Otolith Signatures Research Articles

Isotopic Correlation (δ18O versus δ13C) of Otoliths in Identification of Groundfish Stocks

AbstractThe management of many marine fish stocks suffers from a lack of genetic data or insignificant genetic differentiations, particularly for the commercially important groundfish along the U.S. Pacific coast. In this study, we investigated a large number of otoliths of various groundfish species from the coasts of Washington and Oregon and analyzed them for stable oxygen (18O/16O, or δ18O) and carbon isotope ratios (13C/12C, or δ13C). The isotopic results and correlation of δ18O versus δ13C of Pacific hake Merluccius productus, sablefish Anoplopoma fimbria, Pacific halibut Hippoglossus stenolepis, and Pacific cod Gadus macrocephalus indicated that most of the groundfish we examined had two or more spawning stocks or subpopulations; the only tested species that might belong to a single coastwide stock was yelloweye rockfish Sebastes ruberrimus. Compared with genetic methods of stock identification, the isotopic signatures of otoliths have distinct advantages in determining (1) a time series to separate the different life stages of marine fish and (2) the oceanographic and environmental conditions to which fish are exposed. Thus, stable isotopic signatures in otoliths appear to be an alternative for marine fish stocks when there is little genetic differentiation between populations or no genetic data are available.

Dispersal between Tributary and Main‐Stem Rivers by Juvenile Smallmouth Bass Evaluated Using Otolith Microchemistry

AbstractJuvenile dispersal and the exchange of offspring among fish populations in river–tributary networks are difficult to characterize, but they may play a substantial role in the dynamics of fisheries in these systems. We used geochemical signatures in otoliths to identify the natal origins of young‐of‐year smallmouth bassMicropterus dolomieuin the James River and a tributary, the Maury River, in Virginia. The trace element signatures (Sr:Ca, Rb:Ca, Mg:Ca, and Ba:Ca) in otoliths differed significantly between these two rivers, resulting in approximately 87% accuracy in classifying known‐origin fry to their natal rivers. The variation in otolith composition reflected the variation in water chemistry within the system. We subsequently used classification functions and stock mixture analyses based on these signatures to identify the natal origins of age‐1 juveniles collected from the same rivers 1 year later. The results indicate that approximately 50% of the age‐1 smallmouth bass collected in the James River were spawned in the Maury River, while no evidence for reciprocal main stem‐to‐tributary exchange was found. These results suggest that the downstream movement and dispersal of smallmouth bass is common in their first year of life. Such inputs from tributaries may play an important role in the dynamics of smallmouth bass river fisheries, which are often characterized by substantial annual variation in recruitment. Characterizing such exchange at broader scales will probably be an important step toward linking the spatial ecology of this species with effective management and conservation strategies in river–tributary networks.

Chemical signatures in the otoliths of a coastal marine fish, Menidia menidia, from the northeastern United States: spatial and temporal differences

Knowledge of population structure in marine systems is fundamental to effective management and conservation. The geochemical signature of otoliths may provide a promising natural tag for quantifying population structure in marine fishes. However, the spatial scale at which chemical signatures differ among species from different environments and with different life histories is not yet clear. We examined chemical signatures in the otoliths of juvenile Menidia menidia, a ubiquitous nearshore marine species found along the east coast of North America that undergoes offshore winter migration. Specimens were collected from 16 locations in 2003 and from 9 locations in 2004 between New Jersey and Maine. Otolith geochemistry was analyzed using laser ablation inductively coupled plasma–mass spectrometry and isotope ratio mass spectrometry. Juvenile fish showed significant site-specific differences and were assigned to natal sites with 70% (2003) and 77% (2004) average cross-validated classification accuracies based on elemental (Mg, Mn, Sr, Ba, Cu, and Pb relative to Ca) and isotope (δ13C, δ18O) ratios using quadratic discriminant function analysis. Geochemical signatures showed significant interannual variation, suggesting natural tags are year-class specific. Results suggest enough heterogeneity exists in marine chemical signatures to track movements and determine whether M. menidia return to their natal, nearshore location upon return from winter migration. Because M. menidia show evidence of local adaptation, knowledge of this winter migration will have evolutionary and ecological implications. Geochemical signatures of otoliths will likely serve as a useful tool in species with similar life histories.

