Since 2009, a cetacean presence and distribution long-term monitoring study has been ongoing in the Pelagos Sanctuary, a pelagic marine protected area located in the northwestern Mediterranean Sea. One of the objectives of this study is to assess trends in cetacean presence and distribution to achieve good environmental status (GES) of pelagic marine waters. We assessed the density variability of the fin whale (Balaenoptera physalus) in the Pelagos Sanctuary. We used data collected during summer months (Jun-Sep) from 2009 to 2013, along 2 fixed transects crossing the Pelagos Sanctuary, using ferries as platforms of opportunity. We compared 2 un-corrected indexes: the linear encounter rate and density index, computed applying distance sampling methodology to select the most precise method. We also evaluated the effect of environmental covariates (e.g., platform height, Beaufort sea state, wind speed and direction, cloud cover and rain, longitude, latitude) on abundance and density estimation. Finally, we defined an unbiased index and used it to analyze the temporal and spatial variability of fin whale density in the Pelagos Sanctuary. Fin whale density along the 2 transects varied on a yearly basis, with a peak in 2012 and 2013. Variability also occurred on a monthly basis, with a peak during the first half of the season. A longitudinal and a depth gradient were also evident, confirming differences in species displacement in the area. These results are consistent with previous studies, and update current knowledge of species presence in the area. The protocol tested in this work can be easily applied to the other cetacean species inhabiting the Pelagos Sanctuary providing a novel and cost-effective method to assess long-term trends in cetacean distribution and detect incipient changes in species density. Ó
Background
The Mediterranean subpopulation of fin whale Balaenoptera physalus (Linnaeus, 1758) has recently been listed as Vulnerable by the IUCN Red List of threatened species. The species is also listed as species in need of strict protection under the Habitat Directive and is one of the indicators for the assessment of Good Environmental Status under the MSFD. Reference values on population abundance and trends are needed in order to set the threshold values and to assess the conservation status of the population.
Methods
Yearly summer monitoring using ferries as platform of opportunity was performed since 2008 within the framework of the FLT Med Network. Data were collected along several fixed transects crossing the Western Mediterranean basin and the Adriatic and Ionian region. Species presence, expressed by density recorded along the sampled transects, was inspected for assessing interannual variability together with group size. Generalized Additive Models were used to describe density trends over a 11 years’ period (2008–2018). A spatial multi-scale approach was used to highlight intra-basin differences in species presence and distribution during the years.
Results
Summer presence of fin whales in the western Mediterranean area showed a strong interannual variability, characterized by the alternance of rich and poor years. Small and large groups of fin whales were sighted only during rich years, confirming the favorable feeding condition influencing species presence. Trends highlighted by the GAM can be summarized as positive from 2008 to 2013, and slightly negative from 2014 to 2018. The sub-areas analysis showed a similar pattern, but with a more stable trend during the second period in the Pelagos Sanctuary sub-area, and a negative one in the other two sub-areas. Our findings further confirm the need for an integrated approach foreseeing both, large scale surveys and yearly monitoring at different spatial scales to correct and interpret the basin wide abundance estimates, and to correlate spatial and temporal trends with the ecological and anthropogenic drivers.
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