Energy density of marine pelagic fish eggs

Analysis of the literature on pelagic fish eggs enabled generalizations to be made of their energy densities, because the property of being buoyant in sea water appears to constrain the proximate composition of the eggs and thus to minimize interspecific variation. An energy density of 1.34 J μl⁻¹ of total egg volume is derived for most species spawning eggs without visible oil globules. The energy density of eggs with oil globules is predicted by     x (J μ1¹) where x is the fractional volume of the oil globule.     

Species-specific schooling behaviour of fish in the freshwater pelagic habitat: an observational study

Social living of animals is a broadly occurring phenomenon, although poorly studied in freshwater systems, fish schooling behaviour is an excellent example. The composition of fish schools, species-specific schooling tendencies and preferences of adult fish were studied in the pelagic habitat of the Římov Reservoir, Czech Republic. Video recordings captured over a total of 34 days (16 h per day) in the clear water period of three seasons were analysed. From four species identified as school-forming species – bream, bleak, roach and perch, 40% of the individuals observed formed schools of 3–36 individuals. Although conspecific schools prevailed, 20% of individuals formed heterospecific schools, except bleak that schooled strictly with conspecifics. Schools were composed of individuals of similar body size and life strategy. Heterospecific schools were significantly larger than conspecific schools and showed uneven proportion among species, that is, one species being more abundant when the school dimension increased. Probability of encounter in bleak was lowest and proved highest inclination for schooling. Gregarianism levels depended on species morphology and body size, with larger and morphologically advanced fish tending less to sociability. This indicates that the antipredator function of schooling behaviour is intensified with increasing vulnerability of the species.     

Fluctuations in pelagic fish density linked to ambient conditions

Animal behaviour has been broadly studied for its social or functional aspects but less often for understanding the interactions between organisms and their ambient conditions. A pilot observational study was performed by means of underwater camera to investigate the correlation between environmental factors and the density of fish in the epipelagic habitat of a European temperate reservoir. Explored factors confirmed a positive correlation between water temperature and all observed categories: nonpredatory, predatory, single nonpredatory and schooling fish, as well as freshwater bream and roach. On the other hand, de-trended solar irradiance was found to be negatively correlated with density of nonpredatory fish, freshwater bream and European perch. Sunshine duration was negatively correlated with the density of predatory fish. Precipitation a showed positive relationship with single nonpredatory fish and European perch, whereas wind strength had a negative relationship with density of schooling fish. Furthermore, density of predatory fish was positively correlated with density of single nonpredatory fish and counts of observed clusters. Altogether, findings indicate that fish density is correlated with abiotic factors and the occurrence of predators. This suggests that more ecologically complex studies should be encouraged for better understanding of ecological interactions that drive the structure of aquatic ecosystems.     

Swimming ability and behaviour of post-larvae of a temperate marine fish re-entrained in the pelagic environment

The degree to which behaviour, vertical movement and horizontal transport, in relation to local hydrodynamics, may facilitate secondary dispersal in the water column was studied in post-larval Sillaginodes punctata in Port Phillip Bay, Australia. S. punctata were captured in shallow seagrass beds and released at the surface in three depth zones (1.5, 3 and 7 m) off-shore at each of two sites to mimic the re-entrainment of fish. The behaviour, depth and position of S. punctata were recorded through time. The direction and speed of local currents were described using an S4 current meter and the movement of drogues. Regardless of site, fish immediately oriented toward the bottom, and into the current after release. In shallow water (1.5 m), 86% of fish swam to the bottom within 2 min of release. At one site, the net horizontal displacement of fish was largely unrelated to the speed and direction of local currents; at a second site, fish could not maintain their position against the current, and the net horizontal displacement was related to the speed and direction of currents. In the intermediate depth zone, wide variability in depths of individual fish through time led to an average depth reached by fish that was between the shallow and deep zones. Based on daily increments in the otoliths, however, this variability was not related significantly to the time since entry of fish into Port Phillip Bay. In the deepest depth zone, 81% of fish remained within 1 m of the surface and their horizontal displacement was significantly related to the direction and speed of currents. Secondary dispersal of post-larval fish in the water column may be facilitated by the behaviour and vertical movements of fish, but only if fish reach deeper water, where their displacement (direction and distance) closely resembles local hydrodynamic regimes. In shallow water, fish behaviour and vertical migration actually reduce the potential for secondary dispersal.     

