Body Condition Index Research Articles

Welfare Indicators in Tilapia: An Epidemiological Approach

Interest and concern about rearing methods and their impact on animal welfare have increased. Production evaluation is population-based, and animal welfare analysis should be similar. In fish, the most common welfare indicators are gill state, fin damage, and body condition. The objective of this study was to evaluate the feeding rate effect on the welfare indicators of Oreochromis niloticus using an epidemiological approach. Five growth stages (from 1.2 to 360 g) were studied using four feeding rates as treatments: underfeeding (80%), recommended feeding (100%), and two levels of overfeeding (120% and 140%). The evaluated welfare indicators include the presence of lesions in different body areas and fins, the decrease in body condition index, and their impact on biomass production. Incidence and relative risk were determined for each indicator. Statistically significant associations were found in the indicators of mortality, weight, body condition (K), and presence of evident damage in the caudal and anal fin in all stages. The results showed that the feed rate directly affects the welfare indicators and production. Mortality, weight reduction, K reduction, and caudal and anal fin damage incidence showed to be relevant indicators in all O. niloticus growing stages. As a result of this study, the epidemiological approach seems to be a valuable tool for production. A risk traffic light method is a proposal that could have great potential, with the suggested limits for WI's concerning the individuals present in the culture pond, allowing progressive evaluation and decision-making to correct risky situations.

COMPUTED TOMOGRAPHY FOR MEASURING BODY FAT RESERVES IN THE THREATENED MOJAVE DESERT TORTOISE (GOPHERUS AGASSIZII).

Noninvasive methods for measuring fat reserves in both captive and free-ranging animals are important for monitoring individual and population health, but chelonian anatomy and physiology present challenges to accurate measurements. Standard field-based methods for assessing body condition in Mojave desert tortoises (Gopherus agassizii) involve the qualitative body condition score, which relies on the apparent height of the temporalis muscle relative to the sagittal crest (in addition to other characteristics) and quantitative body condition indices that measure relative mass at size. However, it is unclear how these metrics relate to body fat reserves in this species. The aims of this study were to (1) describe the use of noninvasive computed tomography in measuring body fat volume of Mojave desert tortoises, (2) describe the location of fat reserves, (3) investigate relationships between fat reserves and body condition score and body condition index, and (4) explore whether relative temporalis muscle depth, measured via computed tomography, correlates with body condition score. Body condition scores were assessed for eight captive Mojave desert tortoises prior to euthanasia, and computed tomography was performed postmortem to quantify fat volume and measure temporalis muscle depth. At necropsy, the distribution of fat was documented. Fat volume calculated by computed tomography ranged from 2.83 to 145.38 cm3 (0.07-2.5% body volume). Neither qualitative body condition score nor quantitative body condition index was correlated with fat volume. Bladder content did not compromise body condition index. Body condition score was not correlated with relative temporalis muscle depth. Computed tomography is a noninvasive method for successfully identifying fat reserves and estimating total fat volume in Mojave desert tortoises. The lack of a relationship between computed tomography-determined metrics and commonly used body condition metrics indicates that computed tomography fills a critical gap in the health assessment tool kit for captive and free-ranging Mojave desert tortoises.

Drivers of parasite communities in three sympatric benthic sharks in the Gulf of Naples (central Mediterranean Sea)

Sharks play a key role in the functioning of marine ecosystems and maintenance of trophic web balance, including life cycles of parasites co-occurring in their habitats. We investigated the structure of parasite communities of three sympatric shark species (Etmopterus spinax, Galeus melastomus, and Scyliorhinus canicula) and explored both the influence of host features in shaping the communities and their role as biological indicators of environment stability in the Gulf of Naples (central Mediterranean Sea), a geographical area characterized by strong anthropic pressure. Parasites found were all trophic transmitted helminths with a complex life cycle, except Lernaeopoda galei, that is a ecto-parasite copepod. Communities were all similarly impoverished with 4–5 component species and low values of species richness and diversity. Higher abundance of cestode larvae of the genus Grillotia was found in G. melastomus, although their dominance in all host species suggests that the three sharks have a similar role as intermediate/paratenic hosts in local food webs. Similarly, high abundance of Grillotia larvae could also suggest the occurrence of high abundance of largest top predators in the area. Host morphological (fork length in S. canicula and G. melastomus and body condition index in G. melastomus) and physiological (sex and gonadosomatic and hepatosomatic indices in S. canicula) variables were differently correlated to parasite community structures depending by host species. Potential reasons for the present impoverished parasite communities are discussed.

