The effect of size of food ration on egg production in the female three-spined stickleback, Gasterosteus aculeatus L.

The effect of food level on the egg production of three-spined sticklebacks was studied experimentally. High food levels increased the percentage of fish that matured, and the weight of the fish at maturity. Weight at maturity was positively correlated with the number of spawnings, the mean number and the weight of eggs produced per spawning. Food level had no effect on the size of the eggs, whether size was expressed as wet or dry weight per egg. Fish on the highest food level spawned at shorter intervals than fish at the lower levels. A preliminary model of the effect of food supply on recruitment to a stickleback population is nresented.     

Experimental tests of predation and food hypotheses for population cycles of voles

Pronounced population cycles are characteristic of many herbivorous small mammals in northern latitudes. Although delayed density-dependent effects of predation and food shortage are often proposed as factors driving population cycles, firm evidence for causality is rare because sufficiently replicated, large-scale field experiments are lacking. We conducted two experiments on Microtus voles in four large predator-proof enclosures and four unfenced control areas in western Finland. Predator exclusion induced rapid population growth and increased the peak abundance of voles over 20-fold until the enclosed populations crashed during the second winter due to food shortage. Thereafter, voles introduced to enclosures which had suffered heavy grazing increased to higher densities than voles in previously ungrazed control areas which were exposed to predators. We concluded that predation inhibits an increase in vole populations until predation pressure declines, thus maintaining the low phase of the cycle, but also that population cycles in voles are not primarily driven by plant-herbivore interactions.     

Density-dependent growth and reproduction of the apple snail, Pomacea canaliculata: a density manipulation experiment in a paddy field

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of 15–20 mm shell height into 12 enclosures (pens) of 16 m² at one of five densities – 8, 16, 32, 64, or 128 snails per pen. The survival rate of released snails was 95% and was independent of snail density. The snail density had a significant effect on the growth and egg production of individual snails. This density dependence may have been caused by reduced food availability. The females at high density deposited fewer and smaller egg masses than those at low density, and consequently produced fewer eggs. The females at densities 8 and 16 deposited more than 3000 eggs per female, while the females at density 128 oviposited only 414 eggs. The total egg production per pen was, however, higher at higher snail density. The survival rates of juvenile snails were 21%–37% and were independent of adult density. The juvenile density was positively correlated with the total egg production per pen and hence was higher at higher adult density. However, the density of juveniles larger than 5 mm in shell height, i.e., juveniles that can survive an overwintering period, was not significantly different among density treatments. These results suggest that snail density after the overwintering period is independent of the density in the previous year. Thus, density dependence in growth and reproduction might regulate the population of P. canaliculata in paddies. Received: October 23, 1998 / Accepted: July 16, 1999     

Observations on agonistic behaviour of Clarias gariepinus larvae and juveniles under different densities and feeding frequencies in a controlled environment

Ten-day-old African catfish larvae, Clarias gariepinus , were stocked in aquaria at 5, 13, 22, and 30 larvae per liter with fish at each density being fed three and six times per day over 35 days. Fish were fed with an artificial diet in excess of their satiation requirements at each feeding. Every week total length was determined for samples of 25 fish per tank and all fish were counted at weekly intervals. Behaviour patterns were categorized into timed behaviour (i.e. resting, swimming and browsing), and frequency events e.g. number of contacts, aggressive contacts and number of head or body attacks. Fish at the lowest density grew faster than fish at the other densities. Feeding frequency had no effect on growth. With increasing densities fish less frequently changed timed behaviour patterns while spending a greater percentage of the time swimming. Aggressive behaviour was highest at the lowest density but remained uninfluenced at densities greater than five fish per litre. The high feeding frequency provoked a higher rate of aggressive contacts than the low feeding frequency. A suggested explanation was that these fish were more often in anticipation of food and consequently more active between feedings. In summary, it was shown that density had an effect on duration behaviour patterns such as resting and swimming, while feeding frequency significantly influenced frequency events, in particular the rate of aggressive behaviour. The study contributes to an understanding of factors influencing C. gariepinus juvenile behaviour and suggestions for follow-up experiments are discussed.     

