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The potential utility of otolith microchemistry as an indicator of nursery origins in Pacific halibut (Hippoglossus stenolepis)
Abstract
Identifying the nursery origins of individual Pacific halibut (Hippoglossus stenolepis) is critical to a refined understanding of regional productivity, the effects of environmental forcing on population dynamics, and the spatial impacts of mortality and bycatch. Otolith microchemistry may provide a tool for identifying fish origins, but a precursor to using microchemistry-based assignment models is to evaluate the extent to which there exists detectable spatial structure in these natural tags. Here, we examined assignment accuracy as a function of spatial scale using canonical discriminant function analyses (DFA) applied to microchemical data from age-2 Pacific halibut in the western Gulf of Alaska (GOA) and southeastern Bering Sea (SEBS). Element:calcium ratios were assayed for fourteen trace elements using inductively coupled plasma mass spectrometry; δ¹³C and δ¹⁸O were determined via isotope ratio mass spectrometry. Substantial assignment success (∼75–90 %) was observed at spatial scales that are consistent with stock management. Elemental signatures were defined primarily by δ¹⁸O, δ¹³C, and ⁸⁸Sr:⁴⁸Ca, with a minor contribution from ⁵⁵Mn:⁴⁸Ca. Individuals were most commonly mis-assigned to adjacent locations. However, discontinuities were observed within the western GOA that suggest that elemental signatures do not vary along strictly longitudinal and latitudinal clines. These results highlight the need to exercise caution when attempting to use otolith microchemistry to assign fish to their origins when the baseline elemental data are unable to resolve missing nursery sources from the locations that are included in the discrimination models.
Bycatch of Pacific halibut (Hippoglossus stenolepis) limits many trawl fisheries in Alaska and greatly concerns stakeholders from local communities and fisheries that rely on Pacific halibut. To reduce Pacific halibut mortality, trawlers in the Bering Sea that target flatfish have been developing expedited release procedures to sort Pacific halibut from catches earlier than current regulations allow, while continuing accurate bycatch accounting. We studied survival rates of released Pacific halibut from three trawlers by deploying accelerometer-equipped pop-up satellite archival tags (PSATs) on 160 fish handled under expedited procedures. PSATs recorded and transmitted two metrics indicating swimming activity every 2 h while attached to the fish (for up to 60 days). Analysis of the resulting survival outcomes largely validated current survival-estimation methods, based on structured viability assessments, and found that longer fish length, shorter duration of air exposure, and shorter duration of trawl tow improved predicted Pacific halibut survival. Differences in these results were detected among vessel trips and species targeted by trawling. PSATs provided detailed data from nearly all tagged fish, while exposing fish to conditions experienced by normal releases.
Pacific halibut (Hippoglossus stenolepis) is managed as a single stock throughout the Gulf of Alaska (GOA) and eastern Bering Sea (BS), but biogeographical barriers and the potential for differential impacts of climate change may alter habitat use and distributions, and restrict connectivity between these ecosystems. To improve our understanding of larval dispersal pathways and migrations of young fish within and between GOA and BS, we (a) examined potential pelagic larval dispersal and connectivity between the two basins using an individual‐based biophysical model (IBM) focusing on years with contrasting climatic conditions and (b) tracked movement of fish up to age‐6 years using annual age‐based distributions and a spatiotemporal modeling approach. IBM results suggest that the Aleutian Islands constrain connectivity between GOA and BS, but that large island passes serve as pathways between these ecosystems. The degree of connectivity between GOA and BS is influenced by spawning location such that an estimated 47%–58% of simulated larvae from the westernmost GOA spawning location arrived in the BS, with progressive reductions in connectivity from spawning grounds further east. From the results of spatial modeling of 2‐ to 6‐year‐old fish, we can infer ontogenetic migration from the inshore settlement areas of eastern BS toward Unimak Pass and GOA. The pattern of larval dispersal from GOA to BS, and subsequent post‐settlement migrations back from BS toward GOA, provides evidence of circular, multiple life stage, connectivity between these ecosystems, regardless of climatic variability or year‐class strength.
Anomalously low winter sea ice extent and early retreat in CE 2018 and 2019 challenge previous notions that winter sea ice in the Bering Sea has been stable over the instrumental record, although long-term records remain limited. Here, we use a record of peat cellulose oxygen isotopes from St. Matthew Island along with isotope-enabled general circulation model (IsoGSM) simulations to generate a 5500-year record of Bering Sea winter sea ice extent. Results show that over the last 5500 years, sea ice in the Bering Sea decreased in response to increasing winter insolation and atmospheric CO 2 , suggesting that the North Pacific is highly sensitive to small changes in radiative forcing. We find that CE 2018 sea ice conditions were the lowest of the last 5500 years, and results suggest that sea ice loss may lag changes in CO 2 concentrations by several decades.