Analysis of insulin resistance using the non-linear homeostatic model assessment index in overweight canines.

Background and Aim:Diabetes mellitus is a carbohydrate metabolism disorder produced mainly by a deficit in insulin production or insulin resistance. The homeostatic model assessment (HOMA) is a broad method for estimating insulin resistance and β-cell function. This study aimed to evaluate the stages of insulin resistance using non-linear HOMA index analysis in normoglycemic normal weight and obese canines.Materials and Methods:Insulin resistance was evaluated using the mathematical HOMA non-linear model in canines with different body and glycemic conditions. Forty canines were studied, including 20 normoglycemic normal weight canines and 20 normoglycemic obese canines. Chi-square statistical test was applied, in which the body condition and HOMA non-linear index were evaluated. The Spearman correlation test was conducted to evaluate the glycemic and insulin variables in both types of canines.Results:The Spearman correlation presented a correlation between increased blood glucose levels and insulin in obese canines, with a correlation of 0.79, while no significant changes in insulin were found in normal weight canines with different blood glucose levels, with a correlation of −0.11. The analysis of the non-linear HOMA index showed significant differences between non-linear HOMA insulin resistance in normal weight and obese canines, with a Chi-square statistic of 16.9424 and p = 0.000039. Canine with increased HOMA 2 showed higher levels of insulin with increasing blood glucose compared to those with normal HOMA 2.Conclusion:The HOMA 2 is a marker for evaluating increased insulin resistance in obese dogs and can be used to determine patients at risk for glycemic alterations.

Polecat body size and sex ratio change over time: Impact of invasive competitor or climate warming?

Change in body size is considered to be one of the animal responses to climate warming, although in many cases it is difficult to show whether it is evolutionary or a result of phenotypic plasticity. In this study, we analysed long-term changes in the body size and sex ratio of the European polecat Mustela putorius in relation to two factors: climate warming and competition with the invasive American mink Neovison vison . In the years 1959–2021, the average body mass of polecats inhabiting north-eastern Poland increased from 869 to 1109 g in males and from 449 to 690 g in females, whereas structural body size (the condylobasal length of the skull) increased from 64.4 to 68.0 mm in males and from 55.9 to 59.6 mm in females. The rates of these increases were different for both measures; in consequence, the index of body condition changed non-linearly over time with a high increase in last 30–40 years. The observed increase in polecat body mass and skull size correlated with the rise of mean winter and summer temperatures. We explain the recorded trends by easier access to amphibians (the staple food of polecats in the study area) in mild winters, which increases the survival rate of larger polecats and/or leads to a faster growth rate of subadults in warmer summers. Competition for food between the native polecat and invasive mink could play a role in the increase of polecat body size simultaneously with climate warming, but the timing of recorded changes suggests the second factor to be more important. Our study also confirmed a previous observation of increasing skewed sex-ratio towards polecat males, which, similarly to changes in polecat body size, could have resulted both from climate warming and competition with mink.

Variability in Body Condition and Growth Rates for Rehabilitated Harbor Seal (Phoca vitulina) Pups

In the United States, Canada, and Europe harbor seal (Phoca vitulina) pups are commonly rehabilitated after stranding and then released. Size at release is likely important to post-release survival; however, data have not been compiled to track the body condition of rehabilitated harbor seals at release across the U.S. To better understand spatiotemporal variations in harbor seal morphometrics during rehabilitation and at release, this study retrospectively analyzed body conditions, weights, lengths, and growth rates of rehabilitated harbor seal pups in the U.S. Body condition index (BCI) was calculated, and weight and BCI were modeled regionally and temporally. There was significant variation in weight, length, BCI, and growth rate for rehabilitated and released seals between the East and West Coasts of the U.S. and among different years. Growth rates during rehabilitation were slower than reported for wild pups from birth to weaning. Length at release was not a strong predictor of weight. Because animals of similar weights can have different lengths, weight alone might not be the best criterion for pre-release body condition. A body condition score incorporating weight, length, and possibly other variables such as age or axillary girth could be more informative; however, data on post-release survival are needed to evaluate these options.

