Otolith Core Research Articles

Refined bomb radiocarbon dating of two iconic fishes of the Great Barrier Reef

Refinements to the methodology of bomb radiocarbon dating made it possible to validate age estimates of the humphead wrasse (Cheilinus undulatus) and bumphead parrotfish (Bolbometopon muricatum). Age for these species has been estimated from presumed annual growth zones in otoliths at ~30 and ~40 years respectively. The validity of these estimates was tested using bomb radiocarbon dating on the small and fragile otoliths of these species, and provided an opportunity to refine the method using advanced technologies. A regional Δ14C reference record from hermatypic coral cores from the Great Barrier Reef was assembled and Δ14C measurements from extracted otolith cores of adult otoliths were successful. Validated ages supported the accuracy of growth zone derived ages using sectioned sagittal otoliths.

Integrating genetic and otolith microchemistry data to understand population structure in the Patagonian Hoki (Macruronus magellanicus)

Information from genetic (microsatellites and mtDNA Control Region) and previously collected otolith (trace element fingerprinting of otolith core and edge) markers was jointly interpreted to describe dispersal and gene flow in the Patagonian hoki (Macruronus magellanicus), an intensively harvested marine fish with seasonal migrations between spawning and feeding grounds. Spawning adults from a Chilean (Pacific) spawning site and three feeding ground samples (one from Chile and two temporal samples from the Falkland Islands (Atlantic)) were analysed. The data indicated a high level of Atlantic/Pacific connectivity by means of non-natal homing of individuals to spawning aggregations. Against this background of regional connectivity however, genetic data support the existence of a reproductively isolated population within the overwintering stock. Otolith core results are compatible with reproductive isolation being effected by natal homing to an Atlantic spawning site and/or local adaptation. The discordance between geopolitically defined Atlantic and Pacific management stocks and underlying biocomplexity, and implications for sustainability, are discussed. The study highlights the importance of intraspecific variation in homing behaviours in shaping population structure and the merit of employing complementary analytical approaches.

Microstructure of the otoliths of the glacier lanternfish, Benthosema glaciale

Ageing using otolith microstructure is based on the assumption that growth increments are deposited daily. Individuals of the glacier lanternfish (Benthosema glaciale) were collected in the Flemish Cap (north-west Atlantic) during June–July 2010. A total of 44 individuals (ranging from 28 to 49mm standard length; <2 years old) were selected to analyse otolith microstructure using light microscopy in order to assess the reliability of the use of microincrements to infer the true age of this species. Three distinct zones were distinguished: the larval zone, the postlarval zone and the postmetamorphic zone. The otolith core was, on average, 8±0.7μm. The mean microincrement number in the larval zone was 42±7.5 within a radius of 0.095±0.010mm whereas in the postlarval zone it was 25±3.0 within a segment of 0.104±0.015mm. SEM examinations in a subsample of otoliths showed that the resolution power of the light microscope was sufficient to study both the larval and the postlarval zones. Back-calculation from the time of capture suggests a hatching period from mid-June to mid-December with a peak in September. The spawning time and daily deposition of the microincrements in B. glaciale are discussed.

Stock identification and life history interpretation using trace element signatures in salmon otoliths

Chum salmon (Oncorhynchus keta) in the North Pacific Ocean are anadromous fish, and spend most of their life in the sea until spawning in natal streams. To identify the stock and habitat characteristics of chum salmon, the composition of chemical elements (Ca, Mn, Sr, Zn, and Ba) in otolith was examined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Two main types of analytical work have been carried out; discrete spot analysis and line scan analysis of otolith sections. Salmon otoliths were obtained from the eastern (Canada and USA) and western (Japan and Korea) North Pacific during 1997–1999 spawning seasons. Spot analysis of otolith cores demonstrated significant differences in the element concentration among countries (p = 0.003). Line scanning from the core to the margin showed that Sr concentrations were elevated at the core of the otoliths, decreased during the freshwater stage, increased suddenly at a certain point, and oscillated periodically towards the margin matching with year-ring. The elevated Sr concentration at the core may reflect the maternal contribution to the egg, and the oscillations toward the margin may reflect salinity gradients between onshore/offshore or north/south migrations. The Zn profiles also oscillated and corresponded to the annual ring of the otolith. However, the profiles of Sr and Zn oscillated oppositely after salmon migrated to saline water and the Zn uptake declined toward the rim of the otolith while Sr uptake increased.

