13/02/2012 Recruitment variability in North Sea herring Pelagic RAC - - PDF document

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Recruitment failures in North Sea herring

Ad Corten

Pelagic RAC 8 February 2012

50 100 150

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 0-group x 109

recruitment North Sea herring

Recruitment variability in North Sea herring

1) insufficient egg production 2) long-lived predators 3) oceanic processes 3 possible causes of sustained low recruitment

0,5 1 1,5 2 2,5 50 100 150 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 spawning stock in million tons

recruitent as 0-group x 109

recruitment SSB

Period 1 Period 2

Comparison of two periods with low recruitment

0,5 1 1,5 2 2,5 50 100 150 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 spawning stock in million tons

recruitent as 0-group x 109

recruitment SSB

Period 1 Period 2

Reduced Atlantic inflow

Comparison of two periods with low recruitment

• mackerel
• horse mackerel
• herring ?

Long-lived pelagic predators in Period 2

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Experiments on cannibalisme at RIVO in 1983 (1)

Larvae 15 minutes after ingestion

Experiments on cannibalisme at RIVO in 1983 (2)

Larvae 30 minutes after ingestion

Experiments on cannibalisme at RIVO in 1983 (3)

Larvae 60 minutes after ingestion

Cannibalism in Sardinella maderensis (1) Cannibalism in Sardinella maderensis (2) Drift of larvae and migration of adults after spawning

4W 62 60 56 52 50 54 58 4E 8E 200 m

Shetland Orkney Central North Sea nursery area English Channel spawning area Aberdeen Bank drift larvae adults

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0,5 1 1,5 2 2,5 50 100 150 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 spawning stock in million tons

recruitent as 0-group x 109 recruitment SSB

Cannibalism explains low recruitment in recent period, and high recruitment in 1980 - 1985

High recruitment Low recruitment

Spawning stock and recruitment 1970 - 2009

20 40 60 80 100 120 0,5 1 1,5 2 2,5

recruitment Spawning stock in million tons

1970 - 2009

Hockey stick relationship used by ICES

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

Precautionary approach by ICES

20 40 60 80 100 120 0,5 1 1,5 2 2,5

recruitment Spawning stock in million tons

Blim Btrig Bpa

0,5 1 1,5 2 2,5 50 100 150 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 spawning stock in million tons

recruitent as 0-group x 109

recruitment and spawning stock

recruitment SSB

Blim

SSB dropping below Blim is not the end of the world Cannibalism at high stock sizes is also a management risk

20 40 60 80 100 120 0,5 1 1,5 2 2,5

recruitment Spawning stock in million tons Low egg production Cannibalism

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• Cannibalism offers a simple explanation for the low recruitment in

recent years, and also for the exceptionally good recruitment in the period 1980-1985 when adult stock size was low

• If adult stock size is maintained at levels > 1.5 million tons,

recruitment will probably stay below average and the North Sea herring stock will not produce its maximum yield

• ICES should consider the effects of cannibalism on recruitment,

and its consequences for management

Conclusions

Thank you for your attention

Period 1: scarcity of Atlantic plankton suggests reduced

• ceanic inflow into northwestern North Sea

0.2 0.4 0.6 0.8 1 1.2 1.4 1 2 3 4 5 6 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Metridia lucens Candacia armata Metridia lucens Candacia armata

Period 1

Period 2: no reduced oceanic inflow into North Sea

0.2 0.4 0.6 0.8 1 1.2 1.4 1 2 3 4 5 6 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Metridia lucens Candacia armata Metridia lucens Candacia armata

Period 2

Period 1: reduced Atlantic inflow into the North Sea probably slowed down transport

• f larvae to nursery

areas

4W 62 60 56 52 50 54 58 4E 8E 200 m Shetland Orkney Central North Sea nursery area English Channel spawning area Aberdeen Bank drift larvae

inflow

Spawning stock and recruitment 1960 - 2009

20 40 60 80 100 120

0,5 1 1,5 2 2,5 recruitment Spawning stock in million tons

1960 - 2009

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Changes in stock structure after 1975

Flamborough Whitby West Dogger Dowsing Northeast Bank Sandettie Seine Bay

1955-75 1976-92

Dieppe

Changes in plankton composition in central North Sea

Source: SAHFOS

Distribution of herring larvae in February 2011 Annual yield North Sea herring at two different levels of SSB

200000 400000 600000 800000 1000000 1200000 1400000 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Annual catch in tons Catch at SSB > 1.5 million t Catch at SSB 0.5 – 1.0 million t