Otolith δ18O Research Articles

Hydrodynamic and isotopic niche differentiation between juveniles of two sympatric cryptic bonefishes, Albula vulpes and Albula goreensis

We employed numerical wave models, GIS, and stable isotope analyses of otolith material to identify interspecific differences in habitat and resource use among juveniles of two sympatric and morphologically indistinct bonefishes, A. goreensis and A. vulpes in littoral zones of The Bahamas. Both species occurred in similar water temperatures; however, A. goreensis juveniles occupied habitats characterized by greater wave-driven flow velocities and closer proximity to coral reefs than A. vulpes. Likewise, A. goreensis was present across a broader range of flow environments and sampling stations than A. vulpes, which was typically confined to sheltered, low-flow habitats. The results of stable isotope analyses were consistent with the species’ relationships with environmental parameters, providing support for differential habitat and/or resource utilization. Otolith δ18O did not differ significantly between species, suggesting they experience comparable thermal regimes. However, δ13C varied substantially, with the otoliths of A. goreensis depleted in 13C relative to A. vulpes by approximately 1‰, potentially signifying a greater reliance on pelagic carbon sources by the former, in agreement with observed distinctions in habitat use. In linear models, otolith δ13C was negatively correlated with ambient flow velocity and positively related to distance from coral reef habitats, and these relationships did not vary across species. After accounting for the effects of these variables, species-specific differences in otolith δ13C remained, indicating that other unknown factors contributed to the observed disparities. Collectively, our findings suggest that niche partitioning between A. goreensis and A. vulpes is likely mediated by their differential abilities to compete across various flow environments, likely as a result of divergent behavioral and/or physiological adaptation.

Temperature and depth distribution of Japanese eel eggs estimated using otolith oxygen stable isotopes

Oxygen isotope ratios of the core region of otoliths were examined in Anguilla japonica glass eels collected from two rivers in Japan to verify the possible temperature and depth layer experienced by these eels when they were at the egg stage in their spawning area. To determine the relationship between otolith δ18O values and water temperature, the otoliths of glass eels reared under four different temperatures (15, 20, 25 and 30 °C) were analyzed. The otolith δ18O values showed an inverse relationship to ambient water temperature. Linear regression of the fractionation between otolith oxygen isotopic ratio from the δ18O of seawater and water temperature produced a precisely determined relationship from 15 to 30 °C: δ18Ootolith,PDB − δ18Oseawater,SMOW = −0.153 × T (°C) + 1.418. The δ18O core,PDB values of the otolith core region of the glass eels from the two locations were −2.53 ± 0.12 and −2.59 ± 0.07 respectively, and could be converted to water temperatures of 26.3 ± 0.8 °C and 26.7 ± 0.4 °C, respectively, using the equation and assuming a seawater δ18Oseawater,SMOW = 0.06‰. The water depth corresponding to these temperatures is ∼150 m in the water column in the spawning area of Japanese eels, which corresponds to the upper-most part of the thermocline and chlorophyll maximum in the vertical hydrographic profile. These results were consistent with the field studies that egg development after the beginning of otolith formation and hatching occurs around the upper-most part of thermocline, suggesting that stable isotope micro-analysis is a powerful method to extrapolate unknown spawning ecology of fishes.

Validation of otolith δ18O values as effective natural tags for shelf-scale geolocation of migrating fish

The oxygen isotopic ratio of fish otoliths is increasingly used as a `natural tag' to assess provenance in migratory species, with the assumption that variations in delta O-18 values closely reflect individual ambient experience of temperature and/or salinity. We employed archival tag data and otoliths collected from a shelf-scale study of the spatial dynamics of North Sea plaice Pleuronectes platessa L., to examine the limits of otolith delta O-18-based geolocation of fish during their annual migrations. Detailed intra-annual otolith delta O-18 measurements for 1997-1999 from individuals of 3 distinct sub-stocks with different spawning locations were compared with delta O-18 values predicted at the monthly, seasonal and annual scales, using predicted sub-stock specific temperatures and salinities over the same years. Spatio-temporal variation in expected delta O-18 values (-0.23 to 2.94%) mainly reflected variation in temperature, and among-zone discrimination potential using otolith delta O-18 varied greatly by temporal scale and by time of year. Measured otolith delta O-18 values (-0.71 to 3.09%) largely mirrored seasonally predicted values, but occasionally fell outside expected delta O-18 ranges. Where mismatches were observed, differences among sub-stocks were consistently greater than predicted, suggesting that in plaice, differential sub-stock growth rates and physiological effects during oxygen fractionation enhance geolocation potential using otolith delta O-18. Comparing intra-annual delta O-18 values over several consecutive years for individuals with contrasted migratory patterns corroborated a high degree of feeding and spawning site fidelity irrespective of the sub-stock. Informed interpretation of otolith delta O-18 values can therefore provide relatively detailed fisheries-relevant data not readily obtained by conventional means.