Influence of maternal habitat choice, environment and spatial distribution of juveniles on their propensity for anadromy in a partially anadromous population of rainbow trout (Oncorhynchus mykiss)

AbstractThis study evaluated the importance of the environment and spatial distribution of juvenile fish for the adoption of alternative migratory tactics in a partially anadromous population of rainbow trout (Oncorhynchus mykiss) from the Santa Cruz River. We captured young‐of‐the‐year fish along the river during autumn 2009, 2010 and spring 2010 and determined their maternal origin (anadromous vs. resident) using strontium to calcium ratios in the otolith core. Relative proportion of anadromous offspring, modelled with logistic regression, increased towards headwaters and in areas with deeper channels and larger substrate composition. Body length, modelled with linear multiple regression, varied positively with site depth, water velocity, substrate size and anadromous maternal origin. Based on evidence for limited juvenile movements (<25 km), the spatial extent of this study (240 Rkm) and the identification of large, contrasting reaches along the river, it is likely that the observed spatial distribution of juveniles and their association to sites with coarse substrate composition reflects maternal spawning activity. Results further indicate that anadromous females breed predominantly in middle and upper river sections in areas with coarse substrate. Given that body size in this system has been positively related to propensity for anadromy, we propose that female spawning choice affects their offspring's spatial distribution, providing the adequate physical template for anadromous offspring to reach or maintain larger body sizes and display anadromy themselves. Relevance of this study is also discussed in the context of alterations in response to future dam construction in one of the latest free‐flowing rivers sustaining anadromous O. mykiss.

Coping with connectivity between populations of Merluccius merluccius: An elusive topic

The elemental composition of European hake otoliths was analyzed at the core, settlement mark and otolith edge zones corresponding to the periods of pelagic early life stage, settlement to the bottom, and juvenile life, respectively. Barium, Sr, Mg, Mn and Ca were measured using LA-ICPMS on the sagitta otoliths of hake sampled in 6 different Western Mediterranean areas, separated by tens to hundreds of kilometers. The otolith edge, which should have incorporated the water mass signal of the sampling area, showed a high degree of overlap between geographical areas. This precluded the discrimination of geographically distinct groups. However, there was a clear ontogenetic signal in all the fish, with higher Sr:Ca in the otolith core. Ba:Ca was higher at the settlement mark, while Mn:Ca was overlapped with the core values. The otolith edge had lower ratios for all elements except for Mg. The conflicting effects of the ontogenic patterns and the inconclusive allocation of a geographical signal at the edge, precluded to assign individual hake to a natal origin based on the composition of the core and settlement mark zones. Our results may be explained by high mobility of the juvenile hake in this area of the Mediterranean, or by the relative homogeneity in the water masses for the four elements considered in this study. While this subset is useful for distinguishing major water masses or regional differences between, for example, the Atlantic and the Mediterranean, a larger set of elements could be needed for smaller-scale studies.

Assessing connectivity of a tropical estuarine teleost through otolith elemental profiles

Understanding connectivity between groups of a species is fundamental to effective management and conservation practices, yet is poorly understood for tropical estuarine fishes. Here, age-related trends in otolith elemental chemistry were examined to assess the degree of connectivity of Polydactylus macrochir, a large, non-diadromous, tropical estuarine teleost, across the species' Australian distribution. Elemental signatures (7Li, 43Ca, 55Mn, 88Sr and 138Ba) of transverse sections of otoliths of 3+ yr fish from the 2005 year class collected from 17 locations were sampled using laser ablation inductively coupled plasma mass spectrometry, providing elemental profiles from the otolith core through the first 3 yr of a fish's life. Univariate and cluster analyses revealed differences in elemental signatures of the otolith core among most locations, although similarities were evident among locations in the Gulf of Carpentaria and among 2 neighbouring locations in both Western Australia and the east coast of Queensland. Fewer differences were observed for post-settlement life history stages, although some differences were observed among neighbouring locations separated by as little as 50 km. SIMPROF analyses revealed that 138Ba generally provided the greatest discrimination among locations. Positive correlations were observed between otolith 138Ba concentration of individual fish and river flow indices for material laid down in all age groups, suggesting differences in flow was a significant driver of the observed patterns in 138Ba. Examined in conjunction with complementary studies into the connectivity of the study species, the spatial structuring suggests that P. macrochir populations are susceptible to localised depletion, with limited opportunity for replenishment from neighbouring populations. The results highlight the importance of using multiple, complementary methods for assessing connectivity of aquatic organisms.

