Differences In Otolith Chemistry Research Articles

Application of otolith chemistry at multiple life history stages to assess population structure of Warsaw grouper in the Gulf of Mexico

Chemical markers in otoliths have been used to assess the stock structure of many marine fishes, but these natural markers have yet to be widely evaluated or applied to demersal fishes in offshore habitats where physicochemical gradients are generally less pronounced relative to nearshore waters. To address this, we quantified trace elements (Li, Mg, Mn, Co, Cu, Zn, Sr, Ba) and stable isotopes (δ13C and δ18O) in otoliths of Warsaw grouper Hyporthodus nigritus from 4 regions in the Gulf of Mexico (Texas, Louisiana, Alabama-NW Florida, and SW Florida). Region-specific differences in otolith chemistry were observed, and notable differences in several influential markers (Mn:Ca, Sr:Ca, and Ba:Ca ratios and δ18O) were present, particularly between the most distant regions investigated (Texas/Louisiana and SW Florida). Distinct regional signatures were observed for Warsaw grouper across 3 life history stages: first year (otolith core), most recent years (otolith edge), and lifetime (whole otolith), suggesting that individuals within certain regions share common environmental histories that may represent unique contingents or sub-populations. Findings also demonstrate that spatial variability within these markers was consistent enough to overcome any temporal variability within the geographic domains investigated for all 3 life history stages, highlighting their potential value for assessing the natal origin, exchange, and population structure of this species and potentially other members of the deepwater fish assemblage.

Otolith Chemistry Reveals Natal Region of Larval Capelin in Coastal Newfoundland, Canada

Identifying natal origins of animals is key to determining the relative productivity of natal habitats, dispersal of individuals among local populations (i.e. connectivity), and, ultimately, metapopulation dynamics. As marine fish larvae have a high potential for dispersal, natural tags such as otolith chemistry are often used to determine natal origins. Trace elements may be incorporated into embryonic otoliths while larvae are developing in the egg, resulting in chemical signatures in the pre-hatch region of the otolith that reflect the natal habitat. Our goal was to determine whether the natal origins of 1-3 day old larval capelin (Mallotus villosus), a key forage fish in many northern marine ecosystems, could be determined using otolith chemistry. We sampled larvae from five Newfoundland regions (i.e. embayments: Placentia Bay, St. Mary’s Bay, Witless Bay, Trinity Bay, Notre Dame Bay) during July-August, 2019 to quantify regional differences in otolith chemistry. Additionally, eggs/larvae were field-reared within two regions over multiple years (Notre Dame Bay: 2014, 2015, 2018, 2019; Trinity Bay: 2018, 2019) to quantify interannual variation in region-specific otolith chemistry. Multi-elemental otolith signatures (i.e., Mg, Mn, Zn, Sr, Ba), as determined by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS), differed significantly among regions, with individuals classified into their natal region with 78% success (region-specific success: 68-100%). Classification success into natal region remained high (67-76%) despite interannual variation in otolith trace element concentrations within regions. Characterizing region-specific otolith chemical signatures that reflect natal origins of capelin larvae is the first step in determining the productivity and relative contributions of different regions of coastal Newfoundland to capelin recruitment.

Spatio-temporal resolution of spawning and larval nursery habitats using otolith microchemistry is element dependent

Otolith chemistry is frequently employed in the reconstruction of fish environmental histories. While some elements have been strongly correlated with environmental factors (e.g. salinity, temperature, water chemistry), others may not indicate exogenous factors and simply add endogenous variability to a data set. Several commonly assessed elements were previously identified as being only present in the proteinaceous fraction of endolymph from black bream Acanthopagrus butcheri, suggesting that the choice of elements in otolith multi-elemental fingerprinting could influence their utility as natural environmental markers. To test this hypothesis, we performed several cluster analyses based on different sets of trace element data extracted from both the core and larval region of otoliths of juvenile black bream. We clustered in 3 different ways: (1) all elements analysed; (2) elements identified as being primarily in the salt fraction of the endolymph (i.e. inorganic); and (3) elements identified as being associated with the protein fraction of the endolymph. We subsequently assessed the power of the resulting clusters to resolve cohort identity and/or collection location based on differences in otolith chemistry using multinomial logistic regression. Our results indicate that clustering based solely on salt-fraction elements is best for resolving spatio-temporal variability in spawning sources and larval nursery habitats of black bream.

