Otolith Isotope Research Articles

Spatial subdivision among assemblages of Spanish mackerel, Scomberomorus commerson (Pisces: Scombridae) across northern Australia: implications for fisheries management

ABSTRACTAim This study investigated the use of stable δ13C and δ18O isotopes in the sagittal otolith carbonate of narrow‐barred Spanish mackerel, Scomberomorus commerson, as indicators of population structure across Australia.Location Samples were collected from 25 locations extending from the lower west coast of Western Australia (30°), across northern Australian waters, and to the east coast of Australia (18°) covering a coastline length of approximately 9500 km, including samples from Indonesia.Methods The stable δ13C and δ18O isotopes in the sagittal otolith carbonate of S. commerson were analysed using standard mass spectrometric techniques. The isotope ratios across northern Australian subregions were subjected to an agglomerative hierarchical cluster analysis to define subregions. Isotope ratios within each of the subregions were compared to assess population structure across Australia.Results Cluster analysis separated samples into four subregions: central Western Australia, north Western Australia, northern Australia and the Gulf of Carpentaria and eastern Australia. Isotope signatures for fish from a number of sampling sites from across Australia and Indonesia were significantly different, indicating population separation. No significant differences were found in otolith isotope ratios between sampling times (no temporal variation).Main conclusions Significant differences in the isotopic signatures of S. commerson demonstrate that there is unlikely to be any substantial movement of fish among these spatially discrete adult assemblages. The lack of temporal variation among otolith isotope ratios indicates that S. commerson populations do not undergo longshore spatial shifts in distribution during their life history. The temporal persistence of spatially explicit stable isotopic signatures indicates that, at these spatial scales, the population units sampled comprise functionally distinct management units or separate ‘stocks’ for many of the purposes of fisheries management. The spatial subdivision evident among populations of S. commerson across northern and western Australia indicates that it may be advantageous to consider S. commerson population dynamics and fisheries management from a metapopulation perspective (at least at the regional level).

Atlantic Bluefin Tuna (Thunnus thynnus) Population Dynamics Delineated by Organochlorine Tracers

Atlantic bluefin tuna (ABFT) are highly valued and heavily exploited, and critical uncertainties regarding their population structure hinder effective management. Evidence supports the existence of two breeding populations of ABFT; a western population in the Gulf of Mexico and an eastern population in the Mediterranean Sea; both of which migrate and mix in the North Atlantic. Conventional tagging studies suggest low rates of trans-Atlantic migrations; however, electronic tagging and stable isotopes in otoliths indicate stock mixing up to 57% between management zones delineated by 45 degrees W longitude. Here we show that organochlorine pesticides and polychlorinated biphenyls (PCBs) can be used as tracers of bluefin tuna foraging grounds in the North Atlantic and confirm that stock mixing of juvenile tuna within the U.S. Mid Atlantic Bight is indeed high (33-83% eastern origin), and is likely spatially and temporally variable. We further demonstrate that >10% of the Mediterranean population is migratory, that young bluefin tuna migrate from the Mediterranean to western Atlantic foraging grounds as early as age 1, and then return to the Mediterranean Sea as young as age 5, presumably to breed. The tracer method described here provides a novel means for distinguishing bluefin tuna populations and ontogenetic shifts in migration in the North Atlantic.

Inter-annual variability in isotope and elemental ratios recorded in otoliths of an anadromous fish

Isotope ratios and elemental concentrations in otoliths are often used as natural tags to reconstruct migratory movements and connectivity patterns in marine and anadromous fishes. Although differences in otolith geochemistry have been documented among geographically separated populations, inter-annual variation within locations is less frequently examined. We compared otolith isotope (δ 18O and 87Sr: 86Sr) and elemental ratios (Sr:Ca and Ba:Ca) from several annual cohorts of juvenile American shad ( Alosa sapidissima) in three rivers. These four geochemical signatures distinguished among river-specific populations of this species at both large and small geographic scales, with δ 18O and 87Sr: 86Sr generating the majority of multivariate variation. We found significant variation among years for all variables in two to three rivers. However, the magnitude of variability differed among ratios, with δ 18O ratios showing substantial inter-annual shifts while 87Sr: 86Sr ratios were relatively stable across years. Sr:Ca and Ba:Ca ratios also varied among years. These results imply that investigators using environmentally labile signatures must quantify geochemical signatures for each cohort of interest in order to confidently identify origins of migrants.

