Otolith Chemical Signatures Research Articles

Otoliths as chemical archives through ontogeny reveal distinct migratory strategies of Atlantic halibut within the Gulf of St. Lawrence

Abstract In marine fishes of commercial interest, defining habitat use and migration strategies through ontogeny can help better understand the structure and dynamics of harvested populations and guide their management. The present study relied on otolith chemistry to identify three contingents within the Atlantic halibut (Hippoglossus hippoglossus) stock in the Gulf of St. Lawrence (GSL). We differentiated two chemical signatures from otolith edges, one for shallow (<100 m) and another one for deep (>100 m) waters. By identifying transitions between the deep and shallow habitats, we found that most halibut display migrations from the deep waters to shallow waters during the first 3 years of life. After reaching maturity, most halibut distributing in northern regions of the GSL became full-time residents in deep areas of the GSL. In contrast, halibut found in summer on the shallow plateau of the southern GSL displayed migrating behaviour between shallow (summer) and deep (winter) waters throughout their lives, either on an annual or irregular basis. Overall, our results demonstrate that otolith chemical signatures serve as natural markers of geographically distinct marine environments, facilitating the identification and reconstruction of environmental histories of long-lived marine fishes.

Quantifying larval dispersal portfolio in seabass nurseries using otolith chemical signatures

The temporal asynchronies in larvae production from different spawning areas are fundamental components for ensuring stability and resilience of marine metapopulations. Such a concept, named portfolio effect, supposes that diversifying larval dispersal histories should minimize the risk of recruitment failure by increasing the probability that at least some larvae successfully settle in nursery. Here, we used a reconstructive approach based on otolith chemistry to quantify the larval dispersal portfolio of the European seabass, Dicentrarchus labrax, across six estuarine nursery areas of the northeast Atlantic Ocean. The analysis of natal and trajectory signatures indicated that larvae hatch in distinct environments and then dispersed in water masses featured by contrasting chemical signatures. While some trace elements appeared affected by temporal changes (Mn and Sr), others varied spatially during the larval stage but remained poorly affected by temporal fluctuation and fish physiology (Ba, Cu, Rb and Zn). We then proposed two diversity metrics based on richness and variations of chemical signatures among populations to reflect spatio-temporal diversity in natal origins and larval trajectories (i.e., estimates of dispersal portfolio). Along the French coast, the diversity estimates were maximum in nurseries located at proximity of offshore spawning sites and featured by complex offshore hydrodynamic contexts, such as the Mont St-Michel bay. Finally, our findings indicate that the dispersal portfolio was positively related with the local abundance of seabass juveniles, supporting the assumption that heterogeneity in dispersal history contributes to promote recruitment success in nurseries.

Otolith microchemistry highlights the importance of extensive connectivity for conservation of an iconic migratory fish in a large tropical river basin

Abstract Migratory fishes exert important influences on tropical river food webs, but these species are often most vulnerable to environmental change. Movement patterns of fishes in tropical rivers are also generally poorly understood, hindering conservation efforts. Common snook, Centropomus undecimalis, depend on connected coastal habitats for reproduction and growth and can migrate extensively up rivers when not hindered by barriers to movement. In southern Mexico, they are frequently captured 300 river km or more upstream from the delta of the Usumacinta River. The Usumacinta is a large river system with no mainstem dams, providing an expansive network of connected river, stream and wetland habitats which form a much larger migratory domain that exists in many other systems where common snook have been studied. To assess whether fish captured in different zones were associated with different natal habitats or distinct migratory patterns, variation in common snook otolith chemical signatures was examined in the Usumacinta River from the Gulf of Mexico coast to as far as 600 river km upriver. Otolith microchemistry was useful for characterizing migratory histories of individuals, but there was no clear evidence that fish captured in different river zones used different types of natal habitats. Based on lifetime Sr:Ca in otoliths, a diverse array of movement patterns was evident in the Usumacinta system, with 97% of common snook showing evidence of freshwater habitat use. Prevalent use of riverine habitats by common snook far from the coast reinforces the need to preserve connectivity in the Usumacinta River and other undammed systems supporting migratory species. The natural flow regime and lack of barriers allow for longitudinal and lateral connectivity in this system, providing pathways for migratory species to move extensively and have access to an array of habitats, including productive floodplain lagoons.