Do oil palm plantations provide quality habitat for migratory birds? A case study from Mexico

Habitat loss and degradation represent a major threat to Nearctic-neotropical migratory birds in the tropics. Managed agroecosystems have the potential to mitigate some impacts of land conversion, however, we know little regarding the quality of expanding oil palm plantation habitat for migratory birds in the neotropics. In this study, we used sex and age ratios, body condition, fat deposition, and muscle mass to assess oil palm habitat quality for seven species of migratory songbirds. Specifically, we captured individuals in native forest fragments and oil palm plantations in the state of Tabasco, Mexico, during two winter seasons (2017–2018, 2018–2019), and compared differences in age and sex ratios, and condition indices between habitats. We found that, when differences occurred, most species exhibited indicators of better habitat quality in native forest (older males with higher body condition indices) when compared to oil palm. Our results suggest that age and sex ratios combined with measures of physiological condition can be used to assess habitat quality for Nearctic-neotropical migratory birds overwintering in modified landscapes. Importantly, determining a species’ territorial behavior is key when selecting a given indicator of habitat quality for each species, and in certain cases, more intensive approaches such as estimates of survival, territory size, and food availability may be needed. Our results suggest that management strategies that promote forest-like conditions in oil palm plantations will improve habitat quality for declining populations of Nearctic-neotropical migratory birds.

A half-century of demographic changes in a green turtle (Chelonia mydas) foraging aggregation during an era of seagrass decline

To understand the demographic responses of green turtles to seagrass decline, we examined a data set from study of a mixed-stock foraging aggregation of immature green turtles, Chelonia mydas, collected in Bermuda (32o18’N, − 64o46’W) over five decades. Average turtle size (SCLmin) and mass declined by 22.3% and 58.2%, respectively. Aggregation size structure shifted to smaller sizes and now consists of more small turtles and fewer large turtles. Density (turtles ha−1) increased significantly but biomass (kg ha−1) remained unchanged and low compared to C. mydas biomass observed elsewhere. Green turtles exhibited reduced site fidelity during two portions of the study period, suggesting increased foraging effort. Reduction in turtle body condition index and seagrass coverage occurred from offshore to inshore. Changes in aggregation composition and behavior were consistent with expectations given a documented decline in seagrass availability, combined with increased output from source rookeries. Apparent response to resource decline is traced back to 1976, well before seagrass loss was first documented. Green turtles and their primary food source (Thalassia testudinum) are at the northern limit of their range in Bermuda, where seagrasses would be expected to have a reduced tolerance for natural grazing pressure and increased susceptibility to synergistic stressors, especially temperature, bioturbation and phosphorus limitation. Our results suggest that synergistic stressors, and not green turtles alone, have produced the observed reduction in seagrasses on the Bermuda Platform. Given that seagrass declines have been reported worldwide, our findings may suggest how green turtles will respond elsewhere.

Resting Frequency of Echolocation Calls within a Lesser Horseshoe Bat Population (Southern Poland) and its Relation to Body Size, Condition and Mass

The echolocation calls emitted by stationary bats are characterised by their resting frequency (RF). The ecological role of RF has been widely discussed across the literature concerning the Rhinolophidae family, where it has been found that the RF may vary depending on many factors, although its role in shaping the variability of different populations remains unclear, and the data for many species – includingRhinolophus hipposideros– is scarce. In this study, we aimed to determine how sex, age and biometric parameters (body mass, forearm length and the body condition index) affected the RF in aR. hipposiderospopulation and to investigate the individual variability in this parameter. Bats were captured in front of two Carpathian caves in Southern Poland during the mating season. The echolocation calls of the hand-held bats were recorded, and later their peak frequency was measured with computer software. The analyses showed higher RF values for females than for males, but (in contrast to previous reports) no differences between the age classes were identified. RF did not correlate with any biometric parameters in the studied population, which rules out the possibility of quality characteristic signalling through this parameter. However, we observed significant individual differences in RF within the sex-age groups, which might reflect some communication potential. The discrepancies among the research results available for this topic indicate the need for further studies aimed at investigating the variability of RF and its role across species distribution ranges and their phenology.

