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Otolith elemental signatures reflect residency in coastalwater masses
Mary M. Nishimoto & Libe Washburn &Robert R. Warner & Milton S. Love &Georges L. Paradis
Received: 13 October 2009 /Accepted: 20 July 2010 /Published online: 11 August 2010# The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract We examined variability in otolith chemis-try of wild caught fish in relation to in situtemperature and salinity within the California CurrentSystem. Barium, magnesium, and iron from the mostrecent growth zone in otoliths differentiated pelagicjuvenile shortbelly rockfish (Sebastes jordani) resid-ing in water masses with distinct temperatureand salinity properties from central and southernCalifornia spanning nearly 500 km of coastline. The3-element signature also discriminated fish thatresided in different water masses that were associatedwith mesoscale cyclonic eddy circulation in the SantaBarbara Channel. Variability in otolith chemistryreflected the spatial patterns of both horizontalgradients and vertical gradients in water mass prop-erties related to circulation. Although we found thatthe concentrations of particular elements in otolithswere correlated to ambient temperature or salinity, we
suggest that these parameters are more useful as anidentifying signature of distinct water masses associ-ated with unique otolith signatures rather than asfactors directly affecting otolith chemistry. Otherfactors varying among the water masses or amongthe fish populations residing in the water masses mayalso affect otolith chemistry. We discuss how ocean-ographic phenomena associated with the 19971998El Nio and the persistent, recirculating eddy in theChannel may have affected coastal ocean conditionsand variation in otolith chemistry of fish in the studyarea.
Keywords Otolith microchemistry . Coastalcirculation .Water masses . Early life history
Elemental and isotopic assays of fish otoliths havebeen directed at questions concerning the tracking andmixing of marine and freshwater populations. Infer-ences about fish homing behavior (Secor et al. 1995;Thorrold et al. 2001), connectivity estimates (Millerand Shanks 2004; Standish et al. 2008), and larvaldispersal histories (Swearer et al. 1999; FitzGerald etal. 2004; Hamilton et al. 2008) have been based onotolith chemistry as geographic tags. These applica-tions of otolith chemistry do not necessarily requirethe determination of how element concentrations,salinity, and temperature in the environment are
Environ Biol Fish (2010) 89:341356DOI 10.1007/s10641-010-9698-6
M. M. Nishimoto (*) :M. S. Love :G. L. ParadisMarine Science Institute, University of California,Santa Barbara, CA 93106, USAe-mail: email@example.com
L. WashburnInstitute for Computational Earth System Scienceand Department of Geography, University of California,Santa Barbara, CA 93106, USA
R. R. WarnerEcology, Evolution, and Marine Biology,University of California,Santa Barbara, CA 93106, USA
related to element concentrations in the otolith; whatis required is that the chemical assays of fish otolithsdiscriminate individuals from different locations.However, at least two important limitations inutilizing otolith chemistry to assess the movement ofindividuals and the mixing of populations amonglocations stem from not knowing how environmentalvariability relates to the spatial and temporal variabil-ity in otolith chemistry.
The first limitation is the reliability of site-specificotolith elemental signatures over time. Sufficientconsistency in the spatial variability of otolithelemental signatures allows discrimination amonghabitat sites within a year; however, many studiesthat have compared otolith chemical compositionamong locations over two or more years have foundsignificant interannual variability (Milton et al. 1997;Dove and Kingsford 1998; Patterson et al. 1999;Campana et al. 2000; Gillanders 2002; Hamer et al.2003; Miller and Shanks 2004; Brown 2006). Thistemporal inconsistency is especially problematic if thegoal, for example, is to examine a population overtime to estimate connectivity via migration or larvaldispersal among spatially distinct habitats.
The second limitation is that individuals ofunknown residency can only be assigned to the setof sampled locations that define the spatial variabilityof the signatures. There is an unknown degree of errorin assigning fish of unknown residency to a specificsite of a set of sampled locations, because fishresiding in unsampled locations might share the sameelemental signature as fish from the set of sampledlocations. This can occur even if the spatial variabilityof the otolith elemental signatures defined by a set ofsampled locations is consistent over time and theelemental signatures are good discriminators of thesites sampled.
