Using a shore-based radar system to improve whale-vessel strike risk assessments at the entrance to San Francisco Bay

Globally, vessel strike is a major cause of mortality for large whales, especially where whales are concentrated near busy seaports. Whale-vessel strike risk assessments depend upon a comprehensive understanding of local vessel activity. However, vessel traffic data used for these assessments are typically acquired from Automatic Identification System (AIS) broadcasts by ships, and many vessels do not transmit AIS. Here, we investigated the crucial data gap in these assessments by comparing patterns of AIS and non-AIS vessel activity. To accomplish this, we used an innovative shore-based technology package (ProtectedSeas Marine Monitor, or M2) that repurposed an off-the-shelf marine radar, AIS receiver, and camera to track all vessel traffic. The M2 system positioned at the mouth of the Golden Gate Strait, the entrance to a busy seaport region, San Francisco Bay, CA, USA, provided data for one year of analysis, and we found that 81% of all tracks analyzed were from vessels not transmitting AIS. Non-AIS vessel operation movements were generally more variable, and seasonal trends in non-AIS vessel traffic differed from AIS traffic. The highest speeds reached by non-AIS vessels in clear weather remained consistent even when operating in reduced visibility conditions, such as in dense fog. Our study demonstrated that exclusively using AIS data to model vessel-strike risk can lead to significant underestimations of vessel traffic activity. At the same time, the method used here points to a solution, as this accessible technology can be readily deployed onshore to provide more comprehensive vessel data to produce robust strike risk assessments in coastal or geographically constrained whale habitat.