Otolith Growth Rates Research Articles

Freshwater fish otoliths record signals from both water and physiological processes: new insights from Sr/Ca and Ba/Ca ratios

Using strontium (Sr) and barium (Ba) in otoliths to determine natal origins and understand patterns of fish movements is based on the fundamental assumption that otoliths record water chemistry signals without any major alterations. Although prior studies highlighted that fish physiology can modify the water signal in otoliths, studies for freshwater fish are scarce. We exposed different groups of Atlantic salmon parr Salmo salar to different scenarios of ambient-level variations in Sr/Ca and Ba/Ca ratios and then combined otolith chemical profiles with environmental data (water chemistry and temperature), Fulton's index, and otolith growth rates to assess what factors explain/influence the elemental ratios of Sr and Ba in otoliths. Generalized additive mixed models (GAMMs) using water-based otolith composition, temperature, Fulton's index, and “individual” as explanatory variables allow to demonstrate that water chemistry alone cannot fully explain measured ratios in otoliths, except in scenarios involving significant changes in water chemistry. Other factors (physiological effects) should be accounted for reproducing short and minimal seasonal variations in water composition, considering that inter-individual variability contributes quite significantly in most scenarios.

Exploring European Eel Anguilla anguilla (L.) Habitat Differences Using Otolith Analysis in Central-Western Mediterranean Rivers and Coastal Lagoons from Sardinia

An otolith shape and morphometric analysis was performed on European eel (Anguilla anguilla) subpopulations from five rivers and three coastal lagoons of Sardinia (central-western Mediterranean) to assess the role of different habitats on otolith development. Sagittal otolith shape was described by 11 harmonics from elliptic Fourier descriptors. Comparisons among the harmonics were run through canonical discriminant analyses (CDAs). The CDA reclassification rate (75.7%) demonstrated a spatial environmental discrimination among local eel subpopulations of Sardinia. The Euclidean distance values demonstrated a dissimilarity between the river and lagoon groups. The form factor and roundness shape indices were significantly higher in the river group than in the lagoon group. The distances of the first three rings to the otolith core revealed site-specific otolith development. Moreover, the annual otolith growth rate was faster in the lagoon group than in the river group. The differences among the studied sites in terms of sagittal otolith shape could relate to changes in different local stocks potentially related to environmental peculiarities. Establishing a direct correlation between otolith morphology and environmental factors is challenging, and further studies are needed to investigate the relationship between habitat type/environmental variation and growth/body characteristics of eels. Nevertheless, the achieved results suggest that this method can be considered to be a valuable tool for studying the ontogeny of the European eel.

Variation in Juvenile Salmon Growth Opportunities Across a Shifting Habitat Mosaic

Historically, Chinook Salmon in the California Central Valley reared in the vast wetlands of the Sacramento–San Joaquin Delta. However, more than 95% of floodplain, riparian, and wetland habitats in the Delta have become degraded because of anthropogenic factors such as pollution, introduced species, water diversions, and levees. Despite pronounced habitat loss, previous work using otolith reconstructions has revealed that some juvenile salmon continue to successfully rear for extended periods in the Delta. However, the extent to which the Delta functions to promote salmon growth relative to other habitats remains unknown. In this study, we integrated otolith microstructure (daily increment count and width) and strontium isotope (87Sr/86Sr) records to fill this critical knowledge gap by comparing the growth of natural-origin fall-run Chinook Salmon from the American River that reared in the Delta with those that remained in their natal stream. Using generalized additive models, we compared daily otolith growth rates among rearing habitats (Delta vs. American River) and years (2014 to 2018), encompassing a range of hydrologic conditions. We found that juvenile Chinook Salmon grew faster in the Delta in some years (2016), but slower in the Delta during drought conditions (2014 to 2015). The habitat that featured faster growth rates varied within and among years, suggesting the importance of maintaining a habitat mosaic for juvenile salmonids, particularly in a dynamic environment such as the California Central Valley. Linking otolith chemistry with daily growth increments provides a valuable approach to explore the mechanisms governing interannual variability in growth across habitat types, and a useful tool to quantify the effects of large-scale restoration efforts on native fishes.