Inferring movement patterns of a coral reef fish using oxygen and carbon isotopes in otolith carbonate

Ratios of oxygen and carbon isotopes (δ18O and δ13C) in otolith carbonate provide information on stock separation and migration of fish populations inhabiting isotopically different environments. δ18O and δ13C were analysed from Lethrinus miniatus (Forster 1801) otoliths to elucidate the probability of broad-scale movement in the Great Barrier Reef. Otolith core and edge portions were compared between and within individuals from the same cohort (n=120) to determine whether isotopically different environments were inhabited by juveniles and adults from different latitudes. A comparison between individuals revealed that δ18O and δ13C increased with latitude and were generally higher in edge than in core portions. Similarly, within most individual otoliths, δ18O and δ13C were higher in edge than in core portions. For some individuals, however, there was no difference in δ18O between core and edge portions, potentially signifying self-recruitment to the area or occupation of areas of similar chemistry. The majority of individuals from 19 to 22°S and almost half of individuals from 18°S displayed juvenile δ18O signatures characteristic of a warmer environment, suggesting movement to cooler environments or potential southerly migration of early life stages to adult habitat. In contrast, almost half of adult individuals in the most northern latitude (18°S) appeared to originate from cooler environments, suggesting movement to warmer environments or potential northward migration with ontogeny. These results infer that movement of L. miniatus individuals occurred across different isotopic environments with life stage and was related to latitude. Isotope ratios provided insight into movement patterns of L. miniatus at scales that will be important for delineating management units for this commercially important species.

Integrating microsatellite DNA markers and otolith geochemistry to assess population structure of European hake (Merluccius merluccius)

Population structure and natal origins of European hake were investigated using microsatellite DNA markers and otolith geochemistry data. Five microsatellites were sequenced and otolith core geochemical composition was determined from age-1 hake collected in the northeast Atlantic Ocean and the Mediterranean Sea. Microsatellites provided evidence of a major genetic split in the vicinity of the Strait of Gibraltar, separating the Atlantic and the Mediterranean populations, with the exception of the Gulf of Cádiz. Based on classification models using otolith core geochemical values, individual natal origins were identified, although with an increased error rate. Coupling genotype and otolith data increased the classification accuracy of individuals to their potential natal origins while providing evidence of movement between the northern and southern stock units in the Atlantic Ocean. Information obtained by the two natural markers on population structure of European hake was complementary as the two markers act at different spatio-temporal scales. Otolith geochemistry provides information over an ecological time frame and on a fine spatial scale, while microsatellite DNA markers report on gene flow over evolutionary time scales and therefore act on a broader spatio-temporal resolution. Thus, this study confirmed the value of otolith geochemistry to complement the assessment of early life stage dispersal in populations with high gene flow and low genetic divergence.

Transgenerational effects of anadromy on juvenile growth traits in an introduced population of rainbow trout (Oncorhynchus mykiss)

We determined whether the propensity for anadromy was related to maternal phenotype in a population of partially anadromous rainbow trout (Oncorhynchus mykiss). We identified the maternal phenotype (anadromous versus resident) of wild juveniles from two successive cohorts using stable isotope analysis (δ15N) of muscle tissue and (or) strontium to calcium ratios in the otolith core. We also tested the hypothesis that juvenile size and growth are related to maternal migratory history. For both cohorts, juvenile size at capture and growth, as determined using otolith and scale structural analyses, were strongly related to maternal migratory history. Offspring of anadromous mothers were larger and grew faster than resident offspring. Back-calculated length at age 1 of anadromous and resident adults provided support for a positive association between body size and anadromy, indicating that larger offspring are more prone to displaying anadromy. We conclude that maternal anadromy, which influences adult size and egg quality, affects the propensity of progeny to migrate, thus perpetuating the anadromous tactic across generations and influencing the establishment and persistence of anadromy.

