Fish otolith chemistry influenced by exposure to multiple environmental variables

There is an increasing desire for researchers to use the elemental concentrations in fish otoliths to reconstruct environmental histories of fish. These reconstructions may be plausible due to the unique incorporation of elements into discrete layers of otolith material that correspond to daily growth, and because environmental variables of temperature, salinity, and water chemistry can influence otolith chemistry. However, it is essential to establish exactly how temperature, salinity, and the ambient concentration of elements influence otolith chemistry in order to interpret environmental histories of fish. Using a controlled laboratory experiment we tested the relative and interactive effects of temperature, salinity, and ambient concentration of strontium (Sr) and barium (Ba) on the resulting concentration of Sr and Ba in otoliths of black bream Acanthopagrus butcheri (Munro 1949). Salinity and concentration, and temperature and concentration interacted to affect the elemental concentration of Sr:Ca and Ba:Ca in otoliths. Regression analysis revealed that temperature and ambient concentration contributed most to the trend in otolith chemistry for both elements. Importantly, this is the first experiment to combine three environmental variables and assess their effect on otolith chemistry. Based on these results, it should be possible to use changes in the elemental concentration in otoliths to better reconstruct previous environments of temperature, salinity, and ambient water chemistry, which is especially useful when determining occupancy in habitats such as estuaries that display variable environmental characteristics.     

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.     

Otolith elemental signatures reflect residency in coastal water masses

We examined variability in otolith chemistry of wild caught fish in relation to in situ temperature and salinity within the California Current System. Barium, magnesium, and iron from the most recent growth zone in otoliths differentiated pelagic juvenile shortbelly rockfish (Sebastes jordani) residing in water masses with distinct temperature and salinity properties from central and southern California spanning nearly 500 km of coastline. The 3-element signature also discriminated fish that resided in different water masses that were associated with mesoscale cyclonic eddy circulation in the Santa Barbara Channel. Variability in otolith chemistry reflected the spatial patterns of both horizontal gradients and vertical gradients in water mass properties related to circulation. Although we found that the concentrations of particular elements in otoliths were correlated to ambient temperature or salinity, we suggest that these parameters are more useful as an identifying signature of distinct water masses associated with unique otolith signatures rather than as factors directly affecting otolith chemistry. Other factors varying among the water masses or among the fish populations residing in the water masses may also affect otolith chemistry. We discuss how oceanographic phenomena associated with the 1997–1998 El Niño and the persistent, recirculating eddy in the Channel may have affected coastal ocean conditions and variation in otolith chemistry of fish in the study area.     

Contribution of water chemistry and fish condition to otolith chemistry: comparisons across salinity environments

This study quantified the per cent contribution of water chemistry to otolith chemistry using enriched stable isotopes of strontium (⁸⁶Sr) and barium (¹³⁷Ba). Euryhaline barramundi Lates calcarifer, were reared in marine (salinity 40), estuarine (salinity 20) and freshwater (salinity 0) under different temperature treatments. To calculate the contribution of water to Sr and Ba in otoliths, enriched isotopes in the tank water and otoliths were quantified and fitted to isotope mixing models. Fulton's K and RNA:DNA were also measured to explore the influence of fish condition on sources of element uptake. Water was the predominant source of otolith Sr (between 65 and 99%) and Ba (between 64 and 89%) in all treatments, but contributions varied with temperature (for Ba), or interactively with temperature and salinity (for Sr). Fish condition indices were affected independently by the experimental rearing conditions, as RNA:DNA differed significantly among salinity treatments and Fulton's K was significantly different between temperature treatments. Regression analyses did not detect relations between fish condition and per cent contribution values. General linear models indicated that contributions from water chemistry to otolith chemistry were primarily influenced by temperature and secondly by fish condition, with a relatively minor influence of salinity. These results further the understanding of factors that affect otolith element uptake, highlighting the necessity to consider the influence of environment and fish condition when interpreting otolith element data to reconstruct the environmental histories of fish.     

Incorporation of strontium into otoliths of an estuarine fish

Patterns of Sr/Ca variability in fish otoliths have been widely applied as tracers of movement between freshwater and marine habitats, with the assumption that low salinity habitats correspond to lower otolith levels of Sr/Ca. On the other hand, fluvial estuaries can contain steep gradients in Sr/Ca, and in some estuaries, freshwater values of Sr/Ca can exceed marine values, which are relatively constant across marine habitats. Therefore, to interpret Sr/Ca variability in otoliths of fish that move through estuaries, information is needed about both the incorporation of strontium into otoliths and the nature of the gradient of Sr/Ca in the water. We conducted four experiments to evaluate the incorporation of strontium into fish otoliths under estuarine conditions, using white perch (Morone americana) as a model estuarine fish. One laboratory and the two field experiments tested the relationship between Sr/Ca in the otolith and that in the water. A fourth experiment investigated the effect of salinity, independently of the water chemistry (Sr was manipulated while maintaining a constant salinity and Ca level). All four experiments supported a direct relationship between Sr/Ca in the otolith and the water, across a range of estuarine salinities. Results also indicated that the incorporation of strontium into otoliths of estuarine fishes should be constant across broad gradients of Sr/Ca in estuarine waters. While the experiments supported past applications of tracing estuarine and diadromous movements with otolith Sr/Ca chronologies, we emphasize the need to understand the underlying nature of Sr/Ca gradients in estuaries, which may limit or confound reconstructions of estuarine habitat use.     

