Otolith Elemental Composition Research Articles

Otolith elemental composition indicates differences in the habitat use for larvae and early juveniles of Japanese jack mackerel (Trachurus japonicus) in the waters around Japan

AbstractTrace element analysis of otoliths from core to edge was used to reveal differences in habitats of larvae and early juveniles of Japanese jack mackerel (Trachurus japonicus) caught in the East China Sea (ECS), Pacific Ocean, and Sea of Japan. Multi‐element signatures (Sr:Ca, Mg:Ca, Na:Ca, K:Ca, and Ba:Ca) were analyzed with multivariate statistics to determine whether these element signatures provide insight into regional population structure. The median values of elements: Ca ratios in otolith core region differed significantly among areas, with most elements between the ECS and the Pacific Ocean or Sea of Japan showing significant differences. The Sr:Ca ratios exhibited a V‐shaped trend from the otolith core to the edge, which was likely related to the vertical habitat layer shift of larvae and early juveniles of T. japonicus. Canonical analysis of principal coordinates using the element: Ca ratios and the hatching day were re‐classified according to the sampling area with higher accuracy in the ECS (88–95%) and the Sea of Japan (76–83%) compared to the Pacific Ocean (69–72%). These results indicate that the proportion of eggs, larvae, and early juveniles transported by the Kuroshio Current from the southern ECS—the main spawning ground—to the Pacific Ocean or the Sea of Japan is low and that juveniles distributed in the coastal waters off southwestern Japan may have originated from local spawners. These results suggest that determining the elemental composition of larvae and juveniles of T. japonicus is effective in determining the habitat differences of this species in the three regions. These findings will help to understand population structure and recruitment process of this species around Japan.

Combining natural markers to investigate fish population structure and connectivity

Understanding population connectivity via fish movement and the appropriate scale for management is amplified by the use of tagging techniques. Natural fish markers provide unique signatures that reflect distinct environmental and physiological characteristics of populations, allowing us to determine the degree of connectivity among them. Different markers function within defined spatiotemporal ranges with specific advantages and limitations. We tested whether applying multiple markers would increase the power of assessing population structure and connectivity of black bream Acanthopagrus butcheri from 12 estuaries across southern Australia. We utilised a range of natural markers, including genetics, otolith shape, otolith isotopic composition, and otolith elemental composition, to determine the effectiveness of each marker independently and through integration. For this estuarine-dependent species, combining genetics and otolith-based techniques was complementary in increasing the accuracy of our results but not all marker integrations were consistently beneficial, highlighting the importance of appropriate marker selection. The maximum classification accuracy to collection site of 95% (a combination of genetics, otolith shape, and otolith isotopic composition) emphasised the species’ estuarine dependency and limited connectivity across the sampling range, suggesting location-specific management is needed for this commercially important species.

Evidence of multiple stocks of catfish, Rita rita (Hamilton), from the Ganges Basin on the basis of an integrated analysis of truss morphometrics, otolith microchemistry, and otolith shape

Context Management of Rita rita (Hamilton) in India is hampered by a lack of understanding of possible stock structure. Aim To investigate possible spatial stock structure of Rita rita in the Ganges Basin. Methods Independent analyses of landmark-based truss morphometry, otolith elemental analysis and otolith shape were conducted to investigate the stock structure of the catfish (Rita rita) from the Ganga River and its tributaries Yamuna and Ramganga. Key results Each of the stock-delineation approaches, namely, truss morphometrics, otolith elemental composition and otolith shape, showed significant differences among the fish groups from three sampling locations (P ≤ 0.003). Despite these differences, reclassification analyses showed variable (although all greater than random) reclassification rates (truss morphometry: 78%, otoliths elemental data: 98%, otolith shape: 50%). Conclusions An integrated view of the three analyses suggested spatial structuring and that there may be multiple stocks of Rita rita in the Ganges Basin. Implications Future investigations should consider possible variations in demographic parameters that may affect fisheries management. The multiple-lines-of-evidence approach taken in this study can serve as a template for studies of other species in any region.

Microchemical provenancing of prey remains in cormorant pellets reveals the use of diverse foraging grounds.

