Are artificial reefs effective biomass producers or only concentrating devices ? A case study from the largest Mediterranean artificial reef system Pierre CRESSON*, Sandrine RUITTON, Marie-France FONTAINE, Mélanie OURGAUD, Mireille HARMELIN-VIVIEN Aix-Marseille University , Mediterranean Institute of Oceanography Marseille – France * Present address: IFREMER, Lab Environment and Resources, La Seyne sur Mer
[email protected]
« Attraction vs. production » : A crucial question Artificial reefs : a globally used tool to manage coastal zones and to support small scales fisheries Higher fish biomasses in the vicinity of artificial reefs, with benefits for fisheries
Where did these fishes come from ? Were they attracted or produced ?
© S. Ruitton
The predominant question about artificial reefs in the last decades
« Attraction vs. production » : A crucial question Attraction hypothesis
Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)
« Attraction vs. production » : A crucial question Attraction hypothesis
Production hypothesis
Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)
« Attraction vs. production » : A crucial question Attraction hypothesis
Production hypothesis
Sustainable support to fisheries only from production Adapted from Osenberg et al. (2002), Powers et al. (2002) and Brickhill et al. (2005)
« Attraction vs. production » : A crucial question Underwater surveys of community change
Mathematical modeling Osenberg et al. 2002
Biomass (kg m-3)
© GIS Posidonie
Occurrence (%)
Area of the AR Organic matter production and transfers through trophic network a crucial information but also a knowledge gap (Brickhill et al. 2005) C and N stable isotope ratios
C and N isotopic ratios d15N
Specific isotopic ratio of each OM production C : low enrichment at each trophic level
d13C : OM origin N : high enrichment at each trophic level
d15N : Trophic level +3 à 4 ‰
+1 ‰
d13C
Isotopic ratio integrated in the muscle : mean diet of the last months
Relative positions of all organisms in a trophic network Integrated view of the OM fluxes of a system 7
Largest artificial reef deployment in the Mediterranean 400 AR, of 6 architectural types on a 220 ha area, more than 27 000 m3
8
Main questions 1) Isotopic features of OM sources at the bases of the trophic networks 2) How are these sources used ? Isotopic ratios of benthic invertebrates 3) Fish attraction / biomass production ? Position of fishes in the AR trophic network
Main questions 1) Isotopic features of OM sources at the bases of the trophic networks 2) How are these sources used ? Isotopic ratios of benthic invertebrates 3) Fish attraction / biomass production ? Position of fishes in the AR trophic network
OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here
Predominance of filter feeders Main OM source: Pelagic primary production
OM sources supporting filter-feeders Proportion of the source in the diet of bivalves Nanophytoplankton
Terrestrial inputs
Benthic primary production Phytoplankton
Pelagic primary production : main OM source on artificial reefs Retention of the predominant part of suspended OM (picophytoplankton in oligotrophic waters)
OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here
Predominance of filter feeders Main OM source: Pelagic primary production
Benthic primary production Consumption by grazing or through a detrital pool
OM sources supporting invertebrates Whole dataset : 26 species – 7 taxonomic groups – 5 trophic modes Only main species/ groups presented here
Predators : integration of pelagic and benthic primary production AR production can support an invertebrate trophic network
Predominance of filter feeders Main OM source: Pelagic primary production
Benthic primary production Consumption by grazing or through a detrital pool
OM sources supporting invertebrates
Invertebrates: potential preys for fishes Where do fishes stand in the AR trophic network ?
Fish isotopic ratios
D15N ≈ 7‰ 3 trophic levels
D13C ≈ 2.5 ‰ At least 2 OM sources 16
Conclusion Trophic functionning of artificial reefs Fishes : biomass production based on local OM production or retention
AR = ↗ shelter + ↗ resources for invertebrates
Main OM sources: nanophytoplankton and local benthic PP
Mechanisms involved in carbon use
Mechanisms involved in carbon use
AR deployment enhances : Pelagic- benthic OM fluxes Benthic production OM bacterial recycling
To enhance secondary production
Ecosystem modelling
Conclusion • Effective fish biomass production based on AR primary and secondary productions
• AR : Efficient tools to support sustanabily small scale fisheries if well managed
• Working base for future ecosystem modelling studies and to design efficient management policies
Photo Pierre Cresson 21
Thanks for listening !
Mediteranean raimbow wrasse (Coris 22 julis) on Marseilles’ artificial reefs