Uncertainty regarding the movement and population exchange of Atlantic bluefin tuna (Thunnus thynnus) from the two primary spawning areas (Gulf of Mexico, Mediterranean Sea) is increasingly implicated as a major impediment for the conservation of this species. Here, two mixture methods were applied to natural chemical markers (δ¹⁸O and δ¹³C) in otoliths (ear stones) to comprehensively investigate the nature and degree of transoceanic movement and mixing of eastern and western populations in several areas of the North Atlantic Ocean that potentially represent mixing hotspots. Areas investigated occurred on both sides of the 45°W management boundary as well as waters off the coast of Africa (Morocco, Canary Islands) where both populations are known to occur. Projections of population composition (i.e., natal or nursery origin) from a multinomial logistic regression (MLR) classification method with different probability thresholds were generally in agreement with maximum likelihood estimates from the commonly used mixed-population program HISEA; however, predicted contributions for the less abundant population were occasionally higher for MLR estimates. Both MLR and HISEA clearly showed that mixing of Atlantic bluefin tuna in the Central North Atlantic Ocean was highly variable from year to year with expatriates of eastern or western origin commonly crossing into the other management area. Pronounced transoceanic movement and mixing of western migrants was also present off the coast of Africa, with the occurrence of western migrants in the Canary Islands and Morocco ranging from zero to the majority of the individuals assayed for the years examined. Results indicate highly variable rates of movement and population exchange for Atlantic bluefin tuna, highlighting the need for temporally resolved estimates of natal origin in mixing hotspots to improve population models used to evaluate the status of this threatened species.
Rationale
Otolith strontium:calcium (Sr:Ca) ratios have been extensively used to study fish migration across a river, estuary and ocean at each life stage. However, otolith Sr:Ca ratios only explain partial variations of salinity and quantitative reconstruction of salinity history of fishes remains a challenge. Using a euryhaline fish Lateolabrax japonicus as an example, this study demonstrated an isotopic method of quantitatively reconstructing the salinity histories and habitat uses of the fish.
Methods
Otolith oxygen stable isotopic ratios (δ¹⁸Ooto values) were measured using a mass spectrometer for subsamples sequentially milled from the otolith core to the edge, and otolith Sr:Ca ratios were measured by an electron probe microanalyzer for the comparison. The mean water temperature within the studied area in the time period of each milled subsample was estimated from the Copernicus database. Based on an isotopic fractionation equation, each δ¹⁸Ooto value and water temperature pair determined the water δ¹⁸O value, which was then converted to salinity using published linear equations or an equation that was newly generated for this study.
Results
Individual fish clearly revealed different preferences in habitat use. The retrospectively reconstructed salinity history indicated that most L. japonicus inhabit and can spawn in seawater as well as in brackish estuaries. Few fish lived in freshwater during the young stage based on the analysis of δ¹⁸Ooto profiles; however, otolith Sr:Ca ratios can only reveal freshwater residence and cannot distinguish residence in brackish water or seawater.
Conclusions
This study demonstrated that otolith temporal microstructure and δ¹⁸Ooto values are well‐suited approaches for quantitative reconstruction of salinity histories of the fish. This method can improve the understanding of the habitat uses of other fishes inhabiting diverse habitats among the rivers, estuaries and oceans.
From 2007 to 2013, the southeastern Bering Sea was dominated by extensive sea ice and below-average ocean temperatures. In 2014 there was a shift to reduced sea ice on the southern shelf and above-average ocean temperatures. These conditions continued in 2015 and 2016. During these three years, the spring bloom at mooring site M4 (57.9°N, 168.9°W) occurred primarily in May, which is typical of years without sea ice. At mooring site M2 (56.9°N, 164.1°W) the spring bloom occurred earlier especially in 2016. Higher chlorophyll fluorescence was observed at M4 than at M2. In addition, these three warm years continued the pattern near St. Matthew Island of high concentrations (>1 μM) of nitrite occurring during summer in warm years. Historically, the dominant parameters controlling sea-ice extent are winds and air temperature, with the persistence of frigid, northerly winds in winter and spring resulting in extensive ice. After mid-March 2014 and 2016 there were no cold northerly or northeasterly winds. Cold northerly winds persisted into mid-April in 2015, but did not result in extensive sea ice south of 58°N. The apparent mechanism that helped limit ice on the southeastern shelf was the strong advection of warm water from the Gulf of Alaska through Unimak Pass. This pattern has been uncommon, occurring in only one other year (2003) in a 37-year record of estimated transport through Unimak Pass. During years with no sea ice on the southern shelf (e.g. 2001–2005, 2014–2016), the depth-averaged temperature there was correlated to the previous summers ocean temperature.
Pacific halibut Hippoglossus stenolepis (Schmidt) is presently considered to consist of a single spawning population extending from California through the Bering Sea. However, this satellite tagging investigation suggests that geographic landforms and discontinuities in the continental shelf appear to limit the interchange of mature Pacific halibut among large marine ecosystems and delineate the boundaries of potential spawning components in the Gulf of Alaska and Bering Sea, with smaller components along the Aleutian Islands. The geographic segregation of these spawning components may be reinforced by regional behavioural adaptations and different temperature regimes in each area. These results suggest that the Pacific halibut population may be segregated into somewhat discrete spawning units among which less mixing is likely than that which occurs within them. As such, future stock assessment metrics may be most effective in preserving population function if spawning ecology is treated as a basin-scale process.
We evaluated Atlantic Goliath Groupers, Epinephelus itajara, in their nursery habitats via microchemical analyses of fin rays. Juveniles were sampled from known nursery habitats off southwest Florida, and adults were primarily sampled from a spawning aggregation off southeast, Florida. We collected fin rays using a non-lethal technique that is minimally invasive with no known negative effects on growth or survival. Trace metal constituents in the fin rays were quantified with an inductively coupled plasma mass spectrometer via laser ablation (LA-ICP-MS). Two spatial scales were quantified to test the limitations of grouping individuals based on elemental compositions. On a small spatial scale (i.e., 100s of m), individuals were correctly classified within individual watersheds 64% of the time. On a larger spatial scale (i.e., 10s–100s of km), juveniles were classified with 100% accuracy. Trace metals in adults were analyzed by back-tracking across fin ray annuli to a year in which our previous studies have shown these adults occupied their juvenile habitats (i.e., 2006). These fish were grouped using a measure of dissimilarity and then analyzed to test whether we could reclassify them into these same groupings based solely on the chemical components in their fin rays, which was done with over 84% accuracy. Although juvenile habitats of the adults could not be determined due to the lack of baseline data, classifications were driven by similar elements to those that drove the classification of juveniles, suggesting similar physiological mechanisms. The results highlight the importance of spatial scale for interpreting microchemical analyses on calcified structures in fishes.
