Monitoring cetaceans in European waters

Abstract

1. Monitoring spatial and temporal patterns in cetacean abundance involves a variety of approaches depending upon the target species and the resources available. As a first step, the collection of incidental sightings or strandings information aids the construction of a species list and a rough measure of status and seasonal variation in abundance. These often make use of networks of volunteer observers although the wide variation in abilities and experience means that special attention must be paid to training and to data quality control. More robust monitoring of numbers requires quantification of effort and some correction for factors that influence detectability, such as sea state. 2. The presence of cetaceans may be recorded visually, or indirectly by acoustics. Each has advantages and disadvantages, and their applicability may vary between species. The use of fixed stations tends to allow sustained monitoring at relatively low cost but coverage is limited to the immediate vicinity. For more extensive coverage, mobile platforms are necessary. Platforms of opportunity such as ferries, whale‐watching boats, etc. are often used to survey areas at low cost. These may allow repeat observations to be made over time, but with no control over where the vessel goes, it is typically not possible to sample wide areas, thus limiting abundance estimation. 3. Line transect surveys using dedicated platforms allow representative coverage of areas from which abundance estimates can be made (either using indices or absolute measures derived from density estimation). Assumptions relating to detectability and responsiveness need to be addressed and various methods (such as two‐platform surveys) have been developed to accommodate these. 4. For some cetacean species, mark‐recapture methods can be applied using photo‐identification of recognizable individuals. Again, a number of assumptions are made, particularly relating to recognizability, representativeness of sampling and capture probabilities. Capturing, on film, as many animals in the population as possible helps to reduce the problem of heterogeneity of capture probabilities. Mark‐recapture methods require at least two sampling occasions. If multiple sampling is employed, either open or closed population models can be used. 5. Measuring population change represents a particular challenge for mobile animals such as cetaceans. Changes in ranging patterns may have a large impact on abundance estimates unless very large areas are adequately covered. Power analysis is a useful method to indicate the ability of the data to detect a trend of a given magnitude. Increasingly, spatial modelling using GLMs and GAMs is being used to provide a better understanding of the biotic and hydrographic factors influencing cetacean distribution.

Marine mammal sightings around oil and gas installations in the central North Sea

“…The caveats associated with incidental sighting data are acceptable, as long as data analyses and inferences based on these analyses are made accordingly. Implementation of a more standardized reporting programme to gain additional knowledge on seasonality or species abundance estimates is possible (Evans & Hammond, 2004;Todd, 2016). However, we predict that the additional reporting requirements would inevitably lead to a significant reduction in the number of reports from offshore working volunteers staff and a loss of valuable scientific information (Borja & Elliott, 2013).…”

Abundance, survival and temporary emigration of bottlenose dolphins (Tursiops sp.) off Useless Loop in the western gulf of Shark Bay, Western Australia

“…Correct identification of individuals is a requirement for unbiased parameter estimates (Evans and Hammond 2004;Yoshizaki et al 2009). To ensure this, only excellent-and good-quality photographs and distinctly marked dorsal fins were used for individual identification.…”

Horizontal niche partitioning of humpback and fin whales around the West Antarctic Peninsula: evidence from a concurrent whale and krill survey

“…However, such krill surveys rarely allow for concurrent dedicated cetacean surveys. Cetacean surveys require a constant minimum survey speed, a specified transect design and sufficient area coverage in order to obtain data that can be used for density estimation (Evans and Hammond 2004;Dawson et al 2008). Concurrent marine mammal observations during krill surveys usually only allow for estimation of encounter rates as a measure for relative abundance (e.g.…”