Resolving the early life history of King George whiting (Sillaginodes punctatus: Perciformes) using otolith microstructure and trace element chemistry

Understanding the early life history processes of fish that lead to recruitment is critical for understanding population dynamics. This study explored the early life history of King George whiting (Sillaginodes punctatus) that recruited to an important nursery area in South Australia in 2016 and 2017. The early life history was reconstructed based on the retrospective analysis of otolith microstructure and chemistry for settlement-stage larvae collected fortnightly from July to November. These fish hatched between March and July, but a 3-week period in May led to 52–71% of recruitment. Recruits from successive sampling occasions differed in age, size and growth rate, potentially related to seasonal changes in water temperature and larval food availability. During both years, there were significant changes in otolith elemental chemistry among the groups of recruits that primarily related to changes in Sr:Ca. There are two hypotheses to account for the differences in otolith chemistry: either (1) a single, primary spawning source and within-season environmental change; or (2) multiple spawning sources. Further investigation with oceanographic models of larval dispersal will help differentiate between these. The retrospective analysis of otoliths has improved the understanding of early life history for this important species, with implications for fishery management.

Identifying stock structuring in the sandeel, Ammodytes marinus, from otolith microchemistry

Identifying the scale of population processes is important for sustainable fisheries management. In the North Sea, the lesser sandeel, Ammodytes marinus, is now managed as seven stocks delineated on the basis of biophysical model predictions of the limited scale of larval mixing. This study used otolith chemistry to evaluate whether the stock units defined from these predictions actually reflect the degree of isolation of early life stages. Elemental composition near the core, settlement check and summer opaque zone were examined using laser ablation-inductively coupled plasma mass spectrometry in young of the year, sampled across 8 grounds within three of the main fished stocks in 2011. There were significant spatial differences in otolith chemistry enabling post-settled juveniles to be discriminated among stock areas and even between some grounds. Clustering of near core chemistry pointed to four natal sources, with dominant clusters in each of the three stocks studied and one unique to the Central Western North Sea (sandeel assessment area 4). Moreover, near core chemistry was similar to that of post-settled juveniles at the stock area level. Consistency in the scaling of early life mixing predicted by biophysical models and otolith chemistry supports the present stock units used in assessments.

Spatial and temporal variability in the otolith chemistry of the Brazilian snapper Lutjanus alexandrei from estuarine and coastal environments.

Otolith chemistry of juvenile and adult individuals of the Brazilian snapper Lutjanus alexandrei was measured to assess the utility of natural markers for investigating individual movements. Individuals were collected over a 3-year period (2010-2012) along the north-eastern coast of Brazil from both estuarine (juvenile to sub-adult stages) and coastal (sub-adult to adult stages) areas. Six elements ((7) Li, (24) Mg, (55) Mn, (59) Co, (88) Sr and (137) Ba) were measured in sectioned otoliths of L. alexandrei using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Edge composition analysis indicated that element:Ca ratios in the otoliths of juvenile and sub-adult L. alexandrei from estuaries were not significantly different among the three consecutive years (2010, 2011 and 2012), suggesting that physicochemical conditions within the nursery area investigated were temporally stable. Similarly, apart from two elements (Ba and Co), element:Ca ratios for larger L. alexandrei inhabiting coastal waters were also similar. In contrast, otolith chemistry of similar sized L. alexandrei from estuarine and coastal areas was significantly different (based on recently accreted material). Otolith Mn:Ca and Ba:Ca were both significantly higher for L. alexandrei collected in estuaries compared to fish from adjacent coastal reefs, while the opposite trend was observed for Sr:Ca. Given the pronounced differences in otolith chemistry between estuarine and coastal areas, element:Ca transects were constructed from the core to margin of the otoliths for adults (age 7+ years) collected on reefs to determine the timing of movement (ontogenetic migration) from estuarine to coastal areas. Based on observed patterns of decline for both Mn:Ca and Ba:Ca, it appears that L. alexandrei begin the move to more coastal habitats (i.e. lower element:Ca ratios) after age 2 years. The patterns observed for this species highlight the importance of conserving connectivity between coastal habitats to maintain sustainable fish stocks exploited by artisanal fisheries.

