Otolith Core Research Articles

Identification of potential fish stocks and lifetime movement patterns of Mugil liza Valenciennes 1836 in the Southwestern Atlantic Ocean

The mullet Mugil liza is the Mugilidae that lives southernmost in the western Atlantic Ocean. Knowledge about migration, movements and identification of stocks of this important fishery resource is scarce. Thus, we aim to study movement patterns and to identify the presence of different fish stocks in the southwestern region of the Atlantic Ocean, using cumulative otolith shape morphometric and microchemical analyses of sagittae otoliths. Specimens (n=99) were obtained in four coastal areas: Paranaguá Bay in Brazil, Samborombón Bay, Mar Chiquita Coastal Lagoon, and San Blas Bay in Argentina. Otolith shape indices (circularity, rectangularity, aspect ratio, percentage occupied by sulcus, ellipticity and form factor) were used for stock identification analysis; and otolith microchemistry using LA-ICP-MS (Sr/Ca and Ba/Ca ratios chronological variation) was used for both the analysis of movement behaviors and the identification of fish stocks (otolith edge ratios). Morphometrical indices did not reveal a clear separation among areas. San Blas bay individuals presented otoliths tending to be longer than wider, with a more elliptic shape than the otoliths from other studied areas; also, this area did not share individuals with the most northern one, Paranaguá Bay in Brazil. The analysis of microchemical lifetime profiles revealed three types of behavior pattern: Type I: most frequent use of estuarine environments; Type II: a fluctuating behavior between estuarine and sea/high salinity waters; Type III: most frequent use of sea/high salinity habitats. Otolith edge analysis did not reveal differences among Sr/Ca and Ba/Ca ratios for the different areas. Thus, it cannot be assured that there is more than one stock in the studied region. Mugil liza revealed different environmental migratory behaviors in the Southwestern Atlantic Ocean showing a facultative use of estuarine waters; hence, the species appears to be mostly coastal with the use of low estuaries, as seen also by the Sr/Ca otolith cores ratios; differing from the general mugilid behavior previoulsy described.

Inter-annual variability in otolith chemistry of catfish Genidens barbus from South-western Atlantic estuaries

The catfish Genidens barbus is a commercial species from South America. The aim of the present study was to examine the inter-annual variability in estuary-specific chemical signatures of otolith cores (Ba:Ca, Mg:Ca, Mn:Ca, Sr:Ca and Zn:Ca ratios) for three estuaries from Argentina and Brazil where adults were collected over multiple years. Secondarily, we evaluated whether the percentages of classification of individuals to their natal origin place are affected by the grouping of several cohorts. Most element:Ca ratios were not significantly different among year cohorts. Results from PERMANOVA revealed significant differences in the multi-element signatures of the otolith core between cohorts for the Plata River estuary (PR) (P = 0.006) and the Patos Lagoon (PL) (P = 0.03), while no significant differences (P = 0.9) were found for Paranaguá Bay (PB). The percentages of spatial classification (discriminant function analyses) decreased to between 15.5 and 25% for PR and PL when cohorts were grouped. This work makes it clear that the temporal variation in the chemical signature of the adult catfish otolith core can greatly affect the percentages of spatial classification.

Remarkable vertical shift in residence depth links pelagic larval and demersal adult jellynose fish

Deep-sea fish show diverse migratory behaviors across depths at different life stages. The historical residence depths of jellynose fish Ateleopus japonicus and Ijimaia dofleini (Ateleopodidae) were reconstructed from otolith microstructures and isotopic compositions. δ18O values in the otolith core areas ranged from −0.5 to −1.3‰ among individuals, suggesting that larval and post-larval stages lived in the mixed layer (50–200m). Otolith growth increment widths surged for 10–30 rings around 300–600µm from the core, indicating a fast-growth phase during the early post-larval stage. Fish then migrated downward to 350–800m depth at about 2 months of age, possibly during the post-larval metamorphosis to the juvenile. Otolith growth increments became narrower and otolith δ13C values increased from −5 to −1‰, suggesting a lower growth and metabolic rate when the fish experienced colder water during the downward migration. After arrival at the deepest waters, the fish then migrated upward to the continental margin or upper slopes where the adults persistently resided. A translucent otolith zone was formed after the residence depth shift from the deepest waters to shallower depths, indicating a transition from pelagic to bathydemersal life on the continental shelf or break. The down-and-up shift in residence depth of jellynose fish represents an indirect settlement process to the adult residence depth, which might be associated with a unique post-larval stage moving offshore before the downward migration. The results filled the gap of vertical distributions of jellynose fish from pelagic larvae near the sea surface to the bathydemersal adult dwelling on the continental shelf break.