Otolith microchemistry of tropical diadromous fishes: spatial and migratory dynamics

Otolith microchemistry was applied to quantify migratory variation and the proportion of native Caribbean stream fishes that undergo full or partial marine migration. Strontium and barium water chemistry in four Puerto Rico, U.S.A., rivers was clearly related to a salinity gradient; however, variation in water barium, and thus fish otoliths, was also dependent on river basin. Strontium was the most accurate index of longitudinal migration in tropical diadromous fish otoliths. Among the four species examined, bigmouth sleeper Gobiomorus dormitor, mountain mullet Agonostomus monticola, sirajo goby Sicydium spp. and river goby Awaous banana, most individuals were fully amphidromous, but 9–12% were semi‐amphidromous as recruits, having never experienced marine or estuarine conditions in early life stages and showing no evidence of marine elemental signatures in their otolith core. Populations of one species, G. dormitor, may have contained a small contingent of semi‐amphidromous adults, migratory individuals that periodically occupied marine or estuarine habitats (4%); however, adult migratory elemental signatures may have been confounded with those related to diet and physiology. These findings indicate the plasticity of migratory strategies of tropical diadromous fishes, which may be more variable than simple categorization might suggest.     

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.     

Otolith trace element and stable isotopic compositions differentiate fishes from the Middle Mississippi River, its tributaries, and floodplain lakes

Naturally occurring stable isotope and trace elemental markers in otoliths have emerged as powerful tools for determining natal origins and environmental history of fishes in a variety of marine and freshwater environments. However, few studies have examined the applicability of this technique in large river-floodplain ecosystems. This study evaluated otolith microchemistry and stable isotopic composition as tools for determining environmental history of fishes in the Middle Mississippi River, its tributaries, and floodplain lakes in Illinois and Missouri, USA. Fishes were collected from 14 sites and water samples obtained from 16 sites during summer and fall 2006 and spring 2007. Otolith and water samples were analyzed for stable oxygen isotopic composition (δ¹⁸O) and concentrations of a suite of trace elements; otoliths were also analyzed for carbon isotopic composition (δ¹³C). Tributaries, floodplain lakes, and the Mississippi and Lower Missouri Rivers possessed distinct isotopic and elemental signatures that were reflected in fish otoliths. Fish from tributaries on the Missouri and Illinois sides of the middle Mississippi River could also be distinguished from one another by their elemental and isotopic fingerprints. Linear discriminant function analysis of otolith chemical signatures indicated that fish could be classified back to their environment of capture (Mississippi River, floodplain lake, tributary on the Illinois or Missouri side of the Mississippi River, or lower Missouri River) with 71–100% accuracy. This study demonstrates the potential applicability of otolith microchemistry and stable isotope analyses to determine natal origins and describe environmental history of fishes in the Middle Mississippi River, its tributaries, and floodplain lakes. The ability to reconstruct environmental history of individual fish using naturally occurring isotopic markers in otoliths may also facilitate efforts to quantify nutrient and energy subsidies to the Mississippi River provided by fishes that emigrate from floodplain lakes or tributaries.     

Significance of otolith calcium carbonate crystal structure diversity to microchemistry studies

Otoliths, calcium carbonate (CaCO₃) ear stones of fish, contain a wealth of information about fish life and environmental history yet the CaCO₃ polymorph form the otolith is made of is a critical, but seldom considered, piece of information during otolith analysis. Otolith trace element chemistry data increasingly informs management decisions, but recent work has shown that CaCO₃ polymorphs—aragonite, vaterite, and calcite—can bear on incorporation of trace elements in a non-trivial way. Most fishes are thought to have otoliths of the aragonite CaCO₃ form, but this construct is potentially outdated with many recent literature reports showing otherwise. Our study used previously unpublished neutron diffraction data and reports from published literature to address three objectives: (1) summarize the relative effects of otolith CaCO₃ polymorphism on otolith microchemistry, (2) summarize reports of otolith polymorphs to gain a better understanding of the extent of non-aragonite otoliths among fishes, (3) outline future research needed to align interpretations of microchemistry with our current understanding of otolith polymorph diversity. We found that while aragonite otoliths are the most common, so are exceptions. For example, the ostensibly rare (among species) CaCO₃ form vaterite was reported in at least some otoliths of 40% of the species surveyed. Our work suggests that examination of the CaCO₃ polymorph composition of otoliths should become more common particularly in studies where results will or may be used to inform management decisions. Future research should work to attribute controls on otolith CaCO₃ polymorph expression using a combination of -omics and material characterization approaches to enrich the life history and environmental information output from otoliths and increase our understanding of the assumptions made in otolith trace element chemistry studies.