Hydroclimatic conditions and fishing practices at Late Paleolithic Makhadma 4 (Egypt) inferred from stable isotope analysis of otoliths

The late Paleolithic site of Makhadma 4, located along the Nile River in Upper Egypt, yielded an important ichthyofauna characterized by a very high proportion of tilapia (Oreochromis niloticus). We used isotopic analysis (δ18O) of well-preserved otoliths (“ear stones”) of tilapia to reconstruct the former hydrological conditions, as well as the fishing strategies of the site's inhabitants. Otoliths from two modern fish captured in the Nile River near Esna were also examined to test how accurately tilapia otoliths reflect their ambient environment. All otoliths were sequentially micromilled to recover high resolution isotopic profiles along the main growth axis. Comparison of the modern otolith profiles with environmental data shows that tilapia δ18O values record seasonal variations of the modern Nile hydroclimate but that their values are offset. The archaeological otoliths record very large intra-individual cyclical variations in δ18O values, with relatively consistent amplitude, as well as very high seasonal maximum values (up to +8.3‰), compared with the modern otoliths. The hydrological regime of the water body in which the archaeological fish lived was characterized by a reduced Nile water inflow that could not negate the effect of local evaporation during spring. The reconstructed hydrological conditions are in accordance with a new model of Nilotic behavior that assumes the creation of lakes by damming of the Nile as a result of a high eolian activity during hyper-arid periods of the Late Pleistocene. Although large seasonal evaporation may have resulted in a severe seasonal reduction in the lake's volume and extent, the lake was, nevertheless, maintained for several years. Cyclic variations in otolith δ18O values permit reconstruction of the period of the hydrological cycle during which the fish were captured. Fishing of young individuals occurred mostly after the maximum input of inflow water from the Nile, when evaporitic conditions were at their lowest, i.e. during fall.

Movement, connectivity and population structure of the intertidal fish Lipophrys pholis as revealed by otolith oxygen and carbon stable isotopes

ABSTRACTThe shanny Lipophrys pholis is an intertidal fish commonly found in Portuguese coastal waters. Spawning takes place from early autumn to mid spring, after which demersal eggs hatch and larvae disperse along the coast. Two to three months later, young juveniles return to the tide pools to settle. However, information on fish movement, habitat connectivity and population structure is scarce for this species. One hundred and twenty early juveniles (16–35 mm) were collected in April 2014 from six rocky beaches along the western and south Portuguese coasts (Agudela, Cabo do Mundo, Boa Nova, Peniche, Sines and Olhos de Água). δ18O and δ13C were determined by isotope-ratio mass spectrometry. Data were analysed to determine whether isotopic signatures could be used to assess the degree of separation between individuals collected from different locations. Mean δ13C and δ18O values ranged from −0.02‰ to 1.14‰ and −7.77‰ to −6.62‰, respectively. Both seawater temperature and salinity caused differences in otolith δ18O among the four main sampling areas. The variation among areas in δ13C was most likely related to slight differences in the diet, growth and metabolism of fish. The distinct isotopic signatures, at least for the northern and central areas, suggested low levels of connectivity across large spatial scales during the juvenile stage. Furthermore, similar isotopic signatures within the same area indicated some degree of larval oceanic retention at short spatial scales. This study suggests that stable isotope records in otoliths could provide information about the home residency, movements and habitat connectivity of intertidal fishes.

