NOTE Zimmerman and Nielsen: Measurement of strontium-to-calcium ratios in otoliths of anadromous salmonids 



717 



significant egg development during this extended prespawn- 

 ing freshwater residence led to a dilution of the Sr signature 

 in these anadromous fish. Zimmerman and Reeves (2000, 

 2002) were able to distinguish between resident rainbow 

 trout and summer steelhead in the Deschutes River, Or- 

 egon, by comparing the Sr/Ca ratios in primordia and the 

 first summer of juvenile growth (freshwater growth region). 

 In essence the freshwater gi-owth region acts as a proxy for 

 the freshwater environment and significantly higher Sr/Ca 

 ratios in the primordia suggest an anadromous maternal 

 origin. The greater precision of Sr measures afforded by 

 higher beam currents may be important in distinguishing 

 differences in seasonal ecotypes, such as summer steelhead 

 and spring chinook salmon, or in distinuishing estuary 

 habitats from freshwater and ocean environments. 



These results are applicable only to otolith calcium car- 

 bonate in the mineral form of aragonite. Like Brown and 

 Severin (1999), we have found that crystalline structure 

 affects the distribution of Sr. Vateritic regions should be 

 avoided when measuring Sr/Ca ratios in otoliths. In vater- 

 itic portions of otoliths from chinook salmon and steelhead, 

 Sr is often below our minimum detection limit of 43 ppm, 

 yet the concentration of Ca does not differ from that found 

 in aragonitic otolith regions (Zimmerman, unpubl. data). 



Studies offish migration between marine and freshwater 

 environments are based on the general difference between 

 Sr in marine and freshwater environments. Sr concentra- 

 tions in seawater are generally an order of magnitude 

 greater than in freshwaters (Bagenal et al., 1973; Kalish, 

 19901. Sr is substituted for Ca in the calcium carbonate 

 matrix of the otolith at levels that correspond to those 

 in the environment (Kalish, 1989; Farrell and Campana, 

 1996). Given this relationship, it has become a convention 

 to report Sr as a fraction of Ca (Secor and Rooker, 2000). 

 However, Secor and Rooker (2000) pointed out that Ca is 

 relatively invariant in aragonitic otoliths and rarely var- 

 ies more than 5% within an individual fish. At 8074 points 

 sampled in the primordia, freshwater growth regions, and 

 saltwater gi-owth regions of several species of salmonids 

 the Sr/Ca ratio was entirely driven by differences in Sr 

 (Zimmerman, unpubl. data). At these 8074 points, Sr was 

 highly correlated with the Sr/Ca ratio {r^=99A5%) and Ca 

 was not correlated with the Sr/Ca ratio (r-<0.01%). Given 

 this relationship, increasing precision of Sr is desirable to 

 increase precision of the Sr/Ca ratio. 



Our results suggest that tests of hypotheses related to 

 Sr/Ca ratios can be conducted at higher beam power densi- 

 ties than suggested by Gunn et al. ( 1992 ). High beam power 

 densities resulting from higher beam current and beam 

 diameter of 7 to 10-pm provide greater precision (spatial on 

 the otolith and temporal in the life of the fish) of Sr. This is 

 not true for studies of stock discrimination, such as those 

 described by Thresher ( 1999), that rely on absolute values 

 of multiple elements, including Sr. 



Acknowledgments 



Several people provided unpublished information concern- 

 ing analjftic conditions and detection limits. Gordon Reeves, 



of the U.S. Forest Service Pacific Northwest Research Sta- 

 tion, kindly provided office and laboratory space to CEZ. We 

 thank Eric Volk, Ken Severin, and two anonymous review- 

 ers for comments that improved this manuscript. 



Literature cited 



Babaluk, J. A., N. M. Halden, J. D. Reist, A. H. Kristofferson, 

 J. L. Campbell, and W. J. Teesdale. 



1997. Evidence for non-anadromous behaviour of Arctic 

 charr iSalvelinus alpinus) from Lake Hazen, EUesmere 

 Island, Northwest Territories, Canada, based on scan- 

 ning proton microprobe analysis of otolith strontium dis- 

 tribution. Arctic 50:224-233. 

 Bagenal, T. B., F. J. H. MacKereth, and J. Heron. 



1973. The distinction between brown trout and sea-trout 

 by the strontium content of their scales. J. Fish Biol. 5: 

 555-557. 

 Brown, R., and K. P. Severin. 



1999. Elemental distribution within polymorphic inconnu 

 iStenodus leucichthys) otoliths is affected by crystal 

 structure. Can. J. Fish. Aquat. Sci. 56:1898-1903. 



Campana, S. E., S. R. Thorrold, C. M. Jones, D. Gunther, 

 M. Tubrett., H Longerich, S. Jackson, N. M. Halden, J. M. 

 Kalish, P. Piccoli, H. de Pontual, H. Troadec, J. Panfili, 

 D. H. Secor, K. P Severin, S. H. Sie, R. Thresher, W. J. Teesdale, 

 and J. L. Campbell. 



1997. Comparison of accuracy, precision, and sensitivity in 

 elemental assays of fish otoliths using the electron micro- 

 probe, proton-induced x-ray emission, and laser ablation 

 inductively coupled plasma mass spectrometry. Can. J. 

 Fish. Aquat. Sci. 54:2068-2079. 



Farrell, J., and S. E. Campana. 



1996. Regulation of calcium and strontium deposition on the 

 otoliths of juvenile tilapia, Oreochromis niloticus. Comp. 

 Biochem. Physiol. 115A:103-109. 

 Gunn, J. S., I. R. Harrowfield, C. H. Proctor, and R. E. Thresher 

 1992. Electron probe microanalysis offish otoliths — evalua- 

 tion of techniques for studying age and stock discrimination. 

 J. Exp. Mar Biol. Ecol. 158:1-36. 

 Kafemann, R., S. Alderstein, and R. Neukamm. 



2000. Variation in otolith strontium and calcium ratios as an 

 indicator of life-history strategies of freshwater fish species 

 within a brackish water system. Fish. Res. 46:313-325. 



Kahsh, J. M. 



1989. Otolith microchemistry: validation of the effects of 

 physiology, age and environment on otolith composition. J. 

 Exp. Mar Biol. Ecol. 132:151-178. 



1990. Use of otolith microchemistry to distinguish the progeny 

 of sympatric anadromous and non-anadromous salmonids. 

 Fish. Bull. 88:657-666. 



Kawakami, Y, N. Mochioka, K. Morishita, T. Tajima, 

 H. Nakagawa, H. Toh, and A. Nakazono. 



1998. Factors influencing otolith strontium/calcium ratios in 

 Anquilla japonica elvers. Env. Biol. Fishes 52:299-303. 



Limburg, K. E. 



1995. Otolith strontium traces environmental history of 

 subyearling American shad Alosa sapidissima . Mar Ecol. 

 Prog. Ser 119:25-35. 

 Markowitz, A., D. Grambole, F. Herrmann, W. J. Trompetter, 

 T. Dioses, and R. W. Gauldie. 



2000. Reliable micro-measurement of strontium is the key 

 to cracking the life-history code in the fish otolith. Nucl. 

 Instr and Meth. B. 168:109-116. 