Variation in morphology and life-history strategy of an exploited sparid fish

Defining appropriate management units to balance productivity and yield of exploited species is fundamental to effective resource management. Anecdotal and tag–recapture information related to morphology, movement behaviour and life-history strategy suggest that separate groups of snapper (Chrysophrys auratus) exist in the Hauraki Gulf, New Zealand. To address the existence of discrete groups, we examined morphology, meristics and otolith chemistry from snapper collected throughout the Hauraki Gulf. We also used tag–recapture information, stable isotope analysis and interpreted functional aspects of morphology and meristics data to understand potential life-history strategy differences. Snapper from rocky reef habitats did not display morphology and meristic features distinct from snapper from soft sediment habitats and differences in otolith chemistry and stable isotope ratios could respectively be explained by a locational influence and predominance of kelp in rocky reef food webs. Conversely, snapper collected from a known spawning area had distinct morphological and meristic features consistent with semi–pelagic sparids and stable isotope analysis also indicated a potentially more pelagic and higher trophic-level diet. Maintenance of population complexity such as this is generally beneficial to fish populations, and can be achieved by revisiting the spatial units used for fishery management.

The effect of parasitism by a blood-feeding isopod on the otolith chemistry of host fish

Otolith chemistry is widely used to discriminate fish stocks or populations, although many of the factors that determine trace-element concentrations within the otolith remain poorly understood. We investigated the effect of a blood-feeding isopod ectoparasite, Ceratothoa sp., on the otolith chemistry of yellowtail scad, Trachurus novaezelandiae. We sampled 65 fish from three subpopulations of T. novaezelandiae from Jervis Bay in south-eastern Australia, and used laser ablation (LA)–inductivelycoupled plasma mass spectrometry (ICPMS) to measure otolith lithium (Li) : calcium (Ca), magnesium (Mg) : Ca, strontium (Sr) : Ca and barium (Ba) : Ca from four consecutive summer and winter growth bands. Otoliths of parasitised fish were characterised by significantly lower Li : Ca and Mg : Ca, and higher Sr : Ca, than those of unparasitised individuals from the same subpopulation. The consistency of trends in otolith chemistry across ablation points and among subpopulations suggests that there is a consistent physiological mechanism through which Ceratothoa parasites affect the otolith chemistry of infected individuals. It is likely that a range of physical, metabolic, chemical and behavioural processes act in concert to influence the otolith chemistry of parasitised fish. Given the ubiquitous distribution of parasites in the marine environment, differential rates of parasitism among fish stocks, populations or migratory contingents may be an important but unappreciated factor driving stock- or population-based differences in otolith chemistry.

Use of otolith chemistry to discriminate juvenile Chinook salmon ( Oncorhynchus tshawytscha) from different wild populations and hatcheries in Lake Huron

Chinook salmon ( Oncorhynchus tshawytscha) in Lake Huron consist of wild and hatchery-reared fish distributed among several populations. This study tested whether otolith chemistry can be used to identify the natal origin of Chinook salmon in this system. Concentrations of nine elements (Mg, K, Mn, Fe, Zn, Rb, Sr, Ba, and Pb) in the otoliths of Chinook salmon juveniles from 24 collection sites (17 streams and 7 hatcheries) around Lake Huron were analyzed using laser-ablation inductively-coupled mass spectrometry. Differences in otolith chemistry were found between rearing environments (wild and hatchery), among geological regions (Precambrian, Ordovician, Silurian, Devonian, and Carboniferous), and among collection sites. Discriminant function analysis showed high classification accuracies of juveniles to their rearing environment (wild versus hatchery, 82%), geological region (84%), and collection site (87%) of origin. With these values, there is excellent potential for otolith chemistry to be used to predict the natal origin of adults, and thus inform research and management of Chinook salmon in Lake Huron.