Sex and age‐class differences in feather mite loads in white wagtailsMotacilla alba lugenssuggest self‐regulation of feather mites by birds

In order to investigate the relationship between feather mites and their hosts taking into account host ecology I examined relationships among feather mite load, and sex, age, wing length and body‐condition in the white wagtailMotacilla alba lugens. I captured 288 wagtails in January (the first period) 2002 at a communal roost in Osaka, central Japan. All the wagtails were ringed individually, aged as yearlings (first‐winter) or adults (older) and sexed. I measured wing length, tarsus length and body mass, calculated a body condition index (BCI) and determined the feather mite loads (FML) of each individual by visual examination of the flight feathers of one wing, giving a score using the range 0–5. I recaptured 121 wagtails after an average of 45 days (the second period), and redetermined their FML. FML was not correlated with wing length or body‐condition. Yearlings had higher FML than adults in both periods. Males had greater FML than females in the second period. No significant differences in FML between the first and second periods were detected among yearling females, or adult males, but adult females had significantly lower FML and yearling males had significantly higher FML than the other age–sex classes in the second period. The sexual difference was caused by the lower FML of adult females and the higher FML of yearling males. The BCI of yearling males was the lowest among the four combinations of sex and age‐class. I conclude that wagtails may be able to prevent overpopulation of feather mites on their own plumage. The ability to control mite populations may be related to the condition of the host.

Song as a signal of male identity and quality in the Green-winged Saltator (Saltator similis)

Birdsong is commonly associated with sexual selection. It can influence mate choice through honestly signaling the emitter's quality. Such quality may be reflected, among other factors, in body condition and bite force. The Green-winged Saltator (Saltator similis) is a good model species to assess such relationships because males defend their territories throughout the year and sing a stereotyped song, which is known to be associated with sexual selection. The objective of this work is to assess the relationships among song structure, vocal individuality, and possible fitness indicators (body condition, morphology, and bite force) in captive male Green-winged Saltators. We conducted a linear discriminant analysis (LDA) to assess vocal individuality and to highlight the principal acoustic variables associated with it. We conducted Y-Aware PCAs to evaluate the relationship between the body condition index, bite force, and morphometric and bioacoustic variables. The morphometric variables, particularly head width and length, explained 23% of the bite force variation. The acoustic variation explained 36.17% of body condition index variation, but a small nonsignificant amount of bite force (12%). From the acoustic variables, peak frequency and high frequency of the last syllable, slope of the first syllable, and emission rate showed significant negative relationships with body condition. The LDA model based on acoustic variables had a 97.47% accuracy, although this result might also reflect regional song dialects. The acoustic variables most relevant to individual classification (first syllable duration, last syllable duration, whole-song slope, and whole-song duration) are not the same variables that had a stronger relationship to body condition. Besides that, there can be a weaker mixed signaling, as variables related to frequency modulations were important in both contexts. Our results suggest that the song may act as an honest signal of an individual's body condition in this species.

