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



575 



for AMP (2.95° ±2.47° SD) was larger and 

 to the east of that of TSP {-1.32° ±3.04° 

 SD). The mean latitude bias was relatively 

 small for both AMP (0.56° ±3.50° SD) and 

 TSP (0.10° ±7.26° SD); however both had 

 large variances and thus the estimates were 

 not precise. In several cases, the longitude 

 estimates were within one degree of the 

 true position and there did not appear to 

 be a pattern of over- or underestimating 

 longitude. 



Discussion 



Geolocation estimates determined from ambi- 

 ent light data in high latitudes is equally 

 effective as in lower latitudes. Similar to 

 results from previous geolocation evalua- 

 tions (Welch and Eveson, 1999, 2001; Musyl 

 et al., 2001; Teo et al.. 2004), our longitude 

 estimates were in general more accurate and 

 precise than latitude estimates. Therefore, 

 longitude estimation by light is a promising 

 technique for discerning large-scale move- 

 ment of demersal fishes in coastal Alaska, 

 but latitude estimation determined from 

 light data only will not be adequate for these 

 purposes. 



This study was unique in testing light- 

 based geolocation in depths greater than 60 

 m. The results demonstrate the importance 

 of evaluating geolocation by light for the 

 entire depth range of the species of inter- 

 est. Testing only in the near-surface waters 

 would be misleading because the percent- 

 age of days with estimates from tags at 

 shallower depths was much greater than 

 the percentage of days with estimates from 

 tags at greater depths — the depths which 

 halibut most frequently inhabit (Seitz et 

 al., 2003). 



The accuracies of the longitude estimates 

 in this study were comparable to those at 

 lower latitudes and similar water depth. 

 Errors are discussed in linear distance 

 (Table 1) to account for the fact that a de- 

 gree of longitude varies with latitude and to facilitate 

 comparisons to previous studies. The longitude errors 

 from the tank experiment were generally similar to 

 the errors produced in a comparable experiment where 

 tags were placed on a stationary mooring at a depth of 

 10 m (Welch and Eveson, 1999). The tags submerged 

 at deeper depths in the fixed mooring experiment also 

 showed a longitude error magnitude similar to that of 

 location estimates from tags in the offshore region of 

 the Gulf of Alaska at 50°N, 145°W (Musyl et al., 2001; 

 Welch and Eveson, 2001). The longitude biases were 

 only slightly larger than those from tags on a stationary 

 mooring near Hawaii (Musyl et al., 2001). 



AMP mean error magnitude ( ) 

 5 10 15 20 25 



AMP mean bias (°) 

 -25 -15-5 5 15 



25 



-I 



146 



TSP mean error magnitude (°) 

 5 10 15 20 25 



-25 



TSP mean bias 

 -15-5 5 



15 25 



:[:. 



h- 



146 



Figure 3 



Mean (±SD) positional errors and bias in the fixed mooring experi- 

 ment. Two programs, Argos Message Processor (AMP) and Time 

 Series Processor (TSP), were used to calculate daily longitude (black 

 bars) and latitude (hatched bars). Asterisks (*) indicate mean posi- 

 tional biases that were significantly different from zero in tests with 

 two-way ANOVA. A negative bias indicates that a position estimate 

 was either north or east of the known position, and a positive bias 

 indicates that a position estimate was either south or west of the 

 known position. 



The minimum movement of a fish that was discerned 

 by light-based geolocation in our experiment is the ab- 

 solute sum of the error magnitude and bias. The sum of 

 the error magnitudes and biases of TSP were generally 

 smaller than those of AMP; therefore TSP was a bet- 

 ter estimator of light-based geoposition than AMP and 

 can be used to discern movement at a finer scale. The 

 tank and fixed mooring experiments indicated that lon- 

 gitude estimation by TSP is able to discern movements 

 of approximately ±200 km for depths as great as 150 

 m and AMP is able to discern east-west movements of 

 approximately ±350 km at 150 m deep. Geolocation by 

 light will be able to discern the large-scale movements 