Variation in Juvenile Salmon Growth Opportunities Across a Shifting Habitat Mosaic

Historically, Chinook Salmon in the California Central Valley reared in the vast wetlands of the Sacramento–San Joaquin Delta. However, more than 95% of floodplain, riparian, and wetland habitats in the Delta have become degraded because of anthropogenic factors such as pollution, introduced species, water diversions, and levees. Despite pronounced habitat loss, previous work using otolith reconstructions has revealed that some juvenile salmon continue to successfully rear for extended periods in the Delta. However, the extent to which the Delta functions to promote salmon growth relative to other habitats remains unknown. In this study, we integrated otolith microstructure (daily increment count and width) and strontium isotope (87Sr/86Sr) records to fill this critical knowledge gap by comparing the growth of natural-origin fall-run Chinook Salmon from the American River that reared in the Delta with those that remained in their natal stream. Using generalized additive models, we compared daily otolith growth rates among rearing habitats (Delta vs. American River) and years (2014 to 2018), encompassing a range of hydrologic conditions. We found that juvenile Chinook Salmon grew faster in the Delta in some years (2016), but slower in the Delta during drought conditions (2014 to 2015). The habitat that featured faster growth rates varied within and among years, suggesting the importance of maintaining a habitat mosaic for juvenile salmonids, particularly in a dynamic environment such as the California Central Valley. Linking otolith chemistry with daily growth increments provides a valuable approach to explore the mechanisms governing interannual variability in growth across habitat types, and a useful tool to quantify the effects of large-scale restoration efforts on native fishes.

Age and growth of early-life-stage Atlantic tarpon (Megalops atlanticus) from the northcentral Gulf of Mexico.

Age and growth of early-life-stage Atlantic tarpon Megalops atlanticus collected from Mississippi coastal waters in the northcentral Gulf of Mexico (GOM) are described using otolith microstructure analysis. Tarpon leptocephali (n= 95, 16.0-27.8 mm standard length, LS ) collected from June throughOctober 2013-2018, ranged in age from 22 to 43 days (mean=30.9± 0.5 days). Leptocephalus somatic growth rates ranged 0.46-1.24 mm day-1 (mean=0.76 ± 0.02 mm day-1 ), and leptocephalus otolith growth rates ranged 1.78-3.97 μm day-1 (mean=2.58 ± 0.04 μm day-1 ). Growth rates were inversely correlated to leptocephalus age, indicating the shrinkage phase associated with leptocephalus metamorphosis. Juvenile tarpon (n= 358, 50-359 mm fork length, LF ) were collected from August through December 2007-2018. Juveniles exhibited a positive allometric relationship (adjusted R2 = 0.99, P< 0.001) between length and mass. The age of 100 juveniles (71-277 mm LF ) ranged from 76 to 174 days. Juvenile growth rate was estimated as 1.56 ± 0.11 mm day-1 . Significant (P< 0.001) linear relationships were found between juvenile age and otolith metrics, including otolith mass (R2 = 0.81) and radius (R2 = 0.68). Evaluation of the backcalculated hatch dates suggests that specimens in the collection hatched from late May through mid-September with slight peaks during July and August. A Rao's Spacing Test of Uniformity indicates the presence of significant lunar periodicity in leptocephalus hatch dates (n= 95, U= 250.1, P< 0.05), with 50% of the leptocephali hatched within 5 days (before or after) of the full moon. This study fills critical gaps in the scientific knowledge of tarpon and provides estimates of early-life-history metrics for an iconic game fish at the northernmost extent of its GOM range.

