Secor and Piccoli: Oceanic migration rates of Morone saxatilis. determined by otolith microchemlcal analysis 



63 



odds of recruitment over the lifetime of a fish (Secor. 

 2000a, 2000b, in press). Life-table-based models (e.g.. 

 Goodyear, 1984) depend upon the assumption that an- 

 nual spawning occurs, which remains unsubstantiated 

 for this species. If spawning frequency declines with 

 age, for instance, then generation time and age-at- 

 maximum reproductive value will be substantially over- 

 estimated, which in turn will affect biological reference 

 points (Marshall et al., 2003). 



Electron probe micro-analysis (EPMA) of Sr has 

 been developed as a method to reconstruct individual 

 patterns of migration and habitat use by anadromous 

 populations of striped bass (Secor, 1992). In estuarine 

 environments, strontium is often a reliable tracer of 

 salinity; higher marine concentrations (7 ppm) become 

 diluted in estuarine environments by freshwater inputs 

 when freshwater Sr:Ca end-members (where "end-mem- 

 bers" are the source of a Sr:Ca ratio) are low (Ingram 

 and Sloan, 1992). Kraus and Secor (2004a) surveyed 

 available data and determined that 83% of estuaries 

 have low freshwater end-members, indicating that the 

 ratio of Sr to Ca should be positively (but not neces- 

 sarily linearly) related to salinity in most estuaries. 



In our study we applied EPMA to examine the frac- 

 tion of Chesapeaka Bay striped bass that migrate to 

 ocean waters and the frequency at which females and 

 males undertake spawning runs. We previously used 

 this method to chart age- and sex-specific patterns of 

 Hudson River striped bass (Secor and Piccoli, 1996; Zlo- 

 kovitz and Secor, 1999; Secor et al., 2001). Expectations 

 for ontogenetic rates of emigration (i.e., Kohlenstein, 

 1981) and annual spawning were tested. In addition, 

 we sought evidence for contingent groups ( subpopulation 

 groups with similar lifetime migration patterns; Secor, 

 1999), which we observed previously in Hudson River 

 striped bass. 



Material and methods 



Samples 



Samples collected during spawning runs present the best 

 opportunity to collect a representative sample of mixed 

 age classes, sexes, and migratory behaviors. These sam- 

 ples comprise mostly those ages that have fully recruited 

 to the mature population. We note in our study that 

 this sample incompletely represents migratory behav- 

 iors for those females that have not yet become mature 

 and are not participating in the years spawning run. 

 During the period 15 April-30 May 2000, we obtained 

 samples of 247 male and 122 female striped bass from 

 the upper Chesapeake Bay (N. of 39°00'; n=21), mid- 

 Bay (N. of 37='53WS. of 39°00'; r?=76); Choptank River 

 (/! = 199), Patuxent River (« = 33), and Potomac River 

 (71 = 28). Capture methods were diverse and included 

 the use of gill- and pound-nets (Maryland Department 

 of Natural Resources monitoring), electro-shocking 

 (National Marine Fisheries Service Northeast Center 

 and University of Maryland scientific collections), and 



charter boat angling. All fish were measured (fork length 

 [FL] and weight [g]), sex and diet were determined, 

 and otoliths and scales were collected. Fork lengths 

 ranged from 685 to 1110 mm for females and from 320 

 to 1029 mm for males. 



Otolith Sr:Ca measures 



To conduct EPMA analyses, otoliths (sagittae) were 

 extracted, soaked in \% sodium hypochlorite solution, 

 rinsed with deionized water, and embedded within a 

 resin (Secor et al., 1992). Transverse sections, approxi- 

 mately 1 mm thick, were cut through the otolith cores 

 with a metallurgical wafering saw. The sections were 

 mounted on glass slides, polished on wetted 600-grain 

 sandpaper, and polished again on a slurry of 0.3-^m 

 alumina until their surfaces were free of pits and 

 abrasions, which can cause artifacts in microprobe 

 analysis (Kalish, 1990). Annuli were enumerated based 

 upon standard criteria under optical microscopy (Secor 

 et al., 1995b). Before analysis, otoliths were cleaned 

 ultrasonically and carbon-coated in a high-vacuum 

 evaporator. 



X-ray intensities for Sr and Ca were quantified by 

 using a JEOL 8900 electron probe microanalyzer (Cen- 

 ter for Microscopy and Microanalysis, Univ. Maryland, 

 College Park, MB). Calcite (CaCOg) and strontianite 

 (SrCOg) were used as reference standards and the 

 protocol was checked by using secondary standards 

 containing both Ca and Sr. The details of this analy- 

 sis can be found elsewhere (Secor and Piccoli, 1996). 

 Detection limits for Sr were approximately 230 ppm 

 (±2 standard errors). Four slides, each containing four 

 otolith sections, were loaded into the specimen chamber 

 of the microanalyzer. After initial calibration to Sr and 

 Ca standards (at programmed settings and intervals), 

 transect assignments were made for up to 16 otolith 

 sections. Transects comprised a series of point mea- 

 surements from young to old ages across the sectioned 

 otolith. X-ray maps of otolith structure were collected 

 by using wavelength spectrometers. 



Sr was expressed as a ratio of Ca (Sr:Ca) because of 

 expected competitive interactions between the isotopic 

 species (Kraus and Secor, 2004a). Further Sr:Ca records 

 were converted to salinity exposure profiles according 

 to the model (Secor et al., 1995a): 



''Salinity inhabitance" (sic) (psu) 



40.3 (l-h56.3 e -1523(Sr:Cal )-l. 



r2=0.94 ; n = 54 



where "salinity inhabitance" (sic) is the salinity level 

 (practical salinity units, psu) in the otoliths for the 

 period of time represented for each Sr:Ca datum. 



Oceanic incidence of striped bass 



For our analysis, a subsample of 122 fish (40 males 

 and 82 females) was drawn from the upper Bay (n = lQ], 

 mid-Bay (/?=46), and Choptank River (?i = 66). Esti- 

 mated salinity records for the last year of life (recent 