Elemental signatures in otoliths of hatchery rainbow trout (Oncorhynchus mykiss): distinctiveness and utility for detecting origins and movement

Otolith chemistry in freshwater has considerable potential to reveal patterns of origin and movement, which would benefit traditional fisheries management and provide a valuable tool to curb the spread of invasive and illicitly stocked species. We evaluated the relationship between otolith and water chemistry for five markers (Ba/Ca, Mn/Ca, Sr/Ca, Zn/Ca, and 87Sr/86Sr) in rainbow trout ( Oncorhynchus mykiss ) using the existing hatchery system in Colorado and Wyoming, USA, to provide controlled, seminatural conditions. Otolith Ba/Ca, Sr/Ca, and 87Sr/86Sr reflected ambient levels, whereas Mn/Ca and Zn/Ca did not. Using only the markers correlated with water chemistry, we classified fish to their hatchery of origin with up to 96% accuracy when element and isotope data were used together. Large changes in 87Sr/86Sr were evident in otolith transects, although subtler changes in Sr/Ca were also detectable. Our results suggest the relatively few otolith markers that reflect ambient chemistry can discriminate among locations and track movements well enough to provide valuable insight in a variety of applied contexts.

MARKOV CHAIN MONTE CARLO METHODS FOR ASSIGNING LARVAE TO NATAL SITES USING NATURAL GEOCHEMICAL TAGS

Geochemical signatures deposited in otoliths are a potentially powerful means of identifying the origin and dispersal history of fish. However, current analytical methods for assigning natal origins of fish in mixed-stock analyses require knowledge of the number of potential sources and their characteristic geochemical signatures. Such baseline data are difficult or impossible to obtain for many species. A new approach to this problem can be found in iterative Markov Chain Monte Carlo (MCMC) algorithms that simultaneously estimate population parameters and assign individuals to groups. MCMC procedures only require an estimate of the number of source populations, and post hoc model selection based on the deviance information criterion can be used to infer the correct number of chemically distinct sources. We describe the basics of the MCMC approach and outline the specific decisions required when implementing the technique with otolith geochemical data. We also illustrate the use of the MCMC approach on simulated data and empirical geochemical signatures in otoliths from young-of-the-year and adult weakfish, Cynoscion regalis, from the U.S. Atlantic coast. While we describe how investigators can use MCMC to complement existing analytical tools for use with otolith geochemical data, the MCMC approach is suitable for any mixed-stock problem with a continuous, multivariate data.

Continental-scale variation in otolith geochemistry of juvenile American shad (Alosa sapidissima)

We assembled a comprehensive atlas of geochemical signatures in juvenile American shad ( Alosa sapidissima ) to discriminate natal river origins on a large spatial scale and at a high spatial resolution. Otoliths and (or) water samples were collected from 20 major spawning rivers from Florida to Quebec and were analyzed for elemental (Mg:Ca, Mn:Ca, Sr:Ca, and Ba:Ca) and isotope (87Sr:86Sr and δ18O) ratios. We examined correlations between water chemistry and otolith composition for five rivers where both were sampled. While Sr:Ca, Ba:Ca, 87Sr:86Sr, and δ18O values in otoliths reflected those ratios in ambient waters, Mg:Ca and Mn:Ca ratios in otoliths varied independently of water chemistry. Geochemical signatures were highly distinct among rivers, with an average classification accuracy of 93% using only those variables where otolith values were accurately predicted from water chemistry data. The study represents the largest assembled database of otolith signatures from the entire native range of a species, encompassing approximately 2700 km of coastline and 19 degrees of latitude and including all major extant spawning populations. This database will allow reliable estimates of natal origins of migrating ocean-phase American shad from the 2004 annual cohort in the future.