Evidence of trans-Atlantic movement and natal homing of bluefin tuna from stable isotopes in otoliths

Stable carbon and oxygen isotopes in the otoliths of Atlantic bluefin tuna Thunnus thyn- nus were used to investigate trans-Atlantic movement and natal homing behaviors. Otolith δ 13 C and δ 18 O of yearling bluefin tuna from 6 different year classes (1999 to 2004) were characterized to vali- date the approach and assess inter-annual variation in these markers for individuals from eastern (Mediterranean Sea/eastern Atlantic Ocean) and western (USA Atlantic Ocean) nurseries. Otolith δ 18 O of yearlings from the eastern nursery were enriched relative to their western counterparts, while otolith δ 13 C was relatively similar between nurseries. Temporal variability in otolith δ 13 C and δ 18 O of bluefin tuna was observed; nevertheless, classification success to eastern and western nurseries was relatively high (87%) when all year classes were pooled. Otolith δ 13 C and δ 18 O of yearlings were then used as a baseline sample ('eastern' and 'western' nursery signature) to predict the origin of adoles- cents (20 to 70 kg) from 1 foraging area (USA Atlantic Ocean) and adults (>140 kg) from 1 spawning ground (Mediterranean Sea) based on otolith core signatures (corresponding to yearling period). Mixed-stock analysis indicated that approximately 60% of the adolescent bluefin tuna collected from foraging areas in the USA Atlantic Ocean originated from the eastern nursery, suggesting that sub- stantial trans-Atlantic movement of adolescents from east to west occurred. In addition, natal homing was well developed, with 94% of the adult bluefin tuna collected in the Mediterranean Sea derived from the eastern nursery. Findings from this study suggest that USA fisheries depend upon migrants of Mediterranean origin and that mixing across the 45° W management boundary is substantially higher than assumed previously.

The importance of Colorado River flow to nursery habitats of the Gulf corvina (Cynoscion othonopterus)

We test the hypothesis that Colorado River flow is important in providing nursery habitat for the Gulf corvina (Cynoscion othonopterus), a commercially valuable and endemic fish in the upper Gulf of California. We use oxygen isotopes in otoliths to determine when these fish inhabit isotopically different bodies of water (Gulf of California versus the Colorado Estuary). The δ18O values in the natal otoliths of C. othonopterus, significantly more negative than can be predicted by temperature alone, provide evidence that this species uses the brackish habitats created by flow of the Colorado River. A significant log-linear relationship between the natal δ18O values and the cumulative flow of the Colorado River during natal development confirms use of brackish habitat in years that the Colorado River water reached the Gulf. Natal δ18O values indicate that C. othonopterus seek out estuarine habitats with salinities between 26‰ and 38‰. Reduction in Colorado River flow since the construction of upstream dams has reduced the size of nursery habitat for C. othonopterus. Our results support the hypothesis that declines in commercial landings can be at least partially attributed to reduced river flow. Increased flow would increase nursery habitat and likely benefit recruitment.

Validation of Sr isotopes in otoliths by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS): opening avenues in fisheries science applications

Advances in probe-based mass spectrometry allow for high spatial resolution of elemental and isotopic signatures in fish otoliths that can be used to address fundamental questions in fisheries ecology. Analyses of Chinook salmon (Oncorhynchus tshawytscha) otoliths from two river populations yield identical 87Sr/86Sr ratios using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) and thermal ionization mass spectrometry (TIMS). Results were obtained from freshwater otoliths with low Sr concentrations (300–800 ppm) using high spatial resolution (50 µm) corresponding to temporal histories of ~12 days fish growth. Low natural variation in 87Sr/86Sr among otoliths from the same rivers allows for conservative estimates of external precision of techniques. Thus, we demonstrate that Sr isotope ratios obtained by LA-MC-ICPMS can be accurate and precise, bypassing the time-intensive sample preparation required by microdrilling and TIMS. This technique opens the use of Sr isotopes for broader ecological questions requiring large sample sizes to characterize nursery habitats, metapopulation dynamics, and stock discrimination similar to studies that focus on elemental concentrations, thereby providing a more robust tool for some freshwater and diadromous fishes.

Temperature-dependent fractionation of stable oxygen isotopes in otoliths of juvenile cod (Gadus morhua L.)

Abstract Analysis of stable oxygen isotopes in otoliths is a promising technique for estimating the ambient temperature experienced by fish, but consistent equations relating temperature and fractionation of stable oxygen isotopes in otoliths among different fish species are lacking. Juvenile cod were reared at constant temperatures from 6 to 20°C and the sagittal otoliths were analysed for oxygen isotope values. We determined that temperature-dependent fractionation of oxygen isotopes in the otoliths was close to that reported for inorganic aragonite at low temperatures, but there were deviations from oxygen isotope fractionation equations for otoliths of other species. The linear relationship between oxygen isotope value in the cod otoliths and temperature was determined to be: 1000 Ln α = 16.75(103 TK−1) − 27.09. Temperature estimates with 1°C precision at the 95% probability level require a sample size of ≥5 otoliths. Only an insignificant amount of the variance in the data was due to variance between left and right otolith, and due to repeated measurements of otolith subsamples. This study confirms that stable isotope values of cod otoliths can give precise and accurate estimates of the ambient temperature experienced by fish.