Temperature dependence of δ18O in otolith of juvenile Japanese sardine: Laboratory rearing experiment with micro-scale analysis

We evaluated the use of the stable oxygen isotope (δ18O) in the otolith as a proxy for the temperature history of Japanese sardine Sardinops melanostictus individuals. Japanese sardine juveniles were reared in three different water temperatures over the course of a month. Otolith δ18O (δotolith) was analyzed by extracting the portions formed during the rearing period using a micromill. δ18O of the rearing water (δwater) was also analyzed. A linear relationship between otolith δ18O and ambient water temperature was identified as follows: δotolith−δwater=−0.18*T+2.69 (r2=0.91, p<0.01). This equation is different from that proposed for inorganic aragonite or other Sardinops spp., with resulting application to wild Japanese sardine captured in the Pacific Ocean showing that it estimates a more realistic in situ temperature. Our findings suggest that the Japanese sardine-specific isotopic fractionation equation should be used when interpreting otolith δ18O of the Japanese sardine, and the methods presented here could also be useful to understand the temperature history of other fish species.

Stable oxygen isotope analysis confirms natural recruitment of Lake Michigan-origin lake trout (Salvelinus namaycush) to the adult life stage

Following Lake Michigan’s lake trout (Salvelinus namaycush) population collapse in the mid-1900s, fisheries managers began stocking marked juveniles in 1965 in an effort to restore the population. However, there has been little evidence of naturally produced juveniles recruiting to the adult life stage, but recent collections of unmarked adults in Illinois waters of Lake Michigan suggest otherwise. Here, we provide stable isotopic evidence that confirms recruitment to the adult life stage is occurring in southwestern Lake Michigan. Our results indicate substantial differences in δ18O and δ13C values between hatchery-reared juvenile lake trout and the putative wild adults, effectively eliminating the possibility that unmarked adults were erroneously miss-clipped as hatchery-reared juveniles. We also found significant differences in δ18O signatures between lakes Michigan and Huron and successfully classified over 90% of putative wild adult lake trout as Lake Michigan-origin. Trace element analyses were less effective in distinguishing fish from lakes Huron and Michigan than otolith δ18O. These findings indicate that Lake Michigan’s lake trout restoration program is at least partially effective. Further investigation is required to determine what factors may be facilitating natural reproduction and adult recruitment in southwestern Lake Michigan.

Age validation of Pacific cod (Gadus macrocephalus) using high-resolution stable oxygen isotope (δ 18O) chronologies in otoliths

The Pacific cod (Gadus macrocephalus) fishery in Alaska is large and economically important; in 2014 its ex-vessel value was $203.8 million. Management relies on integrated assessments that employ survey abundance trends and fishery catches as well as age compositions from assessment surveys. However, Pacific cod age determination based on otolith growth zone counts has historically been difficult, adding to the uncertainty in biological reference points. Further, a mismatch exists where modes in fish length frequencies are larger than mean lengths at otolith-based age. To address ageing inaccuracy, we conducted an age validation study using stable oxygen isotopes (δ18O). This approach is based upon the principle that variability in marine carbonate δ18O is inversely related to water temperature, which we independently verified in Pacific cod otoliths (r2=0.74). We sequentially microsampled 40 Pacific cod otoliths from the core to the margin and measured the aragonite δ18O by isotope ratio mass spectrometry. This provided a detailed δ18O life history chronology for each specimen. First, we identified seasonal variation (the cyclical pattern of otolith δ18O values) and determined whether the number of δ18O maxima, considered to represent “true fish age,” was consistent with the age estimated from growth zone counts. Second, we estimated the probability of bias in the ages determined from growth zone counts. Overall, the probability of assigning an age (based on the number of counted growth zones) equal to the true age (number of δ18O maxima) was approximately 61%. However, the probabilities of over- or underestimating the age by 1 year were 25% and 13%, respectively. The probability of over- or underestimating true age by 2 or more years was very low (<2%). The probability of age misclassification was used to correct bias in mean length at age.

