Length-mass Relationship Research Articles

The changing structure of the Lichia amia subpopulations captured by angling in the Zandvlei Estuary, Cape Peninsula, South Africa

The Lichia amia subpopulations of the Zandvlei Estuary were sampled using fly fishing from January 2010 to December 2016. A bloom of Prymnesium parvum decimated the ichthyofauna of Zandvlei in 2012. The mass–length relationship for the pre-bloom data is similar to that developed for L. amia elsewhere in South Africa. Length frequency data in conjunction with the date of capture show seasonal variations in the composition of the sample population. These two factors were used to divide the subpopulation into three components: (i) fish with fork lengths from 400 mm FL to 700 mm FL were present in the sample throughout the year; (ii) fish with < 400 mm FL, regarded as the latest recruits, were absent from the sample between August and February; and (iii) fish with > 700 mm FL, were caught mostly during the months of February to May. These large fish are thought to have matured sexually and leave Zandvlei in late May to join the eastward migration to spawning grounds. The subpopulation growth rate is greater than 20 mm/month in the first year of residence but declines to 10 mm/month or less in their last year.

Length-mass relationships of pond macroinvertebrates do not hold between Southern and Northern Europe.

The lack of reliable data on length-mass relationships, essential to obtain accurate biomass estimates, limits our ability to easily assess secondary production by aquatic invertebrates. In the absence of published equations from similar habitat conditions, authors often borrow equations developed in geographic regions with different climate conditions, which may bias biomass estimates. A literature overview of published size-mass relationships for Portugal and Sweden highlights the need for further data within these biogeographic regions. We increased the number of equations available to Southern and Northern Europe, developing 18 new length-mass relationships for two families and 10 genera in Portugal and Sweden. All equations were published for the first time for these countries, except the genus Asellus. Our length-mass relationships were obtained from specimens collected on a one-time sampling of eight ponds in Portugal and five ponds in Sweden during late spring in 2023. Dry mass (DM) was modelled as a function of body length (BL), using the natural log-linear function with a power model (ln DM = ln a + b × ln BL). The equations obtained were compared with linear mixed models testing the fixed effects of "body length" and "country", as well as their interaction. A comparison of the equations developed in this study showed country-specific differences for all taxa, expect the genus Caenis, indicating a low potential transferability of the equations between Southern and Northern Europe. In contrast, the comparison of the equation obtained for A. aquaticus in this study with an equation published for this taxon in Sweden showed great similarities, suggesting a high transferability. Recommending caution in the borrowing of published length-mass equations, that can differ drastically between different geographic and climatic regions, especially at larger sizes, we provide a series of guidelines and good practices in this field.

Length-mass relationships and seasonal dynamics of Apatania crymophila McLachlan, 1880 (Trichoptera: Apataniidae) caddisfly larvae associated with water moss in the middle reaches of the Yenisei River (Siberia)

Apatania crymophila McLachlan, 1880 is the most abundant species among trichopterans of the Yenisei River. The density of its free-moving larvae associated with water moss in the Yenisei River downstream of the high-pressure Krasnoyarsk Hydroelectric Plant dam was studied from June through November 2022. The biomass of A. crymophila larvae best correlated with case length, and the correlation was well fitted by linear equation; case length was also used to distinguish instars in this study. The abundance and biomass of A. crymophila larvae on water moss increased most rapidly (0.12 and 0.18 day−1, respectively) from early July to early August, when water temperature approached its seasonal maximum. The maximum value of the larval density on water moss was reached in October (1084 larvae per kg of fresh moss). The relationship between larval growth and water temperature suggests that altered environmental conditions in the Yenisei River downstream of the HEP can be considered as a factor favoring the abundancy of A. crymophila.

DNA Methylation Carries Signatures of Sublethal Effects Under Thermal Stress in Loggerhead Sea Turtles.