Presently, inferences are limited to the geographicscale of sampling, the localities sampled, and the timeof sampling within individual studies, because of theuncertainty of changing environmental conditions thataffect the variability in otolith chemistry over timeand space. We turn our attention from the utility ofusing otolith chemistry as a location-based discrimi-nator, and focus on the prospect of utilizing otolithchemistry to identify water mass residency. Amotivation for this study is that if unique elementalsignatures are associated with distinct water masses,then a reference or atlas identifying these relation-
ships might be developed. Otolith chemistry andoceanography might prove useful for reconstructingthe transport histories of individuals and estimatingthe connectivity of populations across broad regionsand over time.
The central goal of this study was to test thehypothesis that variability among otolith trace ele-ment signatures of wild-caught fish was associatedwith in situ water masses. To identify distinct watermasses, we examined temperature and salinity mea-sured during pelagic juvenile fish sampling off centralCalifornia in May 1998 (Sakuma et al. 2000) and offthe southern California coast in June 1998 (Nishimotoand Washburn 2002) spanning a distance of nearly500 km (Fig. 1). The otolith signatures of shortbellyrockfish (Sebastes jordani) from these surveys weredefined by a suite of elements assayed from the outerotolith material grown in the recent days precedingcapture. Otolith sampling included fish found con-centrated in a persistent cyclonic eddy in the westernSanta Barbara Channel (Nishimoto and Washburn2002; Figs. 1b and 2). We examined the variability inotolith chemistry in relation to temperature andsalinity across the study area and the three-dimensional circulation of the eddy.
Materials and methods
Water mass sampling and otolith chemistry
Similar methods were used in the central and southernCalifornia surveys (detailed in Wyllie Echeverria et al.1990; Nishimoto and Washburn 2002; Sakuma et al.2000) to collect the otoliths and oceanographic datafor this study. Fish were collected at night with amodified Cobb mid-water trawl with a 9 mm codendtowed at depth for 15 min at ~5 kmh1 covering~1.5 km (Nishimoto and Washburn 2002). Theopening of the net used in both surveys wasapproximately 10 m wide and 14 m high whentrawling at a headrope depth of 20 m. The depthinterval for each haul was estimated as the distancefrom the mean depth, d0, of the headrope (rope acrossthe top of the net opening) to 14 m below d0 (Table 1).Vertical profiles of potential temperature T andsalinity S (averaged into 1-m depth bins) wereobtained either immediately before or after each haulto at least 200 m or a few meters above shallower
342 Environ Biol Fish (2010) 89:341356
bottom depths. Awater mass where fish were sampledwas defined by the profile within the depth intervalfished.
Otoliths from 68 specimens of shortbelly rockfishsampled from the central and southern Californiacollections were assayed using laser ablation induc-tively coupled plasma mass spectrometry (LA-ICPMS) (Fig. 1, Table 1). The fish ranged from
12.8 mm to 47.2 mm SL. Shortbelly rockfish were themost abundant of the pelagic juvenile rockfishescollected in the survey. Adults of this active,schooling species range from southern British Co-lumbia to southern Baja California, and larvae arefound as much as 400 km from shore (Love et al.2002). Fifty-seven specimens represent all areasdefined by Nishimoto and Washburn (2002) where
Pt. Conception SantaBarbara
124 123 122
123.5 122.5 121.5
121 120.5 120 119.5 119
L I F
N I A
Fig. 1 The study area.Midwater trawling andoceanographic surveys inlate spring 1998 wereconducted off (a) centralCalifornia and (b) southernCalifornia. Shortbellyrockfish specimens andtemperature-salinity profileswere collected in centralCalifornia from stationsnumbered 16, and insouthern California fromstations 724. The eddy(shaded gray area) in thewestern Santa BarbaraChannel was defined bystation dynamic height
shortbelly rockfish were collected during 315 June1998 in southern California (Fig. 1b, Table 1). Elevenotolith samples from central California were collectedduring 1127 May 1998 from the Farallon Islandswest of San Francisco to Cypress Point, Monterey(stations 16 in Fig. 1a and Table 1). The otoliths ofone to four specimens per station were assayed(Table 1).
The otolith samples from the two surveys werehandled some