Otolith elemental fingerprints of juvenile Pacific swordfish Xiphias gladius

The trace element composition of young‐of‐the‐year (YOY) juvenile swordfish Xiphias gladius sagittal otoliths were analysed as a preliminary test of the value of otolith elemental fingerprints for determining swordfish nursery ground origins in the central Pacific Ocean. A suite of five elements (Mg, Zn, Sr, Ba and Pb) was assayed with isotope dilution ICP‐MS; all elemental concentrations were roughly comparable to otoliths of other marine fishes. Multivariate analyses of elemental fingerprints based on Ba and Sr revealed differences between sample sites, and the magnitude of the differences increased with latitudinal separation. With more comprehensive sampling of nursery grounds, it should be possible to identify origin of nursery ground for adult swordfish by analysing the YOY juvenile portion of the sagittal otolith.     

Analysis of otolith chemistry in Nassau grouper (Epinephelus striatus) from the Bahamas and Belize using solution-based ICP-MS

 We examined the utility of otolith minor and trace element chemistry, assayed with inductively coupled plasma mass spectrometry (ICP-MS), as a means of delineating population structure in the Nassau grouper (Epinephelus striatus). We characterized the elemental composition of otoliths collected in 1993 from three locations in Exuma Sound, Bahamas and from Glover Reef, Belize in 1995. A single location in Exuma Sound was sampled in 1994 to test temporal variability in otolith composition. Five elements (Ca, Zn, Sr, Ba and Pb) were routinely detected, at levels significantly above background, by solution-based ICP-MS. Results from analysis of variance of elemental data, expressed as a ratio to Ca, indicated that there were no significant differences among the Exuma locations for any element, but significant variability was found between Glover Reef and the pooled Exuma localities for Zn/Ca, Sr/Ca and Ba/Ca ratios. Significant inter-annual differences at one Exuma Sound location was restricted to Ba/Ca ratios. Discriminant function analysis correctly classified 86% and 95% of the Belize and pooled Exuma sites, respectively. Otoliths from Belize were characterized by low Zn/Ca and high Ba/Ca and Pb/Ca ratios compared to otoliths from fish collected in Exuma Sound. Although differences in Ba levels may be related to upwelling at Glover Reef, more data are needed to definitely link otolith composition with regional differences in water chemistry. Accepted: 15 February 1999     

Asteriscus v. lapillus: comparing the chemistry of two otolith types and their ability to delineate riverine populations of common carp Cyprinus carpio

The chemical composition of common carp Cyprinus carpio asteriscus (vaterite) and lapillus (aragonite) otoliths from the same individual and reflecting the same growth period was measured to (1) determine whether there are differences in the uptake of trace metals (Mg:Ca, Mn:Ca, Sr:Ca and Ba:Ca ) and Sr isotope ratios (⁸⁷Sr:⁸⁶Sr) in co‐precipitating lapilli and asterisci and (2) compare the ability of multi‐element and isotopic signatures from lapilli, asterisci and both otolith types combined to discriminate C. carpio populations over a large spatial scale within a river basin. Depth profile analyses at the otolith edge using laser‐ablation inductively coupled plasma mass spectrometry showed that asterisci were enriched in Mg and Mn and depleted in Sr and Ba relative to lapilli, whilst ⁸⁷Sr:⁸⁶Sr values were nearly identical in both otolith types. Significant spatial differences among capture locations were found when all trace element and Sr isotope ratio data were aggregated into a multi‐element and isotopic signature, regardless of which otolith type was used or if they were used in combination. Discriminatory power was enhanced, however, when data for both otolith types were combined, suggesting that analysis of multiple otolith types may be useful for studies attempting to delineate C. carpio populations at finer spatial or temporal scales.     

