Olivier Le Pape

Global marine primary production constrains fisheries catches

Primary production must constrain the amount of fish and invertebrates available to expanding fisheries; however the degree of limitation has only been demonstrated at regional scales to date. Here we show that phytoplanktonic primary production, estimated from an ocean-colour satellite (SeaWiFS), is related to global fisheries catches at the scale of Large Marine Ecosystems, while accounting for temperature and ecological factors such as ecosystem size and type, species richness, animal body size, and the degree and nature of fisheries exploitation. Indeed we show that global fisheries catches since 1950 have been increasingly constrained by the amount of primary production. The primary production appropriated by current global fisheries is 17-112% higher than that appropriated by sustainable fisheries. Global primary production appears to be declining, in some part due to climate variability and change, with consequences for the near future fisheries catches.

Quantitative description of habitat suitability for the juvenile common sole (Solea solea, L.) in the Bay of Biscay (France) and the contribution of different habitats to the adult population

Abstract This study describes the spatial distribution of young-of-the-year sole based on autumnal beam trawl surveys conducted in the Bay of Biscay (France) during a 15-y period. Previous studies showed that habitat suitability for juvenile sole varies according to physical factors such as bathymetry, sediment structure and river plume influence. These factors, which are known exhaustively for the entire Bay of Biscay from static maps (bathymetry and granulometry) or temporal maps based on a hydrodynamic model (the river plume), were used as descriptors in a generalised linear model of habitat suitability in order to characterise the distribution of juvenile 0-group sole according to delta distribution. This model was used to identify the habitats in which juvenile 0-group sole are concentrated. The respective areas of these habitats were determined from a Geographic Information System (GIS), and their respective contribution to the sole population in the Bay of Biscay was calculated in terms of the estimated number of young fish (GIS area×density derived from the model). Despite the great variability of survey data, this quantitative approach emphasises the highly important role of restricted shallow, muddy estuarine areas as nursery grounds of sole in the Bay of Biscay and demonstrates the relation between interannual variations of nursery habitat capacity (with respect to estuarine extent) and sole recruitment.

Resistance of a coastal ecosystem to increasing eutrophic conditions: the Bay of Brest (France), a semi-enclosed zone of Western Europe

Abstract The Bay of Brest is a semi-enclosed coastal ecosystem receiving high nutrients loading from freshwater inputs. In order to analyse the response of phytoplankton stocks to increasing eutrophic conditions, a survey of the annual cycle of hydrographic properties, nutrients and chlorophyll a concentrations, and carbon uptake rates was performed at four stations in 1993. This database has been compared to earlier measurements performed during several comparable surveys within the last 20 years. As compared to the seventies, a doubled nitrate loading is now entering this ecosystem, which is related to increased agricultural activities on the drainage basins, while the geographical origin of the nitrate input has been modified. As a result of these anthropogenic modifications, summer averaged Si/N stoichiometric balance has decreased during the two last decades but, contrary to what has been observed in other coastal ecosystems, phytoplankton stocks have not increased. Several ecological factors have hindered eutrophication: the high hydrodynamic mixing with adjacent marine waters, caused by the macrotidal regime, induces important nutrients losses, temperature and mostly light limit primary production while Si and P high recycling maintain nitrogen limitation in this ecosystem. Conjunction of these non-anthropogenic factors explains the global stability of phytoplankton stocks.

Relative contributions of different sole and plaice nurseries to the adult population in the Eastern Channel: application of a combined method using generalized linear models and a geographic information system

The Eastern Channel (ICES division 7d) is considered a unit for stock management of the common sole Solea solea and the plaice Pleuronectes platessa. At this scale, the aim of this work is to model the juvenile distribution of these flatfish. The database used, based on different surveys undertaken during the two last decades, includes 4500 coastal beam trawl hauls throughout the Eastern Channel coasts. Multivariate analyses are used to study the interannual fluctuations in flatfish recruitment of the region. This approach allows us to separate the Eastern Channel into homogeneous sectors with regard to these fluctuations. The physical parameters that contribute to the observed juvenile sole and plaice distributions are identified and their distributions are modelled using generalized linear models. The results of these models, and the areas of the different populations derived from a geographic information system, are used to identify the main nursery grounds and to compare their respective importance. This quantitative approach identifies the lack of estuarine dependence of the nursery grounds, and highlights those areas where anthropogenic disturbance could influence these populations.

