Abstract
Knowledge of abundance, trends and distribution of cetacean populations is needed to inform marine conservation efforts, ecosystem models and spatial planning. We compiled a geo-spatial database of published data on cetacean abundance from dedicated visual line-transect surveys and encoded >1100 abundance estimates for 47 species from 430 surveys conducted worldwide from 1975–2005. Our subsequent analyses revealed large spatial, temporal and taxonomic variability and gaps in survey coverage. With the exception of Antarctic waters, survey coverage was biased toward the northern hemisphere, especially US and northern European waters. Overall, <25% of the world's ocean surface was surveyed and only 6% had been covered frequently enough (≥5 times) to allow trend estimation. Almost half the global survey effort, defined as total area (km2) covered by all survey study areas across time, was concentrated in the Eastern Tropical Pacific (ETP). Neither the number of surveys conducted nor the survey effort had increased in recent years. Across species, an average of 10% of a species' predicted range had been covered by at least one survey, but there was considerable variation among species. With the exception of three delphinid species, <1% of all species' ranges had been covered frequently enough for trend analysis. Sperm whales emerged from our analyses as a relatively data-rich species. This is a notoriously difficult species to survey visually, and we use this as an example to illustrate the challenges of using available data from line-transect surveys for the detection of trends or for spatial planning. We propose field and analytical methods to fill in data gaps to improve cetacean conservation efforts.
Highlights
There are many compelling reasons to know how many cetaceans there are in a region, and how they are distributed in space and time
The focus lay on a pre-defined subset of 46 marine mammal species that were of primary interest in the context of the Environmental Risk Management Capability (ERMC) project, which investigated how best to mitigate potential impacts of harmful anthropogenic activities such as intense acoustic disturbance [7]
Heterogeneity of line-transect survey distribution was apparent in a comparison of existing proportional survey coverage and effort per IUCN Marine Region versus the expected coverage based on the size of IUCN Marine
Summary
There are many compelling reasons to know how many cetaceans (whales, dolphins and porpoises) there are in a region, and how they are distributed in space and time. Cetacean populations may be impacted by a wide range of anthropogenic activities, including fishing operations [2], offshore wind farms [3], military sonar exercises [4] or accumulation of marine debris [5]. Many mitigating measures, such as those trying to ensure the sustainability of bycatch [6] or involving the site selection for noiseproducing activities that minimize harmful exposure to acoustically sensitive cetaceans [4,7] rely on good information on cetacean distribution and abundance in geographic areas of interest. We need to calculate biodiversity indices, which include number of species, overall abundance, and species evenness [8]
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