Birth, growth and death rates for enclosed populations of calanoid copepods

Rates of birth, growth and death are inferred for enclosed populationsof the marine copepods Pseudocalanus elongates, Acartia clausiand Temora longicomis. The rates are determined by fitting apopulation dynamics model to the time series of developmentalstage abundances observed over 87 days in four 310 m³ enclosures.Two enclosures were treated with ‘produced’ waterfrom North Sea oil production platforms and two were untreatedcontrols. The pollutant mainly affected recruitment to the populations.Development times varied considerably between enclosures, thoughnot in any apparent relation to the pollutant. Mortality wasmost strongly related to the population size of herring larvaeintroduced to all four enclosures. Levels of secondary productionderived from the model were compared with those calculated directlyfrom population densities.     

Survival, growth and feeding of vendace, Coregonus albula (L.), larvae in net enclosures

Vendace, Coregonus albula (L.), larvae reared in net enclosures anchored in the littoral zone of Lake Ylä-Enonvesi, eastern Finland, were observed for a period of 44 days in order to determine the effects of density on mortality, growth and feeding of the larvae. The ratio of newly-hatched fish to their food was regulated by controlling the number of fish stocked into the enclosures. Prey animals were expected to move freely in and out of the enclosures. The effect of nylon mesh (500 μm) on the penetration of zooplankters into the enclosures was negligible. The influence of possible starvation was determined by analysing the growth of the larvae, their Fulton condition factor, the instantaneous amount of food ingested, and the food composition in their guts. All these indicators were inversely correlated with larval density. Vendace larvae were found to be resistant to mortality from direct starvation. At all densities the survival rate was over 60%. At the time of complete yolk absorption, there was no increase in mortality.     

Effect of food concentration on growth, reproduction and survivorship of Bosmina longirostris (Cladocera)

SUMMARY. 1. Body growth, reproduction and survivorship of Bosmina longirostris were monitored at four food concentrations (0.05, 0.10, 0,25 and 2.50 ing Cl⁻¹) to examine the extent to which food concentration affects these life-history parameters.
2. Food concentration had a significant effect on growth, and most of the reproductive parameters (size and age at maturation, brood size, instar duration and egg development time). More than 60% of animals died before maturation at the lowest food concentration, although most animals survived until maturation at other food concentrations.
3. Change in life-history parameters resulted in a decrease in rate of population growth ( r ) from 0.310 to 0.020 day⁻¹ with decreasing food concentration, showing that this cladoceran is highly food-limited in many lakes.
4. Importance of the effect of food concentration on egg development time was emphasized in relation to analysis of natural populations.     

斜生栅藻密度对新安江萼花臂尾轮虫种群动态的影响

以浓度分别为1.0×106、2.0×106、4.0×106和8.0×106cells·mL-1的斜生栅藻(Scenedesmus obliquus)为轮虫食物,在25℃下,应用群体累积培养和单个体培养法研究了食物浓度对采自新安江水域(屯溪段)的萼花臂尾轮虫的种群动态影响。结果表明:食物浓度对轮虫的种群增长率和休眠卵产量均有显著的影响,均表现为低食物浓度下(≤4.0×106cells·mL-1)萼花臂尾轮虫的种群增长率和休眠卵产量较小且无差异,高食物浓度下(8.0×106cells·mL-1)轮虫的种群增长率和休眠卵产量较大;食物浓度对轮虫的混交雌体百分率和受精率无显著影响;在食物浓度为4.0×106cells·mL-1时萼花臂尾轮虫的净生殖率最大,世代时间最长,而其内禀增长率在食物浓度为1.0×106cells·mL-1时最小;在食物浓度为8.0×106cells·mL-1时平均寿命和生命期望最长。     

