The global estimation of microplastic afloat in the ocean is only approximately 1% of annual global plastic inputs. This reflects fundamental knowledge gaps in the transformation, fragmentation, and fates of microplastics in the ocean. In order to better understand microplastic fragmentation we proceeded to a thorough physicochemical characterization of samples collected from the North Artlantic subtropical gyre during the sea campaign Expedition seventh Continent in May 2014. The results were confronted with a mathematical approach. The introduction of mass distribution in opposition to the size distribution commonly proposed in this area clarify the fragmentation pattern. The mathematical analysis of the mass distribution points out a lack of debris with mass lighter than 1 mg. Characterization by means of microscopy, microtomography, and infrared microscopy gives a better understanding of the behavior of microplastic at sea. Flat pieces of debris (2 to 5 mm in length) typically have one face that is more photodegraded (due to exposure to the sun) and the other with more biofilm, suggesting that they float in a preferred orientation. Smaller debris, with a cubic shape (below 2 mm), seems to roll at sea. All faces are evenly photodegraded and they are less colonized. The breakpoint in the mathematical model and the experimental observation around 2 mm leads to the conclusion that there is a discontinuity in the rate of fragmentation: we hypothesized that the smaller microplastics, the cubic ones mostly, are fragmented much faster than the parallelepipeds.

Investigations of trophic interactions between marine mammals and marine resources typically use trophic models that are based on food composition, energy content of the prey and energy requirements of the predators. Although the diets are known to vary intraspecifically according to year, season, sex and age, this variation is rarely taken into account in published models. The aim of the present study was to assess the diet of the short-beaked common dolphin Delphinus delphis in the Bay of Biscay (France), examine intraspecific dietary variation and explore its effects on calculated daily food intake. Stomach contents were analysed from 71 common dolphins stranded along the French coast between 1999 and 2002. The composition of the fresh fraction of the diet was quantified by occurrence, relative abundance, reconstructed mass and prey size distributions for all prey taxa. Overall, the most important prey species were sardine, anchovy, sprat and horse mackerel, which represented 44.9, 22.6, 8.0 and 5.0% by mass of the fresh diet, respectively. The diet composition displayed significant variation in relation to season and sex/maturity status, in terms of both prey species composition and prey size distributions. Temporal variation in diet composition was interpreted in relation to prey availability in the Bay of Biscay. Because different prey types have different energetic values, changes in diet composition could lead to changes in the estimated total biomass needed to fulfil energetic requirements. In spite of the main prey species varying extensively, estimated daily food intakes changed relatively little because all diets included a high proportion of fat fish (73 to 93% by mass). KEY WORDS: Diet · Delphinus delphis · Common dolphin · Energy requirements · Prey availability · Stomach content analysisResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 354: 277-287, 2008 However, variation in dietary compositions of cetaceans exist and may have different causes, including differential foraging tactics among different segments of the population or spatiotemporal heterogeneity in prey availability (e.g. Young & Cockcroft 1994, Santos et al. 2004aBrown et al. 2001, Page et al. 2005 for pinnipeds). One may expect some degree of ontogeny in foraging abilities as a consequence of the development of sensorial, swimming and diving performances. For similar reasons, highly dimorphic species could also display sex-related differences in diet composition, and females associated with nursed calves could modify their diet either to face the increased energetic demand or as a response to the limited physical performance of their calf. Seasonal patterns in prey availability are mostly related to the recruitment of juveniles in prey populations. However, the energetic costs and benefits of foraging could be more dependent on factors such as prey aggregation or changes in energy density. Many potential prey species show seasonal changes in their distribution patterns and...