Otolith geochemistry does not reflect dispersal history of clownfish larvae

Natural geochemical signatures in calcified structures are commonly employed to retrospectively estimate dispersal pathways of larval fish and invertebrates. However, the accuracy of the approach is generally untested due to the absence of individuals with known dispersal histories. We used genetic parentage analysis (genotyping) to divide 110 new recruits of the orange clownfish, Amphiprion percula, from Kimbe Island, Papua New Guinea, into two groups: “self-recruiters” spawned by parents on Kimbe Island and “immigrants” that had dispersed from distant reefs (>10 km away). Analysis of daily increments in sagittal otoliths found no significant difference in PLDs or otolith growth rates between self-recruiting and immigrant larvae. We also quantified otolith Sr/Ca and Ba/Ca ratios during the larval phase using laser ablation inductively coupled plasma mass spectrometry. Again, we found no significant differences in larval profiles of either element between self-recruits and immigrants. Our results highlight the need for caution when interpreting otolith dispersal histories based on natural geochemical tags in the absence of water chemistry data or known-origin larvae with which to test the discriminatory ability of natural tags.     

Trace metal incorporation in otoliths of pink snapper (Pagrus auratus) as an environmental monitor

Otolith metal concentrations may be related to the environmental exposure history of fish to contamination. Otoliths of pink snapper (Pagrus auratus) collected from the marine basin of Cockburn Sound and offshore near Rottnest Island were analysed by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to measure the concentrations of 11 trace metals. The following metals were investigated using their respective isotopes: aluminum (²⁷Al), calcium (⁴⁴Ca), manganese (⁵⁵Mn), iron (⁵⁷Fe), copper (⁶⁵Cu), zinc (⁶⁶Zn), strontium (⁸⁸Sr), cadmium (¹¹¹Cd), barium (¹³⁸Ba), mercury (²⁰²Hg) and lead (²⁰⁸Pb). Significant differences in otolith metal concentrations were found between the sampling locations for Zn, Cd and Pb. These metals were significantly higher in the otolith edges of the pink snapper captured from the extensive industrial area bordering Cockburn Sound. Life history transects of Zn, Cd and Pb within otoliths of pink snapper sampled from Cockburn Sound typically showed temporal trends that may correspond to the movement of this fish species in and out of this contaminated area during the yearly spawning season.     

Same habitat,different species: otolith microchemistry relationships between migratory and resident species support interspecific natal source classification

We tested the hypothesis that otolith trace elemental signatures (microchemistries) of mottled sculpin Cottus bairdi, slimy sculpin C. cognatus, and juvenile coho salmon Oncorhynchus kisutch were predictive of those of juvenile steelhead O. mykiss across many sites within the Lake Michigan basin. Laser ablation inductively coupled plasma mass spectrometry was used to generate otolith microchemistry signatures for each individual fish. For each species pair, statistical correlations of mean otolith concentrations of Mg, Mn, Cu, Zn, Sr, Ba, and Pb for each site were estimated. Linear equations describing these relationships were used to transform juvenile steelhead otolith microchemistry data to those of each of the other species. Transformed otolith microchemistry data were subjected to random forest classifications developed for mottled sculpin, slimy sculpin, and juvenile coho salmon to assess interspecific natal source assignment accuracies. Steelhead otolith concentrations of Sr were significantly correlated with those of each of the other species, whereas otolith concentrations of Ba and Mn were significantly correlated among some species pairs, but not others. Natal source assignment accuracies of juvenile steelhead to site and watershed generally decreased when otolith microchemistry data were transformed to those of mottled sculpin, slimy sculpin, and coho salmon. Miss-assigned fish often classified into nearby watersheds within larger hydrologic units, leading to higher assignment accuracies at coarser geographical resolutions (75–97% correct assignment to hydrologic unit for each species). These findings suggest that applications of otolith microchemistry data may extend beyond the species from which they are collected.     

