Densities Of Predatory Fish Research Articles

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.

The role of plasticity in the evolution of cryptic pigmentation in a freshwater isopod.

Cryptic pigmentation of prey is often thought to evolve in response to predator-mediated selection, but pigmentation traits can also be plastic, and change with respect to both abiotic and biotic environmental conditions. In such cases, identifying the presence of, and drivers of trait plasticity is useful for understanding the evolution of crypsis. Previous work suggests that cryptic pigmentation of freshwater isopods (Asellus aquaticus) has evolved in response to predation pressure by fish in habitats with varying macrophyte cover and coloration. However, macrophytes can potentially influence the distribution of pigmentation by altering not only habitat-specific predation susceptibility, but also dietary resources and abiotic conditions. The goals of this study were to experimentally test how two putative agents of selection, namely macrophytes and fish, affect the pigmentation of A.aquaticus, and to assess whether pigmentation is plastic, using a diet manipulation in a common garden. We performed two experiments: (a) in an outdoor mesocosm experiment, we investigated how different densities of predatory fish (0/30/60 three-spined stickleback [Gasterosteus aculeatus] per mesocosm) and macrophytes (presence/absence) affected the abundance, pigmentation and body size structure of isopod populations. (b) In a subsequent laboratory experiment, we reared isopods in a common garden experiment on two different food sources (high/low protein content) to test whether variation in pigmentation of isopods can be explained by diet-based developmental plasticity. We found that fish presence strongly reduced isopod densities, particularly in the absence of macrophytes, but had no effect on pigmentation or size structure of the populations. However, we found that isopods showed consistently higher pigmentation in the presence of macrophytes, regardless of fish presence or absence. Our laboratory experiment, in which we manipulated the protein content of the isopods' diet, revealed strong plasticity of pigmentation and weak plasticity of growth rate. The combined results of both experiments suggest that pigmentation of A.aquaticus is a developmentally plastic trait and that multiple environmental factors (e.g. macrophytes, diet and predation) might jointly influence the evolution of cryptic pigmentation of A.aquaticus in nature on relatively short time-scales.

Habitat-specific effects of particle size, current velocity, water depth, and predation risk on size-dependent crayfish distribution

This study assessed effects of abiotic (current velocity, water depth, particle size) and biotic (predation risk for crayfish, size distribution and densities of predatory fish) variables on habitat- and size-specific distribution patterns of lotic crayfish (Orconectes obscurus) using field surveys and tethering experiments. Additionally, particle size manipulations were used with predation assays to assess habitat-specific interactions since the average particle size increased from deep pools to shallow pools to riffles. Large crayfish had the highest densities in deep pools and were associated with increased water depth, whereas small and medium crayfish had the highest densities in shallow pools and were strongly associated with increased particle size and decreased water depth. Regardless of size, crayfish in deep pools had significantly lower survival than in shallow pools and riffles. However, only small crayfish showed consistent differences in predation risk by habitat type and were significantly more vulnerable to predation than larger crayfish. Additionally, large rocky refugia resulted in significantly higher survival of small crayfish in the combined particle manipulation/tethering experiment. Overall, predation appears to be a key mechanism structuring habitat-specific distribution patterns for only small O. obscurus. Large substrates may be particularly important in habitats where both small crayfish density and predation risk are high.

Local and landscape determinants of amphibian communities in urban ponds

Urbanization is currently responsible for widespread declines of amphibian populations globally through the loss, isolation, and degradation of habitat. However, it is not clear how urbanization affects amphibian communities at both local (pond) and landscape scales. We assessed the breeding distribution of frogs in ponds along an urban-rural gradient in Greater Melbourne, Australia, and examined community relationships with habitat quality and landscape context. We sampled frog larvae at 65 ponds on four separate occasions and collected data on local pond and landscape variables. Using Bayesian Poisson regression modeling we found that species richness decreased at ponds surrounded by high densities of human residents and at ponds with high water conductivity, whereas species richness increased substantially at ponds surrounded by a high proportion of green open space. Ordination of individual species presence-absence data by canonical correspondence analysis largely confirmed these findings. Ordination also highlighted the negative influences of pond shading and density of predatory fish, and the positive influence of aquatic vegetation, on community composition. Individual species' responses to urbanization varied. Urbanization had strong negative effects on species that were associated with well-vegetated, sunny, fish-free ponds. Our study highlights the importance of strategic management actions in urban landscapes to improve terrestrial habitat and connectivity around ponds and other wetlands, and local management actions to improve water quality, remove predatory fish, and plant aquatic vegetation at breeding sites.