Stable isotope analyses of otoliths in identification of hatchery origin of Atlantic salmon (Salmo salar) in Maine

We conducted stable oxygen and carbon isotope analyses for otoliths of Atlantic salmon (Salmo salar), in an attempt to develop a reference database on isotopic variability among private and federal hatcheries in Maine which currently support the salmon aquaculture industry and recovery of endangered populations. During the first phase of our study, we collected 40–50 sagittal otoliths of juvenile Atlantic salmon from each of the five hatcheries and analyzed for stable oxygen and carbon isotope ratios (18O/16O or δ18O, and 13C/12C or δ13C). Combination of δ18O and δ13C signatures in otoliths showed that the five hatcheries can be clearly separated and chemically distinguished. By identifying stable isotopic variations of otoliths from different hatchery settings, we were able to establish some isotopic criteria or standards to assign a likelihood that an individual Atlantic salmon came from a specific hatchery within the reference database. If successful, a diagnostic tool that can provide definitive information on identification of the hatchery origin could serve as a novel marking technique, and the chemical method may provide a more effective alternative to DNA analysis for mixed stocks. Overall our isotopic data from otoliths support the hypothesis that there are detectable differences between the five hatcheries, and multiple statistical analyses indicated that we can correctly distinguish individual Atlantic salmon into a hatchery with high confidence.

Geochemical Signatures in Otoliths Record Natal Origins of American Shad

AbstractPopulation connectivity is a critical component in the life history dynamics of anadromous fishes and in the persistence of local populations. We used geochemical signatures in the otoliths of American shadAlosa sapidissimato determine natal origins and estimate rates of straying among river‐specific populations along the U.S. Atlantic coast. Stable isotope (δ13C, δ18O and87Sr:86Sr) and elemental (Mg: Ca, Mn: Ca, Sr: Ca and Ba: Ca) signatures in otoliths of juvenile American shad from rivers from Georgia to New Hampshire varied significantly, allowing for an average of 91% cross‐validated accuracy when classifying individual fish to their natal rivers. We also found significant interannual variability in the geochemical signatures from several rivers, due largely to differences in δ18O values among years. We then used the ground‐truthed geochemical signatures in the otoliths of juvenile American shad to identify the natal origins of spawning adults in the York River system in Virginia. Approximately 6% of the spawning adults collected in the York River were strays from other rivers. Of the remaining fish, 79% were spawned in the Mattaponi River and 21% in the Pamunkey River. The combined results of this and other recent studies suggest that although most American shad spawning in the York River were homing to their natal river, there was much less fidelity to individual tributaries. Small‐scale straying could allow fish spawned in the Mattaponi River to subsidize spawning in the Pamunkey River, which has experienced persistent recruitment failure.

Demonstration and implications of habitat‐specific chemical signatures in otoliths of juvenile summer flounder (Paralichthys dentatus Linnaeus) in North Carolina

The objective of this pilot study was to determine if otolith signatures of juvenile summer flounder Paralichthys dentatus (Linnaeus) were differentiable between beach and marsh habitat types within two North Carolina (NC), U.S.A., estuarine systems. Elemental signatures were differentiable between habitats in both systems, and in one system at smaller spatial scales (<5 km) than have been reported for any estuarine species using otolith microchemistry. These results indicate the potential for using summer flounder otolith elemental analysis to assess the nursery role of habitats within NC estuaries.