Comparison of spatial variation in otolith chemistry of two fish species and relationships with water chemistry and otolith growth

Spatial variation in the chemistry (Mg, Mn, Sr and Ba) of recently deposited otolith material (last 20–30 days of life) was compared between two demersal fish species; snapper Pagrus auratus (Sparidae) and sand flathead Platycephalus bassensis (Platycephalidae), that were collected simultaneously at 12 sites across three bays in Victoria, south-eastern Australia. Otolith chemistry was also compared with ambient water chemistry and among three sampling positions adjacent to the proximal otolith margin. For both species, variation in otolith chemistry among bays was significant for Ba, Mn and Sr; however, differences among bays were only similar between species for Ba and Mn. Only Ba showed significant variation at the site level. Across the 12 sites, mean otolith Ba levels were significantly positively correlated between species. Further, although incorporation rates differed, mean ambient Ba levels for both species were positively correlated with ambient Ba levels. Spatial variation in multi-element otolith chemistry was also broadly similar between species and with multi-element water chemistry. Partition coefficients clearly indicated species-specific incorporation of elements into otoliths. Mg and Mn were consistently higher in snapper than sand flathead otoliths (mean ±s .d ., Mg snapper 22·1 ± 3·8 and sand flathead 9·9 ± 1·5 μg g⁻¹, Mn snapper 4·4 ± 2·6 and sand flathead 0·5 ± 0·3 μg g⁻¹), Sr was generally higher in sand flathead otoliths (sand flathead 1570 ± 235 and snapper 1346 ± 104 μg g⁻¹) and Ba was generally higher in snapper otoliths (snapper 12·1 ± 12·8 and sand flathead 1·8 ± 1·4 μg g⁻¹). For both species, Mg and Mn were higher in the faster accreting regions of the otolith margin, Sr was lower in the slower accreting region and Ba showed negligible variation among the three sampling regions. This pattern was consistent with the higher Mg and Mn, and generally lower Sr observed in the faster accreting snapper otoliths. It is hypothesized that the differences between species in the incorporation of these elements may be at least partly related to differences in metabolic and otolith accretion rate. Although rates of elemental incorporation into otoliths appear species specific, for elements such as Ba where incorporation appears consistently related to ambient concentrations, spatial variation in otolith chemistry should show similarity among co-occurring species.     

Trace elements in the otoliths and dorsal spines of albacore tuna (Thunnus alalunga, Bonnaterre, 1788): An assessment of the effectiveness of cleaning procedures at removing postmortem contamination

Trace element analysis or “elemental fingerprinting” is widely used in stock structure analyses. Postmortem contamination of bony structures can confound the results of microconstituent studies or introduce an additional source of noise to the data, thus reducing the ability of the technique to detect real variation in trace element concentrations. Despite the potential for postmortem contamination during sample preparation, the effectiveness of the procedures used to remove potential contaminants from sectioned otoliths and other calcareous structures prior to laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) has not previously been addressed. Otoliths and dorsal spine sections of albacore tuna (Thunnus alalunga) collected from the North East Atlantic Ocean and the Mediterranean Sea were deliberately contaminated prior to analysis of trace element composition using LA ICP-MS. The effectiveness of three cleaning treatments (rinsing in ultrapure water, 30% hydrogen peroxide and ultrapure 5% nitric acid) at removing this postmortem contamination were compared. Magnesium and strontium were relatively robust to postmortem effects when exposed to contamination at concentrations of 50 ppm and 200 ppm respectively. Soaking in a solution containing Mn, Cs and Ba (50 ppm) caused a marked increase in the detected concentration of each element in both structures. Translucent bands in both structures were more susceptible to contamination. Rinsing in ultrapure water or hydrogen peroxide was not effective at removing Mn, Cs and Ba contamination from either calcareous structure. Washing the otoliths and spines in nitric acid successfully removed postmortem contaminants.The removal of otoliths from tuna damages the appearance of the fish and has an adverse effect on market value. However spines are easily removed, do not affect the appearance or value of the fish and are the most commonly used structure for age determination. A weak but significant correlation was observed between Ba in opaque zones in otoliths and dorsal spines. All other spine to otolith correlations were not significant. The results do not provide support for the use of spines as an alternative to otoliths in trace elemental analyses.     