Spatiotemporal structure of narrow-barred Spanish mackerel (Scomberomorus commerson) from the Red Sea and western Indian Ocean based on otolith micro-chemistry

Though the narrow-barred Spanish mackerel (Scomberomorus commerson) is considered to be migratory, the species is nevertheless thought to be locally overexploited in the northwest Indian Ocean. At the regional level, this local depletion is a major concern for food security. As the population structure and connectivity between sub-populations are poorly understood for this species, we examined the spatio-temporal dynamics of narrow-barred Spanish mackerel via elemental concentrations (P, Mg, Sr and Ba) along otolith transects using LA-ICPMS for samples from 6 sites: Egypt, Djibouti North and South, Somalia, Mozambique and South Africa. For homogeneous size class samples (70–90 cm), otolith chemical signatures immediately preceding capture were used to accurately group individuals sharing a spatial proximity and/or season of capture. Notable differences in otolith edge signatures were found among individuals from north and south of the equator and contrasting cluster compositions from nearby sites in the Gulf of Aden of individuals captured in summer versus winter. Otolith core chemistry identified two spawning chemical compositions. The first common composition was characterized by relatively high concentrations of Sr and lower concentrations of P, Ba and Mg. The second less common spawning chemical composition was particularly rich in P, Ba and Mg and corresponded primarily to individuals caught off Mozambique, Somalia and Djibouti. These results are broadly consistent on one hand with patterns of water mass circulation in the Red Sea and western Indian Ocean and on the other hand with the observed spawning seasons. Though further research using, for example, archival tagging is needed to clarify the mechanisms behind these patterns, these results reveal the potential of otolith chemistry to provide insights into the spatio-temporal dynamics of narrow-barred Spanish mackerel.

Stock structure of the Brazilian sardine Sardinella brasiliensis from Southwest Atlantic Ocean inferred from otolith elemental signatures

The Brazilian sardine (Sardinella brasiliensis) supports the most important pelagic fishery in the southeast-south Brazil. The first studies on the population structure of S. brasiliensis suggested the existence of two stocks taking into account distinct regional somatic growth rates and spawning areas. However, subsequent works were unable to confirm this scenario, and at present for fisheries management purposes a single stock is considered in Brazil. In order to unravel the stock structure of S. brasiliensis, ninety adult individuals (two-year-old fish) were collected between February and October 2018 in three fishing grounds within its distribution area [Rio de Janeiro (RJ), Santa Catarina (SC) and Rio Grande do Sul (RS)]. Multi-elemental signatures of whole otoliths were determined using inductively coupled plasma mass spectrometry and combined with basic biological data (somatic condition, sex and gonadal developmental stages). A generalized linear model found somatic growth rates significantly different across regions. Whole otolith chemical signatures exhibited distinct regional patterns, mainly driven by differences in Ba/Ca, Cu/Ca, Li/Ca, Mg/Ca, Mn/Ca, Sr/Ca, and Zn/Ca. Combining somatic and reproductive data with multi-elemental signatures, the leave-one-out reclassification reassigned the individuals to their areas of origin with a high accuracy (100%, 93% and 87% for RJ, SC and RS, respectively). The results indicate a spatial isolation for the RJ individuals and some degree of intermixing between SC and RS. The data suggest that these fishing grounds should be considered as two stocks (RJ and SC+RS) for fisheries management purposes, although the southern stock is not necessarily homogeneous.