Use of post-bomb radiocarbon dating to validate estimated ages of Pacific bluefin tuna, Thunnus orientalis, of the North Pacific Ocean

Estimation of the age of Pacific bluefin tuna (PBF), Thunnus orientalis, is one of the most important processes involved in assessment of its stock biomass. Recently, the ages of PBF have been estimated by analyzing otolith thin sections. However, the accuracy of this estimated age has not yet been confirmed. In this study, the estimated ages of PBF were validated by a novel use of post-bomb radiocarbon dating. The analysis was based on a metric of the abundance of 14C relative to 12C (Δ14C) in the core portion of the otoliths of PBF. The Δ14C of the otolith core portion was measured by accelerator mass spectrometry and compared with reference values of corals in the western Pacific. The decrease of the Δ14C of the whole otoliths of PBF that were 0–2-years of age from 1982 to 2010 coincided with the decrease of Δ14C reference values. This result suggests that post-bomb radiocarbon dating can be used to estimate ages of adult PBF. The Δ14C of the otolith core portions of fish estimated to be 2–27-years of age gradually declined as birth year approached the present day. This trend was consistent with the trend of the reference values and 10‰ lower. The portions of the otolith formed after 2-years of age remained on the surface of the adult otolith core; these portions would dilute the Δ14C values of the otolith cores of adults and provide an explanation for the depletion. These results support the use of otolith thin sections to estimate PBF ages.

Do Eurasian minnows (Phoxinus phoxinus L.) inhabiting brackish water enter fresh water to reproduce: Evidence from a study on otolith microchemistry

AbstractDynamics of coastal fish assemblages in the Baltic Sea are still rather poorly understood. In particular, little information is available on migrations and movements of the small‐bodied littoral fish species like Eurasian minnow (Phoxinus phoxinus L.). Minnow is considered typically as a river and lake species, but it also inhabits brackish coastal waters of the Baltic Sea. In this study, we investigated movement patterns of brackish water inhabiting minnows using otolith microchemistry. Fish were collected from four different sites (from two bays and two stream mouths) around Saaremaa Island. The results indicated that at least three distinct migration patterns exist: (i) seawater residency, (ii) fast springtime migrations to fresh water or to bays with lower salinities (areas near stream mouths) and (iii) prolonged migrations to fresh water (some cases overwintering in streams). Migration patterns listed above were not evenly distributed among sites, and some individuals did not migrate to fresh water in every year. None of the analysed fish were freshwater residents. Additionally, potentially ontogenetic effects on Mn and Sr concentrations were observed in the otolith core regions, which may have important implications for the interpretation of otolith chemistry data. The overall findings of this study demonstrate that much more complex migration patterns exists in brackish water inhabiting minnow populations than previously thought and at least some individuals do enter freshwater to reproduce.

Age and Stock Analysis Using Otolith Shape in Gags from the Southern Gulf of Mexico

AbstractOn Campeche Bank (CB) in the southern Gulf of Mexico, Gags Mycteroperca microlepis are captured as part of a sequential multispecies fishery. Limited scientific data suggest that the Gag is the third most important commercial species in the region, contributing 21% (total weight) of the overall grouper catch. Neither age nor stock data are currently available for Gags in this region, making it impossible to determine how this species is affected by current fishing practices. Such information is vital for the CB stock since it acts as a genetic source for a mixed Gag population on the west Florida shelf through a constant unidirectional flow of larvae. The goal of this study was to generate baseline age and stock structure data for Gags on CB. Age was quantified from otoliths by counting the number of annuli (indicated by the number of opaque zones from the otolith core to the margin) in thin sections. Otolith shape was analyzed by using discrete wavelet transforms, linear otolith morphometrics, and shape indices. In each of 198 specimens, age was determined based on examination of the right sagittal otolith, and shape analysis was conducted on the left sagittal otolith. Individual age ranged from 4 to 17 years, and otolith opaque band deposition occurred annually between December and February. Using principal components analysis and linear discriminant analysis, two distinct groups were identified (discrimination success = 98%), indicating the presence of substructure in the CB stock of Gags. Otolith analysis clearly assisted in achieving a better understanding of the CB Gag stock and in identifying the complexity within the stock, demonstrating that otolith analysis is a useful tool in characterizing fish populations. The study results highlight the urgent need for Gag stock assessment in this region and for the application of a cautious approach to fishery management given the immense gaps in biological data for this species.Received January 13, 2016; accepted July 18, 2016 Published online October 7, 2016