Natal origin and population connectivity of bigeye and yellowfin tuna in the Pacific Ocean

AbstractNatural chemical markers (stable isotopes and trace elements) in otoliths of bigeye tuna (Thunnus obesus) and yellowfin tuna (T. albacares) were used to investigate their origin and spatial histories in the western and central Pacific Ocean (WCPO). Otolith chemistry of young‐of‐the‐year (YOY) T. obesus and T. albacares from four regions in the WCPO was first determined and used to establish baseline chemical signatures for each region. Spatial variation in stable isotope ratios of YOY T. obesus and T. albacares was detected, with the most noticeable difference being depleted otolith δ18O values for both species from the far west equatorial and west equatorial regions relative to the central equatorial and Hawaii regions. Elemental ratios in otoliths were also quantified for YOY T. obesus and T. albacares collected in 2008, and several showed promise for distinguishing YOY T. obesus (Mg:Ca, Mn:Ca, and Ba:Ca) and T. albacares (Li:Ca and Sr:Ca). The natal origin of age‐1 to age‐2+ T. obesus and T. albacares was then determined for two regions of the WCPO, and mixed‐stock analysis indicated that T. obesus and T. albacares in our west equatorial sample were almost entirely from local production, with a minor contribution from central equatorial waters. Similarly, T. albacares collected in Hawaii were exclusively from local sources; however, a large fraction of T. obesus in Hawaii were classified to the central equatorial region, suggesting that the movement of migrants from outside production zones (i.e., south of Hawaii) are important to Hawaii's domestic fishery.

The imprint of anthropogenic CO2 emissions on Atlantic bluefin tuna otoliths

Otoliths of Atlantic bluefin tuna (Thunnus thynnus) collected from the Mediterranean Sea and North Atlantic Ocean were analyzed to evaluate changes in the seawater isotopic composition over time. We report an annual otolith δ13C record that documents the magnitude of the δ13C depletion in the Mediterranean Sea between 1989 and 2010. Atlantic bluefin tuna in our sample (n=632) ranged from 1 to 22years, and otolith material corresponding to the first year of life (back-calculated birth year) was used to reconstruct seawater isotopic composition. Otolith δ18O remained relatively stable between 1989 and 2010, whereas a statistically significant decrease in δ13C was detected across the time interval investigated, with a rate of decline of 0.05‰yr−1 (−0.94‰ depletion throughout the recorded period). The depletion in otolith δ13C over time was associated with the oceanic uptake of anthropogenically derived CO2.

Fossil otoliths, from the Gulf of Kutch, Western India, as a paleo-archive for the mid- to late-Holocene environment

Abundantly available catfish otoliths excavated from the ruins of the Indus civilization, in the coastal regions of Western India, are expected to preserve a detailed paleoclimatological record of the mid- to late-Holocene. In this study, we analyzed saltwater catfish otoliths recovered from the Gulf of Kutch, Western India. Stable isotopes (oxygen and carbon) and elemental ratios (Ba/Ca and Sr/Ca) were measured by isotope ratio mass spectrometry (IRMS) and high-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), respectively. Oxygen isotopes of a modern otolith (δ18O) indicates the interval when the catfish would have dwelt in the sea and appear to record seasonal temperature variability with a precision of 1 C°. Calculated temperatures from δ18O of a fossil otolith dated to 4.3 cal ka BP indicates that the minimum temperature in winter was ∼2.5 C° lower than that of the present. Although comparison to alkenone results from the northwestern Arabian Sea indicates potential temperature underestimation, further measurements of modern and fossil samples would lead to more precise reconstruction of temperature history during mid- to late- Holocene.

Natural tracers reveal population structure of albacore (Thunnus alalunga) in the eastern North Pacific