Femtosecond laser ablation ICP-MS measurement of otolith Sr:Ca and Ba:Ca composition reveal differential use of freshwater habitats for three amphidromous Sicyopterus (Teleostei: Gobioidei: Sicydiinae) species

The use of freshwater habitats was examined in three amphidromous goby species of the genus Sicyopterus using otolith microchemistry. Two species were endemic to either New Caledonia or Vanuatu whilst the other was widely distributed. Depositional patterns of strontium (Sr) and barium (Ba) in the otolith of adults were analysed with femtosecond laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The Sr:Ca and Ba:Ca results uncovered three different adult behaviours within the freshwater habitat. Some fishes stayed in elevated locations (square profile); others undertook back-and-forth migrations between higher and lower reaches (up-and-down profile), and finally, others stayed in the lower reaches (constant profile). The consequences of these movements to larval survival or competition for food and territory are discussed. This work brings new knowledge on amphidromous behaviour, and it highlights the necessity of multi-elemental analysis to study amphidromy in freshwater systems.     

Differential uses of coral reef habitats by a poorly-known cryptic fish predator

This study used otolith microchemistry to evaluate whether the moray eel Gymnothorax chilospilus uses different habitats throughout its life (mainly juvenile and adult phases). Of the most informative trace elements within otoliths (the twelve isotopes ²³Na, ²⁵Mg, ⁴³Ca, ⁵⁵Mn, ⁵⁹Co, ⁶⁰Ni, ⁶³Cu, ⁶⁶Zn, ⁸⁶Sr, ¹¹¹Cd, ¹³⁸Ba and ²⁰⁸Pb) only three ratios of Ca (Na:Ca, Sr:Ca and Ba:Ca) were informative and therefore used in a multivariate regression-tree analysis. Using a multivariate partitioning, three main phases were described from profiles, including the larval life phase (leptocephali), the intermediate phase (longest section between the larval life phase and the terminal phase) and the terminal phase (final section i.e., the most recent months preceding the death of fish). According to concentrations of the three ratios to Ca, G. chilospilus can be separated into three groups during their larval life stage (very different in Sr and Na), four groups during the intermediate phase (few differences in Sr and Na) and three groups during the terminal phase (differences in Sr), illustrating that G. chilospilus inhabit different habitats during these three phases. Our results showed that the leptocephali encountered different oceanic water masses with fluctuating Sr:Ca ratios during the early larval phase. During the intermediate phase (main part of their life-span), they lived in lagoonal waters such as fringing reefs or reef flats of lagoonal islets, characterized by a lower Sr:Ca ratio. During the latter part of their life, approximately one third of G. chilospilus encountered more oceanic waters close to or at barrier reefs, suggesting possible movements of these fish along a coast-to-ocean gradient.     

The effect of otolith storage methods on the concentrations of elements detected by laser-ablation ICPMS

Otoliths of male (1+ year old) Terubok, tropical shad Tenualosa toli from the Lupar River, Sarawak, were treated in different ways to assess the effect of post-capture storage on element concentrations in the otolith core and edge detected by laser-ablation inductively coupled plasma mass spectrometry (ICPMS). Five treatments were compared: rapid removal and dry storage; fish kept 5 h after capture then otoliths removed; rapid removal and stored in alcohol; fish rozen for 24 h then otoliths removed; and storage of intact head in alcohol for several weeks. Seven elements were detected at the core and edge of otoliths (Li, Na, Mg, Mn, Co, Sr, Ba). Of these, Na, Mg, Co and Ba differed among treatments, being highest in treatments where the otolith remained in the head. Variation in element measurements due to treatment effects was similar to within-otolith variation for the other elements and was least variable in the fresh treatment. The variation in concentration between the core and edge was more significant than other sources of variation (up to 100 times) for some elements. These differences were probably due to physiological factors (ontogenetic) or to yearly changes in water chemistry. The results indicate that the method of otolith storage, especially freezing whole fish, can have a small, but measurable, effect on the concentrations of Na, Mg, Co and Ba.     

