The Evolution of Bottom Trawling Impact on North Sea Flatfish Populations Adriaan Rijnsdorp, Jan Jaap Poos, Georg Engelhard, Katrien Lescrauwaet IFF10, St Malo, 12-16 November 2017
Overview History of exploitation Density-dependent processes Long-term effects of fishing ● Habitats ● Population biology 2
History of exploitation & management Data sources ● Fish remains (archaeological excavations) Ervinck & van Neer
Evidence historic fishing (written sources): rij 16Adriaen Coenen (1578) Visboeck Bennema & Rijnsdorp 2015 Fish Res, 161: 384-399 N Z Beam traw l fishery along the coast
Controversy about use of beam trawls Letter 19 April 1583 to Willem van Oranje “...Netten met Hout openghestelt...hebbende onderaen hangen wel sestien ofte seventien ponden Loots, met groote steenen Adverse effects on seafloor daer beneffens, and food for fish .... Deur de gront gaen strijcken, ....den kleynen Schol ende Bot vangen, nemende mede uyt den gront op het Aes waer mede de Visch ghevoedt wordt, .... “Overfishing” .... Indien sulcks gecontinueert worde, dat men in korten tijdt Depletion of fish stock ghelijcke Visch niet meer vangen en soude” De Groot, 1984 Ocean Management 9: 177-190
Beam trawl fishery in coastal waters (17th – 18th century) Mesdag: Pinken in de branding
Expansion beam trawl fishery (sailing) sail 1850: ± 200 1890: 2200 motor Steam (since 1890)
Changes in catch rate (NL, England) Bumb boats Netherlands Smacks England Posthumus & Rijnsdorp (2016)
Reconstructed stock biomass Rijnsdorp and Millner 1996 ICES JMS 53: 1170-1184 ICES WGNSSK
Density-dependent processes Juvenile phase ● DD mortality after settlement ● DD reduced growth nursery grounds Residual length 0-group Mort alit y: Modin & Phil, 1994. NJSR 32: 331–341 Van der Veer et al. Growt h: Teal et al., 2008 Modin & Phil, 1994. NJSR 32: 331–341 Nash & Geffen, 2007. MEPS 344: Teal et al., 2008 MEPS 358: 219-230 Ciot t i et al., 2013 MEPS 475: 213-232
Density-dependent processes Adult phase Comparison growth rate annuli in otoliths of ● DD-growth? present day samples with Medieval samples Bolle et al (2004) J. Sea Res. 51: 313-328
Density-dependent processes Adult phase ● No dd-efect in reproduction (relative fecundity) 12 Rijnsdorp et al (2005) Can J Fish Aquat Sci 62: 833-843
Changes in maturation reaction norm & maximum body size (Linf) Maturation midpoint Age 4 (cm) 50 Linf= 9 0 cm Linf= Linf= 5 3 cm 40 4 8 cm 30 20 10 0 1890 1910 1930 1950 1970 1990 2010 Grift et al. 2003 MEPS; van Walraven et al. 2010 JSeaR
Food web effects of bottom trawling van Denderen PD, van Kooten T, Rijnsdorp AD. 2013 When does fishing lead to more fish? Community consequences of bottom trawl fisheries in demersal food webs. Proc R Soc B 280: 20131883.
Trawling effects on food availability Prey to fish biomas 0.4 155 a Wt at standardized length (g) 0.3 150 0.2 145 0.1 140 0.0 Pleuronectes platessa 0 2 4 6 8 Trawling intensity (n/ year) 15 Hiddink et al. (2016) J Applied Ecol 53: 1500-1510
Long-term effects of trawling Phenotypic plasticity ● Density-dependence ● Temperature effects ● Eutrophication ● Trawling effects on food availability Changes in sea-bed habitats Evolutionairy changes 16
Fisheries-induced evolution Expectations (modelling evolution of growth, maturation and reproduction) < 0 if t t d w • Evolving traits: Emergent traits: = − − mat 3/4 aw bw ≥ a, c, tmat decrease in max size & size / age at 1st maturity cw if t t , d t mat Genetic traits Start exploitation Mollet et al. (2015) MEPS; Mollet et al. (2016) CJFAS
Managing fisheries-induced evolution Size at maturation (reaction norm) Flat- topped exploitation Mollet et al. (2016) CJFAS 73: 1126-1137
Managing fisheries-induced evolution Protect large / old fish Size at maturation (reaction norm) Full dom e- shaped Half dom e- shaped Flat- topped exploitation Mollet et al. (2016) CJFAS 73: 1126-1137
Conclusions Population regulation • Importance of nursery phase • Density-dependence in adult growth questionable, but what is happening now SSB is at historic high?
Conclusions • Population regulation • Importance of nursery phase • Density-dependence in adult growth questionable, but what is happening now SSB is at historic high? Juvenile growth increased due to eutrophication (and perhaps bottom trawling) • affects size and age at maturation (phenotypic plasticity)
Conclusions • Population regulation • Importance of nursery phase • Density-dependence in adult growth questionable, but what is happening now at historic high SBB? • Juvenile growth increased due to eutrophication (and perhaps bottom trawling) • affects size and age at maturation (phenotypic plasticity) Fisheries-induced evolution • Fisheries selection low until ~ 1880 • Management advise: protect large / old fish
Conclusions • Population regulation • Importance of nursery phase • Density-dependence in adult growth questionable, but what is happening now at historic high SBB? • Juvenile growth increased due to eutrophication (and perhaps bottom trawling) • affects size and age at maturation (phenotypic plasticity) • Fisheries-induced evolution • Fisheries selection low until ~ 1880 • Management advise: protect large / old fish Change in sea-bed habitats
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