Nursery areas and connectivity of the adults anadromous catfish (Genidens barbus) revealed by otolith-core microchemistry in the south-western Atlantic Ocean

The anadromous catfish, Genidens barbus, is a commercial and vulnerable species from South America. The aims of the present study were to assess whether the nursery areas can be discriminated by using microchemical signatures of lapillus otoliths, to assess the accuracy of classifying fish in relation to natal nursery area and to discuss the possibility of existence of homing behaviour. Thus, the otolith-core chemical signatures (barium (Ba):calcium (Ca), lithium (Li):Ca, magnesium (Mg):Ca, manganese (Mn):Ca, strontium (Sr):Ca, and zinc (Zn):Ca ratios) of adult fish were compared among different estuaries (De La Plata River in Argentina, and Patos Lagoon, Paranaguá Bay and Guanabara Bay in Brazil). PERMANOVA analysis showed significant differences in the multi-element signatures of the otolith core among sampling sites for all cohorts (2002, 2003, 2006 and 2007), indicating that the technique has considerable potential for use in future assessments of population connectivity and nursery areas of G. barbus. Via quadratic discriminant function analysis, fish were classified to natal nursery areas with 80–100% cross-validation classification accuracies. These results suggested that a high level of spatial segregation exists in adult catfish life and homing behaviour could not be ruled out on the basis of our data.

Otoliths as elemental tracers of walleye environmental history: insights for interjurisdictional fisheries management

ABSTRACTCarlson AK, Bailey PE, Fincel MJ, Graeb BDS. Otoliths as elemental tracers of walleye environmental history: insights for interjurisdictional fisheries management. Lake Reserve Manage. 32:329–340.Understanding fish natal origins and movement is important for managing interjurisdictional fisheries. We used otolith microchemistry to assess walleye (Sander vitreus) provenance, movement, and natal homing in Lake Oahe, an interjurisdictional reservoir fishery in North Dakota and South Dakota. Historical (1983–1989) water chemistry varied among 6 tributaries for strontium:calcium (Sr:Ca; μmol/mol) and barium:calcium (Ba:Ca) and between 2 main-stem sites for Ba:Ca. Current (2012) water chemistry was spatially heterogeneous for Sr:Ca and Ba:Ca. Both ratios were consistent between the historical and current periods in tributaries and main-stem sites. Sr:Ca and Ba:Ca of age-0 walleye otolith cores varied in proportion to water chemistry, resulting in high reclassification accuracies to habitat types (87%) and individual sites (78%). For adult walleye, contributions of South Dakota natal sites to the North Dakota population (48%, n = 30) and North Dakota natal sites to the South Dakota population (48%, n = 29) were highest for tributaries and embayments (e.g., Moreau and Cannonball rivers, Beaver Bay). Annual downstream movement was more prevalent than upstream movement, particularly during a flood in 2011. An average of 36% of North Dakota walleye and 33% of South Dakota walleye exhibited natal homing each year from 2009 to 2013. Otolith element:Ca ratios are effective natural tracers for evaluating walleye natal origins, movement within and between states, and natal homing. Otolith microchemistry is a tool for interjurisdictional walleye management in reservoirs, providing a methodology for assessing natal origins and movement and designing spatially informed habitat conservation programs and harvest regulations.