No evidence for reduced growth in resident fish species in the era of de-eutrophication in a coastal area in NW Europe

Coastal areas in north-western Europe have been influenced by elevated nutrient levels starting in the 1960s. Due to efficient measures, both nitrate and phosphate levels decreased since the mid-1980s. The co-occurring declines in nutrient loadings and fish productivity are often presumed to be causally linked. We investigated whether four resident fish species (twaite shad, bull-rout, thick-lipped grey mullet and eelpout), that spend the majority of their life in the vicinity of the coast, differed in growth between the historic eutrophication period compared to the recent lower nutrient-level period. Based on Von Bertalanffy growth models of length at age, and the analysis of annual otolith increments, we investigated the difference in sex-specific growth patterns and related these to temperature, eutrophication level (Chlorophyll a), growth window and fish density. In all four species, annual otolith growth rates during the early life stages differed between the two periods, mostly resulting in larger lengths at age in the recent period. All species showed significant correlations between increment size and temperature, explaining the observed period differences. The lack of an effect of total fish biomass provided no evidence for density dependent growth. A correlation with chlorophyll was found in bull-rout, but the relationship was negative, thus not supporting the idea of growth enhanced by high nutrient levels. In conclusion, we found no evidence for reduced growth related to de-eutrophication. Our results indicate that temperature rise due to climate change had a greater impact on growth than reduced food availability due to de-eutrophication. We discuss potential consequences of growth changes for length-based indicators used in management.

Corrigendum to: Identifying physiological and environmental influences on otolith chemistry in a coastal fishery species

Otolith (ear stone) chemistry provides powerful insights into the lives of fish. Although frequently used to reconstruct past environments, the influence of physiology remains unsettled. As such, we investigated the relationships between otolith chemistry, physiological factors and environmental factors in an iconic fishery species, snapper (Chrysophrys auratus). Lifetime otolith profiles were analysed of carbon (δ13C) and oxygen (δ18O) isotopes, and elemental concentrations of lithium (Li:Ca), magnesium (Mg:Ca), manganese (Mn:Ca), strontium (Sr:Ca), and barium (Ba:Ca). Mixed-effects modelling alongside a detailed literature review was used to investigate physiological (age, otolith growth rate, fish size, sex) and environmental influences (sea-surface temperature and chlorophyll-a) on otolith chemistry. Carbon isotopes and magnesium related to physiological factors, suggesting their potential as physiological proxies. Physiology also weakly related to strontium and lithium. By contrast, oxygen isotopes, barium, and manganese (except for natal signatures) were suggested to provide insights into past environments. Our study stresses the importance of consistency in biological characteristics for study designs, and highlights the potential of physiological proxies for distinguishing between populations in uniform water bodies. This study has not only reinforced our confidence in field applications of otolith chemistry, but has furthered our understanding of the influence of physiology.

Identifying physiological and environmental influences on otolith chemistry in a coastal fishery species

Otolith (ear stone) chemistry provides powerful insights into the lives of fish. Although frequently used to reconstruct past environments, the influence of physiology remains unsettled. As such, we investigated the relationships between otolith chemistry, physiological factors and environmental factors in an iconic fishery species, snapper (Chrysophrys auratus). Lifetime otolith profiles were analysed of carbon (δ13C) and oxygen (δ18O) isotopes, and elemental concentrations of lithium (Li:Ca), magnesium (Mg:Ca), manganese (Mn:Ca), strontium (Sr:Ca), and barium (Ba:Ca). Mixed-effects modelling alongside a detailed literature review was used to investigate physiological (age, otolith growth rate, fish size, sex) and environmental influences (sea-surface temperature and chlorophyll-a) on otolith chemistry. Carbon isotopes and magnesium related to physiological factors, suggesting their potential as physiological proxies. Physiology also weakly related to strontium and lithium. By contrast, oxygen isotopes, barium, and manganese (except for natal signatures) were suggested to provide insights into past environments. Our study stresses the importance of consistency in biological characteristics for study designs, and highlights the potential of physiological proxies for distinguishing between populations in uniform water bodies. This study has not only reinforced our confidence in field applications of otolith chemistry, but has furthered our understanding of the influence of physiology.

