WKREF) SSB in million tons 80 1.5 Gdynia, Poland, 29 January 2 February 2007 Bpa 60 1 Blim 40 0.5 20 0 0 1960 1965 1970

SLIDE 1

20 40 60 80 100 120 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 0.5 1 1.5 2 2.5 recruitment as O-ringers in bllions SSB in million tons Blim Bpa

ICES Workshop on Limit and Target Reference Points (WKREF) Gdynia, Poland, 29 January – 2 February 2007 Terms of reference for the meeting review and update the biological basis of limit reference points for fish stocks in the ICES area, taking into account the possible effects of species interactions and regime shifts. As a minimum the limit reference points for North Sea herring, Norwegian spring spawning herring, North Sea cod, Baltic cod and Kattegat sole stocks; Blim: limit stock size

minimum spawning stock required for the production of average

recruitment

r
stock level “below which recruitment is impaired”

Bpa: precautionary stock size

stock size at which the risk of the stock accidentally being below

Blim is minimal

Reference points North Sea herring

Type Value Technical basis Blim 800 000 t below this value poor recruitment has been experienced. Bpa 1 300 000 t part of a harvest control rule based on simulations.

20 40 60 80 100 120 0.5 1 1.5 2 2.5

recruitment spawning stock in million tons A B

SLIDE 2

20 40 60 80 100 120 0.5 1 1.5 2 2.5

recruitment spawning stock in million tons

20 40 60 80 100 120 0.5 1 1.5 2 2.5

recruitment spawning stock in million tons

spawning stock

Blim Bpa

recruitment Example 1 The danger of going below Blim is very high. Consequently, it is wise to set a precautionary Bpa at a sufficiently large distance from

Blim. This will minimise the risk of

stock collapse.

spawning stock

Blim Bpa

recruitment

Example 2 Although recruitment starts declining below Blim, the initial decline is not very steep. The effect on recruitment of reducing the stock a little bit below Blim is small. Consequently, setting an extra safety margin (Bpa) above Blim seems to be unnecessary.

spawning stock

Blim Bpa

recruitment

Example 3 Here we have a dome-shaped

relation. According to the

ICES definition, Blim is defined as the top of the

curve. If we add a safety

margin and put Bpa at some distance to the right side of Blim, we find that recruitment will be reduced in the range

f stock size that we consider

acceptable (to the right-hand side of Bpa.

SLIDE 3

21.37 2 4 6 16 12 10 8 2 20 18 14 8 6 4 14 12 10 16 Spawning stock size x 100,000 tons Recruitment as 0-group in billions 21.37 2 4 6 16 12 10 8 2 20 18 14 8 6 4 14 12 10 16 Spawning stock size x 100,000 tons Recruitment as 0-group in billions

1952-1974

20 40 60 80 100 120 140 1 2 3 4 5 6

500000 1000000 1500000 2000000 0 e+00 4 e+07 8 e+07 SSB Recruitment

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

500000 1000000 1500000 2000000 0 e+ 00 4 e+07 8 e+ 07 SSB Recruitm ent

1 9 6 1 9 6 1 1 9 6 2 1 9 6 3 1 9 6 4 1 9 6 5 1 9 6 6 1 9 6 7 1 9 6 8 1 9 6 9 1 9 7 1 9 7 1 1 9 7 2 1 9 7 3 1 9 7 4 1 9 7 5 1 9 7 6 1 9 7 7 1 9 7 8 1 9 7 9 1 9 8 1 9 8 1 1 9 8 2 1 9 8 3 1 9 8 4 1 9 8 5 1 9 8 6 1 9 8 7 1 9 8 8 1 9 8 9 1 9 9 1 9 9 1 1 9 9 2 1 9 9 3 1 9 9 4 1 9 9 5 1 9 9 61 9 9 7 1 9 9 8 1 9 9 9 2 2 1 2 2 2 3 2 4

SLIDE 4

Conclusion on North Sea herring No basis for changing Blim based on current analysis. SGRECVAP results could be basis for revisiting reference point. The distance between Bpa and Blim defines a risk and should be evaluated in the context of harvest control rules in consultation with stakeholders and managers.