     

Effect of diet on otolith composition in Pomatomus saltatrix, an estuarine piscivore

To test the hypothesis that elemental composition of otoliths (sagittae) could be influenced by differences in natural prey type, young‐of‐the‐year bluefish Pomatomus saltatrix were captured immediately after their migration from oceanic waters into mid‐Atlantic Bight estuaries and fed either shrimp, Crangon septemspinosa and Palaemonetes spp. or fish Menidia menidia under similar temperature and salinity regimes in two separate 60 day experiments. Unlimited rations of fish and shrimp prey were provided in the first experiment which led to differences in bluefish growth rate between the two prey treatments; fish prey was limited in the second experiment to ensure that growth rates of bluefish in the two prey treatments were similar. Concentrations of seven elements in bluefish otoliths were determined using solution‐based inductively coupled plasma mass spectrometry (ICPMS). There was no significant effect of diet on five of the seven elements examined (Na, Mg, K, Ca and Mn). The levels of Sr and Ba in the otoliths of shrimp‐fed bluefish, however, were significantly higher than fish‐fed bluefish in both experiments. Concentrations of Ba in shrimp‐fed bluefish otoliths were double that found in fish‐fed bluefish. The results suggest that diet can explain some of the variation in otolith chemistry previously attributed to physical and chemical properties of the water.     

Extrinsic factors affecting otolith chemistry – implications for interpreting migration patterns in a diadromous fish

Much has been revealed about fish migration, including diadromous behaviour, through the use of otolith chemistry. Manipulative experiments assist with unravelling information on otolith chemical composition and incorporation thereby answering specific questions on diadromous movements. In this study, a laboratory-based experiment was used to determine the relative and interactive effects of salinity and water temperature on the composition of three key elements (Sr, Ba and Mg) within the otolith of a catadromous fish, Percalates novemaculeata, endemic to south-eastern Australia. Otolith incorporation of Sr and Ba was positively related to ambient water concentration, whereas Mg incorporation was not. Sr and Ba increased and decreased significantly across salinity gradients, respectively, with minor positive effects of temperature also being detected. Salinity and temperature interacted to significantly affect the elemental concentration ratios for Ba: Ca in otoliths. Discrimination between fresh water and marine environments shows promise for interpreting P. novemaculeata residency based on these elements alone. However, deciphering finer scale movements within estuarine environments may be difficult. Our data highlights the importance of multifactorial validation experiments and suggests complementary use of multiple approaches for unravelling species-specific patterns of fish movement and habitat use.     

Consistency of elemental and isotope‐ratio patterns across multiple scales from individual fish

Assays of elemental and stable‐isotope ratios across growth increments of scales have the potential to provide a non‐lethal alternative to otolith chemistry for identifying migration and ontogenetic trophic shifts. A central assumption when employing scales as otolith analogues is that any scale from an individual will provide equivalent information about the chemical history of that fish. This assumption was investigated with multiple scales from wild and captive euryhaline Atlantic tarpon Megalops atlanticus from the north‐west Gulf of Mexico. Elemental (Sr:Ca) and isotope‐ratio (δ¹³C and δ¹⁵N) life‐history profiles were compared among multiple scales from each fish. All three chemical proxies showed highly consistent patterns among non‐regenerated scales, while patterns in regenerated scales diverged, indicating rapid regrowth of interior scale material at the onset of regeneration. Patterns of Sr:Ca and δ¹³C covaried, supporting their use as salinity proxies, while δ¹⁵N patterns were consistent with ontogenetic diet shifts. Water samples taken from aquaria holding captive fish were used to calculate partition coefficients for a suite of elements in M. atlanticus scales for future quantification of migratory movements in the region. Together, these results support the assumption that non‐regenerated scales from individual M. atlanticus provide equivalent chemical histories, further validating their use as a viable non‐lethal alternative to otoliths.     