To date, studies of the impacts of climate warming on individuals and populations have mostly focused on mortality and thermal tolerance. In contrast, much less is known about the consequences of sublethal effects, which are more challenging to detect, particularly in wild species with cryptic life histories. This necessitates the development of molecular tools to identify their signatures. In a split-clutch field experiment, we relocated clutches of wild, nesting loggerhead sea turtles (Caretta caretta) to an insitu hatchery. Eggs were then split into two sub-clutches and incubated under shallow or deep conditions, with those in the shallow treatment experiencing significantly higher temperatures in otherwise natural conditions. Although no difference in hatching success was observed between treatments, hatchlings from the shallow, warmer treatment had different length-mass relationships and were weaker at locomotion tests than their siblings incubated in the deep, cooler treatment. To characterise the molecular signatures of these thermal effects, we performed whole genome bisulfite sequencing on blood samples collected upon emergence. We identified 287 differentially methylated sites between hatchlings from different treatments, including on genes with neurodevelopmental, cytoskeletal, and lipid metabolism functions. Taken together, our results show that higher incubation temperatures induce sublethal effects in hatchlings, which are reflected in their DNA methylation status at identified sites. These sites could be used as biomarkers of thermal stress, especially if they are retained across life stages. Overall, this study suggests that global warming reduces hatchling fitness, which has implications for dispersal capacity and ultimately a population's adaptive potential. Conservation efforts for these endangered species and similar climate-threatened taxa will therefore benefit from strategies for monitoring and mitigating exposure to temperatures that induce sublethal effects.

Trait variation in patchy landscapes: Morphology of spotted salamanders (Ambystoma maculatum) varies more within ponds than between ponds.

The influence of intraspecific trait variation on species interactions makes trait-based approaches critical to understanding eco-evolutionary processes. Because species occupy habitats that are patchily distributed in space, species interactions are influenced not just by the degree of intraspecific trait variation but also the relative proportion of trait variation that occurs within- versus between-patches. Advancement in trait-based ecology hinges on understanding how trait variation is distributed within and between habitat patches across the landscape. We sampled larval spotted salamanders (Ambystoma maculatum) across six spatially discrete ponds to quantify within- and between-pond variation in mass, length, and various metrics associated with their relationship (scaling, body condition, shape). Across all traits, within-pond variation contributed more to total observed morphological variation than between-pond variation. Between-pond variation was not negligible, however, and explained 20-41% of total observed variation in measured traits. Between-pond variation was more pronounced in salamander tail morphology compared to head or body morphology, suggesting that pond-level factors more strongly influence tails than other body parts. We also observed differences in mass-length relationships across ponds, both in terms of scaling slopes and intercepts, though differences in the intercepts were much stronger. Preliminary evidence hinted that newly constructed ponds were a driver of the observed differences in mass-length relationships and morphometrics. General pond-level difference in salamander trait covariation suggest that allometric scaling of morphological traits is context dependent in patchy landscapes. Effects of pond age offer the hypothesis that habitat restoration through pond construction is a driver of variation in trait scaling, which managers may leverage to bolster trait diversity.

Length-mass allometries of the larvae of aquatic dipterans: differences between taxa, morphological traits, and methods.

Body mass underpins many ecological processes at the level of individuals, populations, and communities. Often estimated in arthropods from linear morphological traits such as body length or head width, these relationships can vary even between closely related taxa. Length-mass relationships of mosquito (Diptera: Culicidae) larvae are poorly known despite the importance of this family to disease and aquatic ecology. To fill this gap, we measured ontogenetic changes in linear traits (body length, head width, and thorax width) and dry and wet masses and estimated length- and width-mass relationships in larvae of 3 culicid species inhabiting different niches: the tropical Aedes albopictus (Skuse, 1894), the temperate Culex pipiens (Linnaeus, 1758), and the snowmelt Ochlerotatus punctor (Kirby, 1837). We compared our results with published length-mass allometries of other aquatic dipteran larvae. We showed that thorax width and body length, but not head width, reliably predicted body mass for our 3 species. The length-mass allometry slopes in aquatic dipterans varied considerably between and within families but were independent of phylogeny, specimen handling, preservation techniques, and data fitting methods. Slope estimates became less precise with decreasing sample size and size range. To obtain reliable estimates of the allometric slopes, we have thus recommended using data on all larval stages for intraspecific allometries and a wide range of species for interspecific allometries. We also cautioned against the indiscriminate use of length-mass allometries obtained for other taxa or collected at lower taxonomic resolutions, e.g., when using length-mass relationships to estimate biomass production at a given site.

Nonlethally assessing elasmobranch ontogenetic shifts in energetics.