Spatiotemporal analysis of the daily growth traits of the prerecruits of a small pelagic fish in response to environmental drivers

AbstractA comparative analysis of daily growth patterns was carried out for prerecruits (PR) of Engraulis ringens, collected from spring to summer (2015–2016) in northern (18º25'S–24º40'S) and southern nursery areas (32º40'S–41º00'S) off the Chilean coast. Prerecruits ranged from 2.2 to 7.3 cm TL (4.27 ± 0.88) and from 27 to 102 days (54.68 ± 15.92) of age and were hatched mainly in spring 2015. Growth traits were addressed through otolith microstructure analysis to determine the increment width profiles (IW), back‐calculated growth rates (BIGR), and overall absolute growth rates (AGR), whereas the Chl–a and sea‐surface temperature (SST) variations were assessed by analyzing satellite and field‐based data. The IWs (5.42 ± 1.94 µm versus 1.02 ± 0.37 µm), BIGRs (1.02 ± 0.37 versus 0.72 ± 0.26 mm·day−1), and AGRs (0.93 ± 0.15 versus 0.62 ± 0.08 mm·day−1) were significantly higher in the northern than in the southern stocks. The SST ranged from 17°C to 23°C (19.42 ± 1.45) and from 12°C to 18°C (14.81 ± 1.31) and varied significantly between stocks and months, whereas the higher concentration of Chl–α (~8 mg/m3), occurred close to the shallow coastal areas irrespective of stocks. A parabolic relationship between the recent otolith growth rates (ROGR) (as a proxy of recent daily somatic growth rates) and SST recorded at the date of capture estimated a maximum ROGR of 13.4 µm·day−1 at 18°C. These results demonstrated that water temperature seemed to be an important daily growth controller for early juveniles of E. ringens in the Humboldt Current System and that the field‐based growth–temperature relationship obtained will contribute to future potential bioenergetics models for this important forage species.

Putting the pieces together: Recent growth, nutritional condition, and mortality of Engraulis anchoita larvae in the Southwest Atlantic

AbstractDynamics of clupeiform fish populations such as anchovy are frequently impacted by environmental variations which can affect the success of the species recruitment. Herein, we have analyzed recent otolith growth rate, RNA/DNA nutritional condition index (sRD), and mortality rate of argentine anchovy larvae Engraulis anchoita from three different nursery areas in the Southwest Atlantic. We have evaluated the relationship between the environmental variables (abundance of copepod nauplii, temperature, chlorophyll‐a concentration, and abundance of E. anchoita larvae) and larval endogenous variables (size, weight, age, and otolith radius) to sRD and recent growth rate. Fast larval growth rates were observed toward the northern sector of the studied area, characterized by higher temperature. High values of sRD were associated with higher nauplii abundance in the proximity of coastal fronts. The larvae with the lowest growths and lowest minimum values of nutritional condition coincided with the area where there was less abundance of nauplii and higher larval mortality. Larval size and nauplii abundance were positive explanatory factors for both recent growth rate and sRD index. Temperature had a positive effect on recent growth rate and a negative effect on sRD index. This condition index was poorly explained in terms of model fit in comparison with the growth model. The results herein provided could be significant to better understand the recruitment of the species, as to determining favorable areas for the growth and survival of anchovy larvae.

Nutritional condition and otolith growth of Engraulis anchoita larvae: The comparison of two life traits indexes

Abstract Engraulis anchoita larvae were captured in 16 surveys carried out at a fixed sampling station (EPEA) in the Argentinean Sea between 2008 and 2017. Nutritional condition and growth of 214 anchovy larvae were estimated through RNA/DNA index and analysis of otolith microstructure. Standard length, developmental stage, age, weight, otolith radius, recent otolith growth rate (RGR), daily otolith growth rate (DGR), RNA/DNA index (sRD) and a derived index of growth performance were determined for each larva. Significant differences in nutritional condition and otolith growth rates were found among seasons and developmental stages, with slower RGR and lower sRD in larvae captured during winter and in the preflexion stage. On a long time scale, a similar seasonal pattern was observed with DGR. However, the amount of variability explained by ontogeny and seasonality was not equal in both RGR and sRD indexes. RGR was better explained with the analyzed variables, while future studies are required to improve the understanding of the nature of sRD. We observed signs that spring and autumn would be the most propitious season for larval survival in a short time scale, while winter would be the worst one, both in short and in long term scale.

