Bull Trout Movement In Skagit Bay

Mobile tracking and acoustic tags were used to identify marine habitats for anadromous bull trout (Salvelinus confluentus), a threatened, native char found in Skagit Bay, Washington.  Saltwater residency ranged from 36-133 d (mean = 72 d) and habitat surveys were conducted during April-July, the period of saltwater residence for most bull trout.  Fish were found <12 km from the Skagit River and they utilized shoreline areas ranging from 0.1–2.1 km in length.  Bull trout were associated with the shoreline (mean distance from shore = 0.4 km ; 25% of detections <94 m from shore) and shallow water (mean depth = 3.9 m, 25% <2.0 m).  Exact fish positions were difficult to identify because of the shallow, nearshore habitats used and because of our reluctance to approach fish and alter their behavior. Tests of directionality to determine fish locations from a boat suggested that estimates averaged ±14¢ª of the true bearing. Range testing with tags suspended at 0.6 m above the bottom indicated that the maximum distance for code identification varied from 350-800 m for different tag sizes and gain levels.  Maximum decoding distance also varied with bottom slope and tag depth (0.6-3.1 m).  Despite limitations, mobile tracking allowed us to identify critical marine areas used by bull trout and through GIS mapping to describe shoreline, vegetation, and substrate types associated with such habitats.