Piscivorous birds in aquatic ecosystems exert predation pressure on fish populations. But the site‐specific impact on fish populations, including stocked and commercially used fish species, remains disputed. One of the key questions for the management of piscivorous birds and fish is determining the origin of prey and thus which fish populations are targeted by the birds. We addressed this question by provenancing otoliths (earstones) of fish obtained from regurgitated pellets of piscivorous birds by otolith microchemistry analysis. We retrieved otoliths from regurgitated pellets of great cormorants (Phalacrocorax carbo sinensis) collected every 2 weeks for 2 years from breeding and roosting colonies at Chiemsee in Bavaria, Germany, and classified them according to family or species. We collected water samples from Chiemsee and potential surrounding foraging grounds. We measured the strontium (Sr) 87Sr/86Sr isotope ratio and Sr mass fraction of water and otoliths using (laser ablation) inductively coupled plasma‐mass spectrometry. We assigned otoliths from regurgitated pellets to habitat clusters of origin by comparing the Sr isotopic and elemental composition of otoliths and waterbodies. In 36% of cormorant pellets collected at Chiemsee, prey was assigned to waterbodies distinct from Chiemsee. Furthermore, cormorants used different foraging sites during 1 day. Microchemical provenancing of prey remains can contribute to identifying foraging sites of piscivorous birds and to what extend the birds switched among foraging sites.

Otolith morphology and microchemistry fingerprints of European eel, Anguilla anguilla (Linnaeus, 1758) stocks from the Adriatic Basin in Croatia and Montenegro

Morphological and elemental composition of otoliths are a promising tool for fish stock identification. In this regard, European eel has not been sufficiently researched. The aim of this study was to determine otolith shape index values and microchemistry composition of 14 elements in otoliths of European eel between riverine and lacustrine stocks from the Adriatic Basin in Croatia and Montenegro. In terms of morphometry, the shape indices, form factor and circularity of the sagittal otolith were statistically different between lacustrine and riverine stocks. The European eel is considered to have a small otolith relative size. Canonical variate analysis indicated no statistical differences among the concentrations of trace elements in sagittal otoliths between riverine and lacustrine stocks. However, constrained redundancy analysis suggested a stronger relationship of width, total length, females and the silver stage with the Sr/Ca, Cr/Ca, Ba/Ca and K/Ca ratios of specimens from riverine than lacustrine habitats. Gender and stage dependent variations showed that females were better indicators of relationships between microchemistry composition and intrinsic factors. In conclusion, form factor and circularity of the sagittal otolith was able to distinguish between European eel stocks from different habitats. Also, it seems that the European eels' otolith fingerprint is homogenous, species specific and independent of ecological conditions.

Population connectivity in a highly migratory fish, Japanese Spanish mackerel (Scomberomorus niphonius), along the Chinese coast, implications from otolith chemistry

Connectivity is a vital component of metapopulation and fishery ecology, influencing fundamental processes such as population dynamics, evolution, and community responses to climate change. The Japanese Spanish mackerel, Scomberomorus niphonius, is an economically important species in the temperate and subtropical waters of the Northwestern Pacific Ocean, especially in the China seas. As a highly migratory species, the population connectivity of S. niphonius is not only limited to dispersal in the pelagic larval stage but throughout the whole life history, and then determines the natal origin and population structure. However, knowledge about population connectivity is scarce and, at present, is no formal management strategy to ensure the long-term sustainability of the S. niphonius fishery in China. In this study, otolith chemistry was applied to investigate the connectivity among four spawning grounds along a latitudinal gradient spanning much of the known species range in the Yellow Sea and the East China Sea [Qingdao (QD), Lvsi (LS), Xiangshan (XS) and Fuzhou (FZ)]. Otoliths from age-1 spawning or spent individuals were analyzed from the core to the margin by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In each sampled life stage, otolith elements are similar among fish from different spawning grounds, but otolith chemical compositions vary greatly between life stages. Decreasing Ba:Ca ratio indicates ontogenetic migrations from relatively shallow coastal water during larval growth to relatively deep offshore waters with increasing age. The random forest classification and non-metric multi-dimensional scaling show a strong overlap in otolith elemental composition. The hereby results indicate a large-scale connectivity in the life history of S. niphonius: the immature fish mix to a large extent when they feed and overwinter in the extensive offshore waters; adults spawned in the same spawning ground have multiple natal origins and fish caught off QD may even be of FZ origin. Clustering of near core chemistry pointed to three sources, with southern grounds dominated by a single cluster while northern grounds dominated by two or three clusters. From these results, it was suggested that the local mackerel assemblages on which fisheries rely in the Yellow Sea are collectively influenced by the recruitment of all the spawning assemblages along China coastal waters instead of self-sustaining.

