Stable isotope analysis reveals habitat partitioning among marine mammals off the NW African coast and unique trophic niches for two globally threatened species

Abstract

Stable isotope abundances of carbon (δ 13 C) and nitrogen (δ 15 N) in the bone of 13 species of marine mammals from the northwest coast of Africa were investigated to assess their positions in the local trophic web and their preferred habitats. Also, samples of primary producers and potential prey species from the study area were collected to characterise the local isotopic landscape. This characterisation indicated that δ 13 C values increased from offshore to nearshore and that δ 15 N was a good proxy for trophic level. Therefore, the most coastal species were Monachus monachus and Sousa teuszii, whereas the most pelagic were Physeter macrocephalus and Balaenoptera acutorostrata. δ 15 N values indicated that marine mammals located at the lowest trophic level were B. acutorostrata, Stenella coeruleoalba and Delphinus sp., and those occupying the highest trophic level were M. monachus and P. macrocephalus. The trophic level of Orcinus orca was similar to that of M. monachus, suggesting that O. orca preys on fish. Conservation of coastal and threatened species (M. monachus and S. teuszii ) off NW Africa should be a priority because these species, as the main apex predators, cannot be replaced by other marine mammals. KEY WORDS: Marine mammals · Stable isotopes · Trophic ecology · Habitat use · NW Africa Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 416: [295][296][297][298][299][300][301][302][303][304][305][306] 2010 cies, such as the Mediterranean monk seal Monachus monachus (González et al. 1997), the Atlantic humpback dolphin Sousa teuszii (Van Waerebeek et al. 2004) and the African manatee Trichechus senegalensis (Padial & Ibáñez 2005). Mauritania is one of the major African fishing nations and has developed, alongside traditional artisanal fishing, industrial fisheries, becoming one of the major fish exporting African countries (Ababouch 2000). Overfishing, overcapacity, habitat degradation and inequitable access agreements have contributed to the decline in catches throughout West Africa (Atta-Mills et al. 2004, ter Hofstede & DickeyCollas 2006. Assessing the impact of fisheries on the local ecosystem is of great interest (Jouffre & Inejih 2005) but no studies have been conducted involving marine mammals.Available information on the marine mammals of the region is limited to stranding and sighting reports in the case of cetaceans (Duguy 1976, Maigret 1980a,b, 1981, Marchessaux & Campredon 1988, Van Waerebeek et al. 1999, although a few demographic parameters are known for Monachus monachus (González et al. 1997. Nevertheless, the trophic relationships among the marine apex predators of the region have never been investigated, and their possible interactions with the fisheries operating in the area are largely unknown.The methods traditionally used in the study of the trophic ecology of marine mammals have been faecal or stomach-content analyses. These methods present limitations because they only show recently eaten prey, and results can be se...

Foraging dichotomy in loggerhead sea turtles Caretta caretta off northwestern Africa

“…A multivariate cluster analysis was performed to explore the intrinsic pattern of the δ 13 C and δ 15 N values of females, including also the isotopic signature of 3 marine mammal species from Pinela et al (2010) to be used as a reference for neritic and oceanic stable isotope ratios. A nonparametric Mann-Whitney U-test was used to compare the δ …”

“…Fidelity to foraging grounds and dietary specialization is also evident among the neritic foragers identified here, as they exhibited a much higher variability in their stable isotope ratios than that of oceanic foragers. However, even within oceanic foragers there is remarkable variability, for example a group of 6 turtles were extremely depleted in 13 C and 15 N and, hence, likely to exploit the most distant feeding grounds from mainland Africa (Pinela et al 2010).…”

“…Here, we combine previously published information obtained by satellite tracking (Hawkes et al 2006) with information on the stable isotope profile of northwestern Africa (Pinela et al 2010), the concentration of the stable isotopes of carbon and nitrogen in the tissues of nesting females and measurements of carapace length and clutch size to test these predictions.…”

“…The samples were processed at Serveis Cientificotècnics de la Universitat de Barcelona. Isotopic composition was expressed in the standard δ notation in parts per thousand (‰) relative to predefined international standards, V-PDB (Vienna Pee Dee Belemnite) calcium carbonate for δ The concentration of stable isotopes in the bone of marine mammals from northwestern Africa (Pinela et al 2010) was used as a benchmark to support the classification of the turtles as 'neritic' or 'oceanic' foragers.…”

Use of carbon and nitrogen stable isotopes to study the feeding ecology of small coastal cetacean populations in southern Brazil

“…In marine systems, several studies have shown that there are consistent differences between the isotopic composition of animals living in pelagic vs. benthic environments, and among estuarine, nearshore and offshore environments (France 1995, Kurle & Gudmundson 2007, Cherel & Hobson 2007, Newsome et al 2010, Ohizumi & Nobuyuki 2010. Therefore, the use of stable isotopes can help identify the utilization of different habitats by species (Das et al 2003, Capelli et al 2008, Gross et al 2009, Newsome et al 2010, Pinela et al 2010. Additionally, by using this technique, it can be possible to determine if sympatric species explore the same resources and occupy the same trophic level (Cherel et al 2008, Newsome et al 2010.…”