This study was conducted to delineate the stock structure of Sillaginopsis paniijus based on morphometric characters of the species. A total of 194 specimens were collected from the Meghna, Tentulia and Baleswar rivers located in the southern coastal zone of Bangladesh. Data were subjected to univariate ANOVA, multivariate ANOVA, discriminate function analysis (DFA), and principal component analysis. Mean variations of ten morphometric characters; HD, HBD, LBD, PsOL, ED, SnL, SPrDL, HAF, LSDB and LPB showed significant differences (p < 0.05) among 27 morphometric traits that were selected for the study. In DFA, the overall assignments of individuals into their correctly classified original groups were 71.1 and 70.6 % for male and female, respectively. A scatter plot of the first two discriminant functions was used to visually depict the discrimination among the populations. The results showed different stocks of S. panijus in the rivers of Baleswar, Tentulia and Meghna in southwest coast of Bangladesh.
We examined spatial variation in otolith geochemistry as a natural tag in juvenile Atlantic cod (Gadus morhua) to resolve geographic patterns during early life history. Individuals from 54 inshore sites spanned five embayments in eastern Newfoundland. Otolith composition differed at all spatial scales and related inversely to spatial scale. Classification analysis revealed increasing discrimination at coarser spatial scales: site (26%–58%), bay (49%), and coast (76%). Assignment success declined by ∼10% per added site with increasing sampling sites per bay, demonstrating fine-scale (<100 km) variation. When we partitioned environmental variability from observed otolith chemistry using predictive models, assignment success improved by 56%, 14%, and 5% for site, bay, and coast, respectively. Our results demonstrate environmental influence on spatial structure of otolith chemistry and illustrate the importance of resolving baseline variability in otolith chemistry when conducting assignment tests. Collectively, our results describe the potential utility of juvenile otolith composition in evaluating contributions of subpopulations to the Northwest Atlantic cod stock and highlight important limitations imposed by environmental variation at scales less than 100 km.
Assessing the assumption of multivariate normality is required by many parametric multivariate statistical methods, such as MANOVA, linear discriminant analysis, principal component analysis, canonical correlation, etc. It is important to assess multivariate normality in order to proceed with such statistical methods. There are many analytical methods proposed for checking multivariate normality. However, deciding which method to use is a challenging process, since each method may give different results under certain conditions. Hence, we may say that there is no best method, which is valid under any condition, for normality checking. In addition to numerical results, it is very useful to use graphical methods to decide on multivariate normality. Combining the numerical results from several methods with graphical approaches can be useful and provide more reliable decisions. Here, we present an R package, MVN, to assess multivariate normality. It contains the three most widely used multivariate normality tests, including Mardia’s, Henze-Zirkler’s and Royston’s, and graphical approaches, including chi-square Q-Q, perspective and contour plots. It also includes two multivariate outlier detection methods, which are based on robust Mahalanobis distances. Moreover, this package offers functions to check the univariate normality of marginal distributions through both tests and plots. Furthermore, especially for non-R users, we provide a user-friendly web application of the package. This application is available at http://www.biosoft.hacettepe.edu.tr/MVN/.
The chemical composition of calcified structures in fish (otoliths, spines, rays, scales, and vertebrae) has been identified as a useful natural marker of stock structure. This application depends on geographic variation in water chemistry or other factors (e.g., temperature) that influences the chemistry of the calcified structure such that fish that inhabit different environments exhibit differences in their chemical signature. The use of this technique in stock discrimination requires that a number of assumptions must be met, including that groups of interest possess characteristic and reproducible markers and all possible groups contributing to the group mixture have been characterized. In some instances, structural chemistry can permit observation of subtle population structure that may not be detected by genetic applications alone. Chemical analysis of calcified structures is most effective when used as part of a holistic approach to stock identification.
Oxygen isotope ratios from fish otoliths are used to discriminate marine stocks and reconstruct past climate, assuming that variations in otolith δ18O values closely reflect differences in temperature history of fish when accounting for salinity induced variability in water δ18O. To investigate this, we exploited the environmental and migratory data gathered from a decade using archival tags to study the behaviour of adult plaice (Pleuronectes platessa L.) in the North Sea. Based on the tag-derived monthly distributions of the fish and corresponding temperature and salinity estimates modelled across three consecutive years, we first predicted annual otolith δ18O values for three geographically discrete offshore sub-stocks, using three alternative plausible scenarios for otolith growth. Comparison of predicted vs. measured annual δ18O values demonstrated >96% correct prediction of sub-stock membership, irrespective of the otolith growth scenario. Pronounced inter-stock differences in δ18O values, notably in summer, provide a robust marker for reconstructing broad-scale plaice distribution in the North Sea. However, although largely congruent, measured and predicted annual δ18O values of did not fully match. Small, but consistent, offsets were also observed between individual high-resolution otolith δ18O values measured during tag recording time and corresponding δ18O predictions using concomitant tag-recorded temperatures and location-specific salinity estimates. The nature of the shifts differed among sub-stocks, suggesting specific vital effects linked to variation in physiological response to temperature. Therefore, although otolith δ18O in free-ranging fish largely reflects environmental temperature and salinity, we counsel prudence when interpreting otolith δ18O data for stock discrimination or temperature reconstruction until the mechanisms underpinning otolith δ18O signature acquisition, and associated variation, are clarified.