First Insights Into the Growth and Population Structure of Cottoperca trigloides (Perciformes, Bovichtidae) From the Southwestern Atlantic Ocean

The aim of this work was to describe the growth of Cottoperca trigloides, a notothenioid species with a non-Antarctic distribution, and to test the existence of different nursery areas and fish stocks through changes in the otolith elemental composition. Fish were collected during spring 2009 over the Patagonia continental shelf, including the Marine Protected Area Namuncura/Burdwood Bank, in the southwestern Atlantic Ocean. The age and growth analysis were performed by counting marks in sagittae, assuming an annual periodicity of their deposition, and identified 8-year classes (0+ to 7+). Given the size range of the fish, length-at-age data were fitted to the Gompertz growth model TLt = 55.45 [exp ((exp) -0.32 (t – 1.89))], explaining more than 95% of the growth pattern. Moreover, the chemical composition of otolith core and edge areas was analyzed by laser ablation inductively coupled plasma mass spectrometry. The canonical analysis of principal coordinates successfully allocated 72.92% of the fish for the core and 91.67% for the edge area of the otolith, in three groups corresponding to ‘northern Patagonia shelf’, ‘southern Patagonia shelf’ and ‘Marine Protected Area Namuncura/Burdwood Bank’ areas, suggesting a high segregation among them over the Patagonian shelf. Thus, otolith elemental composition has proven to be an efficient approach to identify different nursery areas and stocks for the species. Present results provide new information on the growth and the population structure of C. trigloides, from a geographical area where information on this issue is still scarce, constituting an essential tool to develop conservation principles for the species.

Water and Otolith Chemistry: Implications for Discerning Estuarine Nursery Habitat Use of a Juvenile Flatfish

Variations in otolith elemental composition are widely used to reconstruct fish movements. However, reconstructing habitat use and environmental histories of fishes within estuaries is still a major challenge due to the dynamic nature of these coastal environments. In this study, we performed a laboratory experiment to investigate the effects of variations in salinity (3 levels; 5, 18, 30) and temperature (2 levels; 16oC, 21oC) in the otolith elemental composition (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) of juvenile Senegalese sole Solea senegalensis. Temperature and salinity treatments mirrored the natural conditions of the estuarine habitats generally occupied by juvenile Senegalese sole, thereby providing information on the applicability of otolith microchemistry to reconstruct habitat use patterns within estuarine nurseries, where individual fish move across complex salinity and temperature gradients. While Sr:Ca and Ba:Ca in otoliths were both positively related to salinity, no temperature effect was observed. Partition coefficients, proxies for element incorporation rates increased with increasing salinity for Sr (DSr) and Ba (DBa). In contrast, salinity and temperature had little influence on otolith Mn:Ca and Mg:Ca, supporting physiological control on the incorporation of these elements. Our results are a stepping stone for the interpretation of otolith chemical profiles for fish collected in their natural habitats and contribute to better understanding the processes involved in otolith element incorporation.

Information depths of analytical techniques assessing whitefish otolith chemistry

The elemental composition of otoliths provides historical information on migration and provenance of fish. Due to the complex structure of the otoliths, the depth from which the elemental information originates has to be considered. In this work, three analytical methods often used to assess otolith chemistry are compared. The information depths are calculated for µ-XRF and PIXE and measured for LA-ICP-MS. The information depth in PIXE depends on the energy of the incident particles and on the element to be analysed while in XRF it depends mostly on the element to be analysed as the energy of the incident X-rays usually is high enough to excite atoms at depths of several hundreds of micrometres. If we assume that the otolith is exposed to X-rays from a Rh-tube (20.16 keV) about 50% of the detectable Sr(Kα) X-rays will be emitted from a depth ranging from 0 to 114 µm. In the case of PIXE with 3 MeV protons the corresponding range is 0–15 µm. The information depths in LA-ICP-MS were determined by measuring the depth of the laser-ablated craters or trenches that remained on the otolith surface. The depths measured with a scanning white light interferometer (SWLI) were found to be about 40 µm (spots) and 11 µm (trenches).The correlation between strontium concentrations obtained by spot analysis of whitefish otoliths with PIXE and LA-ICP-MS was excellent (92%), although different spot sizes were used. A comparison of strontium concentration profiles measured with µ-XRF and LA-ICP-MS showed that the higher information depth of µ-XRF in combination with the 52 degree angle of the incident X-rays smoothens out the sharp edges seen in the LA-ICP-MS profiles. Otoliths from whitefish captured in the Baltic Sea (n = 30) were analysed in this comparison of methods. Among these whitefish there are sea spawners, river spawners and stocked fish with different life history and different otolith chemistry.

