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County-funded aerial survey maps boating hot spots and estimates thousands of vessels in San Juan County waters
Summary
San Juan County’s environmental staff presented results of an MRC-funded 2024 aerial survey showing high vessel counts and named hotspots (Sucia Island, Roche Harbour, Friday Harbor and others). The study’s methods and limits were discussed; councilors asked how data can inform mooring-buoy placement and habitat protection.
A county-funded aerial survey of San Juan County waters counted more than 10,000 vessels over eight weekend survey days and used statistical methods to estimate total vessel presence, officials said Monday.
Frances Robertson, marine project manager with the county’s Environmental Stewardship department, told the San Juan County Council that observers recorded 10,196 vessels during the transects and bay counts; statistical adjustment of transect sightings produced an estimated 7,476 vessels across transects, and combining predicted transect totals with bay counts and assuming full marina slips yields a conservative aggregate estimate of roughly 13,294 vessel visits across the eight survey days. Robertson said an average peak summer weekend likely sees about 2,015 vessels in county waters (not counting marina slips).
Robertson described the methods used: line-transect aerial surveys flown at 1,500 feet with observers on both sides of the aircraft, and dedicated bay counts conducted by circling at about 2,000 feet to record vessel type and status (on mooring buoy, anchored, or underway). She said the survey was implemented with a small consulting firm experienced in marine-mammal aerial methods and was funded through the Northwest Straits Commission and the county’s MRC, at a program cost Robertson estimated at about $160,000.
The analysis identified recurring high-density 'hot spots' adjacent to marinas and marine state parks. Robertson pointed to Sucia Island, Prevost and Reed Harbors on Stuart Island, Roche Harbor and Friday Harbor on San Juan Island, Garrison Bay, Spencer Spit on Lopez Island, Fisherman Bay and Port Stanley on Lopez, Cowlitz Bay on Waldron, and Blind Bay on Shaw Island as areas of elevated vessel density. Robertson cautioned that some west-side transects near San Juan Island were shortened because of Canadian airspace restrictions, which likely undercount activity in those areas.
Council members asked specifically whether the data can be used to shape policy on tour and small cruise ships anchoring in sensitive areas. Robertson said the survey did not systematically capture many larger passenger vessels during peak survey windows, but the dataset can still help identify where high recreational-boat density overlaps with priority habitat such as eelgrass and kelp beds and where mooring buoys could reduce anchoring impacts. Robertson and councilors discussed coordination with the Washington Department of Natural Resources on mooring-buoy placement and the need for follow-up, targeted surveys and longer-term monitoring to link vessel presence to habitat decline.
Robertson acknowledged limitations: many small vessels lack AIS, satellite imagery remains costly and limited by cloud cover, and the survey’s transect coverage was constrained by airspace boundaries. She said the study is intended as an evidence base to guide community discussion and agency coordination on mooring buoys, protections for eelgrass and kelp, and restoration priorities.
The council thanked Robertson for the work and asked staff to follow up with DNR and local partners about next steps for management options and possible additional funding for mooring-buoy projects.
The county presentation and council questions indicate an appetite for translating the survey’s hot-spot mapping into targeted management actions, while acknowledging the study’s methodological limits and the need for agency coordination.
