Changes in fish community species and size structure

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1 Changes in fish community species and size structure Claus Stenberg, Josianne G. Støttrup & Per Dolmer Photos: Karsten Dahl & Claus Stenberg

Suitable habitats for many fish species. Primary habitat for certain groups of fish (e.g. wrasse family).

2 Why are boulder reefs important? Offshore cavernous boulder reefs in temperate shallow waters have a high biodiversity and biological production. Suitable habitats for many fish species. Primary habitat for certain groups of fish (e.g. wrasse family). Probably also a vital nursery area for a number of fish species (e.g. cod family). Offshore cavernous boulder a now a rare subtype of the natural habitat Reef at a national and European level. Photo: Karsten Dahl 2 DTU Aqua, Technical University of Denmark

3 Project aims: The overall project: The Blue Reef project aimed to restore/improve the nature quality status for Læsø Tindel reef. Document the effect on biological communities General flora and fauna (AAU) Fish communities (DTU Aqua) 3 DTU Aqua, Technical Photo: University Karsten of Dahl Denmark

Document the effect on biological communities General flora and fauna

Methods The goals were to examine fish biodiversity, abundances and length distribution, food availability and prey selection. Furthermore, fish migration pattern within and around the reef.

4 Methods The goals were to examine fish biodiversity, abundances and length distribution, food availability and prey selection. Furthermore, fish migration pattern within and around the reef. In order to achieve this we used a suite of methodologies BEFORE and AFTER the reef was restored: Multi-meshed gillnets Fyke nets Food availability and stomach analysis (talk by Josianne G. Støttrup) Crab/lobster pots Acoustic tagging of fish and deployment of receivers 4 DTU Aqua, Technical Photo: University Line Reeh of a& Denmark Claus Stenberg

In order to achieve this we used a suite of methodologies BEFORE and AFTER the reef was restored: Multi-meshed gillnets Fyke nets Food

5 Fish abundance Before / After 300 Catch rate multi-mesh size gill net 3.5 Relative change in catch rate CPUE (number gillnet -1 ) p<.0001 Relative change in CPUE Flatfish Gadoids 0.0 Wrasses Other Flatfish Gadoids Wrasses Other Fish group Fish group 5 DTU Aqua, Technical University of Denmark

0 CPUE (number gillnet -1 ) 200 150 100 p<.0001 Relative change in CPUE 2.5 2.0 1.5 1.

6 UW-video June 2012 from semi-stationary video cameras 6 DTU Aqua, Technical University of Denmark

7 Vertical distrubtion patterns of gadoids and wrasses gadoids wrasses 7 DTU Aqua, Technical University of Denmark

8 Catch efficiency Before and After Before high After low 8 DTU Aqua, Technical University of Denmark

9 Tracking of juvenile cod Time Registration of tagged cod Animation video made by Patrizio Mariani, DTU Aqua 9 DTU Aqua, Technical University of Denmark

0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324

10 Regitrations of cod throughout the day/night in July Sunrise Sunset 6 Registrations % Time of day 10 DTU Aqua, Technical University of Denmark

Change in overall size distribution of fish 0.4 0.3 Before Frequency 0.2 0.1 Fish>14 cm 16% 0.0 0.4 0.3 After Frequency 0.

11 Change in overall size distribution of fish Before Frequency Fish>14 cm 16% After Frequency Fish >14 cm 30% DTU Aqua, Technical University of Denmark Fish length (cm)

3 After Frequency 0.2 0.1 Fish >14 cm 30% 0.

12 Abundance of lobster and edible crab 20 Catch rate (number -station ) X Lobster Edible crab 12 DTU Aqua, Technical University of Denmark Homarus gammarus Cancer pagurus

01 2007 2012 0 Lobster Edible crab 12 DTU Aqua,

Conclusions Fish fauna Before / After was dominated of species belonging to the wrasse family (Labridae).

Size distribution patterns in fish changed and to include larger individuals (relative occurrence of fish >14 cm increased

A slight increase was however observed in central parts of the reef.

13 Conclusions Fish fauna Before / After was dominated of species belonging to the wrasse family (Labridae). Cod and saithe (Gadidae) were Before infrequent but increased significantly After (factor 3-4). Size distribution patterns in fish changed and to include larger individuals (relative occurrence of fish >14 cm increased by a factor 2) No significant increase in edible crab and lobster. A slight increase was however observed in central parts of the reef. Activity of cod on the reef showed a clear daily rhythm (active on shallow parts during the night and passive in deeper parts during the day). The restoration of Læsø Trindel have resulted in a biological more well functional reef. 13 DTU Aqua, Technical University of Denmark

Size distribution patterns in fish changed and to include larger individuals (relative occurrence of fish >14 cm increased by a factor 2) No significant increase in edible crab and

all the participants in the field programme

Christensen, Lars Thomsen, Søren Post, Ole

14 Thank you for your attention. We would like to express our gratitude to all the participants in the field programme and lab. work: Loacal fishermen: Poul Olsen and Jørgen Rulle DTU Aqua: Jesper Knudsen, Per Dolmer, Thomas Møller, Stine Kjærulf Andersen, Per Ladewig, Hans-Jørn Aggerholm Christensen, Lars Thomsen, Søren Post, Ole Henriksen, Marie Plambech, Nidia Graff Aarhus University: Karsten Dahl, Jan Damgaard Nielsen and Jens Larsen Copenhagen Univesity: Peter Rask Møller, Tammes Menne and Stine V. Christensen

Per Ladewig, Hans-Jørn Aggerholm Christensen, Lars Thomsen, Søren Post, Ole Henriksen, Marie Plambech, Nidia Graff Aarhus

1 DTU Aqua, Technical University of Denmark

1 DTU Aqua, Technical University of Denmark BlueReef Rebuilding of Marine Cavernous Boulder Reefs in Kattegat Claus Stenberg 1, Karsten Dahl 2, Else Marie Stamphøj 3, Josianne Støttrup 1, Per Dolmer 1