Satellite Imagery for Cetacean Monitoring: Potential and Limitations Across UK and Overseas Territories

A recent study, published in the journal Marine Mammal Science, has comprehensively evaluated the capabilities and constraints of Very High Resolution (VHR) optical satellites for monitoring whales, dolphins, and porpoises (cetaceans) across the United Kingdom and its Overseas Territories (UKOTs). This research meticulously investigated how environmental factors, specifically cloud cover and wind speed, impact optical data quality. The goal was to pinpoint suitable periods and locations for satellite-based observation to effectively complement traditional monitoring methods.

The Evolving Landscape of Cetacean Monitoring

Cetaceans, vital marine mammals, rely on accurate data concerning their population distribution, abundance, and monitoring coverage for effective conservation. Traditional monitoring approaches, such as boat and aerial surveys, face logistical, cost, and environmental constraints. These limitations frequently result in data gaps, especially in remote or difficult-to-access areas.

VHR optical satellite imagery is an emerging technology with the potential to support these methods by detecting both live and stranded cetaceans.

Prior to this study, the operational feasibility of these satellite methods at large spatial and temporal scales, particularly regarding environmental acquisition constraints, had not been thoroughly explored.

Unpacking the Methodology: How Environmental Factors Were Assessed

The study leveraged five years of global ERA5 reanalysis data, specifically focusing on monthly median values of "Total Cloud Cover" (TCC) and "10-meter Wind Speed." This data was utilized at a spatial resolution of 0.25 by 0.25 degrees. Three distinct study areas were chosen to represent diverse climatic zones:

• The British Isles (temperate)
• The Falkland Islands (subpolar)
• The Caribbean region (tropical)

Suitability thresholds for satellite optical imagery and cetacean detection were meticulously defined:

• Cloud Cover: A monthly average TCC of ≤20% was considered "certain" for usable imagery.
• Wind Speed: A threshold of 8 meters per second (equivalent to Beaufort scale 4) was selected, as these conditions allow surface roughness to be minimal enough for cetaceans to be discernible.

Open-source GIS software integrated cloud and wind data to map suitable areas for live whale monitoring. A separate cloud-based framework was deployed to assess stranded cetacean detection along coastal zones.

Seasonal and Regional Suitability: A Detailed Look

The assessment uncovered significant seasonal and spatial variations in the feasibility of satellite monitoring across the study areas.

UK Waters: Live Cetacean Surveys

During boreal spring and summer, low to moderate cloud cover combined with acceptable offshore wind speeds suggests satellites could complement existing live cetacean surveys in remote offshore locations.

However, autumn and winter periods consistently exhibit high winds and extensive cloud cover. These conditions substantially reduce monitoring potential due to rough sea surfaces and unreliable imagery collection.

UK Coastline: Stranded Cetacean Detection

Nearshore areas generally present more feasible conditions year-round, as suitability is primarily governed by cloud cover. Satellites could opportunistically support existing stranding networks, particularly in remote or inaccessible regions such as the Hebridean Islands.

Falkland Islands: Promising Subpolar Conditions

These islands demonstrate more promising conditions overall. During the austral summer, parts of the coastline, including the remote West Island, benefit from lower cloud cover and favorable wind conditions. This could significantly improve strandings monitoring in areas where logistical constraints are prevalent.

Caribbean UKOTs: A Tropical Monitoring Hub

The Caribbean regions offer the greatest promise for year-round operational satellite monitoring.

This is primarily attributed to generally lower cloud cover and moderate winds prevalent in tropical environments, which is invaluable for monitoring important breeding populations of humpback whales and other cetaceans.

Environmental Constraints and the Future of Satellite Monitoring

The study underscored that environmental factors fundamentally constrain satellite observations. Clouds physically block the reflected sunlight required by optical sensors, while wind-generated whitecaps increase reflectance noise, complicating the detection of submerged or surface cetaceans.

VHR optical satellite imagery serves as a complementary tool for cetacean monitoring, optimizing satellite tasking to maximize detection probability and cost-effectiveness.

While environmental challenges such as cloud cover and wind speed limit year-round usability in temperate regions, seasonal windows—particularly during spring and summer—offer viable opportunities for offshore live cetacean detection. This framework is crucial for optimizing satellite tasking and maximizing both detection probability and cost-effectiveness, recognizing the inherent limitations of passive optical sensing. Environmental optical considerations, including cloud presence and sea surface state, remain critical determinants for successful optical satellite cetacean monitoring. Further technological advancements are expected to enhance the future potential of this monitoring approach.