Results of a coastwide tagging study show that ontogenetic migration of Pacific halibut (Hippoglossus stenolepis) continues for larger fish, whereas in recent years the assumption had been that only smaller, younger fish migrated. In 2003–2004, a total of 67 000 Pacific halibut tagged with passive integrated transponder tags were released by the International Pacific Halibut Commission (IPHC) from Oregon to the Bering Sea. Portside scanning recovered over 3000 of these tags. Models were fitted that allowed commercial fishing mortality to be a function of fish length, year, and IPHC regulatory area, while migration probability was a function of area and length. Estimates from the models support the view that exploitation rates were much higher in eastern than western areas prior to the reduction of quotas following new results from a coastwide stock assessment in 2007. We explore possible explanations for differences between tagging and IPHC stock assessment results and note that this research provides confirmation of historical inferences regarding patterns of halibut migration based on conventional tagging.
In spring 2006 and 2007, northern Gulf of Alaska (GOA) shelf waters were
˜1.5°C below average throughout the ˜250 m deep shelf
and the salinity-dependent winter stratification was anomalously weak
due to above (below) average surface (bottom) salinities. Spring 2007
and 2008 temperatures were also ˜-1.5°C below average, but the
anomalies were confined to the upper 100 m due to moderate salt
stratification. Shelf temperatures in these 2 years were among the
lowest observed since the early 1970s, thus interrupting an
approximately 30-year warming trend. We examined winter cooling
processes using historical conductivity-temperature-depth (CTD) profiles
and mooring data from hydrographic station GAK1. The 2006 and 2007
cooling was associated with anomalously strong atmospheric heat loss in
November 2006 and March 2007 and below-average fall runoff, which
weakened winter stratification and allowed the late cooling to penetrate
throughout the water column. In 2007 and 2008, early winter cooling was
weak, fall runoff large, and stratification moderate at 100 m so that
spring temperature anomalies were trapped to the upper 100 m. Analysis
of the 40 year GAK1 CTD record indicates that winter averaged air-sea
heat flux and salinity stratification anomalies explain 81% of the
variation in deep (100-250 m) GOA temperatures. Although the timing and
magnitude of winter runoff influences the shelf temperature
distribution, temperature anomalies are a consequence of
three-dimensional circulation and mixing processes. These involve the
complex, but poorly understood, interplay among the air-sea heat flux;
the ocean heat flux convergences; the stabilizing influence of runoff;
and the destabilizing effects of cooling, vertical mixing, and the
wind-driven cross-shelf buoyancy flux.
Laboratory experiments with juvenile Pacific halibut (Hippoglossus stenolepis Schmidt) (31–150 mm TL) and northern rock sole (Lepidopsetta polyxystra Orr and Matarese) (15–150 mm TL) were conducted to examine relationships between sediment preference, burial performance, and general morphological characteristics. Both species demonstrated significant size-dependent changes in sediment choices during the first year of life. Highest sediment selectivity occurred in the smallest individuals of both species. There was a strong positive relationship between sediment choice and ability to bury quickly and completely, although the choices made by rock sole were less specific than those of comparably sized halibut and were less closely concordant with burial capability. In both species, fish >80 mm TL were relatively nonselective, except that they avoided sediments with the largest grain size (i.e., granules and pebbles). Pacific halibut have narrower, more powerful bodies than rock sole and were stronger burrowers than rock sole. Rock sole were more cryptic than halibut in general locomotion and color-matching capability, and appear to depend to a lesser extent on burial for survival than do halibut. Association with sediment is the first line of defense for juvenile flatfishes, and the relationships shift rapidly with fish size during the first year of benthic life. Therefore, habitat descriptions and models for young post-settlement flatfishes need to be made for narrow size classes.
Inference of population structure and individual ancestry is important for both population genetics and for association studies. With next generation sequencing technologies it is possible to obtain genetic data for all accessible genetic variations in the genome. Existing methods for admixture analysis rely on known genotypes. However, individual genotypes cannot be inferred from low depth sequencing data without introducing errors. This paper presents a new method for inferring individual's ancestry that takes the uncertainty introduced in next generation sequencing data into account. This is achieved by working directly with genotype likelihoods which contains all relevant information of the unobserved genotypes. Using simulations as well as publicly available sequencing data, we demonstrate that the presented method has great accuracy even for very low depth data. At the same time, we demonstrate that applying existing methods to genotypes called from the same data can introduce severe biases. The presented method is implemented in the NGSadmix software available at http://www.popgen.dk/software.
We present empirically based calibrations of our measurements made on a Finnigan MAT252 equipped with a Kiel Device to Vienna Pee Dee belemnite, using an enriched delta18O standard. Calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the Kiel Device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source. After correcting for these biases, the precision of 2200 NBS19 analyses (10-300 mug) is +/-0.07 for delta18O and +/-0.03 for delta13C. We have shared our standard enriched in delta18O with 18 laboratories engaged in paleoceanographic research, producing the first large-scale interlaboratory calibrations for this community. Using correction procedures reported here, water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature-salinity and density variability in the deep ocean.