Population differences in otolith chemistry have a genetic basis in Menidia menidia

The number of studies using otolith chemistry as a tool to reconstruct the environmental history of fishes and to detect population structure continues to rise, despite the fact that factors influencing otolith deposition are not fully understood. Many studies have examined the influence of environmental parameters on otolith composition, but none to date have tested the possible influence of intrinsic factors. Using lab broodstock populations, we examined the influence of genetics and temperature on Mg:Ca, Mn:Ca, Sr:Ca, and Ba:Ca concentrations and partition coefficients in the otoliths of juvenile Atlantic silversides, Menidia menidia . Fish from two populations, South Carolina, USA, and Nova Scotia, Canada, were reared in 15, 21, and 27 °C. We found significant (p < 0.05) differences in otolith Mg:Ca, Mn:Ca, and Ba:Ca ratios as well as in Mg, Mn, and Ba partition coefficients among populations. Such genetic influences on otolith elemental concentrations have important implications for understanding the physiological mechanisms underlying otolith deposition and enhance the utility of otolith chemistry as a marker of fish population structure in the wild.

Discrimination of wild and hatchery trout by natural chronological patterns of elements and isotopes in otoliths using LA‐ICP‐MS

Abstract The potential to discriminate between wild and hatchery trout [Salmo trutta f. fario L. and Oncorhynchus mykiss (Walbaum)] by chronological microchemical patterns of different otolith regions was tested. Central and rim regions of otoliths were analysed for life stage specific elemental patterns of strontium (Sr), sodium (Na) and calcium (Ca) as well as of 87Sr/86Sr ratios using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Classification to life stage specific habitats was achieved with 100% accuracy by 88Sr/43Ca, 23Na/43Ca and 87Sr/86Sr ratios resulting in a discrimination of hatchery fish from wild fish. 87Sr/86Sr ratios and Sr concentration in otoliths were positively correlated with the ratio and amount in the water. Na content in otoliths did not directly reflect the concentration in the water, although the 23Na/43Ca ratio in otoliths contributed to the discrimination. Clear differences in otolith chemistry were found for fish experiencing different geological units or a groundwater‐fed environment in a hatchery. Natural chronological microchemical patterns in otoliths proved to be a valuable tool for discriminating wild and hatchery fish without the necessity to use marking methods. LA‐ICP‐MS is well suited for this application because of its limited need for sample preparation, its potentially high sample throughput and the possibility of simultaneous measurement of multiple elemental and isotopic fingerprints.

Otolith chemistry of juvenile spotted seatrout Cynoscion nebulosus reflects local natal regions of coastal Mississippi, USA

Early juvenile spotted seatrout Cynoscion nebulosus (n = 199) were collected during late summer and autumn 2001 from shoreline habitats within 9 coastal regions bordering Mississippi Sound in the north-central Gulf of Mexico to ascertain how well fish could be spatially classified based on otolith chemistry. Left otoliths were assayed for trace element:Ca ratios of Ba, Li, Mg, Mn and Sr, and right otoliths for δ 13 C and δ 18 O. Significant overall differences in otolith chemistry existed among the 9 regions; 61% of the joint variance in the 7 otolith chemistry variables was explained by the regional factor in a multivariate analysis of variance (MANOVA). All 7 otolith chemistry variables differed significantly among the 9 regions. The isotopes δ 13 C and δ 18 O showed the highest regional affinities, and Li showed the strongest regional association of all the trace elements. Canonical dis- criminant function analysis (CDFA) maximally separated regional groups of early juvenile fish. The first 3 of 7 discriminant functions accounted for 97.5% of the cumulative variance in the 7 otolith chemistry variables. CDF 1 was influenced primarily by δ 18 O and Li, CDF 2 by Mn and δ 13 C, and CDF 3 by Mg and Ba. In the all-inclusive CDFA, 93.4% of cases were classified correctly, and classifica- tion success among regions ranged from 83.3 to 100%. The influence of freshwater discharge from 7 rivers along the Mississippi coastline likely made it possible to detect the relatively fine-scale spatial differences seen in this study, as defined by a mean interregional distance of only 25 km.