Abstract Chemical signatures in otoliths and muscle tissue of albacore (Thunnus alalunga) from two regions of the North Pacific were characterized to examine population structure. Regions included northern (offshore northwest United States and Canada, &amp;gt;40°N) and southern (offshore southwest United States and Mexico, &amp;lt;40°N) areas where albacore have demonstrated region-specific differences in movement and size at age. Juvenile (ages 1–4 years) albacore were collected from each region through recreational and commercial fisheries over a 5-year period (2009–2013). Three different otolith chemistry assays were performed, including whole otoliths (proxy for lifetime signature), otolith edge (recent signature), and otolith core (nursery signature). Whole otolith δ13C and δ18O showed similar δ13C and enriched δ18O (∼0.5‰) values for juvenile albacore collected in the southern region; classification success to the two regions ranged from 78 to 91% during each year (similar age classes). Otolith edge δ13C and δ18O showed similar results as whole otoliths, but no regional differences were found for otolith core chemistry. Incorporation of trace element concentrations from whole otoliths improved discrimination between regions to 90–100% classification success during each year of the study, with significantly higher Ba:Ca, Mg:Ca, and Mn:Ca in albacore otoliths from the northern region. Albacore from the southern region also displayed enriched muscle δ13C (∼1.21‰) and δ15N (∼0.85‰) values relative to fish of the same size range from the northern region. Discrimination success between the two regions across all years using muscle δ13C and δ15N was 91%. Ultimately, results from this study suggest regional separation of albacore in the eastern North Pacific warranting region-specific vital rates in future modelling efforts.

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

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

Listening in on the past: what can otolith δ18O values really tell us about the environmental history of fishes?

Oxygen isotope ratios from fish otoliths are used to discriminate marine stocks and reconstruct past climate, assuming that variations in otolith δ18O values closely reflect differences in temperature history of fish when accounting for salinity induced variability in water δ18O. To investigate this, we exploited the environmental and migratory data gathered from a decade using archival tags to study the behaviour of adult plaice (Pleuronectes platessa L.) in the North Sea. Based on the tag-derived monthly distributions of the fish and corresponding temperature and salinity estimates modelled across three consecutive years, we first predicted annual otolith δ18O values for three geographically discrete offshore sub-stocks, using three alternative plausible scenarios for otolith growth. Comparison of predicted vs. measured annual δ18O values demonstrated >96% correct prediction of sub-stock membership, irrespective of the otolith growth scenario. Pronounced inter-stock differences in δ18O values, notably in summer, provide a robust marker for reconstructing broad-scale plaice distribution in the North Sea. However, although largely congruent, measured and predicted annual δ18O values of did not fully match. Small, but consistent, offsets were also observed between individual high-resolution otolith δ18O values measured during tag recording time and corresponding δ18O predictions using concomitant tag-recorded temperatures and location-specific salinity estimates. The nature of the shifts differed among sub-stocks, suggesting specific vital effects linked to variation in physiological response to temperature. Therefore, although otolith δ18O in free-ranging fish largely reflects environmental temperature and salinity, we counsel prudence when interpreting otolith δ18O data for stock discrimination or temperature reconstruction until the mechanisms underpinning otolith δ18O signature acquisition, and associated variation, are clarified.

Origin of Atlantic bluefin tuna (Thunnus thynnus) in the Bay of Biscay

Abstract We used carbon and oxygen isotope ratios (δ13C and δ18O) in otoliths as a tool for identifying the nursery origin of Atlantic bluefin tuna (Thunnus thynnus) caught in the eastern North Atlantic Ocean (Bay of Biscay). Juvenile and adult bluefin tuna were collected over three consecutive years (2009—2011) using the regional bait boat fleet. Otolith δ13C and δ18O values of bluefin tuna were measured by mass spectrometry, and values were compared with a reference sample of yearling bluefin tuna from eastern (Mediterranean Sea) and western (Gulf of Mexico) nurseries to determine nursery origin. Maximum likelihood estimates based on otolith δ13C and δ18O values indicated that the overall contribution of western migrants to the Bay of Biscay fishery was &amp;lt;1% and varied over the years assessed. A small number of potential western migrants (2.7%) was detected in 2009, and most of these fish appear members of the abundant 2002 and 2003 year classes. In contrast, the Bay of Biscay fishery was composed exclusively (100%) of eastern origin bluefin tuna in 2010 and 2011, suggesting that this fishery is supported almost exclusively by the eastern spawning area but transatlantic western population may contribute to this fishery in a few years.