The origin and development of individual size variation in early pelagic stages of fish

Size variation among individuals born at the same time in a common environment (within cohorts) is a common phenomenon in natural populations. Still, the mechanisms behind the development of such variation and its consequences for population processes are far from clear. We experimentally investigated the development of early within-cohort size variation in larval perch (Perca fluviatilis). Specifically we tested the influence of initial variation, resulting from variation in egg strand size, and intraspecific density for the development of size variation. Variation in egg strand size translated into variation in initial larval size and time of hatching, which, in turn, had effects on growth and development. Perch from the smallest egg strands performed on average equally well independent of density, whereas larvae originating from larger egg strands performed less well under high densities. We related this difference in density dependence to size asymmetries in competitive abilities leading to higher growth rates of groups consisting of initially small individuals under high resource limitation. In contrast, within a single group of larvae, smaller individuals grew substantially slower under high densities whereas large individuals performed equally well independent of density. As a result, size variation among individuals within groups (i.e. originating from the same clutch) increased under high densities. This result may be explained by social interactions or differential timing of diet shifts and a depressed resource base for the initially smaller individuals. It is concluded that to fully appreciate the effects of density-dependent processes on individual size variation and size-dependent growth, consumer feedbacks on resources need to be considered.     

Fetal erythroid cell development: density gradients and size distributions

Electronic size distributions of erythroid cells from fetal C57BL-6J mice during the eleventh through twentieth days of gestation indicate that the erythropoietic cell populations are constantly changing. The mean volume of the liver derived non-nucleated erythroid population decreases from four times the mean adult erythrocyte volume on the thirteenth gestation day to twice the adult erythrocyte volume at birth. The mean volume of the nucleated erythroid cell is about ten times the mean adult erythrocyte volume. The gestation age of an embryo can be determined from blood cell size distributions. The mode of the non-nucleated population and the percentage of each population indicates the gestation age. Size distribution of cells in density gradient fractions apparently indicate two size populations of non-nucleated cells between the thirteenth and fifteenth days. The density of the non-nucleated cells increases during gestation. It is suggested that the decrease in size and increase in density of non-nucleated cells is due to the release of successively smaller reticulocytes from the liver.     

Diversity of juvenile fish assemblages in the pelagic waters of Lebanon (eastern Mediterranean)

Oxygen consumption rates of nauplii of the brine shrimp Artemia franciscana Kellogg 1906 were determined over a range of salinities from 10 to 110 ppm, in temperatures from 0 to 30°C, using a multi-factorial design. The oxygen micro-sensors employed have a fast response time and are capable of accurately measuring oxygen concentrations at temperatures well below 0°C. Oxygen uptake rate ranged from 0.03 to 0.66 μmol O₂ mg⁻¹ h⁻¹ and was sensitive to changes in both salinity and temperature. Temperature was the dominant factor affecting oxygen consumption rates, which showed a significant increase with increasing temperature. A slight decrease was measured in oxygen consumption with increasing salinity related to differential solubility of oxygen in waters of different salinities. Thermal sensitivity of oxygen consumption determined from calculations of Q ₁₀, indicated physiological adaptation of Artemia nauplii to the ranges of temperatures tested. Handling editor: A. van Kerchove     

Shifts in seed size across experimental nitrogen enrichment and plant density gradients

We tested the hypotheses that increased soil resource availability selects for larger seeded plants by: (1) increasing community seed density and seedling competition, (2) increasing light competition, which favours larger seeded species if their additional seed resources are allocated to shoots. We also tested the hypothesis that plants respond to increased nutrient availability by producing larger seeds. In a controlled environment experiment, we grew monocultures of five species of co-occurring annual plant species at two N addition rates and measured seed size and the number of seeds produced. In a second experiment, we sowed seed of the five species together, manipulated nitrogen (N) addition rate and community seed density and measured species performance. In the first experiment, elevated N addition increased seed size, but only in larger seeded species. In the second experiment, high N addition increased community seed production. However, contrary to our hypothesis small seeded species were selected under high seed density, possibly because they germinated and grew more rapidly thus enabling them to pre-empt limiting resources. Larger seeded species were favoured by high N addition, as hypothesised. However, our data suggest that this was due to their seed reserves boosting survival below the denser canopies of fertile conditions rather than via competitive advantages. Our results point to a largely overlooked role of recruitment in determining community response to fertilisation. Although our results may not generalise to all plant communities they suggest that seed traits play a role in community response to both the direct effect of N addition and the stimulation of seed density caused by increased productivity. These results are also consistent with the view that the advantage of large seed size is not a competitive one, but greater survival in the face of environmental hazards.     