Trade‐off between steady and unsteady swimming underlies predator‐driven divergence inGambusia affinis

Differences in predation intensity experienced by organisms can lead to divergent natural selection, driving evolutionary change. Western mosquitofish (Gambusia affinis) exhibit larger caudal regions and higher burst-swimming capabilities when coexisting with higher densities of predatory fish. It is hypothesized that a trade-off between steady (constant-speed cruising; important for acquiring resources) and unsteady (rapid bursts and turns; important for escaping predators) locomotion, combined with divergent selection on locomotor performance (favouring steady swimming in high-competition scenarios of low-predation environments, but unsteady swimming in high-predation localities) has caused such phenotypic divergence. Here, I found that morphological differences had a strong genetic basis, and low-predation fish required less hydromechanical power during steady swimming, leading to increased endurance. I further found individual-level support for cause-and-effect relationships between morphology, swimming kinematics and endurance. Results indicate that mosquitofish populations inhabiting low-predation environments have evolved increased steady-swimming abilities via stiffer bodies, larger anterior body/head regions, smaller caudal regions and greater three-dimensional streamlining.

Foraging interaction between monk seals and large predatory fish in the Northwestern Hawaiian Islands

CRITTERCAMS were deployed on 42 Hawaiian monk seals at French Frigate Shoals, Hawaii, to assess the degree to which interaction with large predatory fish may play a role in their foraging success. Sharks, jacks, and large-bodied snappers were seen near the actively foraging seals approximately 17% of the time. Present at all but the greatest depths (>100 m), the predatory fish were most frequently observed when seals foraged at the summits of neighboring banks. Competing fish were encountered as individuals and in schools (mean 4.5, maximum 46 ind.) and often formed mixed-species assemblages that followed seals around. By traveling with foraging seals, the preda- tory fish could exploit the seals' superior ability to flush cryptic prey from bottom cover. The probing, digging, and rock-flipping activities of seals spook prey into the open, making them available to jacks, sharks, and snappers. The jacks Caranx ignobilis and Seriola dumerili were the most success- ful at obtaining prey items flushed from cover by the seals, but carcharhinid sharks were also seen competing for prey. Some adult seals appeared to feed effectively in prey-rich areas despite the pres- ence of high densities of predatory fish. It is not known if the predatory fish have a larger impact on juvenile seals, the segment of the population with the poorest survivorship.

Oviposition habitat selection for a predator refuge and food source in a mosquito

Abstract. 1. The influence of filamentous algae on oviposition habitat selection by the mosquito Anopheles pseudopunctipennis and the consequences of oviposition decisions on the diet, development, body size, and survival of offspring were examined.2. A natural population of An. pseudopunctipennis in Chiapas, Mexico, oviposited almost exclusively in containers with filamentous algae. Algae represented 47% of the gut contents of mosquito larvae sampled from the natural population. Mosquito larvae fed on an exclusive diet of algae developed as quickly and achieved the same adult body size (wing length) as their conspecifics fed on a standard laboratory diet.3. Multiple regression of survival of mosquito larvae on percentage surface area cover of algae (0–99%) and the density of predatory fish (zero to four fish per container) was best described by a second‐order polynomial model. Increasing fish densities resulted in a reduction in mosquito survival in all algal treatments. The highest incidence of survival was observed at intermediate (66%) algal cover in all treatments.4. The presence of fish significantly extended larval development times whereas algal cover had no significant effect. The presence of fish resulted in emergence of smaller adults due to reduced feeding opportunities and predator avoidance behaviour. Algal cover also affected mosquito wing length but differently at each fish density.5. Oviposition habitat selection improves survival in the presence of predators and feeding opportunities for An. pseudopunctipennis larvae.

Structure of cryptic reef fish assemblages: relationships with habitat characteristics and predator density

Cryptic fish assemblages on temperate rocky reefs in New Zealand are among the most diverse in the world, yet very little is known regarding ecological processes that may affect their density and diversity. The potential effects of reef structural complexity, kelp density, and the increased density of predatory fish on assemblages of small, cryptic reef fishes were examined in northeastern New Zealand. Sampling was conducted in replicated areas inside and outside a marine reserve, where differential densities of predatory fishes were known to occur. There was a strong positive correlation between substratum complexity and the density and diversity of cryptic fish assemblages. While the more common fish species occurred in both kelp Ecklonia radiata forest habitat and rocky reefs grazed by urchins, the composition of less abundant species differed between these 2 habitats. Assemblages in kelp forests were more variable than those in urchin barrens. The sites inside the marine reserve contained, on average, lower densities of cryptic fishes than sites outside the reserve, which might be explained by effects of predators. The effect of the marine reserve appeared to be strongest in the kelp forest habitat, with relatively little difference seen between reserve and non-reserve assemblages in unvegetated habitats. If these observed patterns are found to be consistent at other reserves and at other times, they imply that removal of predators by fishing may have large-scale positive effects on assemblages of small cryptic reef fishes.