The use of otolith chemistry to characterize diadromous migrations

Chemical constituents in otoliths have become a valuable tool for fish ecologists seeking to reconstruct migratory patterns and life-history diversity in a wide range of species worldwide. This approach has proved particularly effective with fishes that move across substantial salinity gradients over the course of their life, including many diadromous species. Freshwater endmembers of several elemental and isotope ratios (e.g. Sr:Ca, Ba:Ca and (87)Sr:(86)Sr) are typically identifiably distinct from marine values, and often differ among freshwater tributaries at fine spatial scales. Because these chemical tags are generally incorporated in proportion to their ambient dissolved concentrations, they can be effective proxies for quantifying the presence, duration and frequency of movements between freshwater and marine habitats. The development of high precision probe-based analytical techniques, such as laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and microbeam methods, has allowed researchers to glean increasingly detailed life-history profiles of these proxies across otoliths. Researchers are also combining multiple chemical proxies in an attempt to refine interpretations of habitat residence patterns. A thorough understanding of the spatial and temporal variation in water chemistry as well as environmental and physiological controls on incorporation of specific elements into otoliths is required for confident estimation of lifetime salinity experience. First some assumptions, methodological considerations and data processing options that are particularly relevant to diadromous otolith chemistry studies are discussed. Insights into diadromous migrations obtained from decades of otolith chemistry research, highlighting the increasingly recognized importance of contingent behaviour and partial migration are then discussed. Finally, areas for future research and the need to integrate otolith chemistry studies into comprehensive assessments of the effects of global environmental change are identified.     

Spatial variability of trace elements in fish otoliths: comparison with dietary items and habitat constituents in seagrass meadows

Juvenile trumpeters Pelates sexlineatus , two main prey items (amphipods and polychaetes) and seagrass constituents (detritus, young and old leaves of Zostera capricorni ) were collected from two sites within six estuaries, to compare the spatial variability of elemental composition (Li, Mn, Sr, Cd, Ba and Pb). Average elemental values were lower in the fish otoliths except for Sr. Significant differences in elemental composition of otoliths were detected among estuaries (Mn and Pb). Spatial differences in concentrations of Cd were found in amphipods and polychaetes; the latter also showed differences for Ba and Pb. Detritus and Z. capricorni leaves showed appreciable spatial differences for most of the elements. Spatial variability between sites was generally high. There were differences in the multivariate composition of trace elements found in the otoliths of fish among estuaries. One estuary was consistently separated from the other estuaries in the ordinations. Significant correlations were detected between concentrations of Mn in otoliths and concentrations in prey, Z. capricorni and detritus. A significant relationship was also detected between Pb in otoliths and detritus. There were significant correlations between concentrations of Mn, Sr and Ba in detritus and that in otoliths. The study demonstrated that trophic transfer and habitat constituents may be considered as a potential source for some of the elemental accumulation in fish otoliths.     

Discrimination of French grunts (Haemulon flavolineatum, Desmarest, 1823) from mangrove and coral reef habitats using otolith microchemistry

Using French Grunts (Haemulon flavolineatum) held captive within mangrove and reef sites, we determined (a) whether otolith microchemical differences existed between mangroves and reefs separated at a biologically relevant spatial scale (0.25-7.1 km), (b) whether patterns in elemental concentrations were consistent across years, and (c) whether it was possible to identify whether a given fish occupied a mangrove as a juvenile.Three sites were established at Great Exuma, Bahamas (two reefs and one mangrove, May 2001) and at Turneffe Atoll, Belize (one reef and two mangroves, August 2001 and 2002). Using concentrations of Sr and Ba, discriminant function analysis (DFA) indicated unique spatial microchemical signatures of fish from each of the three Bahamas sites allowing an average correct classification of 77%. Using concentrations of Sr, Ba, Sn and Pb (2001), and in addition Li, Mg, Cu and Rb (2002), DFA of the three Belize sites indicated an average correct classification of 68% and 85% in 2001 and 2002, respectively. To assess temporal variability in otolith microchemistry, we compared microchemical signatures of Belize fish from 2001 to those from 2002. On average, 42% of fish from 2002 were correctly classified to their captive sites using chemical information from 2001, thus suggesting considerable temporal variability in otolith microchemistry. Finally, to identify whether a given fish occupied a mangrove during its juvenile stage, we ablated the juvenile portion of the otolith taken from reefs in Belize 2002. Results of this analysis indicated that 36% of 39 individuals taken from the reef had a signature more representative of one of the mangrove sites. Although otolith microchemistry varied temporally and our analysis was restricted to the grouping of individuals to only one of three sites, mangroves appeared to contribute to reef populations.     

