Levati, V., Grossi, F., David, L. et al. Winter habitat preferences of cetaceans in the Northwestern mediterranean sea. Marine Biology 172, 72 (2025). https://doi.org/10.1007/s00227-025-04621-8
Ferry Observation
The Northwest Mediterranean Sea has eight whale and dolphin (cetacean) species that call the waters home. While their presence in the summer is well studied, their winter distribution is not as well known. Poor weather conditions such as rough seas and heavy rain limit cetacean detection, making survey work difficult and costly for the data that is collected.
While it’s easy to use presence data collected during the summer months to understand their distribution throughout the year, cetaceans demonstrate seasonality. In other words, their distribution changes with the season. However, we don’t know how their habitat preferences change with the seasons, if it’s driven by food, weather, or something else.
Scientists studied the presence of cetaceans in the Northwest Mediterranean Sea along the coast of Spain, France, and Italy, including Corsica and Sardinia (Figure 1). To avoid the issues that come with winter surveys, they utilized local ferries as observation platforms as the boats transported passengers. This helped reduce costs for chartering boats and avoid cancellations due to bad weather as the ferries maintained their schedules rain or shine. As a result, 12 winters worth of data from 2008 to 2020 were able to be collected.
Cetacean Preference
All eight cetacean species were observed across 281 surveys during the study, but their numbers were less than what is typically observed during the summer. Striped dolphins (Stenella coeruleoalba) were the most common, making up 43% of all cetaceans seen. Fin whales (Balaenoptera physalus) were the second most common sighting making up 31% of observations.
Some species showed regional preference compared to others. Striped dolphins, fin whales, and long-finned pilot whales had significantly higher presence in the western half of the study area compared to the east. On the other hand, bottlenose dolphins were significantly more common in the east than the west. The remaining species including sperm whales, Cuvier’s beaked whale, short-beaked common dolphins, and Risso’s dolphin did not have a significant preference.
Only four of the eight species had enough observations to study their habitat preferences. Depth and sea surface temperature were the most influential variables for these species. Algae concentration and energy in ocean currents (eddy kinetic energy) also had some influence on sperm whales and striped dolphins respectively.
The influence of eddy kinetic energy on striped dolphins could be an indicator of food presence as small fish wouldn’t be able to fight strong currents. They would be concentrated into a small area as currents converge, making it easy eating for the dolphins. The dolphins’ presence in low eddy energy could also be a resting area so they don’t have to fight the current as much. On the other hand, the significant association with sperm whales and certain chlorophyll concentrations is interesting since the whales dive deep for food. The presence of algae could indicate nutrients from the deep are coming to the surface (upwelling). The sperm whales may then use the algae and upwelling as an indicator that there’s concentrated food and dive to find it. However, this needs to be studied more before anything can be said for certain.
Ferry Ingenuity
Using ferries to overcome winter survey complications was really smart. Chartering boats for surveys can be the most expensive part of a study like this. Cancellations due to bad weather can be costly, losing money while not collecting any data. However, the drawback for using a ferry is that it has a set path. This limits the effectiveness of sampling a large area like the Northwest Mediterranean Sea as there are sections of the study area not observed as seen in Figure 1. The researchers note this with the low observations of bottlenose dolphins. The dolphins typically prefer the continental shelf which was minimally traveled compared to the open sea, reducing the quality of analysis for habitat preference.
While the use of the ferries as observation platforms has its limitations, they still helped build a baseline dataset for cetacean presence in the Northwest Mediterranean Sea for the winter season. This included habitat preferences for four species and regional distribution for all eight observed. Future work can continue to use the ferries or maybe look to other methods like passive acoustic buoys that can record whale and dolphin noises as they pass through the area.
I am a recent MSc graduate in marine biology from Bangor University, where I studied population dynamics of elasmobranchs off the coast of Wales. My interests lie in ecological data analysis to understand environmental processes and identify natural patterns. However, nothing beats being in the field and interacting directly with the marine life.
