Seitz et a\ Evaluating light-based geolocation for demersal fishes in high latitudes 



577 



identified sunrise and sunset. This misidentification 

 typically occurred because there were occasional aber- 

 rant light readings. The geolocation software identified 

 these as sunrise, sunset, or both, and therefore gave 

 bogus position estimates. There is an option to override 

 these aberrant sunrise and sunset times when using 

 TSP because the software allows manual selection of 

 sunrise and sunset. For our study, we opted not to do 

 this because we did not want to introduce subjectivity 

 into sunrise and sunset times. We suggest that the soft- 

 ware be modified by the manufacturer to select the next 

 best times for sunrise and sunset so that the investiga- 

 tor may reject aberrant light readings and yet allow the 

 software to objectively choose sunrise and sunset. 



In future studies, we hope to improve geoposition 

 estimates by statistically filtering (Sibert et al., 2003) 

 or smoothing longitude estimates and by incorporating 

 additional sensor data. For example, in conjunction with 

 light data, tag-measured sea-surface temperature (SST) 

 can be compared to remotely sensed SST, to signifi- 

 cantly improve geolocation estimates (Teo et al., 2004). 

 In the case of demersal fish that rarely, if ever, visit 

 the sea surface, maximum daily depth can be used as 

 representative of the total water depth in the region. We 

 can compare the maximum daily depth sampled by an 

 electronic tag to existing bathymetry data to estimate 

 possible daily positions of the fish. We can then combine 

 the geolocation estimated by light-level information 

 with the depth information to yield a most plausible 

 track of daily positions. 



Accurate description of the movement of fish is the 

 cornerstone of sound management plans for ensuring 

 sustainable fisheries in the future (Hunter et al., 2003). 

 Longitude estimation determined from ambient light 

 data may be used to examine large-scale movements of 

 demersal fish in high latitudes. There are several types 

 of electronic tags — some designed for fish as small as 

 15 cm (Arnold and Dewar, 2001). Using this technique, 

 we can describe large-scale spatial dynamics and mi- 

 gration of several commercially important demersal 

 fish species. 



Acknowledgments 



This project was made possible through the expertise of 

 Capt. Harold Kalve of the FV Rocinante during fishing 

 operations; Dave Douglas, Dan Mulcahy DVM, and Julie 

 Meka of USGS-Alaska Science Center; Susan Ingles, 

 Richard Hocking, Pam Parker, Brian Mullaly, Jessica 

 Dunning, and the staff at Alaska Sea Life Center; and 

 Tom Weingartner, David Leech and the crew on the RV 

 Alpha Helix from the University of Alaska Fairbanks. 

 A special thanks to Devin Johnson for statistical assis- 

 tance. Partial funding for this project was provided by 

 the Exxon Valdez Oil Spill (EVOS) Trustee Council (Res- 

 toration Project 478) and the U.S. Geological Survey- 

 Alaska Science Center. The Gulf of Alaska (GAK) 1 

 mooring is funded by the EVOS Trustee Council under 

 a separate grant (Restoration Project 340). The findings 



presented by the authors are their own and not neces- 

 sarily the position of the EVOS Trustee Council. 



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