Otolith Chemistry Discriminates among Hatchery‐Reared and Tributary‐Spawned Salmonines in a Tailwater System

AbstractThe Little Red River tailwater (LRRT) system in Arkansas supports a naturally reproducing population of brown trout Salmo trutta and an economically valuable fishery. Natural reproduction by stocked brook trout Salvelinus fontinalis and rainbow trout Oncorhynchus mykiss is unknown but assumed to be negligible. An artificial tributary to the LRRT was constructed to provide a spawning and nursery refuge for stocked trouts, and shortly thereafter we observed spawning adults and emergent fry in the tributary. As a first step in determining the relative contributions of hatchery‐ and tributary‐produced brook and rainbow trouts recruiting to the LRRT fishery, we distinguished hatchery‐reared fry from tributary‐spawned fry via otolith chemistry. We used laser ablation inductively coupled plasma mass spectrometry to measure the abundances of cations in otoliths. Significant differences between hatchery and wild stocks were found for six element : Ca ratios. A five‐variable (Mg, Mn, Zn, Sr, and Ba) discriminant model was sufficient to distinguish the two brook trout stocks with 100% accuracy, and a two‐variable (Sr and Ba) model discriminated between the two rainbow trout stocks with 94% accuracy. Differences in ambient water chemistry between the two rearing habitats most likely accounted for these distinct otolith signatures. Otolith analysis of brook trout parr of unknown origin captured in the LRRT system suggests that natural reproduction is occurring there and that this stock may be supplemented with tributary‐spawned immigrants. After interannual variation in habitat‐specific otolith chemical signatures is quantified, such an analytical tool would be useful to managers in the LRRT system and elsewhere in determining the relative contributions of various stocks to reproductive and harvested populations.

Elemental analysis of otoliths, fin rays and scales: a comparison of bony structures to provide population and life‐history information for the Arctic grayling (Thymallus arcticus)

Abstract – We used laser ablation inductively coupled plasma mass spectrometry line scans to determine the elemental composition of otoliths, pectoral fin rays and scales of Arctic grayling. Elemental signatures of otoliths and pectoral fin rays effectively provide life‐history information on individual fish, important for management of grayling, and potentially all freshwater teleosts. Bulk elemental signatures measured in the otoliths and fin rays were highly correlated to the stream chemistries where the fish were captured. A surprising result of this study was that fin rays showed the strongest relationship with water chemistry for strontium. Scale strontium concentrations were not correlated to water chemistries suggesting that other physiological mechanisms, or remobilisation, may be influencing the deposition of trace elements within scales. Linear discriminant function analyses for otolith and fin ray elemental signatures (and intriguingly also for scales) separated fish from different rivers for all three structures; thus, this technique can be used effectively as a means to discriminate origin of capture.

Unsupervised Bayesian reconstruction of individual life histories from otolith signatures: case study of Sr:Ca transects of European eel (Anguilla anguilla) otoliths

The reconstruction of individual life histories from chemical otolith measures is stated as an unsupervised signal-processing issue embedded in a Bayesian framework. This computational methodology was applied to a set of 192 European eel (Anguilla anguilla) otoliths. It provided a robust and unsupervised analysis of the individual chronologies of habitat use (either river, estuary, or coastal) from Sr:Ca measures acquired along an otolith growth axis. Links between Sr:Ca values and habitat, age, and season and the likelihood of the transitions from one habitat type to another were modelled. Major movement characteristics such as age at transition between habitats and time spent in each habitat were estimated. As a straightforward output, an unsupervised classification of habitat use patterns showed great variability. Using a hidden Markov model, 37 patterns of habitat use were found, with 20 different patterns accounting for 90% of the sample. In accordance with literature, residence behaviour was observed (28% of the eels). However, about 72% changed habitat once or several times, mainly before age 4. The potential application of this method to any other measures taken along an otolith growth axis to reconstruct individual chronologies gives a new insight in life history tactics analysis.