Control mechanisms of arctic lake ecosystems: a limnocorral experiment

To assess the potential impact of human exploitation on arctic lakes and to determine how these eco systems are regulated we initated a limnocorral experiment in Toolik Lake, Alaska, in the summer of 1983. The limnocorrals were 5 m in diameter and from 5–6 m in depth and were open to the sediments. In 1983 four limnocorrals were deployed in an isolated bay of Toolik Lake within a cross-classified treatment regime of high and low inorganic nitrogen and phosphorus additions and high and low free swimming fish additions. The objective of the nutrient addition was to stimulate phytoplankton growth and determine the extent to which increased plant production was passed through pelagic and benthic food chains. The objective of the fish addition was to determine the impact of fish predation on large-bodied zooplankton, especially the zooplanktivorous copepod Heterocope, then to study the effect of altered Heterocope densities on small-bodied zooplankton species population dynamics. In 1984 two more limnocorrals were deployed, one a low fish, 1 × nutrient addition treatment and the other a no fish, no nutrient treatment. The fish manipulation was changed to confining several fish in cages with the cages held in corrals for varying lengths of time. The addition of inorganic nitrogen and phosphorus dramatically increased phytoplankton productivity. This increase in algal biomass and production greatly altered the light environment and water quality in the nutrient treated limnocorrals. The secchi disk depth in the nutrient treated limnocorrals declined each summer reaching as low as 1 m in 1985. Both oxygen content and pH increased in the nutrient treatment corrals. Corrals not receiving nutrient additions remained near lake concentrations for most water quality parameters. While phytoplankton biomass was stimulated in 1983 phytoplankton growth was not sufficient to draw down all the nitrogen and phosphorus added and these nutrients reached high levels in the last half of the summer. In 1984 phosphorus remained above 20 μg in the nutrient-treated corrals but ammonia dropped to reference levels by day 25. In 1985 both nutrient concentrations rapidly declined to reference levels. Most pelagic components responded to the nutrient additions. Microbial production was stimulated in the nutrient treated limnocorrals and bacterial population sizes built up to nearly 8–10 times those of the reference corrals. However, microheterotrophs soon increased in abundance and apparently grazed down bacteria to reference levels. Phytoplankton population density, as estimated by chlorophyll a determinations, increased dramatically with nutrient addition such that each year the phytoplankton densities were higher than before. Primary productivity was also stimulated and appeared not to be light limited even when phytoplankton densities rose to high levels. In the first two years of the experiment zooplankton densities were little altered by the increased phytoplankton densities. However, by 1985 daphnid densities were quite a bit higher in the high nutrient addition limnocorrals. The benthic community and sediment response was much less affected by nutrient addition. Overall sediment respiration increased in the nutrient treated corrals but underlying sediments seemed little affected. Decomposition of Carex litter was likewise little affected by nutrient addition. Benthic invertebrates were also little impacted by the nutrient addition and increased sedimentation of phytoplankton. However, the response of benthic invertebrates is difficult to assess fully in the current experiment because chironomids, a prominent component of the benthic community, failed to recruit into the limnocorrals and the corrals physically shifted during ice-out in the spring of 1984 disturbing the sediment in several corrals. The fish additions in 1983 of free swimming grayling essentially eliminated large bodied zooplankton, especially Heterocope septentrionalis, from all four limnocorrals. In subsequent summers Heterocope were not so dramatically preyed upon but generally were found in higher densities in the low or no fish treatments. However, either when Heterocope were eliminated in 1983 or were in rough inverse proportion to fish density, altered Heterocope abundance had no obvious affect on small-bodied zooplankton abundance. The fish treatment apparently influenced the zooplankton response to high nutrient addition in 1985. In the high nutrient limnocorrals daphnid populations became very abundant, but in the high fish treatment the daphnid responding was the small-bodied D. longiremis while in the low fish treatment the daphnid responding was the large-bodied D. middendorffiana. Thus we have considerable evidence for bottom up control of phytoplankton density and production. This increased production ultimately, but not for two years, stimulated zooplankton density increases. Increased nutrients had little effect on the benthos or sediments. Fish manipulations influenced large-bodied zooplankton but had little effect on small-bodied zooplankton. Because grayling are predominantly plankton feeders in lakes, no fish effect on benthic invertebrates was expected. Limnocorrals thus seem good systems to study nutrient-phytoplankton interactions. They are not as suitable for benthic invertebrate studies and fish manipulations may be difficult. Most other limnocorral studies were of brief duration; however, in the present study the limnocorrals seemed to perform well over a three year period.     