Evaluation from otolith Sr stable isotope ratios of possible juvenile growth areas of Japanese eels collected from the West Mariana Ridge spawning area

The widely distributed East Asian Japanese eel Anguilla japonica constitutes a single genetically homogeneous population with a single spawning area near the West Mariana Ridge. Otolith 87Sr/86Sr ratios of adults (categorized as “river“, “estuarine” or “sea” eels, according to their habitat use history determined from otolith Sr:Ca ratio analysis) collected from the spawning area in 2008 and 2009 were examined in an attempt to determine their juvenile growth areas. In addition, 87Sr/86Sr ratios of water samples from rivers in China, Taiwan, Korea, and Japan were determined. Otolith 87Sr/86Sr ratios of the “river”, “estuarine” and “sea” eels were 0.707793, 0.708580-0.709944 and 0.709068, respectively, and water sample ratios from China, Korea, Taiwan, and Japan were 0.7104320-0.7141010, 0.7190826-0.7227976, 0.7115523-0.7146914 and 0.706191-0.712484, respectively. “River” and “estuarine” eels, which had otolith 87Sr/86Sr ratios less than 0.7092 (seawater ratio), appeared to have inhabited Japanese rivers and/or estuaries because similarly low isotope ratios were recorded only from Japan. However, the juvenile growth areas of other eels were unknown, as their origins could not be determined from otolith 87Sr/86Sr ratios alone, and required further information regarding otolith elemental and isotope compositions.

Unravelling the population structure of Trachurus picturatus across the NE Atlantic using otolith fingerprinting analyses

Unravelling the population structure of Trachurus picturatus across the NE Atlantic using otolith fingerprinting analyses

Population structure, habitat connectivity and movement patterns of the blue jack mackerel, Trachurus picturatus, in the NE Atlantic: combining natural tags for a rational management of fisheries resources

Population structure, habitat connectivity and movement patterns of the blue jack mackerel, Trachurus picturatus, in the NE Atlantic: combining natural tags for a rational management of fisheries resources

Otolith Microchemistry of Coastal Cutthroat Trout from the Marys and Willamette Rivers

Abstract We sampled the water chemistry and otolith chemistry and structure of juvenile Cutthroat Trout collected in the Marys River and adults collected in the Willamette River to evaluate the potential to identify life-history variation in this population. Differences in water chemistry between the Marys and the Willamette Rivers during the sampling period were reflected in otolith chemistry. We differentiated 2 life-history patterns: juveniles whose mothers appeared to be resident in the Marys River, and juveniles that likely had fluvial mothers from the Willamette River. Otolith elemental composition of both juveniles and adults showed maternal effects on core chemistry, movement, or changes in elemental incorporation related to changes in ontogeny, growth, or seasonal changes in temperature. Water and otolith edge 87Sr/86Sr were consistently lower in the Willamette River than in the Marys River. Mean otolith 87Sr/86Sr during early life provided evidence that some adults reared in tributaries and migr...

Low-fidelity homing behaviour of Biwa salmon Oncorhynchus sp. landlocked in Lake Biwa as inferred from otolith elemental and Sr isotopic compositions

Biwa salmon Oncorhynchus sp. is endemic to Lake Biwa, Japan, where it is an important commercial and recreational fisheries species. However, no information is currently available on its population structure and migration ecology. Therefore, here we evaluated whether otolith Sr/Ca, Ba/Ca and 87Sr/86Sr ratios can be used as natural signatures in Biwa salmon and then used these to determine the natal origins of lake-migration-phase individuals and spawning adults, and the homing ability of spawning adults in the Lake Biwa water system. Quadratic discriminant function analysis demonstrated that the lake-migration school comprised individuals with multiple origins, including rivers to the east, west and north of Lake Biwa, and that the homing rate of spawning adults was low (18 out of 80 individuals), with ca. 78% of fish straying into non-natal rivers. However, this straying behaviour was not spatially random, with fish tending to migrate upstream in rivers neighbouring their natal rivers. The high rate of straying in spawning adults is considered important for establishing and maintaining this species, which is highly adapted to life in the Lake Biwa water system where environmental disturbances often occur.