Effect of temperature and salinity on otolith element incorporation in juvenile gray snapper Lutjanus griseus

Otolith chemistry provides one approach for identifying the relative contribution of juveniles from different nursery habitats to adult populations. The goal of this study was to validate otolith element incorporation by quantifying the relation between otolith and water element concen- trations (Sr/Ca, Mg/Ca, Mn/Ca, and Ba/Ca) as a function of differences in water temperature and salinity using juvenile gray snapper Lutjanus griseus, a reef fish that inhabits estuarine and near- shore habitats as juveniles. We investigated the effects of 20 different temperature (18, 23, 28, 33°C) and salinity (5, 15, 25, 35, 45) combinations on otolith element incorporation (partition coefficient D) in L. griseus. Temperature and salinity had significant effects on DSr but no significant effect on DMg or DMn; however, salinity had a significant effect on DBa. The broad range of temperatures and salin- ities used in the present study encompasses those occupied by juveniles in the wild and therefore provides a realistic test for using otolith chemistry to infer environmental history of individual gray snapper. Element incorporation and the effects of temperature and salinity on element incorporation differ among fish species, limiting development of generalized models aimed at predicting water chemistry from otolith chemistry. Thus, the data presented here underscore the necessity of valida- tion experiments to translate species-specific elemental signatures in otoliths.

Scales of variation in otolith elemental chemistry of juvenile staghorn sculpin (Leptocottus armatus) in three Pacific Northwest estuaries

Although distinct otolith elemental signatures are often observed in fish collected from different estuaries, significant differences are also observed among sites within estuaries. Variation at these smaller spatial scales is not well quantified and has the potential to lead to inappropriate interpretations of otolith elemental data. To quantify variation at multiple scales, the otolith elemental composition (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca, and Pb:Ca) of juvenile staghorn sculpin (Leptocottus armatus Girard, 1854) collected from five sites within three estuaries, the Columbia River (two sites) and Coos Bay (one site), Oregon, and Humboldt Bay, California (two sites), was examined. Using laser ablation-inductively coupled plasma mass spectrometry, each otolith was sampled at three zones: (1) within the primordium, which represents the egg and early larval periods; (2) at the outer edge, which represents the juvenile period just prior to collection; and (3) midway between the primordial and edge samples, which represents the late larval and early juvenile period. There were significant differences in otolith metal-to-calcium ratios at all scales examined. Using multi-element otolith signatures, fish were classified to estuary and site within estuary with relatively high levels of accuracy (av = 70–90%). The largest differences in metal-to-calcium ratios were observed between sites within estuaries (<5 km apart) and the smallest differences were observed among otolith zones. Variation in otolith chemistry may be used to provide information on probable habitat use by estuarine fish but studies must be carefully designed.

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.

Elemental signatures of Acanthochromis polyacanthus otoliths from the Great Barrier Reef have significant temporal, spatial, and between-brood variation

We evaluated the spatial and temporal scales over which otolith signatures varied in a reef fish on the Great Barrier Reef (GBR) using the non-dispersing damselfish Acanthochromis polyacanthus. We found a robust multi-element separation in otolith signatures from reef clusters in the northern and southern GBR. Variance components indicated that this spatial scale accounted for the majority of the variation in two elemental ratios (Ba/Ca and Sr/Ca) over the 2 years of the study. There was also significant variation in elemental signatures between otoliths collected over two consecutive years, as well as within a season. Individual reefs within clusters were less distinguishable based on otolith chemistry and were probably observed by differences within reefs (among sites and broods within sites). These results indicate that it may be difficult to determine the reef of origin for individual fish using otolith chemistry, while determining natal region seems a realistic goal.

Resolution of the early life history of a reef fish using otolith chemistry

We used the elemental signatures in otoliths of the damselfish Pomacentrus coelestis as a proxy for conditions experienced prior to settlement. Fish from the southern Great Barrier Reef (GBR) differed in their pre-settlement otolith chemistry, indicating that they had occupied different water masses during their pelagic stage, thus suggesting multiple larval sources. Fish from reefs in the northern GBR did not differ greatly in their pre-settlement otolith chemistry, suggesting a single larval source. Using “near-natal” signatures, we determined that ~67% of these signatures matched the signature established for Lizard Island (LZ), suggesting LZ could be a source reef. However, these results could also be the result of poor separation among reefs caused by reefs sharing water masses. Otolith chemistry also revealed that 50–70% of all fish examined settled on the reef the day they encountered it, while some fish spent up to 4 days near the reef prior to settlement.