Short‐term Costs and Benefits of Habitat Complexity for a Territorial Fish

An increase in habitat complexity is thought to decrease visibility and the territory size of visually oriented animals. Hence, the addition of physical structure has been viewed as a useful restoration technique to increase the density of territorial species, particularly in stream fishes. However, a decrease in territory size may have a negative effect on the fitness of individual organisms. We attempted to quantify some of the positive and negative effects of increasing habitat structure on the behaviour and growth rate of wild young‐of‐the‐year (YOY) Atlantic salmon. Fish were exposed to one of two habitat treatments in mesh enclosures in Catamaran Brook, New Brunswick: a gravel substrate (low complexity) or a gravel substrate with boulders added (high complexity). Wild‐caught individuals were tagged, weighed and measured before being stocked at densities of 1·m^?2 for 7‐d trials. While fish from high‐complexity treatments benefited from lower rates of aggression, they also had lower foraging rates and smaller territories compared to those in low‐complexity treatments. Specific growth rate, however, did not differ significantly between treatments. While the addition of structure to a habitat may be beneficial at the population level in terms of an increase in population density, our results suggested that individual fish may pay some short‐term costs in these environments. Further research is needed to evaluate the longer term costs and benefits of adding structure to improve the habitat quality for stream salmonids.     

Less is more: density influences the development of behavioural life skills in trout

Theory suggests that habitat structure and population density profoundly influence the phenotypic development of animals. Here, we predicted that reduced rearing density and increased structural complexity promote food search ability, anti-predator response and the ability to forage on novel prey, all behavioural skills important for surviving in the wild. Brown trout were reared at three densities (conventional hatchery density, a fourth of conventional hatchery density and natural density) in tanks with or without structure. Treatment effects on behaviour were studied on trout fry and parr, whereupon 20 trout from each of the six treatment groups were released in an enclosed natural stream and recaptured after 36 days. Fry reared at natural density were faster to find prey in a maze. Moreover, parr reared at natural density were faster to eat novel prey, and showed more efficient anti-predator behaviour than fish reared at higher densities. Furthermore, parr reared at reduced densities were twice as likely to survive in the stream as trout reared at high density. In contrast, we found no clear treatment effects of structure. These novel results suggest that reduced rearing densities can facilitate the development of behavioural life skills in captive animals, thereby increasing their contribution to natural production.     

Food consumption and growth of larvae and juveniles of three cyprinid species at different food levels

Synopsis The relationships between food availability, consumption and growth were analyzed from the onset of feeding to an age of 90 days in three cyprinid species. Fish were held at 20 ± 0.5° C and given two (three) constant rations of approximately 30, (40) or 100% dry body weight (dbw) ind^-1 day^-1. Food consisted of living zooplankton, the size of which correlated with fish size. At high food densities consumption rates decreased rapidly with fish size in all three species. At reduced rations, fish consumed most of the food offered until they were larger than 10 mg dbw. In all species and at each feeding level daily rations consumed increased allometrically with body size. Respiration rate, expressed as routine metabolic rate differed insignificantly between the three species. At high ration levels, growth rates of small bleak, Alburnus alburnus, were distinctly lower than those of roach, Rutilus rutilus, and blue bream, Abramis ballerus. At low food supply all three species grew at similar rates. Assimilation efficiency at low food conditions was approximately twice that of the well-fed groups. If the caloric equivalents of oxygen consumption as measured in well-fed fish are applied to fish fed at low rations their energy budgets do not balance. This indicates the limitations of fish larvae in the partitioning of energy for growth or activity at such conditions.     

Individual space use strategies of wild juvenile Atlantic salmon

Movements of 60 stream-dwelling wild Atlantic salmon Salmo salar (97–118 mm), each tagged with a passive integrated transponder, were monitored during four trials in an enclosed section (24 m long, 45·1 m² total area) of a stream at a range of densities (four, eight, 16 and 32 fish per enclosure). Patterns of space use differed markedly between individuals, with 80% of fish establishing home ranges within 8 days of introduction to the enclosure (settlers) and the remainder continuing to move throughout the length of the enclosure (non-settlers). Although aggressive interactions were quite frequent and dominant fish were observed chasing subordinates, there was considerable overlap of home ranges of settlers at all densities; this was the case even at lower densities at which only a fraction of the enclosure was used by the fish. Thus, rather than adopting fixed territories, the salmon showed a high level of space sharing. Individual fish used the same local area in different ways, ranging from highly localized feeding on drifting food items to a wider-ranging strategy of specialising on benthic food. Among the fish that settled absolute growth rates were inversely related to body size, and at high densities fish lost weight. These results suggest that space use in wild juvenile salmon is more complex than a mosaic of territories, that salmon demonstrate significant variability in individual space use patterns, and that large fish may suffer disproportionately when populations are at the carrying capacity of their environment.     