Temporal and Spatial Variability in Otolith Trace‐Element Signatures of Juvenile Striped Bass from Spawning Locations in Lake Texoma, Oklahoma‐Texas

AbstractWe assessed the potential of trace‐element analysis for identifying spawning locations of striped bass Morone saxatilis in the two tributaries of Lake Texoma, Oklahoma‐Texas, during 2002‐2004. We also assessed the temporal stability of elemental signatures over 3 years. Elemental composition of juvenile striped bass otoliths varied considerably between the two tributary arms of Lake Texoma both within and among years. Overall reclassification rates within each collection year ranged between 81% and 93%. However, because of high interannual variability, classification functions could not reliably predict natal areas for individuals from other year‐classes (19‐62%). Although the mechanisms generating spatial and temporal differences in otolith chemistry are not well understood, spatial differences in otolith chemistry indicate that elemental fingerprints of striped bass from the two natal rivers in the Lake Texoma system provide natural tags of juvenile habitat. The temporal variability we observed in otolith elemental signatures complicates the use of these signatures for determining the natal river of an unknown individual from an unsampled year‐class. However, building a library of trace‐element signatures for yearly cohorts should make it possible to determine natal rivers for adult striped bass from known cohorts.

Temporal and Spatial Variability in Juvenile Red Snapper Otolith Elemental Signatures in the Northern Gulf of Mexico

AbstractWe examined otolith chemistry of age‐0 red snapper Lutjanus campechanus in U.S. waters of the northern Gulf of Mexico (GOM) to determine if otolith elemental signatures could be employed as natural tags to estimate postsettlement population connectivity. Fish (n = 755) belonging to five successive year‐classes (1996‐2000) were sampled from three GOM regions. Solution‐based analysis of otolith chemistry was accomplished with sector field‐inductively coupled plasma‐mass spectrometry (Ba, Mg, Mn, and Sr) and inductively coupled plasma‐optical emissions spectrometry (Ca). Several regional trends observed for individual elements were consistent among year‐classes, yet elemental signatures were significantly different among years as well as among regions (multivariate analysis of variance [MANOVA]; p < 0.001). Jackknifed classification accuracies from linear discriminant function analysis approached 80% (mean = 78.4%) in all years except 1998 (62.4%). Overall, these results indicate otolith elemental signatures may serve as effective region‐specific natural tags of GOM red snapper. Significant differences in otolith chemistry among year‐classes indicates year‐class‐specific analysis is required to derive natural tags. Furthermore, regional differences in elemental signatures may be insufficient in some years to be employed as accurate natural tags.

A field experiment testing for correspondence between trace elements in otoliths and the environment and for evidence of adaptation to prior habitats

Site-specific variation in the trace element composition of fish otoliths can be used to identify fish to source, but the mechanisms controlling elemental composition are poorly understood. Environmental influences on the deposition of barium (Ba), copper (Cu), manganese (Mn), and strontium (Sr) in the otoliths of mudsuckers (Gillichthys mirabilis) were tested using a reciprocal field transplant experiment, in which fish from 3 estuaries were transplanted to each of the 3 estuaries. Fish originating from the 3 estuaries showed no differences in otolith chemistry that might reflect acclimation to past conditions in their home estuary or genetic differences among populations, which simplifies the interpretation of otolith chemistry. Cu and Mn concentrations in otoliths differed according to the site of transplant. Cu in otoliths showed the same pattern of difference among estuaries as did Cu in sediments, but there was no correspondence between Cu in otoliths and dissolved Cu. Ranked differences among estuaries in otolith Mn matched the ranking of estuary-specific differences in dissolved Mn, and there was no correspondence between the concentration of Mn in otoliths and sediments. Fish transplanted to different estuaries showed no differences in otolith concentrations of Ba or Sr, and the concentrations of Ba and Sr in the water column showed a similar lack of difference among estuaries. This study provides field evidence supporting the conclusion that the elemental composition of otoliths reflects environmental conditions to which fish have been recently exposed, but whether that correlation is with trace elements in the sediment or water column can vary.