Low interbasin connectivity in a facultatively diadromous fish: evidence from genetics and otolith chemistry

Southern smelts (Retropinna spp.) in coastal rivers of Australia are facultatively diadromous, with populations potentially containing individuals with diadromous or wholly freshwater life histories. The presence of diadromous individuals is expected to reduce genetic structuring between river basins due to larval dispersal via the sea. We use otolith chemistry to distinguish between diadromous and nondiadromous life histories and population genetics to examine interbasin connectivity resulting from diadromy. Otolith strontium isotope ((87) Sr:(86) Sr) transects identified three main life history patterns: amphidromy, freshwater residency and estuarine/marine residency. Despite the potential for interbasin connectivity via larval mixing in the marine environment, we found unprecedented levels of genetic structure for an amphidromous species. Strong hierarchical structure along putative taxonomic boundaries was detected, along with highly structured populations within groups using microsatellites (FST =0.046-0.181), and mtDNA (ΦST =0.498-0.816). The presence of strong genetic subdivision, despite the fact that many individuals reside in saline water during their early life history, appears incongruous. However, analysis of multielemental signatures in the otolith cores of diadromous fish revealed strong discrimination between river basins, suggesting that diadromous fish spend their early lives within chemically distinct estuaries rather than the more homogenous marine environment, thus avoiding dispersal and maintaining genetic structure.

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.

Evidence for meta-population structure of Sardina pilchardus in the Atlantic Iberian waters from otolith elemental signatures of a strong cohort

The European sardine is one of the most important clupeoid fish captured by the Portuguese and Spanish purse-seine fleets in the Atlantic waters of the Iberian Peninsula. Despite the importance of this species, and substantial decrease in catches over the last two decades, knowledge about sardine movements and connectivity between their juvenile recruitment areas and the adult fishing grounds is scarce. In this study otolith elemental fingerprints (both whole and core) of juvenile recruits (ages 0+ and 1+) and adults (age 3+) collected along the Portuguese coast (North, Central and South regions) were used to demonstrate regional population structure. The whole and otolith core analyses represented the entire life-history prior to capture and the larval/recruitment phases (first 2 months of life), respectively. Molar ratios of Sr/Ca, Mg/Ca, Mn/Ca and Ba/Ca provided location-specific elemental signatures, although otolith elemental compositions were not temporally stable within a year-class, particularly for Mg/Ca and Ba/Ca. Classification accuracy rates obtained from quadratic discriminant function analysis of core and whole otolith chemistry data of adults were relatively high (63% and 79% mean accuracy percentages, respectively). The results support the hypothesis of meta-population structure of the sardine stock in the Atlanto-Iberian waters. However, because there was evidence of some mixing among adult aggregations, and adult aggregations were largely derived from a common northern juvenile recruitment area they cannot be classified as entirely separate stock units for fisheries management purposes. The maximum likelihood analysis suggested for the 2004 cohort that replenishment of adult populations of sardine along the Portuguese coast was mostly derived from a single northern recruitment area.

Insights into mixing and movement of South Pacific albacore Thunnus alalunga derived from trace elements in otoliths

Information on the movement and stock structure of commercially important tunas underpins the effective management of exploited populations. In the case of the South Pacific albacore (Thunnus alalunga) stock, longstanding questions remain regarding the degree of connectivity among larval pools, the migration routes of juveniles and adults and the biophysical factors influencing these processes. We measured trace elements (Li, Mg, Mn, Cu, Sr, Ba, Pb, Ca) in albacore otoliths collected across a broad geographical range in the South Pacific Ocean to address these knowledge gaps. Capture locations in French Polynesia, New Caledonia and New Zealand were discriminated with high accuracy (overall 85% of individuals correctly classified) based on analyses at the otolith edge (reflecting the final <1 month of life) using LA-ICPMS. Spatial comparisons of otolith core chemistry (reflecting the first ∼2 weeks of life post-hatch) from the 2005/06 cohort suggest some mixing of larval pools for fish sampled from New Caledonia and New Zealand, whereas French Polynesian fish may have originated from a chemically and/or geographically distinct larval source. Annual and/or sub-annual cycles in Sr:Ca and Ba:Ca were evident along ablation transects encompassing the full life history of individuals. These patterns may reflect seasonal north-south movements across ocean fronts; however, the vertical behaviours of albacore and the lack of opportunities for controlled experiments on temperature effects and time-lags in elemental incorporation complicates environmental reconstructions based on trace element data alone. Expanding the present analysis across multiple years and regions, and integrating data from several sources (e.g. commercial catch data, tag returns, otolith δ13C and δ18O, ocean circulation models) could help clarify the linkages between environmental factors and mixing and movement patterns in albacore.

