604 



Abstract— The population struc- 

 ture of walleye pollock \Theragra 

 chalcogramma) in the northeastern 

 Pacific Ocean remains unknown. 

 We examined elemental signatures 

 in the otoliths of larval and juvenile 

 pollock from locations in the Bering 

 Sea and Gulf of Alaska to determine 

 if there were significant geographic 

 variations in otolith composition 

 that may be used as natural tags of 

 population affinities. Otoliths were 

 assayed by using both electron probe 

 microanalysis (EPMA) and laser 

 ablation inductively coupled plasma 

 mass spectrometry iICP-MS). Ele- 

 ments measured at the nucleus of 

 otoliths by EPMA and laser abla- 

 tion ICP-MS differed significantly 

 among locations. However, geographic 

 groupings identified by a multivariate 

 statistical approach from EPMA and 

 ICP-MS were dissimilar, indicating 

 that the elements assayed by each 

 technique were controlled by sepa- 

 rate depositional processes within the 

 endolymph. Elemental profiles across 

 the pollock otoliths were generally- 

 consistent at distances up to 100 fim 

 from the nucleus. At distances beyond 

 100 /im, profiles varied significantly 

 but were remarkably consistent 

 among individuals collected at each 

 location. These data may indicate 

 that larvae from various spawning 

 locations are encountering water 

 masses with differing physicochemical 

 properties through their larval lives, 

 and at approximately the same time. 

 Although our results are promising, 

 we require a better understanding of 

 the mechanisms controlling otolith 

 chemistry before it will be possible 

 to reconstruct dispersal pathways of 

 larval pollock based on probe-based 

 analyses of otolith geochemistry. 

 Elemental signatures in otoliths of 

 pollock may allow for the delineation 

 of fine-scale population structure in 

 pollock that has yet to be consistently- 

 revealed by using population genetic 

 approaches. 



Elemental signatures in otoliths of 



larval walleye pollock (Theragra chalcogramma) 



from the northeast Pacific Ocean* 



Jennifer L. FitzGerald 



Simon R. Thorrold 



Biology Department, MS 35 



Woods Hole Oceanographic Institution 



Woods Hole, Massachusetts 02543 



E-mail address (for J L. FitzGerald): ifitzgerald awhoi edu 



Kevin M. Bailey 

 Annette L. Brown 



NOAA Alaska Fisheries Science Center 

 7600 Sand Point Way NE 

 Seattle, Washington 91185 



Kenneth P. Severin 



Department of Geology and Geophysics 



University of Alaska Fairbanks 



P.O. Box 755780 



Fairbanks, Alaska 99775-5780 



Manuscript submitted 4 August 2003 

 to the Sceintific Editor's < X'fice. 



Manuscript approved for publication 

 28 May 2004 by the Scientific Editor. 



Fish. Bull. 102:604-616(20nl 



The "stock" concept is a central tenet 

 of modern fisheries science because it 

 represents the fundamental manage- 

 ment unit of marine fisheries (Begg 

 and Waldman, 1999). This emphasis, 

 in turn, places a premium on accu- 

 rate identification of groups of fish 

 whose population statistics are largely 

 independent of other groups. However, 

 stock identification has often proved 

 problematic in marine fishes. For 

 instance, the stock structure of wall- 

 eye pollock {Theragra chalcogramma* 

 across the North Pacific Ocean has 

 been a topic of investigation for many 

 years. Early studies were based on 

 phenotypic characteristics of pol- 

 lock, such as meristics and morpho- 

 metries (Serobaba. 1977; Hinckley, 

 1987; Temnykh, 1994). Other studies 

 have focused on genotypic markers, 

 such as DNA and allozyme analyses 

 (Grant and Utter, 1980; Mulligan et 

 al., 1992; Shields and Gust, 1995). 

 These approaches resulted in only the 

 broadest characterization of pollock 

 stock structure but have been able 

 to distinguish populations from the 

 eastern and western Pacific (Bailey et 

 al., 1999). Quasi-isolated subpopula- 



tions may be at least demographicallv 

 isolated on smaller spatial scales. For 

 instance, within the Gulf of Alaska, 

 spawning pollock aggregate at specific 

 locations in Shelikof Strait, Prince 

 William Sound, and in the Shumagin 

 Islands region (Bailey et al., 1999). 

 However, the extent of larval dis- 

 persal from the spawning sites and 

 the degree of spawning site fidelity 

 of adult pollock to these locations 

 remains unknown. 



The difficulties associated with de- 

 termining stock structure in fishes 

 are essentially the same ones that 

 currently limit our ability to deter- 

 mine population connectivity in ma- 

 rine systems (Thorrold et al.. 2002). 

 Tag-recapture studies using tags have 

 limited applicability in the case of 

 pollock. Adults are located deep in 

 the water column and are sensitive 

 to barotrauma during the process of 

 being caught, brought to the surface, 

 and tagged. Traditional population 

 genetic approaches may be similarly 



Contribution 11219 from the Woods Hole 

 Oceanographic Institution. Woods Hole, 

 MA 02543. 