Ocean acidification affects somatic and otolith growth relationship in fish: evidence from an in situ study.

Ocean acidification (OA) may have varied effects on fish eco-physiological responses. Most OA studies have been carried out in laboratory conditions without considering the in situ pCO2/pH variability documented for many marine coastal ecosystems. Using a standard otolith ageing technique, we assessed how in situ ocean acidification (ambient, versus end-of-century CO2 levels) can affect somatic and otolith growth, and their relationship in a coastal fish. Somatic and otolith growth rates of juveniles of the ocellated wrasse Symphodus ocellatus living off a Mediterranean CO2 seep increased at the high- pCO2 site. Also, we detected that slower-growing individuals living at ambient pCO2 levels tend to have larger otoliths at the same somatic length (i.e. higher relative size of otoliths to fish body length) than faster-growing conspecifics living under high pCO2 conditions, with this being attributable to the so-called 'growth effect'. Our findings suggest the possibility of contrasting OA effects on fish fitness, with higher somatic growth rate and possibly higher survival associated with smaller relative size of otoliths that could impair fish auditory and vestibular sensitivity.

Ocean acidification promotes otolith growth and calcite deposition in gilthead sea bream (Sparus aurata) larvae

The effects of ocean acidification on otolith crystallization and growth rates were investigated in gilthead sea bream (Sparus aurata) larvae. Larvae were exposed to three different pH levels: pH8.2, pH7.7 and pH7.3 for a period of 18 days post-fertilization. For the first time, we demonstrate that pH has a significant impact on the carbonate polymorph composition, showing calcite in a significant percentage of individuals at low pH. Around 21% of the larvae exposed to pH7.3 showed irregular calcitic otoliths rather than commonly found round aragonitic otoliths. Calcitic otoliths showed a moderate level of heritability suggesting an important role of genetic factors. We also observed significantly larger otoliths in larvae reared at pH7.7 and pH7.3 compared to pH8.2 in both sagittae and lapilli. Our results demonstrate that otolith growth rates in gilthead sea bream larvae increase at low pH while a significant proportion of larvae are prone to the formation of calcitic otoliths at pH7.3.

Pelagic larval duration, growth rate, and population genetic structure of the tidepool snake moray Uropterygius micropterus around the southern Ryukyu Islands, Taiwan, and the central Philippines.

The relationships between pelagic larval duration (PLD) and geographic distribution patterns or population genetic structures of fishes remain obscure and highly variable among species. To further understand the early life history of the tidepool snake moray Uropterygius micropterus and the potential relationship between PLD and population genetic structure of this species, otolith microstructure and population genetics based on concatenated mtDNA sequence (cytochrome b and cytochrome oxidase subunit I, 1,336 bp) were analyzed for 195 specimens collected from eight locations around the southern Ryukyu Islands, Taiwan, and the central Philippines. Eels with longer PLD and lower otolith growth rates were observed at relatively higher latitudes with lower water temperatures (54.6 ± 7.7 days and 1.28 ± 0.16 µm day−1 on Ishigaki Island, Japan, vs. 43.9 ± 4.9 days and 1.60 ± 0.19 µm day−1 on Badian, the Philippines), suggesting that leptocephali grew faster and had shortened pelagic periods in warmer waters. Meanwhile, the eels along the southwest coast of Taiwan had relatively longer PLD (57.9 ± 10.5 days), which might be associated with the more complex ocean current systems compared to their counterparts collected along the east coast of Taiwan (52.6 ± 8.0 days). However, the southwestern and eastern Taiwan groups had similar otolith growth rates (1.33 ± 0.19 µm day−1 vs. 1.36 ± 0.16 µm day−1). Despite the intergroup variation in PLD, genetic analysis revealed fluent gene flow among the tidepool snake morays in the study regions, implying that intraspecies PLD variation had a weak effect on genetic structure. The leptocephalus stage might have ensured the widespread gene flow among the study areas and leptocephalus growth was likely influenced by regional water temperature.