Comparison of growth and condition indices of juvenile flatfish in different coastal nursery grounds

Coastal and estuarine ecosystems in the eastern channel and southern bight of the North Sea provide nursery habitats for juvenile fishes. We examined the age 0-group juveniles of three flatfish speciesSolea solea, Limanda limanda and Pleuronectes platessa, collected in five nursery areas with different characteristics (three sites located near small estuaries and two affected by important inputs of industrial effluents), to evaluate habitat influence on their growth and condition. We measured a biochemical index (RNA:DNA ratio), a morphometric index (Fulton’s K condition index), plus a recent growth index (marginal otolith increment width) on each individual (about 3 months old), collected during surveys in autumn 1999. The three indices displayed few significant differences among the five nursery sites and different patterns for each species. We suggest that the investigated nursery areas provide habitats of equivalent quality for the age 0-group juvenile flatfishes in spite of different anthropogenic disturbances. On the other hand, this study focuses on the importance of using different biological indicators to assess habitat quality and environmental stressors in coastal areas.

Eating up the world’s food web and the human trophic level

Significance Here we combine ecological theory, demography, and socio-economics to calculate the human trophic level (HTL) and position humans in the context of the food web. Trophic levels are a measure of diet composition and are a basic metric in ecology, but have never been calculated for humans. In the global food web, we discover that humans are similar to anchovy or pigs and cannot be considered apex predators. In addition, we show that, although countries have diverse diets, there are just five major groups of countries with similar dietary trends. We find significant links between HTL and important World Bank development indicators, giving insights into the relationship between socio-economic, environmental, and health conditions and changing dietary patterns. Trophic levels are critical for synthesizing species’ diets, depicting energy pathways, understanding food web dynamics and ecosystem functioning, and monitoring ecosystem health. Specifically, trophic levels describe the position of species in a food web, from primary producers to apex predators (range, 1–5). Small differences in trophic level can reflect large differences in diet. Although trophic levels are among the most basic information collected for animals in ecosystems, a human trophic level (HTL) has never been defined. Here, we find a global HTL of 2.21, i.e., the trophic level of anchoveta. This value has increased with time, consistent with the global trend toward diets higher in meat. National HTLs ranging between 2.04 and 2.57 reflect a broad diversity of diet, although cluster analysis of countries with similar dietary trends reveals only five major groups. We find significant links between socio-economic and environmental indicators and global dietary trends. We demonstrate that the HTL is a synthetic index to monitor human diets and provides a baseline to compare diets between countries.

Hydrodynamic prevention of eutrophication in the Bay of Brest (France), a modelling approach

Abstract The Bay of Brest is a semi-enclosed coastal ecosystem where primary production is nutrient-limited, even if huge nutrients loading from tributaries are present. The most striking feature of the bay is the semi-diurnal tidal influence, resulting in large water exchange with the continental shelf. A historical study of the available data has shown the steadiness of this ecosystem during the two last decades in spite of increasing eutrophic conditions. This study has focused on hydrodynamic exchange which is one of the factors supposed to explain the resistance of this ecosystem to eutrophication: this stirring hinders the formation of a persistent upper mixed layer where phytoplankton would be in contact with nutrient-rich brackish waters and available light. Moreover, horizontal tidal currents lead to huge exchanges with the Iroise Sea and, then, to big losses of nutrients and living matter. To study this hydrodynamic influence thoroughly, a physical/biological model of this bay has been developed. This box model, based on the horizontal tidal circulation, has been developed thanks to “ELISE”, an ecological modelling software and, then, tuned and validated on two data sets corresponding with the years 1977 and 1993. The model has allowed us to quantify the influence of hydrodynamics, climatic conditions and biological factors on biogeochemical processes in this ecosystem. It contributes to explain the good resistance of the Bay of Brest ecosystem to eutrophic conditions; both the hydrodynamic properties of this bay and the grazing pressure have prevented it from disturbances caused by high nitrogen loading from the watersheds and explain the steadiness of phytoplankton stocks in spite of increased loading. So, these results allow us to say that, even if nitrogen inputs increase continues, phytoplankton stocks will not increase in significant proportions. Nevertheless, changes in the phytoplanktonic populations may occur if such an enrichment continues.