Water strontium concentrations influence strontium concentrations in the pre-hatch otolith region of capelin (Mallotus villosus)

Unique otolith chemistry signatures have been useful in reconstructing environmental histories of teleost fish, including their use as a natural tag of natal habitat. The ability to use otolith chemistry as a chronological measure of life history, however, assumes that the concentrations of trace elements in the ambient environment are taken up and calcified on the otolith in proportion to their environmental availability. Capelin (Mallotus villosus) is a key forage fish species on the Newfoundland Shelf that migrates from offshore to coastal regions during the summer to spawn. Capelin eggs adhere to the sediment at spawning sites, where they remain throughout incubation. As capelin larvae disperse from spawning/rearing sites immediately upon hatch, the pre-hatch region of capelin otoliths, formed during incubation, indicates natal origin. Chemistry in the otolith pre-hatch region of capelin displays high variability, even in environmentally stable lab-rearing experiments and among individuals within the same family, potentially confounding the ability to characterize otolith chemical signatures from different regions. Here, we aimed to determine whether trace element concentrations in the pre-hatch region of capelin otoliths were influenced when naturally fertilized capelin eggs were reared to hatch under varying concentrations of Barium (Ba; 1×, 6×, 7×, 16×, 27×), Strontium (Sr; 1×, 2×, 4×, 5×, 9×), and Ba+Sr (1×, 5×/1.5×, 5×/3×, 10.5×/3.5×, 19×/5× ambient concentration). Naturally fertilized capelin eggs were collected at a beach spawning site in July 2019 and incubated in control or enriched water treatments until hatch. Sagittal otoliths were removed from larvae from each treatment (n = 20) and analysed for their trace element concentrations (e.g., Mg, Mn, Sr, Ba) using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS). Otolith Sr concentrations from 1 to 2 day old larvae increased significantly with increasing ambient Sr concentrations, and the Sr partition coefficient (0.37 ± 0.01) appeared to be concentration-dependent, decreasing non-linearly with increasing ambient water Sr concentrations. In contrast, otolith Ba concentrations remained consistently high despite environmental enrichment. We estimated high Ba partition coefficients (3.67 ± 0.52), which were calculated using the limits of detection for Ba (i.e., 0.0025 mg/L), due to low ambient Ba concentrations. These high estimated Ba partition coefficients suggested that the pre-hatch otolith Ba concentrations may be primarily maternally derived. Overall, findings suggested that regions varying in ambient water Sr concentrations during capelin egg incubation could result in distinct chemical signatures in the pre-hatch region of capelin otoliths, but variability in water Ba concentrations will not. Future studies could use this variation to investigate population connectivity and productivity, which is currently unknown for capelin.

Temporal stability in the otolith Sr:Ca ratio of the yellow croaker, Larimichthys polyactis (Actinopterygii, Perciformes, Sciaenidae), from the southern Yellow Sea

Otolith chemical signatures are sufficiently stable across time to allow for accurate stock classification. The classification of the southern Yellow Sea population for Larimichthys polyactis (Bleeker, 1877) and its connectivity with others from 1962 is controversial. The study aimed to study the inter-annual variation in otolith strontium:calcium (Sr:Ca) ratios of L. polyactis to determine whether otolith natural tags are representative over long periods and can then be used for population structure classification. Spawning L. polyactis individuals were captured by stow nets in the same site of the southern Yellow Sea coastal waters during April–May in 2003, 2012, and 2013. EPMA (electron probe microanalysis) was used to determine the Sr:Ca ratios of a total of 25 otolith samples. Mann–Whitney U-test was used to test the differences of otolith Sr:Ca ratios from the core to edge for each otolith. One-way ANOVA was performed to compare the mean otolith Sr:Ca values among 2003, 2012, and 2013. Otoliths from 2003, 2012, and 2013 showed similar patterns of Sr:Ca ratios and Sr:Ca ratios could be divided into higher and lower phases in the core and remaining regions, respectively. Inter-annual significant differences for each high or low Sr:Ca phase of otoliths were not observed over short- (between 2012 and 2013) or long-time (between 2003 and 2012, and between 2003 and 2013) scales. Univariate contrasts across the adjacent year and decade classes were statistically similar. The Sr:Ca ratio signatures in the otolith were relatively stable across years and can be used as a reliable natural tag for connectivity assessments and stock identification with little or no genetic differentiation among L. polyactis populations. The short- and long-term temporal stability of otolith Sr:Ca ratios also revealed, the existence of stable L. polyactis stocks in the southern Yellow Sea, consistent with a previous finding of capture survey.