Diet of brown trout in relation to variation in abundance and size of pelagic fish prey

The seasonal diet of a predator, brown trout Salmo trutta [total length ( L _T) 17–69 cm] and simultaneous density and size‐structure of prey populations, vendace Coregonus albula and smelt Osmerus eperlanus (4–16 cm L _T), in a large boreal lake were analysed and compared in 2001 and 2002. The upper L _T limit for consumed prey was c . 40% of the predator L _T. All brown trout, however, preferred small (<10 cm L _T) and avoided large (≥10 cm L _T) prey. The results also suggested that equal densities of similar‐sized (4–10 cm L _T) fish of the two prey species led to random foraging on these species by brown trout, but if either one of the prey species predominated (>50%) in the lake, brown trout shifted to foraging on this species almost exclusively. Brown trout diets thus reflected the density dynamics of the two alternative prey species.     

Structuring of the cyanobacterial community by pelagic fish in subtropical reservoirs: experimental evidence from Australia

1. Subtropical reservoirs of Australia are commonly subject to summer blooms of cyanobacteria. The potential for food web manipulation to control cyanobacterial blooms was investigated in Lake Maroon, south east Queensland using enclosures in which the density of the Australian gudgeon Hypseleotris spp. was manipulated. 2. Zooplankton biomass and community structure were strongly affected by fish density. A size dependent predation effect of Hypseleotris on zooplankton was observed at ambient fish densities, and the community shifted towards a dominance of copepod juveniles and nauplii. Substantial increases in the populations of Ceriodaphnia and calanoid copepods were observed at low fish densities and in the absence of fish. 3. At ambient fish densities total phytoplankton and the proportion of cyanobacteria were maintained at levels similar to those prevailing at day 0. Total phytoplankton and the proportion of cyanobacteria decreased substantially at low fish densities and in the absence of fish. Chlorophytes became dominant in the ‘no fish’ treatment and the grazing‐resistant species Oocystis and Dictyosphaerium were significantly higher than at ambient fish densities. 4. The experiment demonstrated a strong positive relationship between Hypseleotris density and cyanobacteria, and the results suggest that subtropical reservoirs may be suited to food web manipulation as a means of controlling summer cyanobacterial blooms.     

Does the western Irish Sea gyre influence the distribution of pelagic juvenile fish?

The western Irish Sea is an important spawning and nursery ground for many commercially exploited fish. Spawning begins in the Irish coastal regions in early spring, and moves offshore as the season develops. As a result of the onset of seasonal heating in spring, stratification isolates a dome of cold bottom winter water in the deep (>100 m) basin of the western Irish Sea. The resultant density fields drive a cyclonic gyre which dominates the circulation of the region during late spring and summer and is characterized by anticlockwise current speeds which exceed 0.2 ms^–1, after removal of tides. Surveys of pelagic juvenile (0-group) fish in 1994, 1995 and 1996, showed that they were coincident with the centre of the gyre. Physical data from 1994 and 1995, were used to describe the horizontal and vertical structure of the water column and the associated circulation regime. The behaviour and swimming speeds of pelagic juvenile fish were insufficient to explain their apparent shift in abundance away from the coast to the central western Irish Sea. Drifter and current data suggested that their entrainment into the gyre could provide both the transport and retention mechanisms.     

Interacting effects of spatial gradients and fishing gears on characterization of fish assemblages in large reservoirs

Understanding the variation of fish assemblages in reservoirs is crucial for precise interpretations of ecological processes. However, representative samples in such environments are subjected both to spatial zonation (longitudinal and transversal gradients) and limitations of fishing gears. We used a large zoned reservoir (Itaipu, Brazil) to perform a quasi-experiment that assessed the relative role of spatial gradients and fishing methods on fish diversity and composition. Active (cast nets, electrofishing and seines) and passive (gillnets and longlines) fishing gears were employed in lotic, transitional and lentic habitats of the reservoir and three large tributaries. In total 6281 individuals of 101 species were captured. Sampling methods and spatial gradients had an influence on species composition, with significant interactions between these factors. In addition, results produced by a variation partitioning analysis suggested that 21 % of the species richness was explained by the sampling methods and 7 % by the longitudinal gradient; whereas for species composition, 12 and 3 % corresponded to sampling methods and transversal gradients, respectively. Overall, our results suggest that variations among fishing gears is greater than across strong spatial gradients, emphasizing the importance of using multiple and complementary fishing methods in reservoirs for characterization of fish assemblages. The interactions between spatial gradients and sampling methods were higher with transversal than with longitudinal gradients, suggesting that substantial differences in species richness and composition among sampling gears depend to some extent on habitat. The approach used here would be applicable to any large zoned inland waterscape.     

