Methylmercury (MeHg) determinations in hake, its food-chain, and the surrounding waters and sediments allowed us to show that the higher length or age normalized mercury concentrations of Northwestern Mediterranean (Gulf of Lions: GoL) muscle hakes compared to its Northeastern Atlantic (Bay of Biscay: BoB) counterpart are due to both biotic and abiotic differences between their ecosystems. Bioenergetic modeling reveals that the slower growth rate of Mediterranean hake favors the MeHg bioaccumulation in the fish muscle and explains most of the difference between GOL and BoB hake populations. In addition, the waters of the Mediterranean hake habitat favor a higher MeHg exposition, due to the upper position of the thermohalocline, where MeHg is formed. Furthermore, we show that, within the Mediterranean hake population, a major increase in the biomagnification power (the slope of the relationships between logMeHg and δ 15 N), from 0.36 up to 1.12, occurs when individuals enter adulthood, resulting from the combined effects of lowering growth rate and change in feeding habits. Finally, δ 15 N normalized Hg concentrations indicate that the highest Hg concentrations are for hake from the shelf edge and the lowest are for hake from the Rhone prodelta area, suggesting a lower Hg bioavailability in inshore environments, consistent with MeHg distributions in water, sediment, and preys.

International audienceThe Gulf of Lions ecosystemwas described using the Ecopath mass-balancemodel to characterise its structure and functioning and to examine the effects of themultispecific fisheries operating in this area. The model is composed of 40 compartments, including 1 group of seabirds, 2 groups of cetaceans, 18 groups of fish, 12 groups of invertebrates, 5 groups of primary producers, detritus and discards. Input datawere based on several recurrent scientific surveys, two alternative datasets for fishing data, stock assessment outputs, stomach content analyses and published information. Results showed that the functional groups were organised into five trophic levels with the highest one represented by dolphins, anglerfish, Atlantic bluefin tuna, European hake and European conger. European pilchard and European anchovy dominated in terms of fish biomass and catch. Other fish with high biomass such as Atlantic mackerel and blue whiting were highly important in the food web. Seabirds, dolphins and cuttlefish-squids represented keystone species. Important coupled pelagic-demersal-benthic interactions were described. The 7 different fisheries analysed were operating at mean trophic levels situated between 2.6 for small artisanal boats, and 4.1 for purse seines (>24 m) targeting large pelagic fish, indicating an intensively exploited ecosystem. Large trawlers (24-40 m) had the highest impact on most of the groups considered; while purse seines (12-24 m) targeting small pelagic fish had the lowest impact. Preliminary results highlighted the importance of data sources for further ecosystem and fisheries analyses and management scenarios

Mellon-Duval, C., de Pontual, H., Métral, L., and Quemener, L. 2010. Growth of European hake (Merluccius merluccius) in the Gulf of Lions based on conventional tagging. – ICES Journal of Marine Science, 67: 62–70. Growth of European hake was estimated from the results of a conventional tagging study in the Gulf of Lions, the first tagging experiment to have been undertaken on the species in Mediterranean waters. In all, 4277 hake 15–40 cm long were tagged and released on the inshore fishing grounds during spring 2006. The overall recapture rate was 6.5% and times-at-liberty ranged from 1 to 717 d. Growth rate in hake varied with size and sex. The estimated growth parameter (von Bertalanffy k) was estimated as double previously published values based on size frequency distribution in the area. Compared with recent growth parameters derived from the tagging experiments in the Bay of Biscay, k was estimated to be slightly lower in the Gulf of Lions. With this faster growth, hake would mature earlier than previously thought: at age 2 for both sexes, instead of at age 3 or 4 as currently accepted for the Mediterranean. Growth rate by sex decreased to a similar level once fish had attained sexual maturity.

Variations in space and time of juvenile hake diet (5 to 19 cm total length, TL) were investigated in the Gulf of Lions (NW Mediterranean) and related to variation in C and N stable isotope ratios, condition and abundance. Crustaceans (mysids and euphausiids) dominated the diet of the smallest juvenile hake (5 to 9 cm TL), and fishes (sardines and anchovies) of the largest juveniles (15 to 19 cm TL). The transition from a crustacean-to a fish-based diet occurred in medium-sized juveniles (10 to 14 cm TL), which preyed on both crustaceans and fishes (gobiids). These juveniles preyed on fishes when living in shallow waters (30 to 50 m), and crustaceans when located in deep waters (70 to 150 m). Although hake diet did not change over time in shallow waters (fish-based diet), in deep waters it was dominated by mysids and euphausiids in 2002 and natantids in 2003. In the size range analysed, no correlation was found between juvenile hake length and their δ 13 C and δ 15 N values, but a significant correlation with depth was observed, with higher values in shallow waters. The condition factor of medium-sized juveniles did not vary with depth in 2002, but was significantly lower in deep waters in 2003 when they fed on natantids instead of small crustaceans. Abundance of juvenile hake in the Gulf of Lions drastically decreased from 2002 to 2003, particularly in deep waters. The lower condition factor of juvenile hake in deep waters in 2003, probably owing to a lack of suitable food, might have negatively affected their survival. These results support the hypothesis that food resources influence condition and survival of juvenile hake when settled, and thus affect NW Mediterranean fisheries.

Abstract:The relationships between total mercury (Hg) concentration and stable nitrogen isotope ratio (δ 15 N) were evaluated in Mullus barbatus barbatus and M. surmuletus from the Mediterranean Sea and M. barbatus ponticus from the Black Sea. Mercury concentration in fish muscle was six times higher in the two Mediterranean species than in the Black Sea one for similar sized animals. A positive correlation between Hg concentration and δ 15 N occurred in all species. Increase in Hg concentration with δ 15 N was high and similar in the two Mediterranean fishes and much lower in the Black Sea species. Since this was neither related to trophic level difference between species nor to methylmercury (MeHg) concentration differences between the north-western Mediterranean and the Black Sea waters, we suggested that the higher primary production of the Black Sea induced a dilution of MeHg concentration at the base of the food webs.