The Evolution of Bottom Trawling Impact on North Sea Flatfish - - PowerPoint PPT Presentation

The Evolution of Bottom Trawling Impact

• n 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 daer beneffens, .... 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, .... .... Indien sulcks gecontinueert worde, dat men in korten tijdt ghelijcke Visch niet meer vangen en soude” Adverse effects on seafloor and food for fish “Overfishing” Depletion of fish stock

De Groot, 1984 Ocean Management 9: 177-190

Beam trawl fishery in coastal waters (17th – 18th century)

Mesdag: Pinken in de branding

sail motor Steam (since 1890)

Expansion beam trawl fishery (sailing)

1850: ± 200 1890: 2200

Changes in catch rate (NL, England)

Smacks England Bumb boats Netherlands

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

Growt h: 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

Residual length 0-group

Teal et al., 2008 Mort alit y: Modin & Phil, 1994. NJSR 32: 331–341 Van der Veer et al.

Density-dependent processes

• Adult phase
• DD-growth?

Bolle et al (2004) J. Sea Res. 51: 313-328

Comparison growth rate annuli in otoliths of present day samples with Medieval samples

Density-dependent processes

• Adult phase
• No dd-efect in reproduction (relative fecundity)

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Rijnsdorp et al (2005) Can J Fish Aquat Sci 62: 833-843

10 20 30 40 50 1890 1910 1930 1950 1970 1990 2010

Grift et al. 2003 MEPS; van Walraven et al. 2010 JSeaR

Linf= 5 3 cm Linf= 9 0 cm Linf= 4 8 cm

Changes in maturation reaction norm & maximum body size (Linf)

Maturation midpoint Age 4 (cm)

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

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2 4 6 8 140 145 150 155 Pleuronectes platessa a 0.0 0.1 0.2 0.3 0.4

Prey to fish biomas

Wt at standardized length (g)

Trawling intensity (n/ year)

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

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Fisheries-induced evolution

Expectations (modelling evolution of growth, maturation and reproduction)

mat 3/4 mat

if d if , d t t w aw bw cw t t t <  = − −  ≥ 

Mollet et al. (2015) MEPS; Mollet et al. (2016) CJFAS

• Evolving traits:

a, c, tmat Emergent traits: decrease in max size & size / age at 1st maturity

Start exploitation

Genetic traits

Managing fisheries-induced evolution

Mollet et al. (2016) CJFAS 73: 1126-1137

Size at maturation (reaction norm) Flat- topped exploitation

Managing fisheries-induced evolution

Protect large / old fish

Mollet et al. (2016) CJFAS 73: 1126-1137

Size at maturation (reaction norm) Full dom e- shaped Half dom e- shaped Flat- topped exploitation

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