The use of otolith chemistry to determine stock structure of three epinepheline serranid coral reef fishes on the Great Barrier Reef, Australia

We investigated the use of otolith chemistry to identify the stock structure of three epinepheline serranid coral reef fishes: Plectropomus leopardus, Cephalopholis cyanostigma and Epinephelus fasciatus. Chemical signatures of otoliths collected from the three species were compared on a broad regional scale and, for P. leopardus, also on a finer reef scale. The temporal stability in these signals was also examined by comparing two cohorts of P. leopardus, and C. cyanostigma and E. fasciatus from the same cohorts that were collected 2 years before and 2 years after a significant weather disturbance caused by a large tropical cyclone (Cyclone Justin) in March 1997. Results showed differences in otolith chemistry at various spatial scales, as well as between years and individuals collected before and after the cyclone. The results highlight the need to incorporate data from several years in studies using these techniques to discriminate temporary and possibly misleading signals from those that indicate persistent spatial structure in stocks. The difficulties in identifying stocks and stock boundaries in species with spatially complex population structures are discussed, as is the need for complementary analyses of life history characteristics to determine biologically and logistically appropriate management units.

Spatial variation in elemental composition of otoliths of three species of fish (family Sparidae)

Determining the nursery habitat of fishes that have moved from estuarine nursery habitats is difficult. The elemental fingerprints of otoliths of three species of sparids were determined to investigate their utility as a natural tag of the nursery habitat. Juvenile Pagrus auratus (snapper), Rhabdosargus sarba (tarwhine) and Acanthopagrus australis (bream) were collected from two sites in each of 15, six and three estuaries, respectively, and their otoliths analysed by solution-based inductively coupled plasma-mass spectrometry. Significant differences in otolith chemistry were found for all three species of juveniles collected from different estuaries. The same patterns among estuaries were not seen for all species, although it was not possible to sample the same sites within an estuary for all species. For bream, significant differences in otolith chemistry were found among all three estuaries, whereas for tarwhine the six estuaries were separated into three groups. For snapper, a number of estuaries could be separated, but there was some overlap for other estuaries. All three species were collected from the same site within one estuary and their otoliths analysed. Significant differences were found among species, but the implication of this finding remains unclear as the three species show differences in microhabitat use and may also differ in age. Because the elemental fingerprints of juveniles vary among estuaries or groups of estuaries, the nursery or recruitment estuary of adult fish could now be determined by analysing the juvenile region of adult otoliths. Thus, connectivity between estuaries and open coastal populations could be determined. Such information will have major implications for fisheries management because it will provide information on the distance that fish have moved from their recruitment estuary and the number of estuaries that contribute to each adult population.

Identification of Atlantic bluefin tuna (Thunnus thynnus) stocks from putative nurseries using otolith chemistry