Strong population structure deduced from genetics, otolith chemistry and parasite abundances explains vulnerability to localized fishery collapse in a large Sciaenid fish, Protonibeadiacanthus.

As pressure on coastal marine resources is increasing globally, the need to quantitatively assess vulnerable fish stocks is crucial in order to avoid the ecological consequences of stock depletions. Species of Sciaenidae (croakers, drums) are important components of tropical and temperate fisheries and are especially vulnerable to exploitation. The black‐spotted croaker, Protonibea diacanthus, is the only large sciaenid in coastal waters of northern Australia where it is targeted by commercial, recreational and indigenous fishers due to its food value and predictable aggregating behaviour. Localized declines in the abundance of this species have been observed, highlighting the urgent requirement by managers for information on fine‐ and broad‐scale population connectivity. This study examined the population structure of P. diacanthus across north‐western Australia using three complementary methods: genetic variation in microsatellite markers, otolith elemental composition and parasite assemblage composition. The genetic analyses demonstrated that there were at least five genetically distinct populations across the study region, with gene flow most likely restricted by inshore biogeographic barriers such as the Dampier Peninsula. The otolith chemistry and parasite analyses also revealed strong spatial variation among locations within broad‐scale regions, suggesting fine‐scale location fidelity within the lifetimes of individual fish. The complementarity of the three techniques elucidated patterns of connectivity over a range of spatial and temporal scales. We conclude that fisheries stock assessments and management are required at fine scales (100 s of km) to account for the restricted exchange among populations (stocks) and to prevent localized extirpations of this species. Realistic management arrangements may involve the successive closure and opening of fishing areas to reduce fishing pressure.

Trace element-protein interactions in endolymph from the inner ear of fish: implications for environmental reconstructions using fish otolith chemistry.

Otoliths, the biomineralised hearing "ear stones" from the inner ear of fish, grow throughout the lifespan of an individual, with deposition of alternating calciferous and proteinaceous bands occurring daily. Trace element : calcium ratios within daily increments measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are often used in fisheries science to reconstruct environmental histories. There is, however, considerable uncertainty as to which elements are interacting with either the proteinaceous or calciferous zones of the otolith, and thus their utility as indicators of environmental change. To answer this, we used size exclusion chromatography-inductively coupled plasma-mass spectrometry (SEC-ICP-MS) of endolymph, the otolith growth medium, to determine the binding interactions for a range of elements. In addition, we used solution ICP-MS to quantify element concentrations in paired otolith and endolymph samples and determined relative enrichment factors for each. We found 12 elements that are present only in the proteinaceous fraction, 6 that are present only in the salt fraction, and 4 that are present in both. These findings have important implications for the reconstruction of environmental histories based on changes in otolith elemental composition: (1) elements occurring only in the salt fraction are most likely to reflect changes in the physico-chemical environment experienced during life; (2) elements occurring only in the proteinaceous fraction are more likely to reflect physiological rather than environmental events; and (3) elements occurring in both the salt and proteinaceous fractions are likely to be informative about both endogenous and exogenous processes, potentially reducing their utility in environmental reconstructions.

Establishing nursery estuary otolith geochemical tags for Sea Bass (Dicentrarchus labrax): Is temporal stability estuary dependent?

The ability to determine connectivity between juveniles in nursery estuaries and adult populations is an important tool for fisheries management. Otoliths of juvenile fish contain geochemical tags, which reflect the variation in estuarine elemental chemistry, and allow discrimination of their natal and/or nursery estuaries. These tags can be used to investigate connectivity patterns between juveniles and adults. However, inter-annual variability of geochemical tags may limit the accuracy of nursery origin determinations. Otolith elemental composition was used to assign a single cohort of 0-group sea bass Dicentrarchus labrax to their nursery estuary thus establishing an initial baseline for stocks in waters around Ireland. Using a standard LDFA model, high classification accuracies to nursery sites (80–88%) were obtained. Temporal stability of otolith geochemical tags was also investigated to assess if annual sampling is required for connectivity studies. Geochemical tag stability was found to be strongly estuary dependent.

