Morphology and defensive structures in the predator-prey interaction: an experimental study of Parabroteas sarsi (Copepoda,Calanoida) with different cladoceran prey

Parabroteas sarsi is a predaceous calanoid copepod commonly found in South Andes lakes. Feeding experiments were carried out in order to estimate the predation rates and attack patterns on different cladoceran prey. Predation rates were related with prey sizes. The smallest prey, Bosmina longirostris, was ingested up to 5 prey pred^–1 h^–1 while the largest, Daphnia middendorffiana, only at 0.12 prey pred^–1 h^–1. The functional response of P. sarsi differed when confronted with different prey although in all cases, the number of kills increased with prey density. A saturation of ingestion rates at high prey densities was only observed for B. longirostris and Ceriodaphnia dubia juveniles. Remains seldom appeared at the end of the experiments, implying that the predator consumed prey totally. Yet, in all experiments carried out with Daphnia ambigua and D. middendorffiana, remains, including soft parts, were found. Direct observations of attack and of number and types of remains showed interference of handling by tail, helmet, and size of the prey.     

Distribution,production, and age structure of slimy sculpin in an Arctic lake

Synopsis Slimy sculpins (Cottus cognatus) were caught in the littoral region of Toolik Lake, an arctic lake. These sculpins grew slower and lived longer (71 mm at 8⁺ years) than more southerly populations. Sculpin distribution along the slope of the rocky littoral zone was greatest at the 3.5 m depth and coincided with the rock-mud interface. The hypothesis that this preferred area provided both increased prey and reduced predation was advanced. Yearly production estimate of littoral sculpins was 0.40 g · m^–2 · yr^–1 with a P/B ratio of 0.36. This is considerably less than estimates for more southerly populations of the same species.     

Arctic charr in sympatry with burbot: ecological and evolutionary consequences

The trophic niche and parasite infection of Arctic charr (Salvelinus alpinus) were explored in two lakes with sympatric burbot (Lota lota) and two lakes without burbot in subarctic Norway. The CPUE of burbot and charr were similar in one lake, but burbot had a low population density in the other. Burbot were benthivorous in both lakes. Other co-occurring species like brown trout (Salmo trutta), Atlantic salmon parr (Salmo salar), grayling (Thymallus thymallus) and minnow (Phoxinus phoxinus) were also benthivores. At high densities, benthivorous burbot forced the whole Arctic charr population to utilise mainly the limnetic trophic niche. In contrast, at low burbot density or without burbot present, Arctic charr were primarily benthivorous in the littoral zone. Thus, a clear interactive segregation in diet was observed between Arctic charr and burbot at high burbot densities. There was also a high predation pressure from burbot on young Arctic charr along the benthic zones. The extensive use of zooplankton as prey caused a high parasite infection pressure of copepod transmitted Diphyllobothrium spp. larvae, with the potential for high negative impact on the Arctic charr population. As the benthivore trophic niche was occupied by burbot, the ecological opportunities for polymorphism with benthivorous ecotypes or morphs of Arctic charr were probably prevented. Therefore, the sympatry with burbot seems to have large ecological and evolutionary consequences for this Arctic charr population compared with neighbouring lakes where burbot is absent.     

Predator-prey relationships within the pelagic community of Neusiedler See

Neusiedler See, a shallow alkaline lake, has become increasingly eutrophic; this enrichment improved the nutritive situation of the herbivorous zooplankton leading to a higher standing stock. A multiple regression analysis of the long-term development of the crustacean plankton indicates that abiotic factors (i.e. wind, temperature) have the most important impact on the community in spring and autumn, biotic factors (i.e. food, predation) during the summer months. Currently an invertebrate (Leptodora kindti) and two planktivorous fish (Pelecus cultratus and Alburnus alburnus) control the population development of Diaphanosoma mongolianum during summer. L. kindti predation acts on immature stages, whereas the fishes consume adult stages. L. kindti densities of 100 to 200 ind. × m^-3 affect the prey population to a negligible extent; densities between 300 and 500 ind. × m^-3 result in elimination rates of 5% to >40% of the juvenile Diaphanosoma stock. The impacts by the invertebrate predator are pronounced but short-term events. Juvenile and underyearling fish eliminate 4–13% of the zooplankton in the open lake and 5–33% inshore; their predation pressure acts on all younger stages of the crustaceans. Planktivorous fish older than 0 + concentrate on the large food items (adult stages of the crustaceans). A. alburnus is able to consume 6–16% of the zooplankton standing stock during summer. P. cultratus eliminates about 1–49% of D. mongolianum, 1–4% of A. spinosus and 1–31% of L. kindti. From May until August the position of the dominant predator changes, beginning with juvenile fish which are then followed by P. cultratus and then by A. alburnus. Predation by L. kindti is of importance in July and August; at that time it causes suppression of the juveniles of D. mongolianum, but, on the other hand, the older stages of L. kindti are endangered themselves by the predatory impact of planktivorous fish.     

