Azour, Farivar

Bekkevold, Dorte

Christensen, Asbjørn

Deurs, Mikael van

Hüssy, Karin

Lundgaard, Louise Scherffenberg

Mosegaard, Henrik

Møller, Peter Rask

Nielsen, Kristian Ege

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 21, 2017Connectivity, growth and survival in a spatially structured fish population, which iscurrently managed as seven separate stock unitsNielsen, Kristian Ege; Azour, Farivar; Bekkevold, Dorte; Christensen, Asbjørn; Hüssy, Karin; Lundgaard,Louise Scherffenberg; Mosegaard, Henrik; Møller, Peter Rask; van Deurs, MikaelPublication date:2015Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Nielsen, K. E., Azour, F., Bekkevold, D., Christensen, A., Hüssy, K., Lundgaard, L. S., ... Deurs, M. V. (2015).Connectivity, growth and survival in a spatially structured fish population, which is currently managed as sevenseparate stock units. Introduction > Predicting stock recruitment for short lived industrial species can be challenging as they normally enter the fished stock early in life,  thereby limiting the chance to collect biological data before hand. > Nevertheless, predictions of stock recruitment in the industrial fiseries are of major importance since significant propor-tions of the catches are made up of newly recruited fish. > It is generally acknowledged that growth affects the survival of early life stages. For instance, growth determines predation risk and overwinter survival. > However, in spatially structured fish stocks (resident behaviour and habitat attachment), hydrographic dynamic forcing may complicate things considerably, as it affects retention and advection processes and influences the growth conditions experienced by the drifting larvae.  Pressent study > In the present study we have focused on the short lived lesser sandeel (Ammodytes marinus) population in the North Sea .This stock is spatially structured (currently mana-ged as a cluster of sub stocks) with strong affinity to distinct habitats distributed patchily in the North Sea.  > On the Dogger Bank in the North Sea, weights measured during a juvenile-survey and recruiment estimates from stock assessment models indicate that growth and recruit-ment success are interlinked (SEE GRAPH TO THE RIGHT). > We investigated [A] spatial variation in hydrodynamically influenced drift patterns of sandeel larvae, [B] the role of hatch date on growth, and [C] possible linkages between growth in early live and the probability of surviving the first winter.  Methods Location and samples • Larval samples: 1) Dogger Bank, 2) Central banks, 3) North Ea-stern banks. Year-classes 2006, 2008, 2009  • Otoliths: Pre and post winter 0- and 1-group sandeel  from Dogger-bank, year-classes 2008 and 2009 Larval drift  • Age estimates of larvae from otolith daily increments analysis . Age  assumed to represent drift duration. • Backtracking larvae in a hydrographical drift model of the North Sea to estimate and map most possible hatch origin Growth • Growth proxy derived from increase in otolith daily increment width • Hatch date: Catch date minus larval age  • Larval somatic growth proxy: Residuals of larval size—otolith size relationship Overwintering survival • Comparison of otolith growth rates from daily increments of pre– and post winter samples  Results  Conclusions [A ] Results indicate that Dogger Bank is a self-contained reproductive system while central and northern sandeel habitats re-ceived more recruitment from more distant regions, suggesting a source-sink approach to the stock may be appropriate. Im-proved understanding of link between inter-annual variation in hydrodynamics and stock recruitment may proof useful in future attempts to develop spatial explicit forecasts of the recruitment success of this stock.  [B] Larvae hatching late in the season appeared to be growing faster. This advocates for research dealing with the link between stock recruitment and inter-annual variation in timing of primary and secondary production in spring.  [C] There were some indications that growth in early life affected overwinter-survival. However, the results were inconsistent. Al-so the considerable difference between the size of juveniles in December (from scientific sandeel survey) and the size of age-1 sandeel caught in the fishery in April (overwintering lasts until late March—early April) indicates increased overwinter-mortality of the smallest individuals.      [A] Spatial variation in larval drift  Objective: To investigate spatial variation in drift patterns  • Drift patterns are dynamic between areas and years • Dogger Bank is a self-supplying area with a high degree of retention in all study years • Central and North-eastern Banks show higher connectivity with spawning grounds further south and are to a higher degree susceptible to changes in drift patterns • Interannual variability in drift distance and direction is increasing from Dogger Bank to North-easter banks   [C] Overwinter survival and growth Objective: To test whether individuals with faster oto-lith growth rates have higher over-winter survival on Dogger Bank  • Considerable somatic growth (ca. 4cm) occurred during the period from before to after winter • Linear mixed effects model comparison and esti-mation of otolith growth, showed significant higher mean increments for survivors of the winter 2009/2010 caught at the commence of fishery April/Mai compared to juveniles caught before win-ter in Nov/Dec. No such difference was found for the winter 2008/2009.   [B] Larval growth and hatch date  Objective: To test whether otolith growth rate is a pro-xy for larval somatic growth or for hatch date   • Otolith growth showed a significant correlation with somatic growth rate proxy (residuals of the larval size—otolith size relationship), but explaining only 3% of the variance (p= 0.003, r2=0.03) (Not shown).  • Otolith growth was significantly correlated with hatch date, with late hatching larvae showing faster increase in daily increment widths, indicating faster growth (p< 0.001 , r2= 0.18) (Graph below).    Connectivity, growth and survival in a spatially structured fish popula-tion, which is currently managed as seven separate stock units Krisan Ege Nielsena, Farivar Azoura, Dorte Bekkevolda, Asbjørn Christensena, Karin Hüssya, Louise Scherﬀenberg Lundgaarda, Lo&e W. Clausena, Henrik Mosegaarda, Peter Rask Møllerb, Mikael van Deursa a) National Institute of Aquatic Resources, Technical University of Denmark, Jægersborg Alle 1, Charlottenlund Castle, 2920 Charlottenlund, Denmark b) Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, 2100 K øbenhavn , Denmark      Hatch date in relation to otolith growth Lenght distributions before and after the winters of 2008 and 2009 (upper panel) with corresponding oto-lith growth (lower panel). (Dottet line: overall mean. Black lines, sample means)    Dogger Bank Central Banks Northeastern Banks Modelled drift pat-terns of larvae in 2008.  Dogger Bank is self supplied, while recruitment to the central and North-eastern areas drift from southern and possibly coastal are-as. (Ellipse areas cover approximately 70% of possible hatch sites. Red dots: sample posi-tions. Blue shaded are-as: sandeel habitat)   Interlink between recruitment and juvenile growth. (Bubble size indicate spawning stock biomass) Yearly model predictions of the origin (hatch area) of larvae settling at two sites in the North Sea (Dogger Bank (left map) and North Eastern North Sea (right map). The sites are indi-cated with “+” on the maps. Bubble size indicate contributi-on from a given location relative to other locations. The pre-diction assume homogenous distribution of spawning bio-mass across suitable sandeel habitat and spatially invariab-le larval mortality 

Connectivity, growth and survival in a spatially structured fish population, which is currently managed as seven separate stock units

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Connectivity, growth and survival in a spatially structured fish population, which is currently managed as seven separate stock units

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