Variation in the effects of larval history on juvenile performance of a temperate reef fish

For organisms with complex life cycles, the transition between life stages can act as a significant demographic and selective bottleneck. Variation in developmental and growth rates among individuals present in one stage (e.g. larvae), due to initial differences in parental input and/or environmental conditions experienced, can propagate to future stages (e.g. juveniles), and such ‘carry‐over effects’ can shape fitness and phenotypic distributions within a population. However, variation in the strength of carry‐over effects between life stages and the intensity of selective mortality acting on intrinsic variation, and how these might be mediated by environmental variability in natural systems, is poorly known. Here, we evaluate variation in the strength to which larval growth histories can mediate juvenile performance (growth and survival), for a reef fish (Forsterygion lapillum) common to rocky reefs of New Zealand. We used otoliths to reconstruct demographic histories of recently settled fish that were sampled across cohorts, sites and microhabitats. We quantified sources of variation in the strength of carry‐over effects and selective mortality that operate on larval growth histories. We found overall that individuals that grew fast as larvae tended to experience proportional growth advantages as juveniles. However, the strength of growth advantages being maintained into the juvenile period varied among cohorts, sites and microhabitats. Specifically, a stronger growth advantage was found on some microhabitats (e.g. mixed stands of macroalgae) relative to others (e.g. monocultures of Carpophyllum maschalocarpum) for some cohorts and sites only. For other cohorts and sites, the degree of coupling between larval and juvenile growth rates was either indistinguishable between microhabitats or else not evident. Similarly, the intensity of growth‐based selective mortality varied among cohorts, sites and microhabitats: for the cohort and site where carry‐over effects differed between microhabitats, we also observed difference in the intensity to which fish with rapid larval growth rates were favoured. Overall, our results highlight how this spatial and temporal patchiness in extrinsic factors can interact with intrinsic variation of recruiting individuals to have a major influence on the resulting distribution of juveniles and their phenotypic traits.     

The interaction of temperature and fish size on growth of juvenile turbot

Growth rate of tagged juvenile turbot was significantly influenced by the interaction of temperature and fish size. The results suggest the optimum temperature for growth of juvenile turbot in the size range 25–75 g is between 16 and 19°C. Optimal temperature for growth decreased rapidly with increasing size, and is between 13 and 16°C for 100 g turbot. Although individual growth rates varied highly at all times within the temperature treatments, significant size rank correlations were maintained during the experimental period. The study confirms that turbot exhibit ontogenetic variation in temperature optimum, which might partly explain different spatial distribution of juvenile and adult turbot in ocean waters.     

The interaction of temperature and fish size on growth of juvenile halibut

Growth rate of individually tagged juvenile halibut was influenced significantly by the interaction of temperature and fish size. The results suggest an optimum temperature for growth of juvenile halibut in the size range 5–70 g between 12 and 15° C. Overall growth rate was highest at 13° C (1·62% day ⁻¹). At c. 5 g at the beginning of the experiment, fish at 16° C had the highest growth rate (3·2% day ⁻¹), but reduced this rate as they grew bigger. At 9 and 11°p C, growth rates were equal or only slightly lower during the later stages of the experiment, while the fish at 6° C showed significantly lower overall growth rate (0·87% day⁻¹). Optimal temperature for growth decreased rapidly with increasing size, indicating an ontogenetic reduction in optimum temperature for growth. Moreover, a more flattened parabolic regression curve between growth and temperature as size increased indicated reduced temperature dependence with size. Although individual growth rates varied significantly at all times within the experimental temperatures, significant size rank correlations were maintained during the experiment. This indicated an early establishment of a stable size hierarchy within the fish groups. Haematocrit was highest at the highest temperature while Na⁺/K⁺-ATPase activity was inversely related to temperature. There was no difference in plasma Na⁺, Cl⁻ and K⁺ concentrations among the temperature groups.     

