Assessing sources of variation underlying estimates of cetacean diet composition: a simulation study on analysis of harbour porpoise diet in Scottish (UK) waters

There is little previous information on feeding habits of long‐finned pilot whales (Globicephala melas) in the northeast Atlantic. The present study analyzed stomach contents of pilot whales stranded in Portugal (n = 6), Galicia (northwest Spain) (n = 32), and Scotland (United Kingdom) (n = 10), from 1990 to 2011. These animals ranged from 213 to 555 cm in length (24 females, 19 males and 5 of unknown sex). The main prey identified were cephalopods of the families Octopodidae and Ommastrephidae, the former being numerically more important in Iberia (Portugal and Galicia) and the latter more important in Scotland, with Iberian whales also showing a more diverse diet. Multivariate analysis revealed evidence of geographical and seasonal variation in diet. Generalized Additive Modeling results indicated that more octopus (Eledone cirrhosa) were eaten in Iberia than in Scotland, more in the first half of the year, and more in larger whales. Numbers of ommastrephid squids in the stomach decreased over the study period and varied with season and whale length. This study confirms cephalopods as the main prey of pilot whales, as previously reported, although our results also suggest that, in the northeast Atlantic, ommastrephid squid are largely replaced as the main prey by octopods at lower latitudes.

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“…For a discussion of the issue and the consequences of applying different weightings see Pierce et al . () and Tollit et al . ().…”

Section: Methodsmentioning

confidence: 92%

Patterns and trends in the diet of long‐finned pilot whales (Globicephala melas) in the northeast Atlantic

Santos

Monteiro

Vingada

et al. 2013

Marine Mammal Science

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“…Scarcity of information on dolphin diet composition in the study area and more generally in the Mediterranean Sea called for assumptions that, albeit based on a comprehensive review of the available information, may affect the degree of overlap with some fishing gear. Conversely, figures of biomass removal by dolphins are probably robust, uncertainties about population size and energetic requirements being generally more important than uncertainty about diet composition when it comes to estimates of total food consumption (Pierce et al, 2007).…”

Section: Discussionmentioning

confidence: 99%

Biomass removal by dolphins and fisheries in a Mediterranean Sea coastal area: do dolphins have an ecological impact on fisheries?

Bearzi

Agazzi

Gonzalvo

et al. 2010

Aquatic Conservation

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ABSTRACT1. Dolphins are often claimed to compete with fisheries, including through removal of substantial biomass. To calculate the biomass removed by fisheries and the degree of resource overlap with dolphins in a coastal area of Greece, estimates of dolphin abundance based on photographic capture-recapture were combined with an assessment of fishing effort and catch.2. The estimated total biomass consumed annually by local dolphin populations -15 short-beaked common dolphins and 42 common bottlenose dolphins -was 15.5 and 89.8 tonnes, respectively. The total biomass removed by the local fishing fleet (307 fishing boats) was 3469.2 tonnes, i.e. about 33 times greater than that removed by dolphins.3. Dolphins removed 2.9% of the total biomass, fisheries 97.1%. Nine purse seiners (representing only 3% of the active fishing fleet) were responsible for 31.9% of biomass removal. Similarity of biomass composition between dolphins and fisheries was expressed by a Pianka index of 0.46 for common dolphins and 0.66 for bottlenose dolphins.4. Overlap differed according to fishing gear. Common dolphin overlap was higher with purse seiners (0.82), and lower with beach seiners (0.31), bottom trawlers (0.11) and trammel boats (0.06). There was virtually no overlap with longliners (0.02). Bottlenose dolphin overlap was higher with trammel boats (0.89) and bottom trawlers (0.75), and lower with longliners (0.38), purse seiners (0.24) and beach seiners (0.18). There was minimal overlap (0.12) between the two dolphin species.5. This study suggests that ecological interactions between dolphins and fisheries in this coastal area have minor effects on fisheries. Conversely, prey depletion resulting from overfishing can negatively affect dolphins. Fisheries management measures consistent with national and EU legislation are proposed to ensure sustainability and to protect marine biodiversity.

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“…Another source of variation common to all methods is sampling error (Hammond and Rothery , Pierce et al . ). Uncertainty in estimates of diet resulting from sampling error can generally be reduced by increasing sample size, but the inability to obtain a representative sample of the population may not be possible, resulting in bias.…”

Section: Sources Of Variation and Potential Biases Common To All Methodsmentioning

confidence: 97%

Methods of estimating marine mammal diets: A review of validation experiments and sources of bias and uncertainty

Bowen

Iverson

2012

Marine Mammal Science

134

142

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Diet estimation in marine mammals relies on indirect methods including recovery of prey hard parts from stomachs and feces, quantitative fatty acid signature analysis (QFASA), stable isotope mixing models, and identification of prey DNA in stomach contents and feces. Experimental evidence (9 species/13 studies) shows that digestion strongly influences the proportion and size of otoliths that can be recovered in feces. Number correction factors (NCF) and digestion coefficients have been experimentally determined to reduce the biases in fecal analysis. Correction factors and coefficients have not been determined for diet estimated from stomach contents. QFASA estimates which prey species and amounts must have been eaten to account for the fatty acid composition of the predator. Experimental studies on mammals and seabirds (9 species/10 studies) indicate that accurate estimates of diet can be determined using QFASA. Stable isotope mixing models provide rather coarse taxonomic resolution of diet composition. Prey DNA analysis shows promise as a method to estimate the species composition of diet, but further development and testing is needed to validate its use. Obtaining a representative sample from marine mammal populations is a significant challenge. Therefore, the use of complementary methods is recommended to obtain the most informative results.

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Assessing sources of variation underlying estimates of cetacean diet composition: a simulation study on analysis of harbour porpoise diet in Scottish (UK) waters

Cited by 26 publications

References 21 publications

Patterns and trends in the diet of long‐finned pilot whales (Globicephala melas) in the northeast Atlantic

Patterns and trends in the diet of long‐finned pilot whales (Globicephala melas) in the northeast Atlantic

Biomass removal by dolphins and fisheries in a Mediterranean Sea coastal area: do dolphins have an ecological impact on fisheries?

Methods of estimating marine mammal diets: A review of validation experiments and sources of bias and uncertainty

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