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Fishery Bulletin 103(2) 



northern limit of habitat 



search lines for 



search area 



t(1) 



reference longitude 

 for search area t(1) 



reference longitude 



and parallel lines for 



search area t(2) 



reference longitude 



and parallel lines for 



search area t(3) 



southern limit of habitat 



3rd arc defining northern and 



southern extent of search 



area t(3) 



Figure 2 



Definition of terms used to describe the PSAT Tracker algorithm. A search area is a region in a satellite 

 thermal image where a search is conducted for pixels whose temperature values match those recorded by 

 the tag at that time and when it is at the surface. The search area consists of a reference longitude line, 

 defined by the daily calculation of latitude provided by the manufacturer's processed data record and par- 

 allel search lines that provide a hedge on this determination. The search area is uniquely defined by the 

 time at which this calculation was determined. The northern and southern bounds of the search area are 

 determined by either the habitat range or the maximum distance that the tagged fish can swim during 

 each time step. Those pixels underlying the reference and search line, whose temperature best match the 

 temperatures of the selection set of points from the tag, are chosen as candidate points. One candidate point 

 from each search area will eventually define the best track. 



speed of the fish. The latitudinal bounds of the daily 

 search area are constrained in two ways, by the known 

 (or unknown) bounds of the fish's habitat and by its 

 maximum swimming speed. The northern and south- 

 ern bounds of the habitat are entered by the user, and 

 no areas are searched that are beyond these latitudes. 

 These values are meant to be inclusive and can be 

 determined from the literature or estimated by using 

 latitude values provided by light-based geolocation algo- 

 rithms. These bounds are set prior to processing and do 

 not change throughout the processing; in this study the 

 latitude search area was restricted to waters between 

 15 and 50 degrees north. 



Each search area is centered on the light-based lon- 

 gitude estimate (termed the reference longitude). PSAT 

 Tracker does not search every pixel of SST data for 

 matches, but instead searches along parallel lines of 

 longitude on either side of, and including, the reference 



longitude. These lines, termed search lines, are spaced 

 at equal distances from the reference longitude (Fig. 2). 

 The user establishes the extent to which PSAT Tracker 

 searches to the east and west of the reference longitude 

 by choosing the number of search lines as well as their 

 distance of separation. In this study four search lines 

 were drawn on either side of the reference longitude; 

 these parallels were drawn 5 nmi apart resulting in a 40 

 nmi wide daily search area. We refer to each search ac- 

 cording to the time at which the reference longitude was 

 determined, t(i) (where t is the time for which the refer- 

 ence longitude was determined and ; is the index for the 

 sequence of daily search areas in the time series). 



The maximum swimming speed of the fish can also 

 constrain the latitudinal bounds of a daily search area. 

 The farthest a fish can swim in a given time interval is 

 simply the product of its maximum swimming speed and 

 the length of the time interval. Thus, all possible posi- 