Using otolith microchemistry to differentiate between stocked and unstocked Australian bass (Percalates novemaculeata)

Understanding the contribution of stocking to fish populations is an essential part of any program to increase fish numbers and abundance. However, subsequent assessments that aim to quantify the success of fish stocking are rarely considered. Juvenile-sized (<200mm) Australian bass (Percalates novemaculeata), from the upper Snowy River, southern New South Wales (NSW), Australia (assumed to be stocked), wild mature-sized (>400mm) fish from estuarine areas of the Snowy River, juveniles (15–30mm) from a commercial Australian bass hatchery facility known to have supplied fish for past stockings in the Snowy River, and wild fish (>400mm) considered as controls from the Clarence River in northern NSW were used for otolith microchemical analysis. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure Sr:Ca and Ba:Ca ratios at the otolith core and across radial transects to verify natal origins of fish and more specifically, to confirm the origin of fish caught in the river that were assumed to have originated from a recent stocking program. Otolith core Sr:Ca ratios of juvenile fish collected from the upper Snowy River and hatchery fish were not significantly different suggesting they originated from the same source. Significant differences were found in core Sr:Ca and Ba:Ca between known origin hatchery fish and wild fish. Significantly higher Sr:Ca ratios were found at otolith cores compared to the edges in fish collected from the upper Snowy River, suggesting early life was spent in more saline conditions prior to stocking compared to wild fish. Otolith transect data showed a consistent decrease in Sr:Ca ratios and a simultaneous increase in Ba:Ca ratios at approximately 17–59mm from the core, corresponding with the size at which these fish were stocked. This study has demonstrated that otolith microchemistry can successfully distinguish stocked fish from wild recruits and reveals the timing of natural migrations or artificial translocations.

Another New Zealand centenarian: age validation of black cardinalfish (Epigonus telescopus) using lead–radium and bomb radiocarbon dating

Black cardinalfish (Epigonus telescopus, Apogonidae) is an important component of deepsea commercial fishing activity in the New Zealand region. It is estimated to live longer than 100 years on the basis of counts of unvalidated annual growth zones in otoliths. Age-validation procedures for long-lived fishes are often one of the following two techniques: (1) lead–radium disequilibria, which uses the natural decay of radium-226 into lead-210 as a natural clock; or (2) bomb radiocarbon (Δ14C) dating, which relies on the marine signal created by nuclear testing. The high estimated lifespan, as well as the large size of the otolith core region, make E. telescopus an excellent candidate for a combined application of these two independent age-validation techniques. The lead–radium dating using otolith cores indicated that growth-zone counts less than ~60 years were consistent with radiometric ages, whereas higher counts appeared to be under-estimates. There was 95% confidence that maximum age was at least 95 years. The validation indicated that fish aged over 60 years tended to be under-aged by up to 30%. The bomb radiocarbon levels in otolith cores supported age estimates up to ~40 years made from zone counts, and by inference from the zone counts validated with lead–radium dating, longevity exceeds 100 years.

Age and growth of Greenland Halibut (Reinhardtius hippoglossoides) in the Northwest Atlantic: A changing perception based on bomb radiocarbon analyses

Greenland Halibut (Reinhardtius hippoglossoides) is a large circumpolar, right-eyed flounder that supports several international fisheries. Age estimates for this stock have customarily been obtained by counting annuli on the surface of whole otoliths but a matched pair comparison indicated that ages estimated from whole and thin-sectioned otoliths were the same up until age 9 (∼60cm), after which whole otoliths underestimated the true age of the fish by up to 60% and 18 years. Bomb radiocarbon analyses on otolith cores from thin sections were similar to expectations based on the Greenland Halibut reference chronology, indicating that on average, thin sections gave an accurate age for fish in the 57–108cm TL range. Growth curves estimated using thin-sectioned otoliths indicated that Greenland Halibut in this region are slower growing and longer lived than was previously thought. A comparison of growth parameters with other populations of Greenland Halibut showed that although the Pacific and Arctic are faster-growing, the stock off the East Coast of Canada is longer lived. This study represents the first age validation study for this stock in the Northwest Atlantic. Since the fishable portion of the population is largely <60cm, it is possible that these results may not change the current perception of stock status. However, the increased longevity and reduced growth rates will undoubtedly affect other biological parameters and further work should be done to incorporate the new ageing knowledge into the Management Strategy Evaluation for this stock.