Elemental composition of otoliths from Atlantic croaker along an estuarine pollution gradient

Elemental compositions of otoliths of young-of-the-year Atlantic croaker Micropogonias undulatus at stations along a pollution gradient in Galveston Bay, Texas, were compared with those of sediments and livers to establish the level of consonance between otolith and habitat chemistries. Both coupled and uncoupled trends were indicated, with the bulk of the results pointing to the need for additional research to test the working hypothesis that the otolith stores a temporal record of elemental chemistry of resident waters. Lack of simple correlations between elemental concentrations in sediment or liver and in otoliths indicate that other biological and geochemical factors influence otolith composition.     

Ontogenetic and environmental effects on otolith shape variability in three Mediterranean European eel (Anguilla anguilla, L.) local stocks

Otolith morphology is an efficient tool for the discrimination of fish stocks, populations and species when comparative genetic data are not available. Currently, the relationship between environmental factors and otolith shape is poorly characterized for the European eel (Anguilla anguilla), a highly migratory catadromous species constituting a single, randomly mating stock. The present study analyses the differences in otolith morphology between three Mediterranean eel local stocks from different environmental contexts (i.e. two brackish lagoons and one river). The relationship between otolith shape and otolith size was studied by means of Elliptic Fourier analysis and multivariate statistics. Otolith profile was digitally acquired and Cartesian coordinates were extracted. Partial Least Square (PLS) analysis pointed to continuous allometric growth in size and shape in otoliths from all three sites. In the three environments, shape variations occurred during growth as indicated by the presence of a significant and positive relationship between otolith size and the first PLS latent vector (i.e. which bears most of the information regarding otolith outline). Differences between smaller and larger sized otoliths were investigated using PLS Discriminant Analysis (PLSDA) and cluster analysis. Results indicate that otolith shape is highly uniform at smaller than at larger sizes. These shape differences apparently overlap the initial differentiation of the small otolith outlines acquired by eels during the growing phase as elvers in the marine environment. Data were discussed considering that the physical and chemical habitat variability in brackish lagoons and river could underlie a marked change in otolith shape during the animals' growth.     

Listening In on the Past: What Can Otolith δ18O Values Really Tell Us about the Environmental History of Fishes?

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.     

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.     

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

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

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.     

Salinities, not diets, affect strontium/calcium ratios in otoliths of Anguilla japonica

Although otolith Strontium (Sr)/calcium (Ca) ratios have been widely used to reconstruct the past salinity environmental history of anguillid eels, factors affecting the Sr/Ca ratios in otoliths are incompletely understood. Japanese Eel (Anguilla japonica) elvers (mean length 54.7 ± 2.1 mm) were collected in the estuary during their upstream migration and reared at 5 different salinities (0, 5, 15, 25, and 35 psu) and 3 types of feeding conditions (formulated feed, tubifex, and starvation) for 30 days to evaluate the effects of salinity and diets on otolith Sr/Ca ratios. Ca and Sr concentrations in the ambient water significantly increased with salinity (SAL) as [Ca] _water = 15.50SAL − 5.56, and [Sr] _water = 0.21SAL + 0.03, respectively. Sr/Ca ratios in otoliths increased with salinity (SAL) of the rearing water as [(Sr/Ca) × 1000] _otolith = 0.091SAL + 3.790. In diets, Sr/Ca ratios were 4 times higher in tubifex than in formulated feed. However, in otoliths, ANOVA indicated that Sr/Ca ratios did not differ significantly between groups fed on tubifex or formulated feed (p = 0.118). Otolith Sr/Ca ratios were negatively correlated with fish growth rates while the growth rates differed significantly among rearing conditions with different salinities and diets. Partition coefficients of the Sr/Ca ratios from ambient water to fish tissues and otoliths significantly increased with salinity. The Sr/Ca ratios of Japanese Eel otoliths thus were positively correlated with the ambient salinity and decreased with increasing fish growth rate, but was not affected by fish diet.     

Mediterranean juvenile bluefin tuna: life history patterns

As there is a lack of information on the growth and migrations of bluefin tuna, information about them was gathered using the structural and chemical characteristics of their otoliths and mercury levels in body tissues as indicators of physiological and habitat characteristics. The otoliths of juvenile tuna caught in the Spanish Mediterranean littoral were studied. Otolith increments, assumed to be formed daily, were enumerated. Measurements by wavelength dispersive electron microprobe confirmed the presence of strontium in otolith tissue, and an inverse relationship between strontium/calcium (Sr/Ca) concentration ratio and temperature is suggested. Electron microprobe analyses combined with daily increment analyses of otoliths provided life history profiles for individual fish. Additional Sr/Ca concentration ratio data on fish supported the idea that Sr/Ca ratios can provide information on the environmental history of individual fish. Body concentrations of mercury were related to otolith analyses to suggest age structure, critical life history periods, growth environment, stock structure, food web position, and migration history. The techniques applied present an innovative approach to management-related problems, and the combination of chemical analyses with structural analyses promises to expand our knowledge of the life history of migratory fishes.