804 



Fishery Bulletin 90(4). 1992 



Otolith microchemistry and 

 migratory history 



The otolith microchemistry method offers great poten- 

 tial to address questions related to time of maturation 

 and frequency of spawning. A distinct positive shift in 

 Sr/Ca ratio at 5 years in samples MD-1 and MD-3 could 

 be indicative of maturation or onset of coastal migra- 

 tion. Current estimates of age-at-maturation for 

 Chesapeake population females indicate that <30% of 

 females are mature by age 5 years (Maryland DNR 

 1991). Kohlenstein (1981) showed through a tagging 

 study that the majority of female striped bass migrate 

 by 5 years. Lack of a shift in MD-2 might indicate that 

 this individual was a male or had a different migration 

 history. 



All Maryland striped bass samples showed annual 

 peaks and valleys in Sr/Ca ratios. Based on a salinity 

 effect, it can be inferred that valleys represent excur- 

 sions into strontium-poor freshwater habitats. Assum- 

 ing that large, mature adults venture into freshwater 

 or low-salinity habitats to spawn, then spawning fre- 

 quency can be estimated. 



Precision error and spatial resolution of the electron 

 microprobe analysis is critical in the proposed applica- 

 tion of charting individual migratory histories. Preci- 

 sion errors were indicated by differences in Sr/Ca pat- 

 terns and overall level between transects of the same 

 sample (e.g., MD-3). Changing spatial resolution be- 

 tween measurements of 20 and 13f^m permitted 

 greater resolution of seasonal (subannular) patterns. 

 A more complete series of measures along a transect 

 is taken at lower step sizes because gaps between 

 measured points become narrower. This effect could 

 explain variation in mean Sr/Ca levels among transects 

 for the same sample. Alternatively, lack of agreement 

 between transects could indicate machine precision 

 limits in detecting Sr/Ca levels. 



A decline in otolith growth rate wdth age also caused 

 a decrease in spatial resolution (Fig. 4). At a 13fim step 

 size, four or five measurements were taken per annular 

 increment in fish >7 years. Therefore, each measure- 

 ment can correspond to several months of the fish's 

 life. This would explain why nadirs in Sr/Ca ratio rarely 

 approached zero after the 5th annulus. Tagging studies 

 indicate that adult striped bass can migrate between 

 freshwater and coastal habitats within a month (Man- 

 sueti 1961, Chapoton and Sykes 1961, Waldman et al. 

 1990). Peaks and nadirs observed in otolith Sr/Ca ratio 

 may therefore represent temporally-averaged values. 

 Laboratory verification studies are planned to gauge 

 the spatial sensitivity of otolith microchemistry to 

 resolve changes in ambient salinity. 



Other life-history applications 



An ingenious application of the Sr/Ca method to early- 

 life-history consequences of anadromy was made by 

 Kalish (1990). He was able to detect Sr in the core of 

 salmonid otoliths (the earliest deposited material). 

 Under the rationale that maternally-derived protein in- 

 fluenced offspring otolith microchemistry, it was pos- 

 sible to segregate offspring on the basis of whether 

 they originated from eggs spawned by anadromous 

 (high Sr/Ca) or nonanadromous (low Sr/Ca) females. In 

 my study, a single striped bass juvenile of knowTi anad- 

 romous parentage had no detectable Sr in the otolith 

 core. In contrast to salmonid embryos and larvae, 

 striped bass obtain relatively small amounts of mater- 

 nal protein and lipids, and the period of endogenous 

 feeding is considerably shorter. Also, the chorion of 

 striped bass eggs is highly permeable; therefore, am- 

 bient concentrations of Sr could have a greater in- 

 fluence on otolith microchemistry. 



Substantial variation in Sr/Ca ratio occurred for 

 young adult (<5 years old) Chesapeake fish. In all 

 samples, values ranged below detection limits. Pre- 

 sumably these values represent excursions unrelated 

 to spawning by young fish into freshwater systems. 

 Freshwater and slightly saline environments in the 

 upper reaches of Chesapeake Bay tributaries may serve 

 as foraging grounds. Future research could analyze the 

 duration and seasonality of freshwater habitation by 

 fishes that reside in the Chesapeake Bay. 



Further verification studies are needed to establish 

 whether estuarine and marine phases can be distin- 

 guished using Sr/Ca ratios. Ratios tended to continu- 

 ously increase following nadirs, and this pattern could 

 indicate exposure to waters of increasing salinity. A 

 verification study could be carried out by probing the 

 last deposited otolith material (the edge) for Sr/Ca and 

 relating that ratio to the salinity in which the striped 

 bass was sampled in the field or through laboratory 

 rearing studies (Kalish 1989, Townsend et al. 1989, 

 Radtke et al. 1990). A key comparison will be that be- 

 tween samples from estuarine habitats (salinity 5-20 

 ppt) and marine habitats ( > 32 ppt). Should differences 

 be detectable between these groups, then it will be 

 possible to infer detailed patterns of anadromy and 

 related life-history traits. 



Acknowledgments 



Dr. Philip Picoli at the Central Facility for Microanal- 

 ysis, University of Maryland, generously provided 

 expertise and assistance with the electron microprobe. 

 Drs. Ed Houde, John Kalish, and David Townisend gave 

 useful criticisms on earlier versions of this manuscript. 