Otolith δ18O of Pacific bluefin tuna Thunnus orientalis as an indicator of ambient water temperature

To detect the relationship between ambient temperature and otolith stable oxygen isotope (δ18O), Pacific bluefin tuna Thunnus orientalis larvae were reared at 6 different temperatures (between 23 and 28°C at 35 psu; mean δ18O value, +0.31‰) for 2 to 8 d after hatching. For the first time, otolith δ18O was measured using a continuous-flow isotope ratio mass spectrometry system. The linear relationship between otolith δ18O and temperature was determined as follows: δotolith – δwater = –2.70 (T °C) + 5.193 (r2 = 0.806, p < 0.01). The temperature-dependent fractionation of otolith δ18O was close to that reported for inorganic aragonite, indicating that the vital effects of isotopes are small. The otolith δ18O values of juveniles reared at a mean of 26.5°C were not significantly different from those of larvae reared at 26°C. With regard to the 2 pairs of otoliths within an individual fish, the differences in δ18O values between the left and right otoliths were subtle. The otolith δ18O values of larvae reared in δ18O-depleted seawater (24 to 26°C at 32 psu; δ18O, −0.17‰) were lower, but were not significantly different from those of larvae reared in +0.31‰ water, which is the δ18O value of the water in which the spawners were raised. These results suggest that the sample size for the 32 psu experiment was too small and that the rearing durations were too short to affect the otoliths completely. Our results demonstrate that otolith δ18O values of Pacific bluefin tuna larvae can provide precise and accurate estimates of their ambient temperature experience.

Ontogenetic vertical migration of grenadiers revealed by otolith microstructures and stable isotopic composition

Otolith δ18O and δ13C of six species grenadiers were analyzed to reconstruct the historical residing depths and metabolic activity. During the larval to juvenile stage, Spicomacrurus kuronumai, Hymenocephalus lethonemus, and Bathygadus nipponicus gradually migrated from the mixed layer to the thermocline downward over a vertical distance >140m after which they moved within a narrow vertical range for the remaining life. The downward migration distance was less than 65m during the larval stage of Hymenocephalus sp. and Coryphaenoides acrolepis, which showed a second descent period from the thermocline to deeper water as juveniles. Coryphaenoides marginatus stayed at the lower thermocline during the larval stage and the juveniles migrated downward in a relative short distance around 100m and settled in deeper water (>600m), followed by irregular movements over a vertical range of about 200m during juvenile and adult stages. The otolith δ13C profile suggested that fishes (S. kuronumai, H. lethonemus, and B. nipponicus) with a longer migration distance had a higher metabolic rate in their early life-history stages than in the later stages. However, the metabolic rate did not vary for those fishes (H. sp., C. acrolepis and C. marginatus) living within a narrow vertical range during their larval to adult stages. The otolith microchemistry suggested that ontogenetic downward migration was an important strategy for grenadiers linking the life stages between pelagic larvae and benthic settlement. Furthermore, the migration timing and distance for the pelagic larvae varied between species and habitats. S. kuronumai, H. lethonemus, H. sp. and C. acrolepis might metamorphose and settle at the same time, B. nipponicus metamorphosed during migration and C. marginatus migrated as juveniles.

Spatial and temporal trends in stable carbon and oxygen isotope ratios of juvenile winter flounder otoliths

Isotopic ratios of fish otoliths have been used in numerous studies as natural tags or markers to aid in the study of connectivity among fish populations. We investigated the use of spatial and temporal changes in the stable carbon and oxygen isotope ratios of otoliths to differentiate juvenile habitats of winter flounder (Pseudopleuronectes americanus). Young-of-the-year (YOY) juvenile winter flounder were collected annually over a three-year period from 18 stations along the coast of Rhode Island, USA. Sagittal otoliths were removed from fish and analyzed for stable carbon (13C/12C or δ13C) and oxygen (18O/16O or δ18O) isotope ratios using continuous flow isotope ratio mass spectrometry. Differences in isotope ratios were observed among stations and along salinity gradients in the Narragansett Bay estuary and an estuarine river system (Narrow River). Overall, the isotope ratio patterns observed among stations were consistent over the three sampling years; however, differences were noted in isotope ratios and the magnitude of the isotope ratio gradients among years. Significant positive correlations were noted between salinity and δ13C for two of the three years. For each of the three years sampled there was a highly significant positive correlation (2002, r = 0.93, P < 0.01; 2003, r = 0.85, P < 0.01; 2004, r = 0.97, P < 0.01) between δ18O and the salinity of the collection site. Also, there was a significant negative correlation between the number of months of above average river flow and δ18O for the three sampling years (r = 0.99, P < 0.05). These findings suggest that yearly changes in the volume of freshwater inputs to these estuarine habitats may be related to the differences observed in otolith δ18O isotope ratios. Because of these year-to-year differences, sampling of each cohort may be necessary in order to use this isotopic technique for winter flounder connectivity studies.