Investigating ecological segregation among organisms of a given community is challenging, especially when these organisms share similar patterns of distribution, and similar size and morphology. Around the island of Mayotte, a diversified community of at least 4 sympatric delphinids is present year round within a very restricted range: the Indo-Pacific bottlenose dolphin Tursiops aduncus, the spinner dolphin Stenella longirostris, the pantropical spotted dolphin S. attenuata, and the melon-headed whale Peponocephala electra. In addition, the Fraser's dolphin Lagenodelphis hosei makes temporary incursions into peri-insular waters as well. This study aims to assess niche segregation among this tropical dolphin community. We hypothesized that each species occupies its own distinct niche defined by the following axes: habitat, resources and time. We analysed habitat in relation to physiography, behavioural budgets and C and N stable isotope values from skin and blubber samples for each species. The results highlighted that habitat and behavioural budgets were relatively distinct among species, with few exceptions. However, in those species living on the outer reef slope where habitat and behaviour were not well discriminated, stable isotope analyses confirmed that species have different trophic levels (mostly reflected through δ 15 N values) or foraging habitat (mostly reflected through δ 13 C values). This study confirms that the use of multiple methodologies (habitat, behaviour and feeding ecology studies) help in discerning ecological niche segregation, especially when examining closely related species within a common restricted range. KEY WORDS: Tropical dolphins · Ecological niche · Habitats · Stable isotopes · Carbon · Nitrogen · South-west Indian Ocean · Mayotte Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 433: [273][274][275][276][277][278][279][280][281][282][283][284][285][286][287][288] 2011 will bring about competition between members of the same species (intra-specific competition) or between individuals of different species (inter-specific competition) (Roughgarden 1976). Intra-specific competition may be expressed by sex or age related difference in habitat and resource use and has consequences on social structures. Inter-species competition can take various forms, including direct interference (aggressive behaviour) and exploitation-competition, in which individuals indirectly compete for resources (Begon et al. 1986).Investigating segregation processes within communities of organisms that have similar size and morphology has been particularly challenging. In such communities, niche partitioning is difficult to assess as it can occur over small spatial and temporal scales. For example, in species with similar morphology (e.g. body size, jaw or beak shape, etc.), feeding niches are distinct even when feeding occurs in both species within the same area (MacArthur 1958). Niche segregation has been investigated in a number of top marine pred...

The short‐beaked common dolphin, Delphinus delphis, is abundant in both neritic and oceanic habitats. These two domains differ largely in terms of the nature of forage organisms and their availability to surface‐bound top predators, which suggests that the common dolphin should show extensive variability in foraging strategies as a response to these different habitats. However, although its diet is well known over continental shelves, so far, mostly because of sampling issues, nothing has been published on its diet in oceanic habitats. In this study, the diet of sixty‐three common dolphins bycaught in the French albacore tuna driftnet fishery in the summers of 1992–1993 in the oceanic Bay of Biscay was determined from their stomach contents and compared to neritic studies. The diet was dominated by fish (90% by number [N] and 53% by mass of total diet [M]). Cephalopods were also important in the total diet (9%N, 46%M) but were a prey of secondary importance in the fresh fraction (3%N, 10%M), presumably due to longer retention of cephalopod remains in the stomach. Crustaceans were of minor importance. At the species level, the myctophid fish (Notoscopelus kroeyeri) largely dominated the diet. Prey size ranged from 1 to 68 cm, but the majority of preys were from 2 to 30 cm. The prey characteristics and their state of digestion suggest that the common dolphin forages preferentially on small schooling, vertically migrating mesopelagic fauna in the surface layer at dusk and early night. The diet is taxonomically distinct from results obtained in neritic studies but is similar in terms of prey type and the corresponding feeding behavior.

International audienceThe Bay of Biscay (North-East Atlantic) has long been subjected to intense direct and indirect human activities that lead to the excessive degradation and sometimes overexploitation of natural resources. Fisheries management is gradually moving away from single-species assessments to more holistic, multi-species approaches that better respond to the reality of ecosystem processes. Quantitative modelling methods such as Ecopath with Ecosim can be useful tools for planning, implementing and evaluating ecosystem-based fisheries management strategies. The aim of this study was therefore to model the energy fluxes within the food web of this highly pressured ecosystem and to extract practical information required in the diagnosis of ecosystem state/health. A well-described model comprising 30 living and two non-living compartments was successfully constructed with data of local origin, for the Bay of Biscay continental shelf. The same level of aggregation was applied to primary producers, mid-trophic-levels and top-predators boxes. The model was even more general as it encompassed the entire continuum of marine habitats, from benthic to pelagic domains. Output values for most ecosystem attributes indicated a relatively mature and stable ecosystem, with a large proportion of its energy flow originating from detritus. Ecological network analysis also provided evidence that bottom-up processes play a significant role in the population dynamics of upper-trophic-levels and in the global structuring of this marine ecosystem. Finally, a novel metric based on ecosystem production depicted an ecosystem not far from being overexploited. This finding being not entirely consistent over indicators, further analyses based on dynamic simulations are required