Age, growth rate, and otolith growth of polar cod (Boreogadus saida) in two fjords of Svalbard, Kongsfjorden and Rijpfjorden

This work presents biological information for polar cod (Boreogadus saida) collected with a Campelen 1800 shrimp bottom trawl in Kongsfjorden (two stations located in the inner part of the fjord adjacent to the glacier) and Rijpfjorden (one station at the entrance to the fjord) in September and October 2013. The otolith-based ages of polar cod collected in Kongsfjorden (6.1–24 cm total length TL; n = 813) ranged from 0 to 4 years. The growth rate was relatively constant at approximately 4.7 cm year−1 between years 1 and 4, which indicates that growth was fast in the glacier area. The ages of polar cod collected in Rijpfjorden (8.6–15.9 cm TL; n = 64) ranged from 2 to 3 years. The fish from Rijpfjorden were smaller at age than those from Kongsfjorden, and their growth rate between years 2 and 3 (no other age classes were available) was approximately 3.3 cm year−1. In both fjords, males and females were of the same size-at-age and the same weight-at-TL. The small sampling area means that the results on growth rate are not representative of the entire fjords. Instead, the results can be discussed as presenting the possible growth rates of some populations. A strong relationship was identified between otolith size (length and weight) and fish size (TL and TW), with no differences between males and females or the fjords. A significant, strong relationship was also noted between fish and otolith growth rates.

Age and growth rate variation influence the functional relationship between somatic and otolith size

Curves describing the length–otolith size relationships for juveniles and adults of six fish species with widely differing biological characteristics were fitted simultaneously to fish length and otolith size at age, assuming that deviations from those curves are correlated rather than independent. The trajectories of the somatic and otolith growth curves throughout life, which reflect changing ratios of somatic to otolith growth rates, varied markedly among species and resulted in differing trends in the relationships formed between fish and otolith size. Correlations between deviations from predicted values were always positive. Dependence of length on otolith growth rate (i.e., “growth effect”) and “correlated errors in variables” introduce bias into parameter estimates obtained from regressions describing the allometric relationships between fish lengths and otolith sizes. The approach taken in this study to describe somatic and otolith growth accounted for both of these effects and that of age to produce more reliable determinations of the length–otolith size relationships used for back-calculation and assumed when drawing inferences from sclerochronological studies.

Linking temporal changes in the demographic structure and individual growth to the decline in the population of a tropical fish

The exceptional biodiversity and productivity of tropical coastal lagoons can only be preserved by identifying the causes for the decline in the populations living in these vulnerable ecosystems. The Terminos lagoon in Mexico provided an opportunity for studying this issue as some of its fish populations, in particular the Silver Perch (Bairdiella chrysoura), have declined significantly since the 1980s. Fish sampling campaigns carried out over the whole lagoon area in 1979–81 and again in 2006–2011 revealed the mechanisms which may have been responsible for this decline. Based on biometrical data for 295 juveniles and adults from the two periods and on somatic growth derived from 173 otoliths, a study of the temporal changes in the demographic structure and life history traits (individual growth and body condition) made it possible to distinguish the causes of the decline in the B. chrysoura population. Growth models for the lagoon in 1980–1981 and 2006–2011 showed no significant change in the growth parameters of the population over the last 30 years with a logistic model giving an accurate estimate (R2 = 0.66) of the size-at-age for both periods. The decline in the B. chrysoura population could not be explained by an overall decrease in individual size and condition in the lagoon, the average standard length (SL) and Fulton index (FI) having increased slightly since 1980–1981 (4.6 mm and 0.02 for juveniles and 5.42 mm and 0.07 for adults). However, the size structure of the population in the lagoon has changed, with a significant shift in the size distribution of juveniles with a marked reduction in the proportion of juveniles ≤ 60 mm in the captures (90.9% fewer than in 1980–1981). As the otolith growth rate of fish during the first 4 months also decreased significantly between the two sampling periods (−15%), it is suggested that the main reason for the decline in the abundance and biomass of B. chrysoura within this system may be that its habitats are less suitable for fish growth and survival in the initial months after settlement. Environmental conditions in the lagoon appear to allow compensatory growth of the individuals that survive this early demographic bottleneck. The key for the conservation of B. chrysoura probably lies in the identification and restoration of the habitats required by its larvae and juveniles.