Using Seasonal Variation in Otolith Microchemical Composition to Indicate Largemouth Bass and Southern Flounder Residency Patterns across an Estuarine Salinity Gradient

AbstractEstuaries are transitional zones where salinity largely controls the distribution of both freshwater and marine species. However, the degree to which freshwater and marine species use these variable habitats as year‐round residents or transient migrants is largely unknown. We used otolith strontium: calcium ratios (Sr: Ca) as a marker for salinity exposure to reconstruct lifetime salinity exposure plots for individual Largemouth Bass Micropterus salmoides and Southern Flounder Paralichthys lethostigma collected across a seasonally variable estuarine salinity gradient. Initially, we determined that a definable relationship existed between salinity, water chemistry, and otolith Sr: Ca for both species. We then used otolith Sr: Ca profiles to indicate lifetime salinity exposure and subsequently classified each fish as either a freshwater resident, transient (combination of freshwater and estuarine signals in otolith), or estuarine resident. For Southern Flounder we also used Sr: Ca profiles from the otolith core to classify each individual as having either a freshwater core or estuarine or marine core. For Largemouth Bass, most (88%) individuals in the lower estuary were estuarine residents, whereas most (77%) individuals in the upper estuary were freshwater residents. These data support the hypothesis that adult Largemouth Bass in lower portions of the estuary do not migrate to avoid salinity, but rather remain in lower portions of the estuary throughout life. For Southern Flounder, 45% of individuals had estuarine or marine core Sr: Ca signals, while 55% had freshwater Sr: Ca core signals. Combining core and residency classification patterns revealed that three patterns described 95% of the Southern Flounder collected: (1) freshwater core and freshwater resident (16%), (2) freshwater core and estuarine resident (37%), and (3) estuarine or marine core and estuarine resident (42%). These data demonstrate that both tidal freshwater and low‐salinity estuarine habitats are important nursery areas for Southern Flounder.

Population Structure of a Neotropical Migratory Fish: Contrasting Perspectives from Genetics and Otolith Microchemistry

AbstractDeveloping conservation strategies for migratory fishes requires an understanding of connectivity among populations. Neotropical rivers contain diverse and economically important assemblages of migratory fishes, but little is known about the population biology of most species. We examined the population structure of Prochilodus mariae, an abundant migratory fish species found in Venezuelan rivers that plays essential roles in both regional fisheries and ecosystem dynamics. By coupling otolith microchemistry and microsatellite genetic analyses, we were able to evaluate both natal origins of individual fish and genetic structure on a regional level. The chemistry of otolith cores inferred separate breeding grounds for four of six populations, with 75–85% of individuals from each river sharing a natal signature that is distinct from the other populations. In contrast, we detected no genetic structure, indicating that gene flow among these rivers prevents population differentiation. These disparate inferences underscore the complexity of conserving migratory species; otolith data suggest that ensuring fishery sustainability requires recognizing distinct breeding stocks, while gene flow reflects the importance of connectivity across the broader river network on an evolutionary time scale. We conclude that multiple methodological approaches may often be necessary to fully understand the spatial ecology and management needs of migratory fishes and, therefore, also influence local management practices.

Nursery sources and cohort strength of young‐of‐the‐year common carp (Cyprinus carpio) under differing flow regimes in a regulated floodplain river

AbstractOtolith chemistry and standardised catch data were used to identify key nursery sources for common carp (Cyprinus carpio) in the mid‐Murray River region, south‐east Australia, and to quantify annual young‐of‐the‐year (YOY) cohort strength under varying hydrological conditions across 4 years (2005/2006–2008/2009). Analysis of trace element concentrations (i.e., Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) near the otolith core of post‐larval carp (&lt; 1 month of age) resulted in moderate to very high spatial discrimination of nursery sources within years. Otolith elemental concentrations were not stable across years at all nurseries, necessitating resampling of post‐larvae each spawning season to characterise nursery sources. Annual electrofishing surveys detected a strong cohort of YOY carp (4–6 months of age) in 2006, following a period of natural and artificially augmented high flows in the Murray River main channel and tributaries and extended floodplain inundation in the Barmah‐Millewa Forest (BMF). Few YOY were captured in 2007, 2008 or 2009 – years characterised by low flow conditions in the region and no floodplain inundation in BMF. Maximum likelihood estimation of nursery origins indicated that the BMF was the primary source for YOY fish captured from main channel (Torrumbarry Weir) and tributary (lower Goulburn River) sites in 2006, with the lower Goulburn River making a smaller contribution to the YOY cohort. These results demonstrate the importance of hydrology in determining spatial and temporal patterns of carp recruitment in regulated rivers and highlight the need to consider such factors when attempting to optimise ecological outcomes from environmental flow delivery.