In situ life table of a subtropical copepod

• 1 A life table of a subtropical population of Mesocyclops edax(Copepoda: Cyclopoida) was constructed from a cohort of individuals reared in enclosures in a small, eutrophic lake.
• 2 The lake population was sampled concurrently to determine age structure, per capita birth, death and growth rates, body size and clutch size.
• 3 Maturation times of enclosed males and females were 9.3 and 11.4 days, respectively. The population growth rate. r. was estimated to be 0.18 females day^-1 for the enclosed population, but approximated zero on most dates in the lake population. Mortality was low in the enclosures which excluded predators and high in the field population exposed to planktivorous fish and Chaoborus.
• 4 The means and variances of several fitness traits are related to the specific environment. The enclosure approach promises to be a useful tool in the study of zooplankton life history variation.

     

Effects of crowding and availability of food on growth rate of fry in the European and Chinese races of the common carp

Fry of the European and Chinese races of the common carp and interracial crossbred fry were grown together in a series of ponds for 28 days after hatching. The degree of crowding and the amount of available natural food were intentionally varied in these experimental ponds. This resulted in a large variation in mean growth between ponds, ranging from 0·1 to 5·2 g per fish during the 28 days growth period. Under conditions of very slow growth, resulting from severe crowding and lack of natural food, fry of the Chinese carp grew several times faster than European carp. At lower densities and when natural food was more abundant, resulting in a faster rate of growth, this difference between the two races disappeared. Crossbred fry between the Chinese and European carp grew faster than either of their parents under all conditions tested.     

Is fecundity in plaice (Pleuronectes platessa L.) down‐regulated in response to reduced food intake during autumn?

The effect of controlling food intake during the autumn, which is the time of late vitellogenesis, on fecundity, atresia and follicle and ovary growth was examined for plaice Pleuronectes platessa . Eighteen fish were kept in individual pens and either fed on a high or low ration diet. Fish which increased in whole body condition exhibited an increase in carcass condition which means that when food intake is sufficient to maintain whole body condition some resources are used as storage. Follicle growth rate was positively correlated with change in Fulton's condition and total atresia was negatively correlated with change in Fulton's condition. Thus, the rate of vitellogenesis was dependent on the availability of an exogenous food source. Fecundity at the end of the experiment was positively correlated with mass and total length. Food intake had no effect on relative fecundity; however, fish which had a lower food intake lost mass and had a greater intensity of atresia, lowering their absolute fecundity. One fish in a very low condition at the start of the experiment skipped spawning and one fish exhibited a decrease in average follicle diameter during the experiment which is hypothesized to be a prelude to mass atresia.     

Density‐dependent diel activity in stream‐dwelling Arctic charr Salvelinus alpinus

Intraspecific competition plays a significant role in shaping how animals use and share habitats in space and time. However, the way individuals may modify their diel activity in response to increased competition has received limited attention. We used juvenile (age 1+) Arctic charr Salvelinus alpinus to test the prediction that individuals at high population density are more active and distribute their foraging activity over a greater portion of the 24‐h cycle than individuals at low population density. Individually tagged fish were stocked in seminatural stream enclosures at low (2 fish/m²) and high (6 fish/m²) density. During each of two 2‐week experimental rounds, activity of all fish within each enclosure was recorded every 3 h over seven 24‐h cycles. At high density, fish were more active and distributed their activity over a greater portion of the 24‐h cycle, with increased activity particularly at crepuscular times. Fluctuations in ecological conditions (e.g., water temperature and light intensity) also affected activity. Fish at high density grew as fast as fish at low density. This study demonstrates that individuals exhibit a degree of behavioral flexibility in their response to changes in ecological conditions and suggests that intraspecific competition can cause animals to modify temporal aspects of their activity to gain access to resources and maintain growth.     