Hydroacoustic monitoring of the distribution,density and the mass-removal of pelagic fish in a eutrophic lake

A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselkä basin (26 km²) of Lake Vesijärvi to improve the water quality which had remained poor in spite of the termination of nutrient loading. The distribution and density of the fish were studied by hydroacoustics before and during the removal. The initial annual fish density varied between 13 000–21 000 fish ha^–1 in the study area in August 1984–89. The mass-removal of the fish by pelagic trawling took place in 1989–1992. The catch varied annually between 64 and 92 kg ha^–1. Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperlanus (L.)) accounted for c. 85% of the weight of the catch. The mass-removal decreased the pelagic fish density in the Enonselkä basin during the trawling. An increase in the density was observed after the trawling ceased in these years, and the initial density level was reached within one month. The density level after mass-removal remained high compared with oligotrophic lakes. The pelagic fish had a diurnal ascending trend with the decreasing light intensity in August, and the fish were significantly (p < 0.01) higher in the water mass in temperature non-stratified water in autumn than in stratified water in summer. The fish were somewhat deeper in the years of mass-removal than before it. Mass-removal did not affect the individual length of the echosurveyed fish. Fish smaller than c. 15 cm (TS < –44 dB; mainly smelt) were numerically dominant throughout the whole study period.     

Hatching time in spherical,pelagic, marine fish eggs in response to temperature and egg size

Synopsis The relationship between egg diameter ((, mm), temperature (T, °C) and egg development time to hatching (D, in days) was established for approximately spherical, pelagic marine fish eggs as log₁₀D = 7.10 + 0.608 log₁₀ – 4.09 log₁₀ (T + 26), which explains 82% of the variance of a data set of 140 cases, covering 84 species of teleost fishes, for temperatures from 2.8 to 29.5° C and eggs of 0.6 to 3.4 mm diameter.From this relationship it appears that temperature has 4.7 times as much effect on development time as egg diameter when all variables are expressed in standard deviation units. A discussion of these and related factors is given.ICLARM Contribution No. 308.     

Multi-scale recruitment patterns and effects on local population size of a temperate reef fish

Recruitment of the temperate reef fish Coris julis was studied across the Azores Archipelago (central North Atlantic), over four consecutive recruitment seasons and at three spatial scales: between islands (separated by 100s of km), sites within islands (separated by 10s of km) and transects within sites (separated by 10s of m). At the largest scale ( i.e . between islands) spatial recruitment patterns were highly variable, suggesting the influence of stochastic processes. Recruitment was spatially consistent within islands, even though magnitude was unpredictable between years, indicating that processes at meso-scales are probably more deterministic. Recruits settled randomly at the transect scale, probably reflecting habitat homogeneity. It was proposed that large and island-scale patterns reflect larval availability, driven by physical and biological processes occurring in the plankton. No evidence was found for a density-dependent relationship between newly settled and 2 week settled C. julis nor between cumulative recruitment and young-of-the-year. It appears that adult density is limited by larval supply (pre-settlement regulation) at low recruitment sites, and determined by post-settlement, density-dependent processes at high recruitment sites. This work is one of few to investigate multiple spatial and temporal scales of recruitment for a coastal fish species inhabiting isolated, temperate oceanic islands and hence, provides a novel comparison to the many studies of recruitment on coral reefs and other, more connected systems.     

A bounded null model explains juvenile tree community structure along light availability gradients in a temperate rain forest

Ecologists have proposed that tree species may coexist by specialising on light environments associated with gaps of different sizes. Remarkably few community‐level studies, however, have actually examined juvenile tree distributions along light availability gradients. Here we describe distributions of juvenile trees in relation to canopy openness in a temperate rainforest, and test the hypothesis that competitive sorting causes coexisting species to overlap less in light environment occupancy than would be expected by chance. Average overlap of species’ interquartile ranges on the canopy openness gradient was tested against a bounded domain null model of community structure which used range‐size criteria to constrain random placement of species optima. Microsite availability was strongly skewed towards low light, with 43% of microsites occurring at <5% canopy openness. We therefore transformed canopy openness values to ranks, so that equal intervals on the transformed gradient represented equal areas of microsite availability. We then calculated the interquartile range (25–75%) of sample ranks occupied by juveniles of each species. About half the assemblage was non‐randomly distributed in relation to canopy openness, providing evidence of niche expression. Average overlap of species’ interquartile ranges did not depart significantly from that predicted by the bounded null model, indicating that community structure in relation to canopy openness was mainly explained by a mid‐domain effect. As predicted by the null model, species’ interquartile range mid‐points were concentrated in the centre of the rank‐transformed gradient, and species richness (overlap of interquartile ranges) peaked close to the median light environment. Most species therefore had intermediate light requirements. The apparent lack of constraints on pairwise overlap suggest that differences in light use are not a prerequisite for tree species coexistence. As far as we are aware, this is the first study to identify a mid‐domain effect on a resource availability axis.     

The response of fish size and species diversity to environmental gradients in two Neotropical coastal streams

Hydrobiologia - This study investigated the effects of environmental variation on fish diversity patterns in two coastal streams in northwestern Ecuador. Specifically, we examined the role of...