Distinguishing aquaculture and wild yellowtail kingfish via natural elemental signatures in otoliths

Naturally occurring elemental signatures (or composition) of otoliths may enable aquaculture and wild-caught yellowtail kingfish (Seriola lalandi) to be distinguished from one another, so that in the event of aquaculture escapes, escaped fish could be identified. Yellowtail kingfish were obtained from aquaculture ventures in three regions of Spencer Gulf and from nine areas external to aquaculture throughout the inner and outer Spencer Gulf, South Australia. Elemental signatures (Mg, Mn, Sr, Ba) of otoliths were analysed via LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry). A highly significant difference was detected between elemental signatures of the aquaculture and wild-caught fish, but this difference was largely attributable to wild fish from Port Augusta being different from the other groupings. For individual elements, it was difficult to detect differences between aquaculture and wild-caught fish, as there was considerable variation between locations within each group. When comparisons between aquaculture and wild fish were made of the inner and outer Spencer Gulf, aquaculture fish could be distinguished from wild fish and classified correctly with a high degree of accuracy (82–100%); the same level of accuracy was not achieved with wild fish. Comparison of elemental transects showed considerable variability between locations, with no features distinguishing aquaculture or wild-caught fish. It would be beneficial to combine naturally occurring signatures with other identification approaches (e.g. artificial elemental signatures) to accurately distinguish aquaculture from wild-caught fish. The impacts of escaped fish could then be determined.

Coastal Origin of Common Snook, Centropomus undecimalis, in Florida Bay

We used the elemental signatures of otoliths to investigate the coastal origin of common snook (Centropomus undecimalis) in Florida Bay, Florida and evaluate current management boundaries. We examined juvenile otoliths from Florida’s Atlantic and Gulf of Mexico (Gulf) populations and determined that there were significant differences in several elemental ratios (Mn/Ca, Cu/Ca, Sr/Ca, Ba/Ca). In addition, a discriminant function analysis (DFA) indicated a significant separation between the juveniles from each coast and otoliths were never misclassified by coast, indicating a distinct difference in their otolith chemistry. Using only juvenile otoliths to derive a calibration function, a separate DFA indicated that the adults from Florida Bay likely originated from both coasts of Florida in roughly equal proportions. Although these preliminary results contradict tagging studies, they concur with genetic studies suggesting that both east and west coast populations contribute to the common snook found in Florida Bay.

The use of otolith chemistry to determine stock structure of three epinepheline serranid coral reef fishes on the Great Barrier Reef, Australia

We investigated the use of otolith chemistry to identify the stock structure of three epinepheline serranid coral reef fishes: Plectropomus leopardus, Cephalopholis cyanostigma and Epinephelus fasciatus. Chemical signatures of otoliths collected from the three species were compared on a broad regional scale and, for P. leopardus, also on a finer reef scale. The temporal stability in these signals was also examined by comparing two cohorts of P. leopardus, and C. cyanostigma and E. fasciatus from the same cohorts that were collected 2 years before and 2 years after a significant weather disturbance caused by a large tropical cyclone (Cyclone Justin) in March 1997. Results showed differences in otolith chemistry at various spatial scales, as well as between years and individuals collected before and after the cyclone. The results highlight the need to incorporate data from several years in studies using these techniques to discriminate temporary and possibly misleading signals from those that indicate persistent spatial structure in stocks. The difficulties in identifying stocks and stock boundaries in species with spatially complex population structures are discussed, as is the need for complementary analyses of life history characteristics to determine biologically and logistically appropriate management units.