The ability to identify past patterns of salinity habitat use in coastal fishes is viewed as a critical development in evaluating nursery habitats and their role in population dynamics. The utility of otolith tracers (delta C-13, delta O-18, and Sr/Ca) as proxies for environmental salinity was tested for the estuarine-dependent juvenile white perch Morone americana. Analysis of water samples revealed a positive relationship between the salinity gradient and delta O-18, delta C-13, and Sr/Ca values of water in the Patuxent River estuary. Similarly, analysis of otolith material from young-of-the-year white perch (2001, 2004, 2005) revealed a positive relationship between salinity and otolith VC, 8180, and Sr/Ca values. In classifying fish to their known salinity habitat, 8180 and Sr/Ca were moderately accurate tracers (53 to 79% and 75% correct classification, respectively), and delta C-13 provided near complete discrimination between habitats (93 to 100% correct classification). Further, delta C-13 exhibited the lowest inter-annual variability and the largest range of response across salinity habitats. Thus, across estuaries, it is expected that resolution and reliability of salinity histories of juvenile white perch will be improved through the application of stable isotopes as tracers of salinity history.
Marine ecologists commonly use discriminant function analysis (DFA) to evaluate the similarity of distinct populations and to classify individuals of unknown origin to known populations. However, investigators using DFA must account for (1) the possibility of correct classification due to chance alone, and (2) the influence of prior probabilities of group membership on classification results. A search of the recent otolith chemistry literature showed that these two concerns are sometimes ignored, so we used simulated data sets to explore the potential pitfalls of such oversights. We found that when estimating reclassification success for a training data set, small sample sizes or unbalanced sampling designs can produce remarkably high reclassification success rates by chance alone, especially when prior probabilities are estimated from sample size. When using a training data set to classify unknown individuals, maximum likelihood estimation of mixture proportions and group membership afforded up to 20% improvement over DFA with uninformative priors when groups contributed to the sample unequally. Given these results, we recommend the use of (1) randomization tests to estimate the probability that reclassification success is better than random, and (2) maximum likelihood estimation of mixture proportions in place of uninformative priors.
Pop-up archival transmitting (PAT) tags were used to study the fall migration of halibut in the Gulf of Alaska (GOA). We tagged 6 Pacific halibut Hippoglossus stenolepis on summer feeding grounds in the eastern GOA and another 6 in the western GOA from June 13 to August 6, 2002. The tags were programed to be released from the fish on January 15, 2003, at the height of the winter spawning season: 10 tags successfully detached, transmitted archived environmental data (depth and temperature), and generated accurate latitude-longitude coordinates shortly after pop-up; 2 tags deployed off SE Alaska were lost. The tags revealed that 6 fish had moved a considerable distance (>200 km) between tagging and pop-up, and all of these had moved northward to some extent. The longest of the observed migrations was from the southern Alaska Peninsula to Yakutat Bay, a linear displacement of 1153 km; 4 fish showed little evidence of geographic displacement, exhibiting migrations that ranged only from 30 to 69 km. Although 2 fish had moved inshore by the end of the tagging period, all other fish had moved offshore regardless of their overall migration distance. The precise timing of offshore movements varied, beginning as early as August and as late as January. These observations generally corroborate conventional tagging, indicating migration of halibut toward winter spawning grounds in the northern GOA, and movement of fish to deep water in fall. However, no single stereotypic migration behavior was apparent, and a variety of vertical movement patterns and temperature profiles were observed. Halibut spent most time in waters of 5 to 7°C, but experienced temperatures ranging from 2.6 to 11.6°C. Depth observations ranged from 0 to 736 m, with summertime activity concentrated in depths from 0 to 400 m, and halibut that exhibited offshore movement were typically observed at 300 to 700 m by mid-winter. Vertical movement (short-period changes in depth) varied among fish and over time, with some fish displaying little vertical activity, others displaying short periods of activity, and still others displaying considerable activity throughout their time at liberty.
Elemental signatures in both the natal and post-settlement regions of otoliths from newly settled Pomacentrus coelestis were measured from multiple sites, reefs, years and reef clusters at different latitudes on the Great Barrier Reef, Australia. Significant differences in latitude and year were found for Sr/Ca and Ba/Ca in the post-settlement portion of the otoliths, and discriminant function analyses identified clear separations in otolith signatures from different reef clusters and years. The spatial and annual variation found in the post-settlement signatures was similar to that determined for the brooding damselfish Acanthochromis polyacanthus, and suggests that these 2 species record environmental variation in a similar manner. This similarity also extended to reefs, although some reefs showed persistent trends for individual elemental ratios. However, the natal portion of the otoliths proved less useful in discriminating groups, and appeared to be decoupled from the post-settlement portion of the otoliths. Such a decoupling could be the result of differences in the food source during the natal period, protein content of the 2 otolith portions which could potentially influence affinity for trace elements, or ontogenetic changes in the depositional process itself. Our findings indicate that robust elemental signatures are likely to be found among clusters of reefs rather than individual reefs, which has important ramifications for future studies attempting to discriminate reef fish populations.
