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Fishery Bulletin 104(4) 



of Pacific halibut because this species performs spawn- 

 ing migrations of over 1100 km (Loher^). Additionally, 

 with recovery rates as high as 50% in area-specific con- 

 ventional tagging experiments (Kaimmer, 2000), TSP 

 can could be used for a large portion of tag recoveries 

 in future experiments. 



At the largest scale, we were able to discern with 

 confidence whether the wild Pacific halibut in this study 

 were in the Gulf of Alaska or Bering Sea. Individual 

 estimates were subject to occasional large errors and 

 therefore caution should be practiced when using these 

 estimates to represent the true position of fish. Examin- 

 ing patterns in estimates is more useful for determining 

 locations. To reach the Bering Sea (west of 157°W), a 

 Pacific halibut would have to migrate from the Gulf 

 of Alaska through False Pass (163. 5°W), which is the 

 eastern-most connection between the two areas. The 

 wild Pacific halibut in our study appeared to remain 

 within the Gulf of Alaska, because fewer than S'/f of the 

 longitude estimates were to the west of 163. 5°W, and 

 those appeared to be erroneous because adjacent esti- 

 mates did not consistently corroborate them. Trends in 

 longitude estimates did not provide sufficient evidence 

 to indicate that aiiy of the wild Pacific halibut swam to 

 the Bering Sea. 



A variety of uncontrollable factors can cause intrinsic 

 and extrinsic errors in geolocation estimates. The pre- 

 dominant source of intrinsic error is refraction in the 

 earth's atmosphere that is caused when light travels 

 through the atmosphere and is bent by air and other 

 molecules (Schaefer and Liller, 1990). This error limits 

 the absolute accuracy of the estimates to a constant 

 0.32° longitude and a minimum of 0.7° latitude (Hill 



^ Loher, T. 2005. Personal commun. Int. Pac. Halibut. 

 Comm. PO Box 95009, Seattle, WA 98145-2009. 



and Braun, 2001). Extrinsically, light levels may be 

 drastically influenced by changing external conditions, 

 such as waves, water turbidity, diving behavior of the 

 animal, biofouling, and cloud cover (Metcalfe, 2001). In 

 particular, the Alaska coastal region frequently experi- 

 ences large changes in weather systems that change 

 cloud cover and sea-state on a daily, or even hourly, 

 basis. One final consideration for errors is that accu- 

 rate location estimates rely on unobstructed horizons. 

 If the horizon is obstructed, such as by the mountains 

 surrounding the coast of Alaska, it alters the time(s) of 

 apparent sunrise (and sunset), thus affecting geoloca- 

 tion estimates. The tank experiment was conducted in 

 a deep, north-south fjord whose walls obstructed the 

 horizon, and the fixed mooring experiment was adjacent 

 to an island on the east and to steep coastal mountains 

 to the west. Undoubtedly, these false horizons accounted 

 for part of the errors and bias. 



One shortcoming discovered in the fixed mooring 

 experiment was a conspicuous gap in longitude and 

 latitude estimates from December to June at 146 m. 

 This six-month gap was probably the result of low am- 

 bient light levels during the winter associated with 

 high latitudes. It is unknown why the gap lasted into 

 the summer when ambient light drastically increased. 

 However, for practical application in studies of Pacific 

 halibut migration, light-based geolocation estimates 

 will capture some individual migrations to the spawn- 

 ing grounds as some Pacific halibut begin migrating in 

 October and arrive on the continental slope by early 

 November (Seitz et al., 2003). 



We may be able to increase the number of location 

 estimates with some fine-tuning of both software types. 

 Several days were rejected because of poor light read- 

 ings. However, some days had smoothly sloping sunrise 

 and sunset events that appeared to be sufficient for 

 accurate geolocation estimates, but the software mis- 



