Global warming and fish stocks: Winter spawning of Baltic sprat (Sprattus sprattus) as a possible future scenario Gerd Kraus1, Volker Mohrholz3, Rüdiger Voss1, Miriam Dickmann3, Hans-Harald Hinrichsen1, Christian Möllmann2, Friedrich W. Köster2, Dietrich Schnack1, Jens-Peter Herrmann4, Andreas Lehmann1, Axel Teming4, Daniel Stepputtis1, Jürgen Alheit3 1Leibniz Institute of Marine Sciences, Kiel, Germany 2Danish Institute for Fisheries Research, Charlottenlund, Denmark 3Institute for Baltic Sea Research, Rostock-Warnemünde, Germany 4Institute of Hydrobiology and Fishery Science, Hamburg, Germany

THE BALTIC SEA Gotland Basin Gdansk Deep Bornholm Basin m to North Sea Study area

Approach Resolve the timing of gonadal maturation and spawning of Bornholm Basin sprat at high resolution and covering the full annual reproduction cycleResolve the timing of gonadal maturation and spawning of Bornholm Basin sprat at high resolution and covering the full annual reproduction cycle Parallel investigation of egg and larval abundance in the field to resolve egg and larval productionParallel investigation of egg and larval abundance in the field to resolve egg and larval production Corresponding 3D-hydrographic measure- ments to resolve ambient conditions for the spawning stock, eggs and larvaeCorresponding 3D-hydrographic measure- ments to resolve ambient conditions for the spawning stock, eggs and larvae

Sampling Temporal coverage: April 2002 to April 2003April 2002 to April Surveys in 2-8 week intervals14 Surveys in 2-8 week intervalsFish: 4 Hydroacoustic Transects4 Hydroacoustic Transects Min. 2 trawl hauls per ICES square (~15nm horizontal resolution)Min. 2 trawl hauls per ICES square (~15nm horizontal resolution) Sprat sexed & maturity staged per haul and cm10-20 Sprat sexed & maturity staged per haul and cm Calculation of weighted average date specific sex ratios and maturity stagesCalculation of weighted average date specific sex ratios and maturity stagesIchthyoplankton: 45 Bongo net stations (500 µm)45 Bongo net stations (500 µm) ~10nm grid point distance~10nm grid point distance Stage specific abundanceStage specific abundance values for eggs and larvae values for eggs and larvaeHydrography: Time series measurementsTime series measurements -Dars Sill and Central Bornholm Basin -Dars Sill and Central Bornholm Basin Synoptic 3D-hydrographic measurementsSynoptic 3D-hydrographic measurements -Vertical resolution <1m -Vertical resolution <1m -Horizonal resolution ~ 10 nm -Horizonal resolution ~ 10 nm

Reproduction cycle of Baltic sprat I JanFebMarAprMayJunJulAugSepOctNovDec Spawning intensity Available energy Temperature

Sprat gonadal maturation cycle during 2002 / 2003

Temperature time series in the Central Bornholm Basin annual temperature minimum in the intermediate layer SST August

54° 42.00‘ N 12° 42.00‘ E TS patterns at the Darss Sill ACB

Distribution of the youngest egg stage during November 2002 Temperature surface derived from maximum temperaturesTemperature surface derived from maximum temperatures No eggs in areas with max. temperatures below 12°CNo eggs in areas with max. temperatures below 12°C Majority of sprat distributed in >11°C layerMajority of sprat distributed in >11°C layer Hydroacoustic Abundance Temperature [°C] Depth [m]

Distribution of the youngest egg stage during January 2003 Uneven egg distribution, larger abundances on the southern and south-eastern slopes of the BasinUneven egg distribution, larger abundances on the southern and south-eastern slopes of the Basin No eggs in areas with max. temperatures below 7°CNo eggs in areas with max. temperatures below 7°C Majority of sprat distributed in >6°C layerMajority of sprat distributed in >6°C layer Depth [m] Temperature [°C] Hydroacoustic Abundance

Distribution of the youngest egg stage during March 2002 Largest egg abundances in the southern Central BasinLargest egg abundances in the southern Central Basin No eggs in areas with max. temperatures below 6.0°CNo eggs in areas with max. temperatures below 6.0°C Temperatures below 3-4 °C avoided by spratTemperatures below 3-4 °C avoided by sprat Majority of sprat distributed in >6°C layerMajority of sprat distributed in >6°C layer Hydroacoustic Abundance Temperature [°C] Depth [m]

Sprat egg abundance: Seasonal dynamics 2002 / 2003 Peak spawning 2002 in mid April (150 eggs Ia/m²) Peak spawning 2002 in mid April (150 eggs Ia/m²) Spawning ceased by mid July 2002 (0.2 eggs Ia/m²)Spawning ceased by mid July 2002 (0.2 eggs Ia/m²) Spawning restarted in November 2002 (1.2 eggs Ia/m²), reached 4.5 eggs Ia/m² by mid January 2003Spawning restarted in November 2002 (1.2 eggs Ia/m²), reached 4.5 eggs Ia/m² by mid January 2003 Contribution of winter spawning to annual egg production is negligibleContribution of winter spawning to annual egg production is negligible Early spawning season in 2003 (140 eggs Ia/m² in mid March)Early spawning season in 2003 (140 eggs Ia/m² in mid March)

Sprat larval abundance and feeding: Seasonal dynamics 2002 / Peak larval abundance 2002 in May (45 larvae/m²) Peak larval abundance 2002 in May (45 larvae/m²) Winter spawning yielded low larval abundance (0.5 – 1.0 larvae/m²)Winter spawning yielded low larval abundance (0.5 – 1.0 larvae/m²) No feeding in March 2003No feeding in March 2003 Low larval abundance combined with low feeding intensity in winter 2002/2003Low larval abundance combined with low feeding intensity in winter 2002/2003

JanFebMarAprMayJunJulAugSepOctNovDec Reproduction cycle of Baltic sprat II Spawning intensity Available energy Temperature ?

Seasonal condition dynamics of sprat 2002 / 2003 Average Weight at mean length during spawning time considerably higher in 2003Average Weight at mean length during spawning time considerably higher in 2003 More Pseudocalanus available in winter/spring 2003More Pseudocalanus available in winter/spring 2003 JanFebMarchApril Abundance [(n/m²)/1000] Pseudocalanus C4-Adult Date Weight at mean length 9,0 9,3 9,6 9,9 10,2 10,5 10,8 11,1 11,4 11,7 12,0 12,3 12,6 Mean Apr-Jul [g]

Conclusions Sprat winter spawning, a possible effect of the 2002 warm summer inflow, is evidenced by maturity and egg abundance dataSprat winter spawning, a possible effect of the 2002 warm summer inflow, is evidenced by maturity and egg abundance data The contribution of winter spawning to annual egg and larval production is negligibleThe contribution of winter spawning to annual egg and larval production is negligible A possible depletion of energy reserves due to winter spawning could be compensatedA possible depletion of energy reserves due to winter spawning could be compensated The sprat spawning season started earlier in 2003 than in 2002 in the remaining warm summer inflow waterThe sprat spawning season started earlier in 2003 than in 2002 in the remaining warm summer inflow water The high condition and early spawning in 2003 may be an effect of increased Pseudocalanus abundance in winter spring 2002/2003 The high condition and early spawning in 2003 may be an effect of increased Pseudocalanus abundance in winter spring 2002/2003 Winter spawning may be critical to sprat reproduction under food limited winter/spring situations Winter spawning may be critical to sprat reproduction under food limited winter/spring situations

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