Retracing migration pattern in reproductive and non-reproductive female kutum Rutilus frisii,in south Caspian Sea,using otolith microchemistry

Because trace elements of otoliths are considered a natural marker capable of recognizing the chemical composition of ambient water and fish migration history, these elements could be potentially used to analyse the movement of reproductive (R) and non-reproductive (NR) mature-sized fish. Supposedly, it is not essential for NR individuals to migrate to rivers for spawning because they do not have developed gonads. To investigate the potential differences in migration history between female R and NR kutum, Rutilus frisii, in the southwest waters of the Caspian Sea, the ratios of Sr, Ba, Mg, Na, K and P to Ca in otoliths (from the core to the edge) were examined using laser ablation–inductively coupled plasma–mass spectrometry. In NR fish, a significant increase in Sr:Ca ratio in the otoliths' growth rings, likely due to greater seawater residency, and an increase in Ba:Ca ratio in the last two rings were observed. Increased Ba:Ca ratio could be due to the movement of NR mature-sized fish to the coastal zones for foraging. Seasonal physiological factors such as gonad maturation and spawning activity are more likely to be involved in differences in the other elemental ratios (Mg, Na, K and P). These results suggest that microchemical analyses of growth rings of otolith can be used as a valuable tool for better understanding the movement pattern of different types of adult fish, which could be completed with data from other methods like tagging.     

Small-scale spatial variation in the elemental composition of otoliths of Stegastes nigricans (Pomacentridae) in French Polynesia

Solution-based inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine if Stegastes nigricans collected from 15 sites in French Polynesia could be distinguished by the trace element composition of their otoliths. A total of 293 adults were collected by spearing and their otoliths were analysed. We found that elemental signatures differed significantly among sites within and between the islands of Tahiti and Moorea (p<0.001), primarily due to variation in concentrations of the elements Ba, Ca, Li, Mg, Mn, Na, Sr and Y. The otoliths of fish collected within Papeete Harbour in Tahiti had distinctive elemental signatures characterised by relatively high concentrations of Mn. Otoliths of these fish could be distinguished from others that were collected only a small distance (200 m) from the harbour. This is the first time that differences in chemical composition of otoliths have been reported at such small spatial scales and this trait may prove useful for the studies of connectivity of populations at within reef scales.     

Assessment of spawning site fidelity in interior Fraser River Coho salmon Oncorhynchus kisutch using otolith microchemistry,in British Columbia,Canada

Coho Salmon Oncorhynchus kisutch show fidelity to natal spawning watersheds. Fine-scale homing, however, within rivers is not well understood. Interior Fraser Coho (IFC) salmon eggs were incubated at known spawning locations in the Coldwater River, two main stem sites and one-off channel pond site, providing otolith reference data for comparison to otolith signatures for returning adults using laser ablation inductively coupled plasma mass spectrometry. Elemental ratios for Ba:Ca and Sr:Ca in otoliths of juvenile O. kisutch differed significantly among the spawning locations examined. Juvenile otolith data were used to conduct a linear discriminant analysis to assess fine-scale homing in adults. Juvenile data were all assigned to the location where they had been incubated, producing a robust data set used to compare adult otoliths and define natal locations based on elemental signatures in otoliths of adult spawners. Homing and straying were apparent at the reach level; 57.1% of adults returned to their natal spawning locations, while 42.9% strayed to other spawning sites within the Coldwater River. Straying to novel incubation sites at the reach scale demonstrated plasticity in homing within a watershed.     