The fish otolith (earstone) has long been known as a timekeeper, but interest in its use as a metabolically inert environmental recorder has accelerated in recent years. In part due to technological advances, applications such as stock identification, determination of migration pathways, reconstruction of temperature and salinity history, age validation, detection of anadromy, use as a natural tag and chemical mass marking have been developed, some of which are difficult or impossible to implement using alternative techniques. Microsampling and the latest advances in beam-based probes allow many elemental assays to be coupled with daily or annual growth increments, thus providing a detailed chronological record of the environment. However, few workers have critically assessed the assumptions upon which the environmental reconstructions are based, or considered the possibility that elemental incorporation into the otolith may proceed differently than that into other calcified structures. This paper reviews current applications and their assumptions and suggests future directions. Particular attention is given to the premises that the elemental and isotopic composition of the otolith reflects that of the environment, and the effect of physiological filters on the resulting composition. The roles of temperature, elemental uptake into the fish and the process of otolith crystallization are also assessed. Drawing upon recent advances in geochemistry and paleoclimate research as points of contrast, it appears that not all applications of otolith chemistry are firmly leased, although others are destined to become powerful and perhaps routine tools for the mainstream fish biologist.
The eastern Pacific halibut (Hippoglossus stenolepis ) fishery is prosecuted over a nine-month season with a provision to cease harvests if stock declines to historically-observed minimum spawn- ing biomass. The industry has requested to extend fishing into winter, but little information exists regarding potential impacts on spawning aggregations or effective spawning biomass. A strictly an- nual spawning cycle is presumed, but some adults fail to undertake the offshore migration associated with continental slope spawning. We examined depth records of halibut tagged with Pop-up Archival Transmitting (PAT) tags for evidence of offshore seasonal migration (n = 72). For tags that were physically recovered (n = 16) we identified the occurrence of abrupt (~100 m) mid-winter ascents, believed to be egg release. The active spawning season, defined by occurrence of these rises, lasted from 27 December-8 March, at bottom depths of 278-594 m. Eighteen percent of tagged halibut remained onshore. Thirty-one percent of fish with detailed archival records did not exhibit spawning rises, including all fish that remained onshore. Correcting for the possibility that some were likely immature, the data suggest that ~10% of the mature fish do not participate in the spawning migration and 10-15% that migrate to deep water may not actively spawn. The data suggest that opening the commercial fishery in early spring would likely subject actively spawning fish to fishing mortality, and could truncate the effective spawning period. Natural rates of skip-spawning and fisheries-in- duced reduction of the spawning period relative to suitable larval rearing conditions could introduce temporal and regional variance into levels of effective spawning biomass and warrant further inves- tigation.
Fish ecologists have used geochemical values in otoliths to examine habitat use, migration, and population connectivity for decades. However, it remains difficult to determine an unambiguous dietary δ¹³C signature from bulk analysis of otolith. Studies to date have focused on the aragonite component of otoliths with less attention paid to the organic fraction. We describe the application of compound-specific stable isotope analysis (SIA) to analyze amino acid (AA) δ¹³C values from small amounts (<1 mg) of otolith powder. We examined δ¹³C values of otolith and muscle AAs from a reef-associated snapper (Lutjanus ehrenbergii (Peters, 1869)) collected along a carbon isotope gradient (isoscape) from seagrass beds to coral reefs. Carbon isotope values in otolith and muscle samples were highly correlated within and among coastal habitats. Moreover, δ¹³C values of otolith AAs provided a purely dietary record that avoided dilution from dissolved inorganic carbon. Otolith AAs served as a robust tracer of δ¹³C values at the base of the food web, making compound-specific SIA a powerful tool for dietary reconstructions and tracking the movement of fishes across isoscapes.
Stable carbon isotope values of otoliths can provide long-term records of the environmental conditions experienced by fish. However, because otoliths contain carbon from metabolic sources, it is necessary to determine the proportion of metabolically derived carbon to properly interpret otolith delta(13)C values. To evaluate the relative influence of dietary carbon and the dissolved inorganic carbon (DIC) of ambient seawater on the delta(13)C isotopic composition of fish otoliths, juvenile red drum Sciaenops ocellatus were raised for 6 and 9 mo in tanks with flow-through ambient seawater and fed diets differing by 2.1 parts per thousand (p < 0.001) in carbon isotope composition (delta(13)C). At the end of the experiment, muscle tissue from the 2 treatments reflected the isotopic composition of the diets and differed by 2.1 parts per thousand (p < 0.001). Muscle tissue from both groups was enriched by 1.5 parts per thousand in (13)C relative to the respective diets. The carbon isotope values of otoliths were enriched by similar to 16 to 17 parts per thousand relative to the respective diets, indicating that DIC was the dominant source (85 to 92%) for otolith carbon relative to carbon derived from diet. However, otoliths from the 2 treatments differed by 1.26 parts per thousand and expressed 60% of the difference in the isotope values of the diets. This result indicates that the nutritional characteristics of food may be a factor influencing the incorporation of metabolic carbon in the otolith. This is one of the first studies to provide evidence that the nutritional value of food influences otolith delta(13)C value.
Otolith elemental composition is a commonly used tool in the classification of fish stocks. Lower detection limits associated with trace element analysis techniques are the source of much debate among fisheries biologists. Using empirical data from Atlantic herring (Clupea harengus) from two separate origins in the British Isles, we addressed methods of treating data at or falling below detection limits and the implications of each treatment for discrimination and classification analysis. Estimating actual values of non-detected concentrations, through imputation, is now a plausible and simply applied strategy and should be considered as an alternative to constant replacement. We compare a robust regression approach to estimating values for non-detects alongside a variety of constant replacement methods. We also show the value of a highly censored variable in understanding population discrimination; despite 40% of readings for one element being non-detects, we found that its removal was detrimental to the discriminatory power of the analysis.