Fingerprints of lagoonal life: Migration of the marine flatfish Solea solea assessed by stable isotopes and otolith microchemistry

The commercially important marine flatfish common sole (Solea solea) facultatively uses NW Mediterranean lagoons as nurseries. To assess the imprint left by the lagoonal passage, muscle carbon (C) and nitrogen (N) isotope values of S. solea juveniles caught in Mauguio lagoon in spring (shortly after arrival from the sea) and in autumn (before the return to the sea) were compared with values of juveniles from adjacent coastal marine nurseries. In addition, in the lagoon, sole otolith stable isotope (C and oxygen (O)) and elemental (11 elements) composition in spring and autumn, and the stable isotope composition (C and N) of organic matter sources in autumn, were determined. Overall, our data indicate that a distinct lagoonal signature existed. Specifically, lagoon soles showed a strong enrichment in muscle tissue 15N (>6‰) compared to their coastal relatives, likely linked to sewage inputs (see below), and a depletion in 13C (1–2‰), indicative of higher importance of 13C depleted terrestrial POM in the lagoon compared to coastal nurseries. In addition, over the time spent in the lagoon, sole otolith δ13C and δ18O values and otolith elemental composition changed significantly. Analysis of the lagoon sole foodweb based on C and N isotopes placed sediment particulate organic matter (POM) at the base. Seagrasses, formerly common but in decline in Mauguio lagoon, played a minor role in the detritus cycle. The very strong 15N enrichment of the entire foodweb (+7 to +11‰) compared to little impacted lagoons and coastal areas testified of important human sewage inputs. Regarding the S. solea migration, the analysis of higher turnover and fast growth muscle tissue and metabolically inert and slower growth otoliths indicated that soles arrived at least several weeks prior to capture in spring, and that no migrations took place in summer. In the autumn, the high muscle δ15N value acquired in Mauguio lagoon would be a good marker of recent return to the sea, whereas altered otolith δ18O values and elemental ratios hold promise as long-term markers. The combination of several complementary tracers from muscle and otoliths may present the chance to distinguish between fish from specific lagoons and coastal nurseries in the future.

High-latitude climate variability and its effect on fisheries resources as revealed by fossil cod otoliths

Abstract Geffen, A. J., Høie, H., Folkvord, A., Hufthammer, A. K., Andersson, C., Ninnemann, U., Pedersen, R. B., and Nedreaas, K. 2011. High-latitude climate variability and its effect on fisheries resources as revealed by fossil cod otoliths. – ICES Journal of Marine Science, 68: 1081–1089. Cod (Gadus morhua) otoliths from archaeological sites in northern Norway were analysed to reconstruct the temperature regime and determine the age structure, growth, and population identity of the fish harvested. Otoliths were selected from late- and post-medieval sites (700–300 years ago) to evaluate historical changes in the geographic region that matches the present-day stocks of Northeast Arctic cod (NEAC) and Norwegian coastal cod (NCC). Seasonal temperature cycles were reconstructed from stable isotope (δ18O) measurements along transects representing fish ages 1.5–3 years old. Reconstructions of the size, age, and growth characteristics of individual fish were based on otolith growth increments. The geographical source and stock identity of the individuals were estimated based on otolith elemental composition and otolith growth features. Both NCC and NEAC fish were represented at Måsøy and Vanna. The results indicate that fishing at Vanna exploited NEAC during their spawning migration, compared with fishing at Måsøy, which was restricted to more coastal fish. Fish growth patterns appeared to be affected by changes in the temperature regimes as estimated from otolith δ18O and back-calculated fish length-at-age, with evident differences between pre- and post-1600 periods.