AbstractChemical signatures in the otoliths of teleost fishes represent natural tags that may reflect differences in the chemical and physical characteristics of an individuals' environment. Otolith chemistry of Atlantic bluefin tuna (Thunnus thynnus) was quantified to assess the feasibility of using these natural tags to discriminate juveniles (age 0 and age 1) from putative nurseries. A suite of six elements (Li, Mg, Ca, Mn, Sr and Ba) was measured in whole otoliths using solution‐based inductively coupled plasma mass spectrometry. Otolith chemistry of age‐1 T. thynnus collected from the two primary nurseries in the Mediterranean Sea and western Atlantic Ocean differed significantly, with a cross‐validated classification accuracy of 85%. Spatial and temporal variation in otolith chemistry was evaluated for age‐0 T. thynnus collected from three nurseries within the Mediterranean Sea: Alboran Sea (Spain), Ligurian Sea (northern Italy), and Tyrrhenian Sea (southern Italy). Distinct differences in otolith chemistry were detected among Mediterranean nurseries and classification accuracies ranged from 62 to 80%. Interannual trends in otolith chemistry were observed between year classes of age‐0 T. thynnus in the Alboran Sea; however, no differences were detected between year classes in the Tyrrhenian Sea. Age‐0 and age‐1 T. thynnus collected from the same region (Ligurian Sea) were also compared and distinct differences in otolith chemistry were observed, indicating ontogenetic shifts in habitat or elemental discrimination. Findings suggest that otolith chemistry of juvenile T. thynnus from different nurseries are distinct and chemical signatures show some degree of temporal persistence, indicating the technique has considerable potential for use in future assessments of population connectivity and stock structure of T. thynnus.

Otolith chemistry of juvenile snapper Pagrus auratus in Victorian waters: natural chemical tags and their temporal variation

Determining contributions of juveniles from different areas to adult populations is diffi- cult using traditional methods such as manual tagging and recapture. Environmental variation can, however, produce natural chemical tags in the otoliths of juvenile fish. Spatial variation in the otolith chemistry (Ba, Sr, Mn) of age 0+ snapper collected from inlets along the coast of Victoria, south- eastern Australia, enabled characterisation of otolith chemical tags for specific areas. In particular, a tag based largely on high Ba levels was specific to snapper from the largest juvenile area and most important fishery, Port Phillip Bay. This tag enabled accurate discrimination between snapper from this bay (98 and 85%) and other Victorian nursery areas in the 2000 and 2001 year classes, respec- tively. Differences in classification accuracy between these year classes were due to lower Ba levels in otoliths from some sites within Port Phillip Bay in 2001. Variation in otolith chemical tags between adjacent sampling months during the recruitment period, although significant, did not confound spa- tial discrimination between Port Phillip Bay and other inlets. Likewise, variation in otolith chemical tags between adjacent year classes, while greater than between months, did not greatly confound spatial discrimination. Comparisons across 5 year classes over a 9 yr period for Port Phillip Bay, how- ever, showed substantial differences in the elemental tag for this Bay. Differences in otolith chemistry between year classes were driven by variation in Ba and Sr levels, whereas between-month differ- ences were due to Mn variation. Future classifications of adults to juvenile nursery areas will require chemical tags characterised from juveniles of the same year class(es) as the adults being classified. Small-scale (mo) temporal mismatches between the sampling of juvenile and adult otoliths of the same year class should not influence the accuracy with which adults are classified to juvenile origins.

Temporal and spatial variability in elemental composition of otoliths: implications for determining stock identity and connectivity of populations

Interannual variability in elemental composition of otoliths may confound spatial interpretations. The elemental fingerprints of otoliths of juvenile fish were determined for fish collected from 12 to 15 estuaries in each of three consecutive recruitment years to determine temporal variation in otolith chemistry for each estuary. It was also examined whether there is overlap in elemental fingerprints of fish collected in different years and from different estuaries that may confound subsequent spatial comparisons. Significant differences in otolith chemistry were found among years for individual elements (lithium, manganese, strontium, and barium) and for multi-element fingerprints. Some estuaries showed large variation in multi-element fingerprints among years, whereas others showed little variation among years. There was some overlap of elemental fingerprints of different estuaries, but these were not always for fish collected in the same year. The significant spatial and temporal variation in elemental fingerprints meant that it was possible to confound spatial differences with temporal differences. Therefore, if the natal estuary of the adults is to be determined, a library of elemental fingerprints needs to be built up over time for each estuary rather than a single year-class of juveniles being used as the elemental fingerprint for a number of year-classes of adults.