Rearing conditions and habitat use of white seabass (Atractoscion nobilis) in the northeastern Pacific based on otolith isotopic composition

White seabass, Atractoscion nobilis, is an important coastal resource throughout both California and Baja California, but whether this species comprises a single or multiple subpopulations in the northeastern Pacific is not known. The aim of this study was to infer larval rearing habitats and population structure of white seabass by sampling adults from three regions spanning a latitudinal temperature gradient and a distance of over 1000 km, and analyzing the isotopic composition (δ18O and δ13C) of otolith aragonite corresponding to the larval, juvenile and adult stages. Otolith cores revealed high isotopic variability and no significant differences among regions, suggesting overlapping rearing conditions during the larval stage, the potential for long distance dispersal or migration or selective mortality of larvae at higher temperatures. Back-calculated temperatures of aragonite precipitation derived using regional salinity-δw relationships and local salinity estimates also did not differ significantly. However, there were significant differences between the δ18O values of the first seasonal growth ring of age 0 fish as well as back-calculated aragonite precipitation temperatures, suggesting the presence of two potentially discrete subpopulations divided by Punta Eugenia (27°N) along the central Baja California peninsula. These findings are consistent with regional oceanographic patterns and are critical for understanding white seabass population structure, and provide information needed for the implementation of appropriate management strategies.

Pilot Test of Elemental Fingerprint in Otolith Core as a Natural Biological Tag to Discriminate among Gymnocypris przewalskii Stocks in Lake Qinghai in Qinghai-Tibet Plateau, China

Naked or scaleless carp, Gymnocypris przewalskii as reported by Kessler in 1876 is the unique economic fish species and play an important ecological function to maintain the health ecosystem of Qinghai Lake watershed. The natural resource of naked carp, however, has sharply decreased during the past decades. Understanding the stock structure of this endangered fish is prerequisite to make a scientific management strategy to restore its stock resources. Otolith fingerprint has been widely applied to identify and discriminate fish stocks. In the present work, a pilot test was performed to evaluate the possibility of applying trace element fingerprint of otolith core as a natural tag to discriminate among stocks of two inflowing river and releasing artificially hatched and nursed individuals of naked carp. Results showed that otolith core elemental fingerprinting can successfully discriminate the three analyzed naked carp groups which will provide a robust tool for conservation and future development of aquaculture industry of this economic fish in Qinghai-Tibet plateau.

Otolith chemistry as an indicator of movements of albacore (Thunnus alalunga) in the North Atlantic Ocean

Albacore (Thunnus alalunga) in the North Atlantic Ocean is currently managed as a single well-mixed stock, although this assumption remains contentious. We measured stable isotopes (δ13C and δ18O) and trace elements (Mg, Mn, Sr, Ba) in otoliths of albacore collected from two feeding grounds, namely the Bay of Biscay and Atlantic offshore waters, and compared them among sampling locations and life history stages. Measurements in otolith core, post-core and edge were used to determine whether albacore from these two regions have the same nursery origin and migratory patterns. We found no clear evidence of distinct nursery grounds based on otolith core chemistry, but Sr:Ca and Mg:Ca were different in the post-core portions of albacore from the two locations, suggesting residency in different regions during the early juvenile stage. Otolith edge chemistry, particularly stable isotopes and Sr:Ca, proved to be a valuable tool for classifying individuals to their capture locations. Annual cycles of Sr:Ca ratios were visible along life history transects, likely reflecting migratory patterns between water masses of differing salinity, but the timing of Sr:Ca cycles differed between the two groups. Differentiation in trace element concentrations in the otolith post-core and the timing of Sr:Ca cycles suggest the occurrence of two migratory contingents of albacore in the north-east Atlantic Ocean.

Image-enhanced burnt otoliths, bomb radiocarbon and the growth dynamics of redfish (Sebastes mentella and S. fasciatus) off the eastern coast of Canada

Many past attempts to age deep-water redfish (Sebastes mentella) and Acadian redfish (S. fasciatus) in the north-west Atlantic have been stymied by inappropriate ageing methods, the absence of age validation and the failure to differentiate among species. Herein we report substantial improvements in methods for ageing Sebastes spp. by linking the established ‘crack and burn’ method to modern sectioning and image-enhancement protocols. Bomb radiocarbon assays of the otolith core and monitoring of year-class progression confirmed the accuracy of the resulting age determinations to an age of 46 years. The use of microsatellite DNA to confirm species identity eliminated past confusion caused by species mixtures. Age determinations of 1252 redfish from the eastern coast of Canada demonstrated the presence of significant differences in growth rate and longevity both between the two redfish species and among populations and stocks, with a maximum observed longevity of 70 years. Even within species and stocks, an individual fish with a fork length of 38cm could be anywhere between 15 and 50 years of age, highlighting a near cessation of somatic growth after sexual maturation. In keeping with other deep-water species, sustainable management will require more attention to the low productivity expected of redfish stocks, rather than the high initial biomass that can support short-term but high catch rates.