Impact of fish predation on coexisting Daphnia taxa: a partial test of the temporal hybrid superiority hypothesis

Fish predation was tested as a factor mediating the coexistence of Daphnia taxa in the shallow, hypertrophic Lake Blankaart. Naturally co-occurring populations of D. galeata and the hybrid D. galeata x cucullata were subjected to different levels of fish predation in in situ enclosures. In control enclosures without fish, the largest taxon D. galeata rapidly became dominant over the intermediate sized D. galeata x cucullata, mainly as a result of higher birth rates. In enclosures with fish, population densities of D. galeata dropped relative to D. galeata x cucullata, due to higher mortality rates. These results are in concordance with the `temporal hybrid superiority hypothesis', and can be explained by a higher vulnerability of the large and more conspicuous D. galeata to the size selective predation exerted by visually hunting planktivorous fishes. After approximately one month, however, population growth rates of D. galeata and D. galeata x cucullata in the enclosures with fish converged, due to a relative reduction in the mortality rate of D. galeata. This suggests that, in the presence of fish, D. galeata may co-exist with hybrids due to a decrease in its relative vulnerability to visual predation with time. Indeed, both D. galeata and the hybrid showed strong reductions in adult body size in the enclosures with fish, but this size reduction tended to be stronger in D. galeata than in D. galeata x cucullata. In addition, turbidity increased in the enclosures with fish and may additionally have reduced the relative advantage of D. galeata x cucullata with regard to mortality caused by visual predation.     

Competition in Bembicium auratum (Gastropoda) and its effect on microalgal standing stock in mangrove muds

Summary Field experiments were undertaken on the intertidal gastropod Bembicium auratum, which is one of the dominant organisms in sheltered bays and mangroves in New South Wales. Animals were caged at various densities ranging from natural levels to about four times normal density. Increased density resulted in increased mortality (particularly of juveniles), reduced body weight, and (less conclusively) a decline in growth rate. Chlorophyll levels of the mangrove mud within the cages were monitored as an index of food availability, and increased rapidly in the control cages in the absence of Bembicium, remained steady at normal Bembicium densities, and declined at higher densities. Parallel experiments were conducted on the effects of substratum on Bembicium. One series of animals was allowed access to a hard substratum and the other only to mud. Body weights and survivorship were proportionally lower in animals deprived of a hard substratum. Juveniles were particularly susceptible to a shortage of food and more dependent on a hard substratum than adults. The population structure also suggests a high juvenile mortality but high adult survival and longevity. Bembicium is very abundant low on the shore but numbers decline and body weights increase higher up. Chlorophyll levels and amounts of hard substratum also decline up the shore. Possibly juveniles settle selectively (or survive better) low on the shore where oysters provide a dense hard substratum. Adults are less dependent on the hard substratum and may disperse up the shore, explaining the patterns of abundance and size.As Bembicium limits its own food supply and seemingly is limited by it, predation is unlikely to play an important role in restricting population density.     

Experimental evidence that ecological effects of an invasive fish are reduced at high densities

Understanding the relationship between invasive species density and ecological impact is a pressing topic in ecology, with implications for environmental management and policy. Although it is widely assumed that invasive species impact will increase with density, theory suggests interspecific competition may diminish at high densities due to increased intraspecific interactions. To test this theory, we experimentally examined intra- and interspecific interactions between a globally invasive fish, round goby (Neogobius melanostomus), and three native species at different round goby densities in a tributary of the Laurentian Great Lakes. Eighteen 2.25 m² enclosures were stocked with native fish species at natural abundances, while round gobies were stocked at three different densities: 0 m^?2, 2.7 m^?2, and 10.7 m^?2. After 52 days, native fish growth rate was significantly reduced in the low density goby treatment, while growth in the high density goby treatment mirrored the goby-free treatment for two of three native species. Invertebrate density and gut content weight of native fishes did not differ among treatments. Conversely, gut content weight and growth of round gobies were lower in the high goby density treatment, suggesting interactions between round gobies and native fishes are mediated by interference competition amongst gobies. Our experiment provides evidence that invasive species effects may diminish at high densities, possibly due to increased intraspecific interactions. This is consistent with some ecological theory, and cautions against the assumption that invasive species at moderate densities have low impact.