Body condition is an important proxy for the overall health and energetic status of fishes. The classically used Fulton's condition factor requires length and mass measurements, but mass can be difficult to obtain in large species. Girth measurements can replace mass for wild pelagic sharks. However, girth-calculated condition has not been validated against Fulton's condition factor intraspecifically, across ontogeny or reproduction, or in a controlled setting. We used the epaulette shark (Hemiscyllium ocellatum), because they are amenable to captive reproduction, to track fine-scale body condition changes across life stages, oviparous reproduction and between condition indices. We measured four girths, total length and mass of 16 captive epaulette sharks across 1 year and tracked female reproduction daily. We also collected length and mass data from an additional 72 wild-caught sharks and 155 sharks from five previous studies and two public aquaria to examine the relationship between length and mass for this species. Even though data were derived from a variety of sources, a predictable length-mass relationship (R2 = 0.990) was achievable, indicating that combining data from a variety of sources could help overcome knowledge gaps regarding basic life history characteristics. We also found that condition factor decreased during early life stages, then increased again into adulthood, with predictable changes across the female reproductive cycle. Finally, we determined that both Fulton's and girth condition analyses were comparable. Outcomes from this study uniquely provide body condition changes across the complete life history, including fine-scale female reproductive stages, and validate the use of girths as a nonlethal whole-organism energetic assessment for fishes.

The effect of preservation on biomass and length estimates and its variation within and between two mayfly species

Biomass estimation offers insight into aquatic ecosystem processes and represents a powerful tool for measuring biomass in communities. However, the accuracy of this estimate depends considerably on the sample preservation method and biological traits of species. We quantified the effect of commonly used preservatives (70% ethanol, 4% formaldehyde, freezing), preservation time and initial body length on the estimation bias of body length, wet mass and dry mass in two mayfly species with dorsoventrally flattened (Rhithrogena carpatoalpina) and cylindrically elongated (Habroleptoides confusa) bodies. Freezing caused the largest relative bias in the length estimates (up to 18%). Both ethanol and freezing led to significant underestimation of wet mass of preserved animals (up to 26%). The most severe biases were observed in dry weights, where all preservation methods reduced body mass by up to 89%. Importantly, initial body mass influenced the bias caused by preservation, and the larger animals were less affected by mass losses. In contrast, larger individuals were subject to greater length reduction. Additionally, the length–mass relationship for R. carpatoalpina, based on fresh mass of living animals, was significantly different from relationships based on preserved animals. Overall, formaldehyde caused the least biased estimates of length and biomass in both examined species. This case study indicates that direct measurements of unpreserved individuals or length–mass equations based on fresh material are likely the best ways to avoid severe underestimation of biomass in aquatic insects. However, more studies covering a broader range of taxa are needed to reach general conclusions.

How to estimate body condition in large lizards? Argentine black and white tegu (Salvator merianae, Duméril and Bibron, 1839) as a case study.

Body condition is a measure of the health and fitness of an organism represented by available energy stores, typically fat. Direct measurements of fat are difficult to obtain non-invasively, thus body condition is usually estimated by calculating body condition indices (BCIs) using mass and length. The utility of BCIs is contingent on the relationship of BCIs and fat, thereby validation studies should be performed to select the best performing BCI before application in ecological investigations. We evaluated 11 BCIs in 883 Argentine black and white tegus (Salvator merianae) removed from their non-native range in South Florida, United States. Because the length-mass relationship in tegus is allometric, a segmented linear regression model was fit to the relationship between mass and length to define size classes. We evaluated percent, residual, and scaled fat and determined percent fat was the best measure of fat, because it was the least-associated with snout-vent length (SVL). We evaluated performance of BCIs with the full dataset and within size classes and identified Fulton's K as the best performing BCI for our sampled population, explaining up to 19% of the variation in fat content. Overall, we found that BCIs: 1) maintained relatively weak relationships with measures of fat and 2) splitting data into size classes reduced the strength of the relationship (i.e., bias) between percent fat and SVL but did not improve the performance of BCIs. We postulate that the weak performance of BCIs in our dataset was likely due to the weak association of fat with SVL, the body plan and life-history traits of tegus, and potentially inadequate accounting of available energy resources. We caution against assuming that BCIs are strong indicators of body condition across species and suggest that validation studies be implemented, or that alternative or complimentary measures of health or fitness should be considered.