Relationships of Brain Glucocorticoid Receptors and Commonly Used Stress Parameters with Body Condition of Juvenile American Alligators (Alligator mississippiensis)

Crocodilians in aquacultural settings exhibit variations in growth and condition. The underlying cause of this has yet to be elucidated, but corticosterone (CORT, the main crocodilian stress hormone) is thought to play a role in this. In this study, we measured baseline plasma CORT, heterophil-lymphocyte (H/L) ratios, relative glucocorticoid receptor (GR) levels in the forebrain, and body condition index (BCI) of juvenile American alligators (Alligator mississippiensis). Both univariate and multivariate analyses were used to investigate underlying relationships. In univariate analysis, BCI was negatively related with GR levels, but not by plasma CORT. Multivariate analysis revealed that both relative forebrain GR levels and plasma CORT were negatively related with alligator BCI: alligators with higher body condition had lower levels of CORT and GRs. However, H/L ratios were unrelated to BCI. Our results implicate dysregulation of the hypothalamic-pituitary-adrenal axis as an important factor influencing crocodilian body condition.

Long duration advertisement calls of nesting male plainfin midshipman fish are honest indicators of size and condition.

The plainfin midshipman fish (Porichthys notatus) has long served as a model organism for neuroethology research on acoustic communication and related social behaviors. Type I or 'singing' males produce highly stereotyped, periodic advertisement calls that are the longest known uninterrupted vertebrate vocalizations. Despite the extensive literature on the acoustic behaviour of this species, it remains unclear whether reproductive males signal their quality via their highly energetic, multiharmonic advertisement calls. Here, we recorded the advertisement calls of 22 reproductive type I males at night in a controlled laboratory setting in which males were housed in aquaria maintained at a constant temperature (13.9±0.3°C). The duration of the advertisement calls from type I males was observed to increase from the first call of the night to the middle call after which call duration remained steady until the early morning hours and first light. A strong positive correlation was observed between loudness (sound pressure level and maximum sound pressure level) of the advertisement call and body size (mass and standard length; rs>0.8). In addition, an asymptotic relationship was observed between the harmonic frequencies (f0-f10) of the advertisement calls and male body condition, with harmonic frequencies initially increasing with body condition indices, but then plateauing when body condition measures were high. Taken together, our results suggest that type I male advertisement calls provide reliable honest information about male quality regarding size and body condition. Such condition-dependent information of calling males could potentially be used by receptive females to help facilitate mate choice decisions.

Plumage and Fat Condition Scores as Well-Being Assessment Indicators in a Small Passerine Bird, the Zebra Finch (Taeniopygia guttata).

The well-being of animals kept by humans in laboratories, farms, or as pets should always be of the highest importance, and a prerequisite for this is adequate housing. To assess the potential indicators for animal well-being, ideally simple, non-invasive, and reliable methods are necessary. Here, we propose a novel plumage scoring system for small songbirds, using the example of the Zebra Finch, and examine its reliability in comparison with a well-known body condition index, that is, the fat score. We used up to five different observers of different experience levels to assess inter- and intra-observer reliability of the proposed plumage score and also the fat score. We found substantial inter-observer reliability for the proposed novel plumage score, and lower inter-observer reliability for the fat score, which seems to require more training of observers. The intra-observer reliability of the experienced observer who trained the others also showed a very strong reliability for the plumage score and for the fat score. Thus, we conclude that our proposed novel plumage score is a simple, reliable, and non-invasive way to estimate an important indicator of captive Zebra Finches' well-being. Furthermore, the plumage score can be reliably taught to other observers. The plumage score, maybe in combination with the fat score, may be an important tool to reliably assess well-being on a regular basis in captive populations in zoos, laboratories, or pet stocks.

Sample point variation in gastrointestinal bacteria of migrating Northern Saw-whet Owls (Aegolius acadicus) of eastern North America