Otolith chemical fingerprints of skipjack tuna (Katsuwonus pelamis) in the Indian Ocean: First insights into stock structure delineation.

The chemical composition of otoliths (earbones) can provide valuable information about stock structure and connectivity patterns among marine fish. For that, chemical signatures must be sufficiently distinct to allow accurate classification of an unknown fish to their area of origin. Here we have examined the suitability of otolith microchemistry as a tool to better understand the spatial dynamics of skipjack tuna (Katsuwonus pelamis), a highly valuable commercial species for which uncertainties remain regarding its stock structure in the Indian Ocean. For this aim, we have compared the early life otolith chemical composition of young-of-the-year (<6 months) skipjack tuna captured from the three main nursery areas of the equatorial Indian Ocean (West, Central and East). Elemental (Li:Ca, Sr:Ca, Ba:Ca, Mg:Ca and Mn:Ca) and stable isotopic (δ13C, δ18O) signatures were used, from individuals captured in 2018 and 2019. Otolith Sr:Ca, Ba:Ca, Mg:Ca and δ18O significantly differed among fish from different nurseries, but, in general, the chemical signatures of the three nursery areas largely overlapped. Multivariate analyses of otolith chemical signatures revealed low geographic separation among Central and Eastern nurseries, achieving a maximum overall random forest cross validated classification success of 51%. Cohort effect on otolith trace element signatures was also detected, indicating that variations in chemical signatures associated with seasonal changes in oceanographic conditions must be well understood, particularly for species with several reproductive peaks throughout the year. Otolith microchemistry in conjunction with other techniques (e.g., genetics, particle tracking) should be further investigated to resolve skipjack stock structure, which will ultimately contribute to the sustainable management of this stock in the Indian Ocean.

Discrimination of yellowfin tuna Thunnus albacares between nursery areas in the Indian Ocean using otolith chemistry

Yellowfin tuna Thunnus albacares is a highly exploited species in the Indian Ocean. Yet, its stock structure is still not well understood, hindering assessment of the stock at a suitable spatial scale for management. Here, young-of-the-year (&lt;4 mo) yellowfin tuna otoliths were collected in 2018 and 2019, from 4 major nursery areas in the Indian Ocean: Madagascar, Seychelles-Somalia, Maldives and Sumatra. First, direct age estimates were made in a subset of otoliths by visually counting microincrements to identify the portion of the otolith corresponding to the larval stage. We then developed 2-dimensional maps of trace element concentrations to examine spatial distribution of elements across otolith transverse sections. Different distribution patterns were observed among the elements analysed; Li, Sr and Ba were enriched in the portion of the otolith representing early life, whereas Mn and Mg concentrations were heterogeneous across growth bands. Last, we analysed inter-annual and regional variation in otolith chemical composition using both trace elements (Li, Mg, Sr, Ba and Mn) and stable isotopes (δ13C and δ18O). Significant regional variation in otolith chemical signatures was detected among nurseries, except between Madagascar and Seychelles-Somalia. Otolith δ13C and δ18O were important drivers of differentiation between western (Madagascar and Seychelles-Somalia), Maldives and Sumatra nurseries, whereas the elemental signatures were cohort specific. Overall nursery assignment accuracies were 69-71%. The present study demonstrates that baseline chemical signatures in the otoliths of yellowfin tuna are regionally distinct and can be used as a natural tag to investigate the nursery origin of older individuals in the Indian Ocean.

Otolith Chemistry Reveals Natal Region of Larval Capelin in Coastal Newfoundland, Canada