CONDITION FACTOR AND LENGTH-MASS RELATIONSHIP OF THE MAIN FISH SPECIES IN THE BANDAMA RIVER DOWNSTREAM OF THE TAABO DAM (COTE DIVOIRE)

Length-mass relationship and condition factor are important tools used in biology, ecology and stock assessment for the management and conservation of fish populations and also to assess the level of disturbance of an aquatic ecosystem. The present study aims at providing baseline data on the length-mass relationship and condition factor of sixteen (16) fish species landed at NDènou, downstream of the Taabo dam, prior to the impoundment of the dam under construction on this part of the Bandama River. The fish were collected monthly from november 2019 to october 2020.They were measured (standard length in cm) and weighed (mass in g). The data were processed in Excel and Statistica 7.1 software. A total of 16047 fish were examined. The standard lengths of these fish varied from 3.50 to 40 cm with masses ranging from 2.87 to 1400 g. The values of the allometry coefficient ranged from 1.64 for Hemichromisbimaculatus to 2.99 for Labeocoubie. Fourteen (14) fish species studied have negative allometric growth (b &lt; 3 p &lt; 0.05). Distichodusrostratus and Labeocoubie species have isometric growth. The condition factor has values between 0.83 ± 0.39 (Mormyropsanguilloides) and 3.65 ± 0.74 (Coptodonhybrid).

Differences in the morphological body condition index of sea turtles between species and size classes

AbstractThe body condition of animals is an important indicator of their habitats and the effects of anthropogenic activities and pollution. Body condition indices calculated from morphometric measurements have been widely employed as they are easy to use and inexpensive. In sea turtles, Fulton's condition index, calculated as the bodyweight divided by the cube of straight carapace length (SCL), has been commonly used and it has been proposed that an index of ≥1.2 indicates a good body condition. However, comparing Fulton's condition index between different species and size classes is problematic as it does not consider the mass-length relationship. In this study, we conducted a meta-analysis to evaluate the differences between sea turtles. A literature review indicated that most studies reported the SCL-based Fulton's condition index for green turtles (Chelonia mydas), followed by loggerhead turtles (Caretta caretta) and hawksbill turtles (Eretmochelys imbricata). Therefore, we compared the values reported for healthy turtles of these three species. Meta-analysis supported the adequacy of 1.2 as a threshold in juvenile and adult green turtles and large juvenile and adult loggerhead turtles. High Fulton's condition index values were found for hatchlings and post-hatchlings of all three species and small loggerhead turtle juveniles. Low Fulton's condition index values were found for hawksbill turtles, particularly small juveniles. The differences in the Fulton's condition index between species and size classes indicated that it should be used carefully as a threshold for health condition evaluation.

Reduced lifetime fitness (growth, body condition and survivability) of hatchery-reared tiger pufferfish Takifugu rubripes compared to wild counterparts.

Tiger pufferfish Takifugu rubripes (order Tetraodontiformes, family Tetraodontidae) is a highly exploited species and stocks continue to decline, although hatchery-reared juveniles have been released since 1965 for stock enhancement. To determine why the stock has not recovered through hatchery-release practices, this study investigated and compared the population characteristics of wild and hatchery-origin fish. The length-mass relationship showed that hatchery-origin fish were skinnier, with males weighing less than 90% of the mass of wild males of the same length. The hepatosomatic index tended to be lower in hatchery-origin fish. Age was estimated using the otolith-based method, and the estimates were more accurate and precise than those obtained by the conventional vertebra-based method. At the age of 2.9 years, an age at which specimens were the most abundant in catches, hatchery-origin males weighed only 67% of wild males. The maximum observed age was 12 years for wild fish and 5 years for hatchery-origin fish. The instantaneous total mortality rates of hatchery-origin fish were more than twice as high as those of wild fish. In summary, the hatchery-origin fish had poor health status, poor growth and high mortality, and their fitness in natural environments was therefore hypothesized to be low throughout life.

Allometric relationships of ecologically important Antarctic and Arctic zooplankton and fish species

Allometric relationships between body properties of animals are useful for a wide variety of purposes, such as estimation of biomass, growth, population structure, bioenergetic modelling and carbon flux studies. This study summarizes allometric relationships of zooplankton and nekton species that play major roles in polar marine food webs. Measurements were performed on 639 individuals of 15 species sampled during three expeditions in the Southern Ocean (winter and summer) and 2374 individuals of 14 species sampled during three expeditions in the Arctic Ocean (spring and summer). The information provided by this study fills current knowledge gaps on relationships between length and wet/dry mass of understudied animals, such as various gelatinous zooplankton, and of animals from understudied seasons and maturity stages, for example, for the krill Thysanoessa macrura and larval Euphausia superba caught in winter. Comparisons show that there is intra-specific variation in length–mass relationships of several species depending on season, e.g. for the amphipod Themisto libellula. To investigate the potential use of generalized regression models, comparisons between sexes, maturity stages or age classes were performed and are discussed, such as for the several krill species and T. libellula. Regression model comparisons on age classes of the fish E. antarctica were inconclusive about their general use. Other allometric measurements performed on carapaces, eyes, heads, telsons, tails and otoliths provided models that proved to be useful for estimating length or mass in, e.g. diet studies. In some cases, the suitability of these models may depend on species or developmental stages.