Can dietary reliance on Pacific salmon eggs create otolith Sr/Ca signatures that mimic anadromy in resident salmonids?

Many aspects of the ecology, growth, life history, and population dynamics of fishes differ between anadromous populations and those residing exclusively in freshwater habitats. Analysis of the elemental composition of otoliths (ear stones) is commonly used to indicate the migration history of individuals, relying on the differences in ambient concentrations of calcium, strontium, and barium and their subsequent incorporation into calcified structures. Dietary contribution to otolith chemistry is often overlooked, but in this study we report results consistent with the possibility that reliance on food resources derived from the ocean via Pacific salmon can produce otolith Sr/Ca ratios suggesting anadromy in freshwater resident fish. Dolly Varden, Salvelinus malma, from the Iliamna River, Alaska feed very heavily on eggs and other tissues from sockeye salmon, Oncorhynchus nerka, and their otoliths had Sr/Ca ratios typical of fish making seasonal migrations to marine waters but the Ba/Ca ratios were consistent with residence in fresh water. Water samples from the river provided no indication that the elevated Sr concentrations came from the river. A simulation of otolith chemistry resulting from Sr incorporation from both water and diet across a range of published values in salmonids indicates that a diet of salmon eggs and tissues can produce marine Sr/Ca ratios while fish remain in fresh water. Without experimental evidence such as a controlled diet study these results are suggestive but not conclusive. Nevertheless, they send a cautionary note that in some cases heavy reliance on marine-derived food sources might affect otolith microchemistry, creating an appearance of anadromy in fish that did not leave fresh water.

Quantitative electron microprobe mapping of otoliths suggests elemental incorporation is affected by organic matrices: implications for the interpretation of otolith chemistry

In an effort to understand the mechanism of otolith elemental incorporation, the distribution of strontium (Sr) and sulfur (S) in otoliths of Platycephalus bassensis was investigated in conjunction with otolith growth patterns. Optimisation of electron probe microanalysis (EPMA) quantitative mapping achieved both high spatial resolution (<3µm) and two-dimensional visualisation of the fine scale Sr and S distributions in otoliths of P. bassensis with minimal damage. Electron backscatter diffraction (EBSD) mapping confirmed that grain growth is aligned with the otolith c-axis, with grain orientation independent of both otolith elemental composition and growth patterns. Results showed a linear correlation between Sr and S distribution (R2=0.86), and a clear association with the otolith growth patterns determined by scanning electron microscopy. Further examination by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) showed that incorporation of Mg and Ba appeared independent of both S distribution and the growth patterns. The results suggest that element incorporation into the otolith is linked to the organic composition in the endolymph during mineralisation, and the organic matrices may assist, in part, the uptake of Sr. Thus, these findings may have significant implications for the interpretation of otolith Sr chemistry.

Quantifying physiological influences on otolith microchemistry

Summary Trace element concentrations in fish earstones (‘otoliths’) are widely used to discriminate spatially discrete populations or individuals of marine fish, based on a commonly held assumption that physiological influences on otolith composition are minor, and thus variations in otolith elemental chemistry primarily reflect changes in ambient water chemistry. We carried out a long‐term (1‐year) experiment, serially sampling seawater, blood plasma and otoliths of mature and immature European plaice (Pleuronectes platessa L.) to test relationships between otolith chemistry and environmental and physiological variables. Seasonal variations in otolith elemental composition did not track seawater concentrations, but instead reflected physiological controls on metal transport and biokinetics, which are likely moderated by ambient temperature. The influence of physiological factors on otolith composition was particularly evident in Sr/Ca ratios, the most widely used elemental marker in applied otolith microchemistry studies. Reproduction also triggered specific variations in otolith and blood plasma metal chemistry, especially Zn/Ca ratios in female fish, which could potentially serve as retrospective spawning indicators. The influence of physiology on the trace metal composition of otoliths may explain the success of microchemical stock discrimination in relatively homogenous marine environments, but could complicate alternative uses for trace element compositions in biominerals of higher organisms.