Identifying natal origins of animals is key to determining the relative productivity of natal habitats, dispersal of individuals among local populations (i.e. connectivity), and, ultimately, metapopulation dynamics. As marine fish larvae have a high potential for dispersal, natural tags such as otolith chemistry are often used to determine natal origins. Trace elements may be incorporated into embryonic otoliths while larvae are developing in the egg, resulting in chemical signatures in the pre-hatch region of the otolith that reflect the natal habitat. Our goal was to determine whether the natal origins of 1-3 day old larval capelin (Mallotus villosus), a key forage fish in many northern marine ecosystems, could be determined using otolith chemistry. We sampled larvae from five Newfoundland regions (i.e. embayments: Placentia Bay, St. Mary’s Bay, Witless Bay, Trinity Bay, Notre Dame Bay) during July-August, 2019 to quantify regional differences in otolith chemistry. Additionally, eggs/larvae were field-reared within two regions over multiple years (Notre Dame Bay: 2014, 2015, 2018, 2019; Trinity Bay: 2018, 2019) to quantify interannual variation in region-specific otolith chemistry. Multi-elemental otolith signatures (i.e., Mg, Mn, Zn, Sr, Ba), as determined by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS), differed significantly among regions, with individuals classified into their natal region with 78% success (region-specific success: 68-100%). Classification success into natal region remained high (67-76%) despite interannual variation in otolith trace element concentrations within regions. Characterizing region-specific otolith chemical signatures that reflect natal origins of capelin larvae is the first step in determining the productivity and relative contributions of different regions of coastal Newfoundland to capelin recruitment.

Population structure and dynamics of the Atlantic mackerel (Scomber scombrus) in the North Atlantic inferred from otolith chemical and shape signatures

The Atlantic mackerel, Scomber scombrus, is an economically important and widely distributed fish species in the North Atlantic, currently considered to comprise two stocks: the North-West Atlantic (NWA) and the North-East Atlantic (NEA). Each stock is composed of different spawning components which involve temporal and spatial movements driven by the environment. Thus, resolving the species population structure and dynamics presents a challenge to scientists. In this study, the stock structure of S. scombrus was evaluated using otolith shape and chemical signatures. One hundred and eighty individuals of the same cohort (age 3), caught between January and February of 2018 from six key locations in the North Atlantic, were used. Individuals were collected from the two spawning components in the NWA stock, the Canadian Northern component (NWA-N) and the US Southern component (NWA-S); and from the three spawning components in the NEA stock, the North Sea (NEA-NS), the Western (NEA-W) and the Southern components (NEA-S), plus, an overlapping area of these last two components, the Bay of Biscay (NEA-BB). Combined otolith signatures fully discriminated the NEA and NWA stocks (100% of reclassification success, indicates distinct population-units) and discriminated the components within each stock with high reclassification percentages (100% and 68% for the overall reclassification of the NWA and NEA components, respectively). These data suggest that NWA stock should be regard as two distinct population-units for fisheries management purposes, confirms the complex metapopulational structure of the NEA stock, and calls for the need of continuous evaluation of these complex stocks in order to achieve a sustainable exploitation.

To sea or not to sea? Multiple lines of evidence reveal the contribution of non‐diadromous recruitment for supporting endemic fish populations within New Zealand's longest river

Abstract Otolith microchemistry was used to identify marine‐ versus freshwater‐derived recruitment of three native freshwater fish species belonging to the southern hemisphere family Galaxiidae, in New Zealand's longest river system, the Waikato River. Water chemistry data for trace elements and 87Sr/86Sr isotope ratios were collected from five lentic and 10 lotic water bodies throughout the lower river floodplain. Potential spawning sites for galaxiids were compared with values obtained by laser ablation inductively coupled mass spectrometry (LA‐ICPMS) depth profiling of young‐of‐the‐year otoliths sampled from fish in nine lower river catchment sites. Otolith chemical signatures from the larval rearing period indicated that catchment‐scale recruitment for two species, Galaxias argenteus (Gmelin, 1789) and Galaxias fasciatus Gray, 1842, was driven predominantly by non‐diadromous recruitment from one lake (Lake Waahi). In contrast, diadromous recruitment appeared to be more common for Galaxias maculatus (Jenyns, 1842); however, non‐diadromous specimens were also identified for the first time from a New Zealand river. Reversing lake outlet flows linked to river stage appears be important in facilitating the dispersal of rheotactic larvae out of lakes, suggesting that lake outflow management at key times could be used to sustain this ecologically important function. This study highlights that some water bodies can supply a disproportionately large number of recruits to support fish populations within the wider riverscape. Identifying these water bodies and managing them to sustain recruitment is key to the conservation of non‐diadromous Galaxiidae in this modified lowland environment