Influence of Green Tides in Coastal Nursery Grounds on the Habitat Selection and Individual Performance of Juvenile Fish

Coastal ecosystems, which provide numerous essential ecological functions for fish, are threatened by the proliferation of green macroalgae that significantly modify habitat conditions in intertidal areas. Understanding the influence of green tides on the nursery function of these ecosystems is essential to determine their potential effects on fish recruitment success. In this study, the influence of green tides on juvenile fish was examined in an intertidal sandy beach area, the Bay of Saint-Brieuc (Northwestern France), during two annual cycles of green tides with varying levels of intensity. The responses of three nursery-dependent fish species, the pelagic Sprattus sprattus (L.), the demersal Dicentrarchus labrax (L.) and the benthic Pleuronectes platessa L., were analysed to determine the effects of green tides according to species-specific habitat niche and behaviour. The responses to this perturbation were investigated based on habitat selection and a comparison of individual performance between a control and an impacted site. Several indices on different integrative scales were examined to evaluate these responses (antioxidant defence capacity, muscle total lipid, morphometric condition and growth). Based on these analyses, green tides affect juvenile fish differently according to macroalgal density and species-specific tolerance, which is linked to their capacity to move and to their distribution in the water column. A decreasing gradient of sensitivity was observed from benthic to demersal and pelagic fish species. At low densities of green macroalgae, the three species stayed at the impacted site and the growth of plaice was reduced. At medium macroalgal densities, plaice disappeared from the impacted site and the growth of sea bass and the muscle total lipid content of sprat were reduced. Finally, when high macroalgal densities were reached, none of the studied species were captured at the impacted site. Hence, sites affected by green tides are less favourable nursery grounds for all the studied species, with species-specific effects related to macroalgal density.

Influence of Hydr ocarbons Exposure on Survival, Growth and Condition of Juvenile Flatfish: A Mesocosm Experiment

Abstract Juveniles of numerous commercial marine flatfish species use coastal and estuarine habitats as nurseries. Hence, they are likely to be exposed to a number of anthropogenic stressors such as accidental and chronic exposure to chemical contaminants. Little is known about their response to such pollutants at the individual level and about the consequences on their population dynamics. Mesocosm experiments were conducted to determine whether short (24 h) but high exposure to petroleum hydrocarbons (1/1000 v: v water: fuel), similar to what happened after an oil spill on coastal areas, affects survival and biological (growth, body condition and lipid reserve) performances of juvenile common sole, which live on near shore and estuarine nursery grounds. Results demonstrated that this type of exposure significantly reduce survival, growth (size, recent otolith increment and body condition), and especially energy storage (triacylglycerol to free sterol ratio) of the juvenile fish on the medium-term (three months after the exposure). These medium-term consequences affect future recruitment of this longlived species.

Conflicts in the coastal zone: human impacts on commercially important fish species utilizing coastal habitat

Coastal ecosystems are ecologically, culturally, and economically important, and hence are under pressure from diverse human activities. We reviewed the literature for existing evidence of effects of human-induced habitat changes on exploited fish utilizing coastal habitats. We focused on fish species of the Northeast Atlantic for which fisheries advice is provided by International Council for the Exploration of the Sea (ICES) and which utilize coastal habitats for at least one life-history stage (LHS). We found that 92% of these species are impacted by human activity in at least one LHS while utilizing coastal habitat and 38% in multiple stages. Anthropogenic pressures most commonly shown to impact these fish species were toxicants and pollutants (75% of species). Eutrophication and anoxia, invasive species, and physical coastal development affected about half of the species (58, 54, and 42% of species, respectively), while indirect fishing impacts affected a minority (17% of species). Moreover, 71% of the ICES advice species that utilize coastal habitats face impacts from more than one pressure, implying cumulative effects. Given that three-fourths of the commercial landings come from fish species utilizing coastal habitats, there is an obvious need for a better understanding of the impacts that human activities cause in these habitats for the development of ecosystem-based fisheries management.