Oecologia (2002) 133:380–388 DOI 10.1007/s00442-002-1032-3

COMMUNITY ECOLOGY

Chantal Salen-Picard · Audrey M. Darnaude Denise Arlhac · Mireille L. Harmelin-Vivien

Fluctuations of macrobenthic populations: a link between climate-driven river run-off and sole fishery yields in the Gulf of Lions Received: 30 January 2002 / Accepted: 23 July 2002 / Published online: 6 September 2002 © Springer-Verlag 2002

Abstract The Rhone river is the most important input to the Mediterranean Sea, responsible for 50% of the primary productivity of the Gulf of Lions. A highly variable amount of 1–23×106 t year–1 of terrestrial material is exported to the sea by the Rhone and stocked on the continental shelf for the most part. Soft-bottom communities off the Rhone delta were dominated by polychaetes both in species richness and abundance, and exhibited strong temporal fluctuations mainly related to flooding events. Floods caused pulses of organic matter followed, with different time lags, by peaks of polychaetes. Opportunistic, short-lived species, such as Mediomastus sp. and Aricidea claudiae, exhibited high short-term peaks in density and biomass a few months after flooding events. Conversely, long-lived species, such as Laonice cirrata and Sternaspis scutata, peaked in density and biomass with a time lag of 1–3 years, and their population increase lasted for a few years. The common sole, Solea solea, is a voracious predator of polychaetes which represent >80% of its prey. A positive correlation was found between the mean annual discharge of the Rhone river and the annual commercial landings of S. solea with a time lag of 5 years in the two fishing harbours (Sete and Martigues) located close to the Rhone delta. The long-term increase in food (i.e. polychaete density and biomass) after flooding events might favour the different stages of the sole life cycle, enhancing its population size for several years. Fluctuations of sole fishery yields in the Gulf of Lions could be influenced by climate, as the Rhone river flow is related to the North Atlantic Oscillation that drives precipitation over Western Europe.

C. Salen-Picard (✉) · A.M. Darnaude · D. Arlhac M.L. Harmelin-Vivien Centre d’Océanologie de Marseille, CNRS UMR 6540, Université de la Méditerranée, Station Marine d’Endoume, 13007 Marseille, France e-mail: [email protected] Fax: +33-4-91041635

Keywords Polychaetes · Terrestrial organic matter · Flatfish landings · North Atlantic Oscillation · Western Europe

Introduction Rivers are known to enhance the productivity of marine coastal areas through the input of nutrients and organic matter of terrestrial origin. The river plume-sea boundaries are sites of high nutrient concentrations, biomass and primary production (Largier 1993). The role played by land-based run-off in increasing coastal fishery production has been recognized in many places (e.g. Kerr and Ryder 1992) and was thought to be of particular importance for oligotrophic or semi-enclosed seas such as the Mediterranean (Caddy 1993, 2000). Generally, scientists have attempted to link fish abundance to planktonic primary production, that may itself be related to oceanic climate fluctuations (Cushing 1982; Nielsen and Richardson 1996). The relationships between primary production and pelagic fisheries, through an increase in organic matter fluxes along the pelagic food webs, have been demonstrated for various fish species throughout the world, even if the relationship is not a simple one (Cushing 1995; Yañez et al. 1998). The link between primary productivity and demersal fisheries is far less evident. If an increase in planktonic production may influence the pelagic larval stages of fish and therefore favour their recruitment level (Leggett and Deblois 1994; Bailey et al. 1995), an increase in benthic food resources is likely to influence the benthic stages of demersal fish species. The hypothesis that fluctuations in demersal fisheries could be linked to fluctuations in benthic food resources was suggested by different authors (Horwood 1993; Millner and Whiting 1996), but rarely demonstrated. Changes in benthic communities have often been related to changes in food availability due to fluctuations of phytoplanktonic production (Beukema 1991; Josefson et al. 1993; Frid et al. 1996; Josefson and Conley 1997). However, few studies have taken into account the role

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played by the terrestrial particulate organic matter (POM) in enhancing coastal benthic community dynamics through its possible assimilation by depositivorous organisms (Salen-Picard et al. 1997; Salen-Picard and Arlhac, in press). We hypothesize here that fluctuations of macrobenthic communities, linked to terrestrial inputs in coastal areas influenced by river run-off, could be partly the cause of fluctuations of some demersal fisheries, as benthic invertebrates represent the main food source for numerous fish species. The aim of this paper was therefore to look for relationships between the respective fluctuations of the Rhone river discharge, the benthic communities living off its delta and landings of the common sole, Solea solea (L.), in the Gulf of Lions, using long-term data series available in this part of the north-western Mediterranean. Because correlation is not necessarily causality, we used data on the biology and life-history cycles of the invertebrates and fish to explain the patterns observed.

tal individual density), molluscs (6–50%) and small crustaceans, mostly cumaceans and amphipods (6–17%). The community of the Vase Terrigène Côtière (VTC) (coastal terrigenous silt) extends from 15–25 m down to 100–120 m depth on muddy bottoms (median silt diameter