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Stability of elemental signatures in the scales of spawning weakfish, Cynoscion regalis Brian K. Wells, Simon R. Thorrold, and Cynthia M. Jones

Abstract: We quantified elemental signatures in scales of ages-1 and -2 weakfish, Cynoscion regalis, collected during the spawning season in Pamlico Sound, Chesapeake Bay, and Delaware Bay in 1998. We compared these signatures with elemental signatures from scales of juvenile weakfish collected while still resident in natal estuaries at five locations along the Atlantic coast in 1996 and 1997. Although Mg/Ca and Mn/Ca were lower in the juvenile portion of scales from adults compared with scales from juvenile fish, Sr/Ca and Ba/Ca were similar in the three age groups. We compared scale and otolith chemistries from juveniles and adults to determine if relative concentrations of elements/Ca in scales remained consistent, even if absolute levels were altered. Scale Mn/Ca and Ba/Ca remained correlated with those in otoliths of adult fish. Finally, we examined the ability of elemental signatures in scales to act as natural tags of natal estuaries in spawning weakfish. Allocation of fish to natal estuaries based on geochemical signatures in scales and otoliths from age-1 fish was similar; however, allocation was different for age-2 fish. Elemental signatures in scales degraded after the juvenile period and after maturation were insufficiently stable for use as a natural tag of natal origins in weakfish. Résumé : Nous avons quantifié les signatures d’éléments chimiques dans les écailles d’acoupas royaux, Cynoscion regalis, d’âges 1 et 2, récoltés durant la saison de fraye dans le détroit de Pamlico et dans les baies de Chesapeake et de Delaware en 1998. Nous avons comparé ces signatures à celles d’écailles d’acoupas juvéniles récoltés alors qu’ils résidaient toujours dans les estuaires où ils sont nés, à cinq sites le long de la côte atlantique en 1996 et 1997. Bien que les rapports Mg/Ca et Mn/Ca soient plus bas dans la portion juvénile des écailles des adultes par comparaison aux rapports mesurés chez les jeunes poissons, les rapports Sr/Ca et Ba/Ca sont semblables dans les trois classes d’âge. Nous avons comparé les données chimiques des écailles et des otolithes des jeunes et des adultes pour voir si les concentrations relatives des éléments demeurent stables par rapport au Ca, même si les concentrations absolues ont changé. Les rapports Mn/Ca et Ba/Ca des écailles restent reliés à ceux des otolithes chez les adultes. Enfin, nous avons évalué le potentiel des signatures d’éléments chimiques dans les écailles pour servir d’étiquette naturelle pour identifier l’estuaire d’origine des acoupas sur les frayères. Les identifications des estuaires d’origine d’après les signatures géochimiques des écailles sont similaires à celles des otolithes chez les poissons d’âge 1, mais elles sont différentes à l’âge 2. Les signatures d’éléments chimiques dans les écailles se dégradent après la période juvénile et, après la maturation, elles n’ont plus la stabilité nécessaire pour servir d’étiquette naturelle des sites d’origine des acoupas. [Traduit par la Rédaction]

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Introduction Geochemical tracers in the calcified structures of fishes and marine invertebrates are becoming more commonly used as natural tags of population affinities and natal origins (e.g., Pender and Griffin 1996; DiBacco and Levin 2000; Thorrold et al. 2001). The use of isotopic and elemental markers as natural tags is based on the assumption that variations in the physical and chemical environment are recorded in skeletal tissue of individuals that are geographically isolated for at least some part of their life histories. Otoliths are generally considered the best option for use of geochemical signatures

as natural tags in fish populations because concentrations of at least some elements in otoliths are highly correlated with environmental levels (Bath et al. 2000; Wells et al. 2003) and because of the unique patterns of daily and annual increments visible in sectioned otoliths (Campana and Thorrold 2001). However, otolith methods also have significant disadvantages because the fish must be sacrificed and mutilated to recover the otoliths. Scales may represent a nonlethal, nondestructive alternative to otoliths for use as a natural tag of natal origins. Concentrations of at least some elements in fish scales are highly correlated with environmental chemistry (Wells et al. 2000a,

Received 9 August 2002. Accepted 8 March 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 13 May 2003. J17038 B.K. Wells,1,2 S.R. Thorrold,3 and C.M. Jones. Center for Quantitative Fisheries Ecology, Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529, U.S.A. 1

Corresponding author (e-mail: [email protected]). Present address: NOAA/NMFS, 110 Shaffer Rd., Santa Cruz, CA 95060, U.S.A. 3 Present address: Biology Department, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, U.S.A. 2

Can. J. Fish. Aquat. Sci. 60: 361–369 (2003)

J:\cjfas\cjfas60\cjfas6004\F03-028.vp Tuesday, May 13, 2003 10:21:32 AM

doi: 10.1139/F03-028

© 2003 NRC Canada

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2003). For example, Wells et al. (2000b) demonstrated that elemental signatures in scales of juvenile weakfish (Cynoscion regalis) were specific to natal estuaries along the east coast of the United States, which was similar to results based on the otoliths from the same fish (Thorrold et al. 1998). However, the usefulness of scale geochemistry as a natural marker is dependent on the stability of the elemental signatures to the age when determination of natal location is desired. Although otoliths are generally considered to be metabolically inert (Campana 1999), the stability of scale chemistry has not been properly tested. It is known, for instance, that scales are vulnerable to resorption during times of increased calcium demand (Bilton and Robins 1971; Bilton 1975). Scales may also continue to crystallize after circuli are initially formed (Fouda 1979). The aim of our research was to determine if scale chemistry was sufficiently stable to be used as a natural tag of natal origins of adult weakfish. Our analysis of scale chemistry stability was in two parts. First, we examined correlations between the core chemistry of otoliths and scales from adult (1- and 2-year-old) weakfish. We then estimated the natal origins of adult weakfish based on elemental signatures in scales. The accuracy of these assignments was tested by comparing the results with estimates obtained from geochemical signatures in otoliths from the same fish.