We characterize bacteria of the gastrointestinal tract of 28 Northern Saw-whet Owls (Aegolius acadicus) netted on the Mohonk Preserve near New Paltz, New York, during autumn of 2017. We used selective culture media to determine the presence and assess the point infection level of Escherichia coli, Pseudomonas aeruginosa, Salmonella spp., and Staphylococcus aureus in the buccal, cloaca, feces, and pellets of Northern Saw-whet Owls. We tested for associations between bacteria type, sample point location, and infection levels and then assessed the influence of bacterial infection on the health of owls using multivariate analysis of mass/wing chord body condition index estimates and heterophil to lymphocyte ratios. All Northern Saw-whet Owls tested positive for at least one type of bacteria. We found an inverse relationship between the number of bacteria types detected per individual and the percentage of Northern Saw-whet Owls affected, with 46.43%, 32.14%, and 3.57% of individuals showing presence of 2, 3, and 4 types of bacteria, respectively. Multivariate analysis revealed a significantly heightened presence of S. aureus in the buccal cavity, and Spearman's rho analysis revealed positive correlations between site colonization levels and bacterial species levels. We found no significant relationship between bacterial load and mass/wing chord body condition indexes or heterophil to lymphocyte ratios. We did, however, find a positive relationship between heterophil to lymphocyte ratios and Leucocytozoon infections. Providing baseline information of common microflora can help future research recognize potentially harmful gastrointestinal pathogens. Therein, as the first study to characterize bacteria of the Northern Saw-whet Owl, this research adds to the information portfolio of the species and may benefit future research efforts.

Effects of microplastics (PVC, PMMA) on the mussel Semimytilus algosus differ only at high concentrations from those of natural microparticles (clay, celite)

Laboratory exposure studies allow to investigate the impact of microplastics on marine biota, but commonly lack a procedural control, i.e. assessing the effects of natural microparticles. In two experiments with the mussel Semimytilus algosus, we compared the effects of clay vs. polyvinyl chloride (PVC) and celite vs. polymethyl-methacrylate (PMMA), respectively, at concentrations of 1.5, 15 and 150 mg l−1. After more than 60 days, no effects on respiration and clearance rates, mortality and byssus strength were observed. However, in mussels exposed to PVC the Body Condition Index was 34% lower at 150 mg l−1 than at 1.5 mg l−1. Furthermore, at 15 mg l−1, mussels exposed to microplastics produced over 40% less byssus than those exposed to natural microparticles. This suggests that mussels react differently to natural microparticles and to microplastics, but only at high particle loads that exceed current environmental microplastic concentrations by orders of magnitude.

A longitudinal analysis of the growth rate and mass of tail feathers in a great tit population: ontogeny, genetic effects and relationship between both traits

Feathers have a diversity of functions in birds and are costly to produce, so their growth rate and mass can be reliable indicators of nutritional condition at the time of production. Despite the potential for feather metrics to advance our understanding of foraging, they are underused in avian ecology. One reason for this is the difficulty of interpreting whether individual variation is driven by ontogenetic, genetic or environmental effects, which is exacerbated by the fact that most analyses have been done on cross‐sectional data. We addressed this deficit using a longitudinal dataset of tail feathers collected from great titsParus majorto test for ontogenetic and genetic effects on growth rate, mass and length, while controlling for body/feather size differences and other confounding factors. First, we found that the type of moult episode and experimentally‐induced replacement differentially affected the length, mass and growth of feathers, providing evidence of an ontogenetic effect that should be considered when comparing these feather traits across individuals as a measure of condition. Second, we detected moderate to high repeatability and heritability values from parent–offspring regression for these three feather traits, which are suggestive of an underlying genetic component of variation. Third, we used a mean centring within‐individual approach to test whether feather growth rate and feather mass (length‐corrected) are indeed positively correlated with each other as overlapping indicators of body condition in birds, and found that this association, although positive, is weak and only significant between individuals. This suggests that both metrics are not so intimately linked as originally thought, and probably have different sensitivities to variation in foraging performance and ecological conditions. Together with the higher plasticity of feather growth rate compared to feather mass, our results support the idea that feather growth rate is better suited for examining short‐term responses to environmental variation.