Experimental evidence of complex relationships between the ambient salinity and the strontium signature of fish otoliths

The otolith strontium:calcium ratio (Sr:Ca) has been widely used to assess the connectivity between fish populations in ocean, estuarine and freshwater environments as the concentration of Sr in the otoliths is strongly correlated with water salinity. This correlation was tested experimentally in hypersaline conditions by submitting the extremely euryhaline tilapia species Sarotherodon melanotheron heudelotii (Cichlidae), found throughout West African continental waters and commonly used as an aquaculture species, to a strong salinity gradient (15–106). Experimental and control individuals were reared from birth in a closed system at a constant salinity of 10 and injected with oxytetracycline (OTC) to mark the otoliths at the beginning of the experiment. Randomly selected control fish were maintained for 75days at salinities of 10–20. The remaining experimental fish were acclimated to a salinity above 100 which was reduced by 10 each week to a salinity of 20. The salinity and temperature of the water were recorded every day and the Sr concentrations in the water were measured weekly by solution-based ICP-MS. The fish from the control and experimental groups were sampled weekly and otolith transverse sections were prepared for Sr:Ca measurements by laser ablation ICP-MS. No significant difference in the otolith growth rates after OTC marking was found between the control and experimental groups (ANCOVA, p=0.63), showing that the experimental design did not affect fish growth. The Sr concentration in the water was closely related to ambient salinity (positive linear regression, R2=0.96). For most of the fish tested (~80%), the relationship between otolith Sr:Ca and salinity was positive but nonlinear (power law, R2=0.77 on log–log plot). However, about 20% of individuals from both the control and the experimental groups showed consistently low Sr:Ca ratios irrespective of the salinity, suggesting that the Sr incorporation into the otoliths in these fish was strongly regulated. This shows that there is high variability between individuals in the regulation of Sr incorporation by a euryhaline species and indicates that otolith datasets for ecological applications should be interpreted with caution.

Getting a Good Start in Life? A Comparative Analysis of the Quality of Lagoons as Juvenile Habitats for the Gilthead Seabream Sparus aurata in the Gulf of Lions

Temperate coastal lagoons are considered key habitats for several highly prized marine fishes, which colonise them as nurseries. Lagoons can, however, exhibit diverse abiotic and biotic conditions, with potential consequences for their quality as habitats. To investigate this, we compared size, body condition (Fulton’s condition factor K and lipid classes), past growth rate (from otoliths) and sources of food web organic matter (OM), among group 0 juveniles of the gilthead sea bream Sparus aurata L. 1758, captured at the end of their first summer of residence in four lagoons of the Gulf of Lions in the NW Mediterranean (Mauguio, Thau, Bages and Salses-Leucate). These lagoons have different environmental conditions and freshwater inputs. Although age was similar for all lagoons, juveniles from Mauguio and Bages were significantly larger and heavier than those from Thau and Salses-Leucate. They were also in better condition, with higher white muscle triacylglycerol/sterol ratio (mean ± SD 35.7 ± 20.1 in Mauguio and 23.2 ± 9.8 in Bages versus 15.1 ± 15.2 in Thau and 7.4 ± 7.9 in Salses-Leucate). All exhibited similar otolith growth rates for their larval marine phase (2.8 ± 0.4 μm day−1), but significant differences were found for the lagoon phase, with higher values in Mauguio and Bages (10.1 ± 0.9 and 9.7 ± 1.0 μm day−1, respectively) than those in Thau and Salses-Leucate (8.4 ± 1.2 and 8.9 ± 0.8 μm day−1, respectively). White muscle stable isotope analysis revealed that terrestrial carbon use by the juveniles was >33 % in Mauguio and <5 % in Salses-Leucate, with intermediate values (~15 %) in Thau and Bages. Although these effects on fish condition and growth rate may relate in part to differences in water salinity and dissolved oxygen in the four lagoons, it is probable that they are mostly related to differences in food web enrichment with terrestrial OM.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish.

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3- that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.