Transgenerational marking of brown troutSalmo trutta f.f.,using an84Sr spike

The maternal transmission of a naturally occurring and non-toxic minor isotope of strontium (84Sr) to the central otolith region of the progeny of a typical European freshwater fish species, brown trout, Salmo trutta f.f. L. was accomplished. The focus was to apply minimum doses of a non-toxic solution at physiological salt concentration to minimise potential adverse effects on the fish. Female spawners were intraperitoneally injected with doses of 12.5 and 30.2 μg 84Sr kg−1 fish. Eggs were stripped and the resulting progeny were reared in a hatchery for about 1 year before sampling the otoliths. Strontium isotope ratios in the otoliths of the offspring were measured by cross-sectional line scans using laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Otolith cores of marked juveniles showed a significantly elevated 84Sr/86Sr ratio compared with control fish and to otolith regions created after hatching. Mass marking of cohorts of progeny from individually spiked brown trout with Sr isotopes is therefore possible for dispersal and migration studies without the necessity of handling eggs and applying other larval marking methods like immersion.

Single population and common natal origin for Adriatic Scomber scombrus stocks: evidence from an integrated approach

AbstractPapetti, C., Di Franco, A., Zane, L., Guidetti, P., De Simone, V., Spizzotin, M., Zorica, B., Čikeš Keč, V. and Mazzoldi, C. 2013. Single population and common natal origin for Adriatic Scomber scombrus stocks: evidence from an integrated approach – ICES Journal of Marine Science, 70: 387–398. In order to implement proper fishery management strategies aimed at avoiding stock declines, information about connectivity among stocks and populations is critically required. In this perspective, the present study investigated population structure of the Atlantic mackerel Scomber scombrus in the northern–central Adriatic Sea by integrating multiple approaches (analysis of fisheries data, population genetics, and otolith chemistry). Monthly data of fishery landings indicate a latitudinal trend along the western Adriatic coast, with Atlantic mackerel disappearing from the northern waters in winter, corresponding to the reproductive season. Population genetic analyses by genotyping of eight microsatellites clearly point to the presence of a single panmictic population in the northern–central Adriatic Sea. Otolith cores of samples from the northern–central Adriatic were chemically homogeneous, suggesting a common spawning ground. These results strongly suggest that Atlantic mackerel perform an autumn–winter migration in the northern–central Adriatic Sea, from the northern to the central sector, to reach a single spawning ground, and that a single population is present in this area. Considering that S. scombrus has shown a marked decline in the last 40 years in the Adriatic, this study highlights a potential high vulnerability to collapse by overfishing for the Atlantic mackerel stocks in this geographic area.

The use of otolith microchemistry to investigate spawning patterns of European anchovy: A case study in the eastern Ligurian Sea (NW Mediterranean)

Spawning patterns of European anchovy were investigated in the eastern Ligurian Sea (NW Mediterranean) by using otolith microchemistry. Otolith cores of 1-year-old anchovies showed a homogeneous chemical composition, while those of 2 years old anchovies separated into five different groups. This suggests the occurrence of single vs. multiple spawning areas related to the two annual cohorts. Patterns of core microchemistry of 2 years old anchovies support the hypothesis of different spawning areas, possibly characterised by different salinity regimes, in the eastern Ligurian Sea (e.g. typical marine habitats like shelf edges vs. lower salinity locations like river mouths). Results suggest that: (1) stocks of European anchovy could depend on different spawning areas characterised by different environmental conditions; (2) patterns may change from year to year, and this could affect the annual risk of stock fluctuations or collapse; and (3) the use of otolith microchemistry is a promising approach for investigating spawning patterns of European anchovy.