Studies investigating population structure and mixed-stock composition of fish populations frequently use otolith chemistry as a natural tool for discerning stocks. Current methods for estimating mixed-stock composition, however, assume complete accuracy in the training data, which is often not the case. Here we present a method for estimating mixed-stock composition using multivariate continuous data that accounts for uncertainty in the training data. Application of the method to previously reported data for natal homing in weakfish (Cynoscion regalis) and simulations based on these data revealed that for sample sizes greater than about 30, the present method provides results that are quite similar to those of previous methods. An advantage of this Bayesian approach over other methods, however, is the ease with which functionals of the model, such as migration distance and direction, can be calculated. It also provides simple means of visualizing spatial structure in the classification probabilities and migration patterns.
Analysis of otolith strontium (Sr) or strontium-to-calcium (Sr:Ca) ratios provides a powerful tool to reconstruct the chronology of migration among salinity environments for diadromous salmonids. Although use of this method has been validated by examination of known individuals and translocation experiments, it has never been validated under controlled experimental conditions. In this study, incorporation of otolith Sr was tested across a range of salinities and resulting levels of ambient Sr and Ca concentrations in juvenile chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), sockeye salmon (Oncorhynchus nerka), rainbow trout (Oncorhynchus rnykiss), and Arctic char (Salvelinus alpinus). Experimental water was mixed, using stream water and seawater as end members, to create experimental salinities of 0.1, 6.3, 12.7, 18.6, 25.5, and 33.0 psu. Otolith Sr and Sr:Ca ratios were significantly related to salinity for all species (r2 range: 0.80-0.91) but provide only enough predictive resolution to discriminate among fresh water, brackish water, and saltwater residency. These results validate the use of otolith Sr:Ca ratios to broadly discriminate salinity histories encountered by salmonids but highlight the need for further research concerning the influence of osmoregulation and physiological changes associated with smoking on otolith microchemistry.
For many marine ecosystems, uncertainty about nursery locations and the spatial dynamics of juvenile fish impedes our understanding of fish production. Walleye pollock Gadus chalcogrammus occur throughout the coastal North Pacific Ocean and support some of the world's largest fisheries. We used otolith microchemistry to answer questions about whether cohorts of young-of-The-year (age-0) walleye pollock are spatially split in the western Gulf of Alaska (GOA). Demographics indicate a possible cohort split between habitat influenced by the Alaska Coastal Current (ACC) (Semidi regions) and habitat more influenced by oceanic influxes (Kodiak region). We used a stratified-random sampling design to select 204 age-0 juveniles collected with a small-mesh trawl during late summer 2007 and 2011. Laser ablation-inductively coupled plasma mass spectrometry was then used to measure the composition of elements assimilated into their otoliths within 1 wk of capture (otolith edge) and over their life histories (otolith edge to core). Otolith edge chemistry varied by region of capture, primarily in strontium:calcium (Sr:Ca), barium:Ca (Ba:Ca), and manganese: Ca (Mn:Ca). Semidi-region otoliths were discriminated from Kodiak otoliths by lower Sr:Ca and higher Ba:Ca and Mn:Ca with 78% (2007) and 79% (2011) success. We estimated that exchange between these 2 habitats was limited for ≥3 (2007) and ≥7 (2011) weeks, sufficient to explain observed demographic differences. We hypothesize that a Semidi-Kodiak split buffers the western GOA population against losses due to density-dependent mechanisms and downstream transport.
This is the second of two parts of an article on Quantitation in Elemental Analysis which grew out of Professor Kaiser's participation as a speaker at the 22nd Annual Summer Symposium on Analytical Chemistry. This meeting was held June 11 to 13, 1969, at the University of Georgia. The first part of this article appeared in the February issue of ANALYTICAL CHEMISTRY, p 24 A.
The ability to determine connectivity between juveniles in nursery estuaries and adult populations is an important tool for fisheries management. Otoliths of juvenile fish contain geochemical tags, which reflect the variation in estuarine elemental chemistry, and allow discrimination of their natal and/or nursery estuaries. These tags can be used to investigate connectivity patterns between juveniles and adults. However, inter-annual variability of geochemical tags may limit the accuracy of nursery origin determinations. Otolith elemental composition was used to assign a single cohort of 0-group sea bass Dicentrarchus labrax to their nursery estuary thus establishing an initial baseline for stocks in waters around Ireland. Using a standard LDFA model, high classification accuracies to nursery sites (80–88%) were obtained. Temporal stability of otolith geochemical tags was also investigated to assess if annual sampling is required for connectivity studies. Geochemical tag stability was found to be strongly estuary dependent.
Connectivity is a central issue in the development, sustainability and effectiveness of networks of Marine Protected Areas (MPAs). In populations with site attached adults, connectivity is limited to dispersal in the pelagic larval stage. Whilst biophysical models have been widely used to infer early dispersal, empirical evidence through sources such as otolith microchemistry can provide a means of evaluating model predictions. In the present study, connectivity in the lesser sandeel, Ammodytes marinus, was investigated using LA-ICP-MS otolith microchemistry. Otoliths from juveniles (age 0) were examined from four Scottish spawning areas predicted to differ in terms of larval retention rates and connectivity based on past biophysical models. There were significant spatial differences in otolith post-settled juvenile chemistry among locations at a scale of 100–400 km. Differences in near core chemistry pointed to three chemically distinct natal sources, as identified by a cluster analysis, contributing to settlement locations.