Fator de condição relativo e modelo preditivo para a presença do caracol invasor Achatina (Lissachatina) fulica em Sergipe, Nordeste do Brasil

Abstract: Achatina fulica is among the world’s 100 most impactive invasive species, and is now found in almost all Brazilian states, including Sergipe. This exotic snail is known to have negative impacts, not only on the environment, due primarily to the rapid growth of its populations, but also on public health, given that it is an intermediate host of nematodes that cause zoonotic diseases. However, relatively little is known of the development of this snail, including its relative condition factor. We investigated the occurrence of A. fulica in 24 municipalities distributed in the eight subregions of the state of Sergipe in the dry and rainy season. Furthermore, we present here a predictive model for the occurrence of A. fulica based on the variation in climate and soil chemistry. This snail was more frequent on soils with a pH of 6.5-7.5. A negative correlation was found between the growth of A. fulica and the soil pH, then, the more acidic the soil, the more allometric the growth of A. fulica. The relative condition factor indicated differences in the development pattern of A. fulica among the eight subregions. The influence of rain in increasing the frequency of A. fulica showed a significant correlation. As well, higher temperatures influenced the resting behavior of A. fulica. The mathematical model used to identify the potential presence of A. fulica presented a high degree of agreement. This is the first ecological study of A. fulica to verify the association between the body mass-length relationship and the relative condition factor, and the results indicate that the development of this exotic land snail in Sergipe is influenced by climatic factors and principally, the soil pH. The predictive mathematical model provides valuable insights into the biotic and abiotic factors associated with the presence of A. fulica, and the influence of climatic variables and the chemical parameters of the soil on the occurrence of this species. These findings provide important guidelines for the development of measures for the control of A. fulica populations, which will contribute to both public and environment health.

Length–mass relationships of Australian aquatic invertebrates

AbstractLength–mass relationships for macroinvertebrates are used in many ecological studies; however, few length–mass relationships are available for Australian freshwater macroinvertebrates. In this study, we generated 22 new length–mass relationships from 2148 specimens spread across four orders, nine families, three subfamilies, 12 genera and two species. We also qualitatively compare our length–mass equations with others from Australia, New Zealand, North America and Europe.

Thinking Inside the Box: Comparative Limb Bone Shape in Emydid Turtles

Limb bone morphology often correlates with functional demands placed on animals by the environment. Comparisons of limb bone allometry in functionally divergent turtle taxa indicate highly specialized lineages show extensive flattening of the humerus. In Sea Turtles this contributes to flipper-shaped forelimb morphology that facilitates lift-based swimming (i.e., underwater flight). In contrast robust humeri and femora in terrestrial tortoises may reflect specializations for resisting high torsional loads during terrestrial walking and digging. However, it is unknown whether allometric patterns of ecomorphological divergence can be detected among more closely related lineages within clades that encompass species with diverse ecological habits. To test whether limb bone size and shape vary among closely related taxa that live in divergent habitats, we used phylogenetic comparative methods to assess scaling patterns and overall morphology of the humerus and femur of 27 emydid turtle species representing four genera: Graptemys (semiaquatic), Pseudemys (semiaquatic), Trachemys (semiaquatic), and Terrapene (terrestrial). In general, we found that limb bones of emydid taxa scale isometrically for most length–diameter and length–mass relationships. However, terrestrial Terrapene (box turtles) species exhibit relatively short femora compared with species from the more aquatic genus Trachemys (sliders). The relatively small limb bones of box turtles may promote limb withdrawal into the shell, but without mechanical costs because of high safety factors of turtle limb bones. The comparatively robust forelimb and hindlimb dimensions of Graptemys (map turtles) relative to other emydid clades may reflect habitat and foraging pressures in this group.

