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Fishery Bulletin 102(4) 



on elemental composition, the spatial scale on which 

 the data were gathered was different. Our laser ablation 

 ICP-MS method required that we ablate a 70 /im x 70 

 /jm raster, or a 720-/im line, in order to enable sufficient 

 time to generate precise estimates of otolith composition. 

 The EPMA analysis was less destructive than laser ab- 

 lation ICP-MS, and therefore it was possible to sample 

 individual points at a much finer spatial resolution 

 (~5 fim), albeit with considerably less sensitivity and pre- 

 cision. For instance, using EPMA we were able to sam- 

 ple five points across a transect ending approximately 

 90 urn from the nucleus. In contrast, only a single ras- 

 ter could be sampled along this profile with laser abla- 

 tion ICP-MS. Although the diameter of laser probes 

 is approaching that of EPMA, ICP-MS is unlikely to 

 match the spatial resolution of EPMA without further 

 development of truly simultaneous mass analyzers such 

 as time-of-flight ICP mass spectrometry (Mahoney et 

 al., 1996). However, we were able to program the laser 

 probe to trace out growth increments once the otolith 

 radius had reached 120 fim and we found that the total 

 length of a daily ring was approximately 700 /jm. This 

 finding, in turn, allowed us to construct elemental pro- 

 files at reasonable spatial resolution across the otoliths 

 of larval pollock without sacrificing instrument preci- 

 sion by limiting acquisition times. Although it has not 

 been used before with otoliths, our approach provides 

 significant advantages over previous methods of using 

 a raster to create elemental profiles (e.g., Thorrold et 

 al., 1997; Thorrold and Shuttleworth 2000). 



Previous work on pollock otolith chemistry was some- 

 what successful at distinguishing fish from locations 

 in the Bering Sea and the Gulf of Alaska. Severin et 

 al. (1995) used EPMA to sample the outer margin of 

 otoliths from juvenile pollock collected along the Alaska 

 Peninsula in the Gulf of Alaska and in Bristol Bay. We 

 generated elemental profiles across otoliths from the 

 nucleus out to approximately 90 /im for the EPMA sam- 

 ples, and up to 600 /mi for the laser ablation ICP-MS as- 

 says. The profiles revealed some interesting differences 

 between the elements assayed by each instrument. For 

 instance, only one of the elements (K) from the EPMA 

 analysis showed a significant interaction between pro- 

 file and location, yet significant profile x location interac- 

 tions were detected for Mn/Ca, Sr/Ca, and Ba/Ca ratios 

 with laser ablation ICP-MS. We were also struck by 

 the similarity of profiles from individuals sampled at 

 the same location, as evidenced by the size of standard 

 errors around mean values at specific distances across 

 the otolith. For instance, the extended profiles from pol- 

 lock collected in Bristol Bay and Yakutat show indepen- 

 dent patterns of variation for all three elements from 

 the nucleus out to 600 /jm. Taken together, these data 

 indicate that larvae from several spawning locations 

 are indeed encountering water masses with differing 

 physicochemical properties through their larval lives, 

 and at approximately the same time. We lack, however, 

 a sufficient understanding of the mechanisms control- 

 ling otolith chemistry to be able to relate the profiles 

 to specific properties of different water masses in the 



study area. This knowledge will be necessary before it 

 is possible to reconstruct dispersal pathways of larval 

 pollock based on probe-based analyses of otolith geo- 

 chemistry. Nonetheless, the among-location variability 

 in elemental profiles revealed by both instruments is 

 encouraging and justifies further investigations of oto- 

 lith geochemistry in larval pollock. 



Past attempts at identifying stock structure of wall- 

 eye pollock in the North Pacific Ocean based on genetic 

 techniques have been inconclusive (Bailey et al., 1999). 

 In the most recent study, Olsen et al. (2002) were un- 

 able to distinguish between pollock from the Kamchat- 

 ka Peninsula and several locations within the Gulf of 

 Alaska based on three polymorphic microsatellite loci. 

 Allozyme and MtDNA markers showed significant differ- 

 ences between North American and Asian populations, 

 and among Gulf of Alaska locations. These data were 

 difficult to reconcile because both markers showed tem- 

 poral instability within locations. Adult tagging studies 

 shed little light on the population structure of pollock 

 because they address questions of repeat spawning, 

 whereby adult fish return to the same area to spawn in 

 subsequent years, rather than homing to natal spawn- 

 ing locations (Tsugi, 1989). It has proved impossible, 

 except in rare circumstances (Jones et al., 1999), to 

 artificially mark larvae before they are dispersed from 

 spawning grounds, and therefore natural geochemical 

 tags remain the most promising avenue for determining 

 natal origins in walleye pollock. The ability to determine 

 natal origins of individual fish is critical in the case of 

 migratory marine fishes because it allows quantification 

 of population connectivity through straying of adults as 

 well as through larval dispersal (Thorrold et al.. 2001). 

 These data, in turn, identify the spatial extent of fish 

 stocks that are demographically isolated or alternatively 

 provide connectivity rates that are necessary to param- 

 eterize spatially explicit models if the species is usefully 

 viewed as a metapopulation (Hanski and Gilpin, 1997; 

 Smedbol and Wroblewski, 2002). 



In summary, the elemental composition of otolith 

 material deposited during early larval life in walleye 

 pollock differed significantly among locations in the 

 Gulf of Alaska and Bering Sea. These results imply that 

 the larvae originated from different spawning locations, 

 not that they constitute separate stocks. Nonetheless, 

 these data represent the necessary first steps in using 

 elemental signatures in otoliths as natural tags of natal 

 origins in walleye pollock. Elemental profiles across 

 otoliths were also unique to specific locations, suggest- 

 ing that individuals collected at a location had expe- 

 rienced similar environmental conditions throughout 

 their larval lives. This observation raised the possibility 

 of reconstructing larval dispersal pathways based on 

 high-resolution sampling of otolith chemistry. Although 

 further work is needed to understand the processes 

 influencing elemental uptake in pollock otoliths, we sug- 

 gest that the potential information available from such 

 studies would be invaluable for effective management 

 of commercial pollock fisheries (Bailey et al., 1999). The 

 approach appears to be particularly appropriate for in- 