Reproductive cycle of the golden carpet shell (Polititapes aureus) in the Ria Formosa lagoon (southern Portugal)

AbstractThe present study described the gametogenesis and assessed the pattern of energy storage throughout the reproductive cycle of the golden carpet shell (Polititapes aureus) from the Ria Formosa lagoon (southern Portugal). Monthly sampling was performed for two years (March 2016–February 2018) and the study was based on gonad histology, complemented by the estimation of the mean gonadal index, body condition index and biochemical composition. The species’ reproductive cycle presented a seasonal pattern, with a resting period mainly between October and January and gametogenesis beginning around February–March. The spawning season of P. aureus was shorter in 2016 (June–September) than in 2017 (May–October). Furthermore, ripe individuals were very scarce (1.3%) in 2017 compared with the previous year (11.4%). Mean gonadal index (GI) reflected the species’ reproductive cycle and the body condition index (CI) and biochemical composition of the individuals exhibited high variation between years. Regarding the biochemical composition, proteins ranged between 190.6 and 595.2 μg mg−1, glycogen from 5.7 to 102.6 μg mg−1 and total lipids between 31.6 and 80.7 μg mg−1. The reproduction of P. aureus was strongly influenced by fluctuations in both seawater temperature and chlorophyll a, as reflected through the temporal variation in the gonadal cycle, condition index and biochemical composition. Overall, the information gathered in this study is valuable to propose scientifically based harvesting management measures for the long-term sustainable exploitation of this shellfish resource, further reinforcing the importance of implementing adaptive fishery management strategies to cope with global climate change.

Natural variability and potential use of melanomacrophage centres as indicators of pollution in fish species from the NW Mediterranean Sea

The present study provides a baseline assessment of the prevalence and densities of splenic melanomacrophage centres (MMCs) in 18 fish species from the NW Mediterranean Sea related to spatiotemporal and environmental factors and fish traits. Their correlation with other established health indicators, such as body condition indices (condition factor, hepatosomatic and gonadosomatic indices), parasite community descriptors and histological assessment of target organs (gills, liver and spleen) is also assessed. Despite MMCs variability is mainly attributed to the species identity and fish size, their potential use as generic biomarkers of health condition is pointed out for certain species (e.g. Spicara maena and Micromesistius poutassou) in which an increased response was identified, and whose potential drivers are discussed. Most importantly, present results provide a comprehensive assessment of MMCs in the fish community for future studies in the area.

Quantitative meta-analysis reveals no association between mercury contamination and body condition in birds.

Mercury contamination is a major threat to the global environment, and is still increasing in some regions despite international regulations. The methylated form of mercury is hazardous to biota, yet its sublethal effects are difficult to detect in wildlife. Body condition can vary in response to stressors, but previous studies have shown mixed effects of mercury on body condition in wildlife. Using birds as study organisms, we provide the first quantitative synthesis of the effect of mercury on body condition in animals. In addition, we explored the influence of intrinsic, extrinsic and methodological factors potentially explaining cross-study heterogeneity in results. We considered experimental and correlative studies carried out in adult birds and chicks, and mercury exposure inferred from blood and feathers. Most experimental investigations (90%) showed a significant relationship between mercury concentrations and body condition. Experimental exposure to mercury disrupted nutrient (fat) metabolism, metabolic rates, and food intake, resulting in either positive or negative associations with body condition. Correlative studies also showed either positive or negative associations, of which only 14% were statistically significant. Therefore, the overall effect of mercury concentrations on body condition was null in both experimental (estimate±SE=0.262± 0.309, 20 effect sizes, five species) and correlative studies (-0.011± 0.020, 315 effect sizes, 145 species). The single and interactive effects of age class and tissue type were accounted for in meta-analytic models of the correlative data set, since chicks and adults, as well as blood and feathers, are known to behave differently in terms of mercury accumulation and health effects. Of the 15 moderators tested, only wintering status explained cross-study heterogeneity in the correlative data set: free-ranging wintering birds were more likely to show a negative association between mercury and body condition. However, wintering effect sizes were limited to passerines, further studies should thus confirm this trend in other taxa. Collectively, our results suggest that (i) effects of mercury on body condition are weak and mostly detectable under controlled conditions, and (ii) body condition indices are unreliable indicators of mercury sublethal effects in the wild. Food availability, feeding rates and other sources of variation that are challenging to quantify likely confound the association between mercury and body condition in natura. Future studies could explore the metabolic effects of mercury further using designs that allow for the estimation and/or manipulation of food intake in both wild and captive birds, especially in under-represented life-history stages such as migration and overwintering.