Sedentary fish as indicators of changes in the river flow rate after impoundment

The natural flood pulse of the Amazon basin has been altered progressively by the installation of hydroelectric dams, which have had a potentially adverse impact on the local fauna. Fish are sensitive to environmental changes, and one the way to assess how they respond is through the analysis of life history traits, such as growth patterns and reproduction. Here, we evaluated the life history traits of two sedentary cichlids, Geophagus argyrostictus and Geophagus altifrons, in the middle Xingu River (upstream and downstream sectors) pre- and post-impoundment including the flood and low water hydrological seasons. The mass-length relationship, condition factor (K), gonadossomatic index (GSI), breeding season, gonadal stages, and the length at 50% sexual maturity (L50) were analyzed. Data on river flow rates and the characteristics of the fish were collected during the pre- and post-impoundment periods. Post-impoundment, the discharge of the Xingu decreased in downstream sector. During the pre-impoundment period, the K of G. argyrostictus and female G. altifrons was lower in the upstream sector in both seasons, but higher in the downstream sector to G. argyrostictus. The GSI was lower in the post-impoundment period in both sectors and seasons in G. argyrostictus, whereas in the G. altifrons females, it was higher in the downstream sector in the low water season, but lower in the upstream sector in both seasons. In G. argyrostictus, spawning occurred in a single phase and was restricted to the low water season in both periods, whereas in G. altifrons, spawning was multiple. There was a reduction in the frequency of immature individuals in the upstream sector post-impoundment. The L50 was lower, principally in the upstream sector, in both species. Our results indicate that the growth patterns and reproduction of the two populations has been impacted, especially in the endemic G. argyrostictus. The construction of dams thus appears to place endemic species under a greatest risk of extirpation. These results provide important insights for the improvement of fish resource monitoring and the development of adequate management techniques.

MEDI: Macronutrient Extraction and Determination from invertebrates, a rapid, cheap and streamlined protocol.

Macronutrients, comprising carbohydrates, proteins and lipids, underpin many ecological processes, but their quantification in ecological studies is often inaccurate and laborious, requiring large investments of time and bulk samples, which make individual‐level studies impossible. This study presents Macronutrient Extraction and Determination from Invertebrates (MEDI), a protocol for the direct, rapid and relatively low‐cost determination of macronutrient content from single small macroinvertebrates.Macronutrients were extracted by a sequential process of soaking in 1:12 chloroform:methanol solution to remove lipid and then solubilising tissue in 0.1 M NaOH. Proteins, carbohydrates and lipids were determined by colorimetric assays from the same individual specimens.The limits of detection of MEDI with the equipment and conditions used were 0.067, 0.065 and 0.006 mg/ml for proteins, carbohydrates and lipids respectively. Adjusting the volume of reagents used for extraction and determination can broaden the range of concentrations that can be detected. MEDI successfully identified taxonomic differences in macronutrient content between five insect species.Macronutrient Extraction and Determination from Invertebrates can directly and rapidly determine macronutrient content in tiny (dry mass ~3 mg) and much larger individual invertebrates. Using MEDI, the total macronutrient content of over 50 macroinvertebrates can be determined within around 3 days of collection at a cost of ~$1.35 per sample.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

Length-mass relationships for macroinvertebrates in the Choghakhor international wetland, Iran

This study investigates length-mass relationships for 17 families of freshwater macroinvertebrates collected in the Choghakhor Wetland (Central Iran). Body length, head width, distance between eyes and mass (dry weight and wet weight) were used to estimate biomass with linear, log-linear and exponential models. The results show better performance of the log-linear model (ANOVA, p = 0.03), as compared to the other two (ANOVA, p > 0.05). A cross-validation test demonstrated that all three models performed reasonably well, in terms of both statistics and the accuracy of prediction. However, for more than 60% of the relationships the p value for the log-linear model was greater than for the other two, suggesting that the accuracy of this model is in general superior. Body length was generally demonstrated to be a good indicator to estimate biomass. However, for some taxa, the measurement of sclerotized structures (i.e., the distance between the eyes and the head width) were also found to be suitable biomass indicators. The latter is beneficial for the estimation of biomass with individuals that have been kept for a long time under laboratory conditions and/or those that show damaged parts of their bodies. The present study provides the first set of length-mass relationships for macroinvertebrates in the Middle-East region, and its findings are expected to contribute to the estimation of biomass in aquatic environments that are affected by semi-arid conditions and different degrees of anthropogenic stress.