Temperature and salinity influence the chemistry in the pre-hatch otolith region of capelin, Mallotus villosus, during lab and field egg incubation experiments

Identifying the natal origin of marine fish is important to understand connectivity and productivity among populations residing in different habitats. Capelin, Mallotus villosus, is a key marine forage fish species that spawns at both beach (warmer, less saline) and deep-water (15–40 m; cooler, more saline) habitats along the northeast Newfoundland coast. Currently, the contribution from each habitat to the spawning population is unknown. Previous research, where capelin eggs from known families were lab-reared, identified family-based otolith chemical signatures in the pre-hatch (embryonic) region of larvae. In this study, we investigated whether temperature and salinity influenced embryonic otolith chemistry to determine whether variation in environmental conditions would result in habitat-specific signatures, thereby overwhelming family-based signatures and allowing the identification of natal origin. Capelin eggs from many families were incubated together under controlled temperature (4, 8, 10 °C) and salinity (10, 20, 30 psu) treatments in the lab and uncontrolled conditions within each habitat in the field. Elemental concentrations (i.e., Sr, Ba, Mg, Mn) in the pre-hatch region of 1-day old larvae were quantified via LA ICP-MS. Elemental concentrations varied among individuals reared under identical conditions, likely due to family-based chemical signatures. Despite this variation, mean elemental concentrations differed when mean temperatures varied by ≥4 °C between treatments (lab) and rearing habitats (field), resulting in high treatment- and habitat-specific classification success (~73–88%) of individuals. In contrast, embryonic otolith chemistry did not vary consistently with salinity. Temperature differs consistently between rearing habitats and, thus, these findings suggest that when capelin rearing habitats differ by ≥4 °C within a year, chemical signatures in the embryonic otolith may be used to determine the natal origin of individuals.

Application of Otolith Chemistry to Investigate the Origin and State‐Straying of Steelhead in Lake Erie Tributaries

AbstractIn Lake Erie, the fishery for steelhead Oncorhynchus mykiss is overwhelmingly dominated by stocking from state agencies in Michigan, New York, Pennsylvania, and Ohio. Managers that stock steelhead may become concerned if a sizeable portion of the fish they stock do not return to the waters in which they were released and instead stray to other states (“state‐straying”). During fall 2009, spring 2010, and spring and fall 2015, we evaluated the origin and state‐straying of adult steelhead in five annually stocked tributaries of Lake Erie. We also investigated spatial differences in the origin and state‐straying of adult steelhead at different stream locations in two tributaries during 2015. Otolith chemistry signatures were first used to discriminate among yearling steelhead from each of the state hatcheries that stock Lake Erie and wild juveniles from Cattaraugus Creek, New York, and the Grand River, Ontario, resulting in a mean jackknifed classification accuracy of 88% (range = 72–100%). Otolith chemistry analysis was then performed on unknown‐origin adult steelhead collected in Lake Erie tributaries during the fall and spring spawning runs, and natal sources were identified by using source‐specific otolith chemical signatures. State‐strays averaged 46% (range = 13–88%) of the adult fish collected, and high percentages of strays were identified in Chautauqua Creek, New York (72%), and Cattaraugus Creek (88%). Steelhead collections in these New York tributaries during a second year revealed that the percentage of strays present was consistently high, and large proportions of strays were identified in both upstream and downstream locations. These results suggest that state‐straying is widespread in Lake Erie tributaries and that strays make up a large proportion of New York's Lake Erie tributary fishery. Strategies to reduce straying may include practicing upstream releases and reducing the number of hatchery fish that are released in some states.