Response of littoral invertebrates to reduction of fish density: simultaneous experiments in ponds with different fish assemblages

1. We experimentally reduced densities of predatory fish in replicated 2 m2 areas of the littoral zone in two ponds to test whether density and biomass of invertebrates would respond to release from fish predation. The ponds are of similar size and in close proximity, but support different fish assemblages: bluegills (Lepomis macrochirus Rafinesque) and largemouth bass (Micropterus salmoides (Lacepede)) in one pond, and bluespotted sunfish (Enneacanthus gloriosus (Holbrook)) and chain pickerel (Esox niger Lesueur) in the other. Fish densities were reduced to less than 15% of ambient levels in both experiments.2. In the bluegill–bass pond, density and biomass of most invertebrate taxa and size classes were unaffected by the fish manipulation. Total invertebrate densities did not differ significantly between fish treatments, but total invertebrate biomass was significantly greater where fish density was reduced, averaging 30% higher over the course of the study. Likewise, manipulation of fish in the bluespotted sunfish–pickerel pond had few significant effects on individual taxa and size classes. There were no significant effects on total invertebrate abundance in the bluespotted sunfish–pickerel pond.3. Our results provide direct experimental evidence consistent with the collective evidence from previous work, suggesting that the impact of fish predation on density and biomass of invertebrate prey in littoral habitats is variable, but generally weak. Invertebrates that coexist successfully with fish in littoral systems probably are adept at taking advantage of refugia offered by the structurally complex physical environment.

The influence of fish predation on the abundance cycles of an algal turf invertebrate fauna.

A fauna of small invertebrates in a perennial algal turf habitat showed marked seasonal fluctuations in abundance. These were characterised by a summer (December) peak in the abundance of gammarid amphipods and an autumn (April) peak in polychaete densities. Invertebrate abundance was lowest during the period February-March which coincided with an influx of very high densities of juveniles (0+ year class) of the sparid fish Chrysophrys auratus. Both juvenile C. auratus and also the mullid Upeneichthys porosus achieved their highest densities over coralline turf areas and fed on the associated invertebrates. Gammarid amphipods constituted the main food items of each species. All these observations suggested a key role of fish predators in determining the observed seasonal patterns of invertebrate abundance.This hypothesis was investigated by the use of replicated fish exclusion shields and cages which excluded both species from the algal turf and also permitted an assessment of Upeneichthys porosus feeding in the absence of Chrysophrys auratus. The experiment ran from November until June and covered the period of invertebrate and fish abundance changes. Little evidence of a fish predation effect on either the densities of or the timing of abundance peaks of the invertebrate fauna was detected. Sediment analysis of the cage and control experimental sites revealed no evidence of a cage effect on the microhabitat. It is suggested that seasonal changes in the abundance of invertebrates occurs independently of the high densities of predatory fish recorded in this habitat.

The influence of predation and conspecific adults on the abundance of juvenile Evechinus chloroticus (Echinoidea:Echinometridae).

Reefs dominated by red algae, associated with high echinoid densities, are consistent features of the shallow subtidal around northeastern New Zealand. Factors determining the abundance of juvenile Evechinus chloroticus were investigated in such a habitat. Using a factorial design, a field experiment was used to assess the influence of predators and adult E. chloroticus on juvenile abundance. The use of 2 m2 exclusion cages enhanced juvenile E. chloroticus abundance over a 16 month period, an effect independent of conspecific adults. We attributed this effect to the exclusion of benthic-feeding, predatory fish. Several species forage over the study area at high densities and are known from gut content analysis to prey on juvenile E. chloroticus in the field. Invertebrate predators are at very low densities in the area. The possibility of caging and site artefacts confounding this interpretation is discussed.Adult E. chloroticus did not directly affect conspecific juvenile densities during the experimental period. However their removal produced a significant change in community structure toward one dominated by macroscopic brown algae. Echinoid removal led to rapid recruitment of laminarian and fucoid algae, predominantly Ecklonia radiata and Sargassum sinclairii. In addition, densities of herbivorous gastropods, particularly the limpet Cellana stellifera decreased in the echinoid exclusion area, as did the feeding rates of predatory fish. The consequences of E. chloroticus removal may be dependent upon the size of the area from which they are excluded.Despite the high densities of predatory fish, a low though consistent number of juvenile E. chloroticus escape predation. We suggest that these represent sufficient input into the adult grazing population to maintain the habitat. This interpretation argues against a key role for predators in structuring shallow water reef communities in northeastern New Zealand.