GMU 423 - Henry, Washington

Henry Island is approximately 1.6 square miles (4.1 km²) and consists of two forested ridges — one roughly 3 miles long and one about 1.5 miles — connected by a low sand-bar isthmus, giving the island an H-shape when viewed from above. Elevations are low, generally under 200 feet, with the highest point on the larger northern ridge (Big Henry). The isthmus supports a rare saltwater marsh ecosystem. Coastline is varied, with rocky shoreline, sand beaches at Open Bay (south) and Mosquito Pass (west), and dense second-growth conifer forest covering the interior ridges. The island lies in the rain shadow of the Olympic Mountains, receiving roughly 25–30 inches of precipitation annually.

Henry Island is nearly entirely private land. The San Juan Preservation Trust protects 78 acres — including the isthmus and flanking shoreline — as the Henry Island Preserve, accessible only by small watercraft landing at Open Bay or Mosquito Pass beaches, with 1 mile of hiking trails (pedestrian only, no vehicles). The preserve prohibits hunting without special permission. No road access exists; the island has no ferry service and no public dock. Hunters must arrive by private boat or kayak. All private land requires written landowner permission per San Juan County ordinance.

Interior ridges support second-growth Douglas fir, western red cedar, and madrone forest with salal, sword fern, and Oregon grape understory — classic Pacific Northwest island deer habitat. The isthmus saltwater marsh provides salt-tolerant vegetation including glasswort, pickleweed, and sedges. Shoreline zones transition to rocky outcrops and grassy bluffs. The rain-shadow climate produces drier summer conditions than the mainland, favoring open grassy glades on south-facing slopes.

Black-tailed deer on Henry Island are a resident island population with no migration corridor to the mainland. Deer are year-round residents confined to the island's ~1.6 sq mi land area. Populations are subject to periodic Adenovirus Hemorrhagic Disease (AHD) events that have caused significant die-offs across the San Juan archipelago. The small, closed population means density can fluctuate sharply following disease events. Deer use the conifer forest ridges for cover and the isthmus/open areas for foraging. No elk or bear are present on the island.

San Juan County prohibits centerfire and rimfire rifles county-wide; all hunting in GMU 423 is restricted to archery, muzzleloader, shotgun, or handgun (.40 cal / 10mm or larger). One deer per hunter per license year (general limit); no second-deer special permits are currently listed for GMU 423 in WDFW regulations (unlike larger islands such as Orcas, San Juan, Lopez, and Blakely). GMU 423 is in WDFW Region 4, District 13. WDFW does not publish standalone harvest statistics for GMU 423 due to the very low hunter participation; the unit is effectively hunt-by-permission-only on private land. AHD outbreaks (most recently documented across the San Juan archipelago in 2020–2022) have caused population fluctuations. No public trailheads, state wildlife areas, or DNR lands are present on Henry Island. BLM's San Juan Islands National Monument (~1,000 acres spread across the archipelago) does not include Henry Island. The San Juan Preservation Trust preserve restricts hunting. Hunters should verify current access and trespass rules with WDFW Region 4 and local landowners before planning any hunt.