Use of otolith chemistry to examine patterns of diadromy in the threatened Australian grayling Prototroctes maraena

Otolith chemical analyses (proton-induced X-ray emission and laser ablation inductively coupled plasma mass spectrometry) were used to examine patterns of diadromy in Australian grayling Prototroctes maraena collected from three rivers in south-eastern Australia. Concentrations of Sr:Ca and Ba:Ca were measured in the sagittal otoliths of 25 fish and in water samples collected on two occasions from multiple sites within freshwater, estuarine and marine reaches of the rivers. The results provide evidence of marine residency during the juvenile phase, with high Sr:Ca and low Ba:Ca in the inner region of the otoliths that were consistent across all three rivers. This general pattern was apparent for all individuals examined, suggesting that diadromous migration may be an obligate aspect of the life history. The chemical signatures of the outer regions of the otoliths were distinct between the three rivers, whilst there was no difference in the inner regions of the otoliths. This suggests that juveniles from different rivers may reside in a relatively homogenous chemical environment, such as the sea, and populations in coastal Victorian rivers may share a common marine recruitment source.     

Spatial and temporal variations in otolith chemistry and relationships with water chemistry: a useful tool to distinguish Atlantic salmon Salmo salar parr from different natal streams

Otolith elemental (Sr:Ca, Ba:Ca, Mn:Ca, Mg:Ca and Rb:Ca) and isotopic (⁸⁷Sr:⁸⁶Sr) profiles from several annual cohorts of juvenile Atlantic salmon Salmo salar were related to the physico‐chemical characteristics (chemical signatures, flow rate, temperature and conductivity) of their natal rivers over an annual hydrological cycle. Only Sr:Ca, Ba:Ca and ⁸⁷Sr:⁸⁶Sr in otoliths were determined by their respective ratios in the ambient water. Sr:Ca ratios in stream waters fluctuated strongly on a seasonal basis, but these fluctuations, mainly driven by water flow regimes, were not recorded in the otoliths. Otolith Sr:Ca ratios remained constant during freshwater residency at a given site and were exclusively related to water Sr:Ca ratios during low flow periods. While interannual differences in otolith elemental composition among rivers were observed, this variability was minor compared to geographic variability and did not limit classification of juveniles to their natal stream. Success in discriminating fish from different sites was greatest using Sr isotopes as it remained relatively constant across years at a given location.     

Spatio-temporal variation in the elemental compositions of otoliths of southern bluefin tuna Thunnus maccoyii in the Indian Ocean and its ecological implication

The elements Na, Mg, Mn, Ca, Sr and Ba in otoliths of southern bluefin tuna Thunnus maccoyii , collected from their feeding ground in the central Indian Ocean and spawning ground between southern Java and north-western Australia were measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and compared among sampling locations and developmental stages. The Na, Mg and Mn to Ca concentration ratios were significantly higher at the larval stage than at the adult stage, and the ratio reached a peak at the first inflection point of the otolith, mean ± s.d. 43·3 ± 4·9 days after hatching and decreased sharply to a low level thereafter. The temporal change of the elements:Ca ratios in the first inflection point corresponded to the life stage transition from larva to juvenile, indicating that the uptake rate of elements from ambient waters was significantly influenced by the ontogenetic change in the fish. The elemental composition at the otolith edge differed significantly in sub-adults on the feeding grounds and adults on the spawning grounds. Thus, the otolith elemental composition can be used as a biological tracer to study the time of the ontogenetic shift and to reconstruct the past migratory environmental history of T. maccoyii . In addition, the elemental composition of the otolith core of the adult was similar between feeding and spawning grounds, indicating that the fish in the Indian Ocean had the same larval origin, which is consistent with the single spawning population hypothesis.