Blue marlin (Makaira nigricans) longevity estimates confirmed with bomb radiocarbon dating

The longevity of blue marlin (Makaira nigricans) remains unresolved. The use of fin spines and sagittal otoliths for age reading has led to unconfirmed longevity estimates near 20–30 years. Age validation has been elusive because large individuals are uncommonly caught, and a technique that can be applied to structures that provide estimates of age was absent. The use of otolith chemical signatures has been limited by sagittal otoliths that are very small — whole otolith mass of adult blue marlin rarely exceeds 10 mg for the largest fish. Recent advances in the detection limits of radiocarbon (14C) with accelerator mass spectrometry — coupled with recently acquired knowledge of marine bomb 14C signals spanning the tropical Pacific Ocean — have led to an opportunity to age blue marlin from small amounts of otolith material. In this study, otoliths from a recently collected 1245 lb (565 kg) female blue marlin at a measured 146-inch (371 cm) lower jaw fork length were analyzed for 14C. Estimated longevity was either 12–21 or 32–44 years on the basis of bomb 14C dating. Using multiple lines of evidence, it was determined that the young age scenario was most likely, with evidence for an age close to 20 years using a series of deductions in the bomb 14C dating method.

The influence of high pCO2 on otolith shape, chemical and carbon isotope composition of six coastal fish species in a Mediterranean shallow CO2 vent

Naturally acidified environments, such as CO2 vents, are important sites to evaluate the potential effects of increased ocean acidification on marine ecosystems and biota. Here we assessed the effect of high CO2/low pH on otolith shape and chemical composition of six coastal fish species (Chromis chromis, Coris julis, Diplodus vulgaris, Gobius bucchichi, Sarpa salpa, Symphodus ocellatus) in a Mediterranean shallow CO2 vent. Taking into consideration the major and trace elements found near the vent and the gradient of dissolved inorganic carbon, we compared the otolith chemical signatures of fish exposed long-term to elevated CO2 emissions and reduced pH (mean pH 7.8) against fish living in two control sites (mean pH 8.2). A number of element:Ca ratios (Na:Ca, Mg:Ca, Mn:Ca, Cu:Ca, Zn:Ca, Sr:Ca, Ba:Ca and Pb:Ca), along with isotope ratios, were measured in otoliths (δ13C and δ18O) and water (δ13CDIC) samples. Additionally, we performed otolith outline shape and morphometric analysis to evaluate the effect of high CO2/low pH. We observed species-specific responses with regards to both shape and chemical signatures. Significant differences among sites were found in otolith shape (elliptical Fourier descriptors) of G. bucchichi and D. vulgaris. Elemental and isotopic signatures were also significantly different in these site attached species, though not for the other four. Overall, the carbon isotopic composition seems a good proxy to follow pH gradient in naturally acidified area. Ultimately, besides improving our knowledge of the effects of high CO2/low pH on otoliths, the present results contribute to our understanding on their use as natural tags.

Potential sources of red snapper (Lutjanus campechanus) recruits estimated with Markov Chain Monte Carlo analysis of otolith chemical signatures

Otolith chemical signatures were used to estimate the number of likely nursery sources that contributed recruits to a suite of red snapper (Lutjanus campechanus) year-classes sampled in 2012 in US Atlantic Ocean waters from southern Florida (28°N) to North Carolina (34°N). Otoliths from subadult and adult fish (n=139; ages 2–5 years) were cored and their chemical constituents analysed for δ13C, δ18O, as well as the elemental ratios of Ba:Ca, Mg:Ca, Mn:Ca and Sr:Ca. Results from multiple linear regression analyses indicated that there was significant latitudinal variation for δ13C, Ba:Ca, Mg:Ca and Mn:Ca. Therefore, these variables were used to parameterise Markov Chain Monte Carlo (MCMC) models computed to estimate the most likely number of nursery sources to each age class. Results from MCMC models indicated that between two and seven nursery sources were equally plausible among the four age classes examined, but the likely number of nursery sources declined for fish aged 4 and 5 years because of apparent mixing between more northern and more southern signatures. Overall, there is evidence to reject the null hypothesis that a single nursery source contributed recruits among the age classes examined, but increased sample size from a broader geographic range may be required to refine estimates of the likely number of nursery sources.