Recruitment sources of invasive Bighead carp (Hypopthalmichthys nobilis) and Silver carp (H. molitrix) inhabiting the Illinois River

Knowledge of natal environments and dispersal of invasive Bighead carp (Hypopthalmichthys nobilis) and Silver carp (H. molitrix) inhabiting the Illinois River would be valuable for directing population reduction efforts intended to supplement electrical barriers in the Chicago Sanitary and Ship Canal and limit the probability of these species invading the Great Lakes. However, the extent to which Bighead carp and Silver carp (collectively referred to as bigheaded carps) stocks in the Illinois River are derived from recruits that originate within the Illinois River itself versus immigrants from the Mississippi and Missouri rivers is unknown. Bigheaded carps are also known to use connected floodplain lakes during early life, but the contribution of these habitats to recruitment of Bighead and Silver carps in the Illinois River is also unknown. The aim of this study was to identify natal environment of adult bigheaded carps collected from the Illinois River during 2010–2011 using stable isotope and trace element analyses of otolith cores. Both water and otolith strontium:calcium ratios (Sr:Ca) and water and otolith oxygen isotope ratios (expressed as δ18O) were strongly correlated for known-origin bigheaded carps, consistent with other fish species. Most Bighead and Silver carps collected from the Illinois River used river channel rather than floodplain lake habitats during early life. The majority of adult Silver carp originated in the Illinois River, although 11–39 % were immigrants from the Missouri or middle Mississippi Rivers. In contrast, 97 % of the Bighead carp originated in the Illinois River. Our results indicate that efforts to substantially reduce abundance of bigheaded carps in the Illinois River drainage should continue to focus on the Illinois River itself, but will likely need to be expanded to include the middle Mississippi and Missouri Rivers for sustainable control of Silver carp.

Monthly variability of self-recruitment for a coral reef damselfish

Understanding the dynamics of marine populations is critical to managing marine systems effectively and requires information on patterns of population dispersal and connectivity that are still poorly known. We used transgenerational marking to study larval dispersal of the humbug damselfish, Dascyllus aruanus, in the patchy reef seascape of the southwest Lagoon of New Caledonia (SWL), southwest tropical pacific. The adult population of a patch reef located in the central part of the SWL was injected repeatedly with an enriched 137Ba solution to ensure mass production of marked larvae over two successive reproductive seasons. Multiple cohorts of newly settled larvae were sampled, and their otolith core was analyzed by laser ablation inductively coupled plasma mass spectrometry to assess the seasonal and interannual variability of self-recruitment at the central reef. Connectivity between this reef and ten neighboring reefs was also estimated. Analysis of >1200 settlers indicated that self-recruitment varied significantly between months (ranging from 0 to 68 %) and years (21 % in 2011 and 0 % in 2012). However, variable self-recruitment did not always correspond to variable numbers of self-recruits. Therefore, whereas self-recruitment is undoubtedly a good indication of the degree of population openness, it may not indicate local population persistence. Finally, being the first self-recruitment study to include such a large number of settlers, our study reveals that the threshold used to determine marked individuals significantly affects perceived self-recruitment and connectivity rates and, therefore, must be carefully chosen.