Methods Study species Weakfish support recreational and commercial fisheries along the Atlantic coast of the United States. Adults follow a seasonal migration, moving south and offshore during the fall and winter and north and inshore during the spring and summer spawning season (Nesbit 1954; Wilk 1979). Spawning occurs throughout the species range in estuarine and nearshore waters. Larvae remain in their respective nursery areas through spring and summer by using selective tidal transport (Rowe and Epifanio 1994). In the fall, juveniles migrate from estuaries to coastal marine waters south of Cape Hatteras, North Carolina, U.S.A., before reinvading estuaries the following spring. Most weakfish reach sexual maturity at age 1, with remaining individuals becoming sexually mature at age 2 (Lowerre-Barbieri et al. 1996). Importantly, in the context of the present study, each fish experiences at least two distinctly different environments as juveniles in the estuaries and adults in the coastal marine zone. Therefore, the stability of the natural elemental signatures in the scales of juvenile weakfish can be inferred from a lack of significant alteration of the estuarine elemental signature while resident in the marine environment. Quantification of the natal-estuary elemental signatures In a previous study (Wells et al. 2000b), we quantified elemental signatures (Mg/Ca, Mn/Ca, Sr/Ca, and Ba/Ca (Mg, magnesium; Ca, calcium; Mn, manganese; Sr, strontium; Ba, barium)) in scales of juvenile (age-0+) weakfish from the 1996 and 1997 cohorts using laser ablation inductively coupled plasma mass spectrometry (ICP-MS). We found significant differences in elemental signatures among five estuaries along the Atlantic coast of the United States (Doboy Sound, Pamlico Sound, Chesapeake Bay, Delaware Bay, and Peconic

Can. J. Fish. Aquat. Sci. Vol. 60, 2003

Bay). However, before these signatures could be used to determine natal origins of adult fish, we first needed to assess the stability of these signatures in adult weakfish. Adult data collection Age-1 and age-2 weakfish were collected between June and September 1998 from Pamlico Sound, Chesapeake Bay, and Delaware Bay using a long-haul seine, a pound net, and a 10m otter trawl, respectively (Fig. 1). Ages were determined from validated annuli in thin-sectioned otoliths (LowerreBarbieri et al. 1994). Scales were removed from the fish and decontaminated for elemental analyses. The cleaning process included sonication for 5 min in 3% ultrapure hydrogen peroxide to loosen organic debris followed by two rounds of washing with an acid-washed electric rotary toothbrush and Milli-Q water (Millipore, Billerica, Mass.). This methodology was identical with the preparation of juvenile scales for elemental analysis in our previous study. Scales were then secured onto petrographic slides with mounting tape for elemental analysis by laser ablation ICP-MS. Otoliths from the same fish were analyzed as part of a larger study examining natal homing of adult weakfish. Details on the otolith geochemical analyses can be found in Thorrold et al. (1998) and Thorrold et al. (2001). Chemical analysis of adult scales The elemental composition of scales from adult weakfish was analyzed using a Thermo Finnigan Element2 sector field ICP-MS (San Jose, Calif.) and a New Wave LUV266 laser ablation system (Fremont, Calif.) operating at 266 nm (Thorrold and Shuttleworth 2000). A square 0.3-mm2 raster was ablated through the osseous layer from the core toward the anterior edge representing the same period of growth assayed in the juvenile scales so that direct comparisons could be made with the juvenile scales assayed in our earlier study (Wells et al. 2000b). The ablated material was swept by a carrier gas (He) into a dual-inlet quartz spray chamber. The He stream was then mixed with a wet aerosol (1% HNO3) from a 20 µL·min–1 self-aspirating nebulizer. A total of five isotopes were quantified (25Mg, 48Ca, 55Mn, 86Sr, and 138Ba), all in analog mode and in low resolution (r = 300). A blank sample consisting of the 1% HNO3 wet aerosol and a matrix-matched laboratory standard were introduced every 10 samples through the nebulizer to account for deposition on cones and changes in elemental mass bias through time. Measurement precision (relative standard deviation) of our lab standard, uncorrected for variations in elemental mass bias, was as follows (N = 33): Mg/Ca, 4.6%; Mn/Ca, 1.1%; Sr/Ca, 1.7%; Ba/Ca, 2.8%. These estimates are likely to be conservative because scale samples were corrected for mass bias using the standard. We calculated detection limits as 3σ values of 1% HNO3 sample blanks (N = 33) that were run throughout the analyses. These limits were