Effect of somatic and otolith growth rate on stable isotopic composition of early juvenile cod ( Gadus morhua L) otoliths

The relative amounts of the stable isotopes of carbon and oxygen in fish otoliths can be used to reveal the environmental history experienced by the fish. This requires that the relative amounts of the isotopes are deposited in equilibrium with the surrounding environment, or that the offset from this equilibrium is known and can be quantified. It is known that carbon isotopes in biogenic carbonates are a mixture of carbon from the seawater and metabolically derived carbon, but the effect of the somatic growth rate of the fish is still unclear. The possible effect of otolith growth rate and fractionation of both carbon and oxygen isotopes are also not established. We carried out a controlled laboratory experiment where we reared cod ( Gadus morhua L.) larvae and early juveniles at two temperatures (6 and 10 °C) and generated different growth rates within each temperature by manipulation of prey levels. The otoliths of the resulting fish were analysed for carbon and oxygen isotopes. We found no effect of otolith precipitation rate on fractionation of either carbon or oxygen isotopes. However, there was a depletion of 13C in the otoliths of fish with elevated metabolism. The proportion of metabolically derived carbon in the otoliths was estimated to be 28–32%. Our results suggest that measurements of oxygen isotopes in otoliths can be a reliable tool to estimate ambient temperature since the oxygen isotopes seem to be deposited in the otoliths independently of kinetic and metabolic effects. Fractionation of carbon isotopes in otoliths on the other hand can give valuable insight into metabolism and feeding pattern of fish.

Reconstructing the lives of fish using Sr isotopes in otoliths

For many species, understanding the processes underlying variation in life history strategies is limited by the difficulty of tracking individuals throughout their lives. Within the rapidly expanding field of otolith microchemistry, novel approaches are being combined with state-of-the-art analytical techniques to provide new and valuable information about the environmental history of fishes. However, no approach to date allows the reconstruction of fish movements at high temporal resolution (weeks to months) over relatively small spatial scales (1–10 km). We used micromilling techniques to extract strontium (Sr) isotopic signatures from the otoliths of four returning Atlantic salmon (Salmo salar) adults. Distinct Sr isotopic signatures were detectable from four life cycle stages, including prefeeding hatchery development, rearing stream growth, smolt out-migration, and ocean residence. High-resolution analyses of Sr isotope records establish that natal stream signatures are recoverable and show that both site fidelity within the freshwater stage and the timing of migration vary considerably among individuals. Results made possible with this approach provide insight into a long-standing debate on the mobility of salmon during their nonmigratory stage. The ability to resolve flexible behaviors of salmon increases our understanding of their population biology and conservation needs.

Stable d13C and d18O isotopes in otoliths of haddock Melanogrammus aeglefinus from the northwest Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 216:223-233 (2001) - doi:10.3354/meps216223 Stable d13C and d18O isotopes in otoliths of haddock Melanogrammus aeglefinus from the northwest Atlantic Ocean Gavin A. Begg*, Christopher R. Weidman** National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543, USA *Present addresses: CRC Reef Research Center, James Cook University, Townsville, Queensland 4811, Australia **Woods Hole Oceanographic Institution, Department of Geology and Geophysics, McLean 200A, Woods Hole, Massachusetts 02543, USA. E-mail: cweidman@whoi.edu ABSTRACT: Analyzing stable isotopes deposited in fish otoliths can provide insights into the environmental variation that individual fish experience throughout their life history which, in turn, can indicate their home range, movements, and underlying stock structure. We investigated the use of stable δ13C and δ18O isotopes in haddock Melanogrammus aeglefinus otoliths as indicators of environmental variation and stock structure in the northwest Atlantic Ocean. Samples were collected between 1995 and 1998 from individual spawning components on Georges Bank, the Gulf of Maine, and along the Scotian Shelf. Samples from 1 yr- and 4 yr-old haddock were analyzed from an integrated and chronological life history perspective, respectively. Annual, seasonal, and ontogenetic variation in otolith isotopes indicated that haddock undergo shifts in distribution throughout their life history, possibly as a means of compensating for variable environmental conditions; this underlines the difficulty of temperature reconstructions from mobile organisms with optimal temperature ranges. KEY WORDS: Otolith isotopic composition · Carbon · Oxygen · Georges Bank · Retrospective environmental regimes · Stock identification · Fisheries management Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 216. Online publication date: July 06, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.