The influence function is used to develop criteria for detecting outliers in discriminant analysis. For Mahalanobis' D2, the influence function is a quadratic function of the deviation of the discriminant score for the perturbed observation from the discriminant score for the mean of the corresponding group. A χ2 approximation to the null distribution of the influence function values appears to be suitable for graphical representation.
Spawning in Greenland halibut (Reinhardtius hippoglossoides) occurs along the continental slope and in submarine canyons in the eastern Bering Sea. It is assumed that these bathymetric features and their associated circulation patterns deliver eggs and larvae to suitable nursery habitats over the continental shelf. However, there have been no directed field studies examining spawning areas or transport of Greenland halibut early life stages in the Bering Sea, nor is it known how large-scale oceanographic forcing modulates specific physical mechanisms of delivery. The present study was undertaken to: better define spawning areas of Greenland halibut, examine development and distribution of larvae, and understand the influence of climate variations on interannual patterns of transport, distribution and abundance. Eggs were found in Bering and Pribilof Canyons and over the adjacent slope in February and early March, confirming that spawning occurs in these regions. Larvae were present over the slope, outer shelf and middle shelf in winter and spring, and settled juveniles were collected over the shelf in September. Oceanographic modeling approaches that simulate larval advection from spawning to nursery habitats indicate that depth-discrete variations in transport pathways from submarine canyons to the adjacent shelf contribute to interannual variability in transport trajectories. Overall, our results highlight specific physical mechanisms of delivery that are modulated by large-scale atmospheric and oceanographic forcing, potentially varying the degree of slope–shelf connectivity for Greenland halibut and other slope-spawning species.
Otolith microchemistry is a powerful tool for fisheries biology, but many applications stretch the limits of analytical techniques. Sources of measurement error were evaluated with a methodological comparison of solution and laser ablation inductively coupled plasma mass spectrometry at two laboratories, using otoliths and certified reference materials NIES-22 and FEBS-1. The intercalibration exercise was effective for Na, Ca, Zn, Cu, Sr, Ba, and Pb. Measurements of Sr were most robust, i.e. sensitivity, accuracy, and precision were least affected by laboratory, technique or sample type. Calcium and Ba measurements varied more between laboratories than between techniques. Lead and Cu measurements were more dependable across laboratory and technique while Zn measurements were more sensitive to these effects. Because the measurement of otolith elements may vary with methodology, it is imperative to consider the sensitivity, accuracy and precision for each application as the suite of elements of interest may differ. The development of reference materials has been the first step in standardization of the procedures, but there is a need for wider ranging calibration and comparison studies.
Comparison of δ18O and δ13C data from otoliths with estimates of equilibrium aragonite indicate that 18O, but not 13C, was deposited near equilibrium, confirming the utility of otolith oxygen isotopes as an indicator of ambient temperature. There was a strong correlation between δ13C and temperature and, concomitantly, a strong relationship between δ18O and δ13C. Evidence is presented to support the importance of metabolic effects in mediating 13C disequilibria and a strong relationship between metabolic rate (VO2) and δ13C is hypothesized. Fishes with low metabolic rates or those living at low temperatures had 13C deposited near to equilibrium; fishes living at higher temperatures showed extreme depletion in otolith 13C. Trends were also evident in δC values in juveniles and adults of a species with the juveniles, presumably with a higher metabolic rate, being more depleted in 13C. -from Author
Previous models have predicted that when mortality increases with age, older individuals should invest more of their resources in reproduction and produce less dispersive offspring, as both their future reproductive value and their prospect of competing with their own sib decline. Those models assumed stable population sizes. We here study for the first time the evolution of age-specific reproductive effort and of age-specific offspring dispersal rate in a metapopulation with extinction-recolonization dynamics and juvenile dispersal. Our model explores the evolutionary consequences of disequilibrium in the age structure of individuals in local populations, generated by disturbances. Life-history decisions are then shaped both by changes with age in individual performances, and by changes in ecological conditions, as young and old individuals do not live on average in the same environments. Lower juvenile dispersal favours the evolution of higher reproductive effort in young adults in a metapopulation with extinction-recolonization compared with a well-mixed population. Contrary to previous predictions for stable structured populations, we find that offspring dispersal should generally increase with maternal age. This is because young individuals, who are overrepresented in recently colonized populations, should allocate more to reproduction and less to dispersal as a strategy to exploit abundant recruitment opportunities in such populations.
The relationship between water temperature, growth rate, and otolith isotopic ratios was measured for juvenile plaice ( Pleuronectes platessa ) reared at two temperatures (11 and 17°C) and two feeding regimes (1 and 3 prey items·ml −1 ). The otolith isotope ratios in individual fish ranged from −2 to −4 for carbon isotope ratios (δ 13 C) and from 0.2 to 1.9 for oxygen isotope ratios (δ 18 O). The otolith oxygen isotope ratios were significantly affected by water temperature, but not by feeding level, and there were no significant synergistic effects. The fractionation of oxygen isotopes during otolith growth was independent of individual growth rate. Carbon isotope ratios were not significantly affected by food ration or water temperature, but were related to fish growth rate. The carbon isotope ratios were negatively correlated with fish length in the colder water treatments, and tended to increase with fish length in the warm water treatments. The laboratory-determined relationship between otolith oxygen isotope ratio and water temperature was applied to individuals of five species (plaice, cod, whiting, haddock, gurnard) collected in a single trawl sample. The otolith derived temperatures often overestimated measured water temperatures. The difference between real and estimated water temperatures varied between species, and the closest fit was for field-caught plaice.
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 (delta C-13, delta O-18) 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.