Application of Otolith Chemical Signatures to Estimate Population Connectivity of Red Snapper in the Western Gulf of Mexico

AbstractOtolith chemical signatures of Red Snapper Lutjanus campechanus from six nursery regions were used to estimate the sources of recruits to four sampling regions in the western Gulf of Mexico (Gulf) and to estimate whether postsettlement mixing of Red Snapper occurs between the U.S. and Mexican portions of the western Gulf. In a previous study, region‐specific otolith signatures (element : Ca ratios: Ba:Ca, Mg:Ca, Mn:Ca, Sr:Ca, and Li:Ca; stable isotope delta values: δ13C and δ18O) were developed based on age‐0 Red Snapper (2005–2007 year‐classes) sampled from the six nursery areas. In the present study, subadult and adult Red Snapper (ages 1–3) belonging to those same year‐classes were collected from four sampling regions within the western Gulf (two regions in U.S. waters; two regions along the Mexican continental shelf) during summer in 2006–2008. Left sagittal otoliths were used to age subadults and adults to the corresponding nursery year‐classes, and right sagittal otoliths were cored for chemical analysis. Off the southwestern U.S. coast, the sampled age‐1–3 Red Snapper included locally derived recruits as well as recruits from the northwestern Gulf nursery region. However, analytical results were inconclusive with respect to estimating the connectivity between Red Snapper populations in U.S. and Mexican waters of the western Gulf.Received May 4, 2015; accepted August 25, 2015

Interspecific variation in juvenile snapper otolith chemical signatures in the northern Gulf of Mexico

The objective of this study was to evaluate whether age-0 lane snapper Lutjanus synagris otolith chemical signatures could serve as accurate proxies for those of its congener, red snapper L. campechanus, among northern Gulf of Mexico (GOM) nursery regions. Red (n = 90) and lane (n = 53) snappers were sampled from 3 regions of the northern GOM in fall 2005, and their otolith chemistry was analyzed with sector field-inductively coupled plasma-mass spectrom- etry (Ba:Ca, Mg:Ca, Mn:Ca, Sr:Ca, Li:Ca) or stable isotope ratio-mass spectrometry (δ 13 C and δ 18 O). Chemical signatures were significantly different among regions (MANOVA, p < 0.001) and between species (MANOVA, p = 0.029), with the species effect being driven by significant differ- ences in 4 of the 7 constituents analyzed (ANOVA, p < 0.036). The significant region effect per- sisted (MANOVA, p < 0.001), but the species effect was non-significant (MANOVA, p = 0.964) when constituent values were normalized to species-specific means. Mean regional classification accuracies from linear discriminant functions computed with otolith constituent data were 84% for lane snapper and 80% for red snapper whether data were normalized or not. Maximum likelihood models parameterized with normalized lane snapper otolith chemistry data estimated red snapper regional composition reasonably well among mixed-region samples (mean error = 9.7% among models). Therefore, it appears age-0 lane snapper otolith chemical signatures can serve as accu- rate proxies for those of red snapper in the northern GOM. These results have broader implica- tions for deriving natural tags based on otolith chemistry for fishes that may have low abundance in parts of their range.

Assigning origins in a potentially mixed‐stock recreational sea trout (Salmo trutta) fishery

AbstractThe anadromous, or sea‐going, life history form of brown trout, or sea trout ( Salmo trutta), may lead to potential mixing of populations while foraging at sea. In this article, we assess the potential that multiple populations are using common semi‐enclosed estuaries and quantify the potential levels of straying (i.e. dispersal) of foreign‐produced individuals into populations by using otolith chemical signatures as natural ‘tags’. To do so, we created a database of juvenile fish otolith chemistry (a marker of freshwater production) from four rivers and compared the chemistry of harvested fish in two estuaries important to anglers, the Renews River and Chance Cove Brook, to the database. A discriminant function analysis revealed significant differences in the otolith chemistry of juvenile fish inhabiting the four rivers with a 97% cross‐validated accuracy when classifying individual juveniles to their natal river, indicating our baseline was robust. When assigning adults caught over 3 years (2007–2009) in the recreational fishery in the Renews River estuary, it was determined that over 95% of the fish caught each year originated from Renews River. In contrast, harvested fish in Chance Cove during 2009 were disproportionately comprised of fish produced in Renews River, suggesting the potential for source‐sink population dynamics in Newfoundland. Taken as a whole, these results indicate limited population mixing in nearshore estuaries of this region, but also highlight the potential for some populations to subsidise the harvest by anglers in different areas.