Natural tracers reveal population structure of albacore (Thunnus alalunga) in the eastern North Pacific

Abstract Chemical signatures in otoliths and muscle tissue of albacore (Thunnus alalunga) from two regions of the North Pacific were characterized to examine population structure. Regions included northern (offshore northwest United States and Canada, &amp;gt;40°N) and southern (offshore southwest United States and Mexico, &amp;lt;40°N) areas where albacore have demonstrated region-specific differences in movement and size at age. Juvenile (ages 1–4 years) albacore were collected from each region through recreational and commercial fisheries over a 5-year period (2009–2013). Three different otolith chemistry assays were performed, including whole otoliths (proxy for lifetime signature), otolith edge (recent signature), and otolith core (nursery signature). Whole otolith δ13C and δ18O showed similar δ13C and enriched δ18O (∼0.5‰) values for juvenile albacore collected in the southern region; classification success to the two regions ranged from 78 to 91% during each year (similar age classes). Otolith edge δ13C and δ18O showed similar results as whole otoliths, but no regional differences were found for otolith core chemistry. Incorporation of trace element concentrations from whole otoliths improved discrimination between regions to 90–100% classification success during each year of the study, with significantly higher Ba:Ca, Mg:Ca, and Mn:Ca in albacore otoliths from the northern region. Albacore from the southern region also displayed enriched muscle δ13C (∼1.21‰) and δ15N (∼0.85‰) values relative to fish of the same size range from the northern region. Discrimination success between the two regions across all years using muscle δ13C and δ15N was 91%. Ultimately, results from this study suggest regional separation of albacore in the eastern North Pacific warranting region-specific vital rates in future modelling efforts.

Examining the utility of bulk otolith δ13C to describe diet in wild-caught black rockfish Sebastes melanops

Otolith carbon isotope δ13C values may provide temporally resolved diet proxies in fish. If otolith δ13C values reflect diet, isotope values from recent otolith and muscle tissue should correlate and known ontogenetic diet shifts should be reflected in comparisons between otolith material deposited during different life history stages. We analyzed paired otolith and muscle samples for δ13C from black rockfish Sebastes melanops to examine the potential of otoliths to reflect diet in small (200−299 mm fork length) and large (≥300 mm) fish. We found a significant positive regression between δ13C values from recent (~12 mo) otolith material and muscle in large fish, but not in small fish. Within individual otoliths, δ13C values were enriched by ~3‰ in recent otolith edge material compared to age-0 otolith core material and were consistent with known nearshore−offshore gradients in δ13C values at the base of the food web. Bulk otolith δ13C appeared to provide a broad indicator of dietary carbon sources, but variation in metabolism and dissolved inorganic carbon δ13C among and within individuals likely influences otolith δ13C as well and limits precision. Nevertheless, the results are promising and bulk otolith δ13C may be an appropriate tool to examine large trophic and ecosystem level shifts that have occurred concurrently with changes in habitat, commercial fishing, invasive species, climate change, and other direct or indirect human impacts using historic or ancient otoliths. Future studies should continue to consider the utility of bulk otolith δ13C to describe diet in other marine fish using this simple approach.

Patterns of larval source distribution and mixing in early life stages of Pacific cod (Gadus macrocephalus) in the southeastern Bering Sea

Effective and sustainable management depends on knowledge of spawning locations and their relative contributions to marine fish populations. Pacific cod (Gadus macrocephalus) in the southeastern Bering Sea aggregate at discrete spawning locations but there is little information on patterns of larval dispersal and the relative contribution of specific spawning areas to nursery habitats. Age-0 Pacific cod from two cohorts (2006 and 2008) were examined to address the following questions: (1) does size, age, and otolith chemistry vary among known capture locations; (2) can variation in elemental composition of the otolith cores (early larval signatures) be used to infer the number of chemically distinct sources contributing to juvenile recruits in the Bering Sea; and (3) to what extent are juvenile collection locations represented by groups of fish with similar chemical histories throughout their early life history? Hierarchical cluster (HCA) and discriminant function analyses (DFA) were used to examine variation in otolith chemistry at discrete periods throughout the early life history. HCA identified five chemically distinct groups of larvae in the 2006 cohort and three groups in 2008; however, three sources accounted for 80–100% of the juveniles in each year. DFA of early larval signatures indicated that there were non-random spatial distributions of early larvae in both years, which may reflect interannual variation in regional oceanography. There was also a detectable and substantial level of coherence in chemical signatures within groups of fish throughout the early life history. The variation in elemental signatures throughout the early life history (hatch to capture) indicates that otolith chemical analysis could be an effective tool to further clarify larval sources and dispersal, identify juvenile nursery habitats, and estimate the contributions of juvenile nursery habitats to the adult population within the southeastern Bering Sea.