A comparison of Visual Prey Detection Among Species of Piscivorous Salmonids: Effects of Light and Low Turbidities

Differences in reaction distance to prey fish by piscivorous salmonids can alter predator–prey interactions under different visual conditions. We compared reaction distances of three piscivorous salmonids commonly found in western lakes: cutthroat trout, Oncorhynchus clarki utah, rainbow trout, O. mykiss, and the nonnative lake char, Salvelinus namaycush. Reaction distances to salmonid prey were measured as functions of light and turbidity in a controlled laboratory setting. In addition, predation rates and swimming speeds of lake char preying on juvenile cutthroat trout were measured experimentally under a range of light levels. Reaction distances for cutthroat trout and rainbow trout increased rapidly as light levels increased, reaching relatively constant reaction distances at higher light levels. Reaction distances for lake char were similar to cutthroat trout and rainbow trout at the lower light levels; however, lake char reaction distances continued to increase with increasing light intensity to asymptote at distances 65% higher than those for both cutthroat and rainbow trout. Predation rates by lake char were low for the darkest light levels, increased rapidly under low light levels (0.50–0.75lx), and then declined to an intermediate rate at all higher light levels. Swimming speeds by lake char also increased rapidly from extremely low light conditions to a peak and declined to an intermediate level at light levels above 1.00lx. These results suggest that, above the saturation intensity threshold, piscivorous lake char react to fish prey at greater distances than do cutthroat trout and rainbow trout. These differences may help explain the decline of native trout following the introductions of nonnative lake char in lakes and reservoirs of western North America.     

Partitioning of food resources by sculpins in Lake Michigan

Synopsis Resource partitioning was studied in two benthic Lake Michigan fishes, the deepwater sculpin,Myoxocephalus thompsoni, and the slimy sculpin,Cottus cognatus, that exhibit nearly disjunct distributions along a hypolimnetic depth gradient. Fish were collected in an area of sympatry over two 24 h periods. These sculpins exhibited food segregation—slimy sculpins ate primarilyPontoporeia affinis, deepwater sculpins ate bothP. affinis andMysis relicta — and their depth segregation was associated with the reported abundance of these prey. Different feeding behaviors may be responsible for this association, since slimy sculpins consumed intermediate size ranges ofPontoporeia and deepwater sculpins consumedPontoporeia in a pattern similar to the ambient size distribution. Neither fish showed a daily activity cycle, and both appear food-limited.     

Takvatn Through 20 Years: Long-term Effects of an Experimental Mass Removal of Arctic Charr, Salvelinus Alpinus, From a Subarctic Lake

Between 1984 and 1989, the experimental removal of 31 tons (666000 fish) of stunted Arctic charr, Salvelinus alpinus, from Takvatn in northern Norway, had strong effects on the populations of Arctic charr, brown trout, Salmo trutta, and three-spined sticklebacks, Gasterosteus aculeatus. The littoral catch per unit effort (CPUE) of charr had decreased by 90% in 1990 and then increased to about 50% of the initial level by 1994 while the pelagic CPUE had decreased to zero. Growth in both charr and trout greatly improved when the charr density had decreased, and large fish of both species appeared in the catches. These large fish became predators on small charr in the littoral zone. The incidence of trout increased from below 1% to 15% from 1988 to 1999 after a brief peak at 30% in 1992 and 1993. The charr population attained a bimodal size distribution and did not return to the stunted state during the 10 years following the intensive fishing period. The mass removal experiment showed that it is possible to change the structure of a charr population by intensive fishing. Predation on small charr from cannibals and large trout was probably essential for maintaining the new population structure. An increase in the growth of young charr from 1995 to 1997 was related to a high consumption of Daphnia and Eurycercus. Rapid changes in the growth of charr followed the density fluctuations in sticklebacks, which show large annual variations in this system; the rapid changes in charr growth were probably caused by variations in the competition intensity for cladoceran prey between young charr and sticklebacks. Twenty years of data has provided important information, but even more time is needed to follow the long-term trends in northern lakes such as Takvatn.     

Distribution of Gammarus lacustris Sars (Amphipoda, Gammaridae) in Lake Shira (Khakasia, Siberia) and laboratory study of its growth characteristics

Spatial distribution of Gammarus lacustris in Lake Shira and growth of young specimens that feed on lake biota were studied. The amphipods have been shown to inhabit the littoral, sublittoral and the upper aphytal zones of the lake on stony-sandy soil and silted sand. The young and adult individuals stay apart. The young live in submerged or semi-submerged vegetation in the littoral, the adults in the sublittoral and upper aphytal zones. Maximum density of amphipods was encountered in the areas influenced by human activity.The feeding experiments revealed that the lake plankton is a more important food source for G. lacustris than any other food species. The specific growth rate measured was 0.039 d^–1, with a length increment 0.095 mm d^–1.     

Nitrogen uptake from prey and substrate as affected by prey capture level and plant reproductive status in four carnivorous plant species

Uptake of nitrogen from prey and substrate and partitioning of prey-derived nitrogen were studied in the carnivorous plant species Pinguicula alpina, P. villosa, P. vulgaris and Drosera rotundifolia in a subarctic environment. Efficiency in nitrogen uptake from prey was evaluated by tracing ¹⁵N from ¹⁵N-enriched Drosophila flies fed to the plants. The in situ uptake efficiency differed somewhat between species and ranged from 29 to 41% of prey N. This efficiency was not affected by different feeding levels or plant reproductive status (flowering or non-flowering). A test of the amount of N absorbed from prey caught on flower stalks of Pinguicula villosa and P. vulgaris showed that both species took up little of what was available in prey (2.5% or less). The uptake efficiency found in greenhouse grown plants was higher than in plants in situ (40–50% vs. 30–40% respectively). This could probably best be explained by the absence of rain and a higher temperature in the greenhouse. The prey-derived ¹⁵N was traced to reproductive organs and winter buds. Non-flowering individuals allocated 58–97% of the N derived from prey to their winter buds. Flowering individuals allocated 17–43% of the N income from prey to reproduction, while 34–71% were allocated to buds. Root uptake of nitrogen was stimulated by increased prey capture. This increase in uptake of nitrogen from the substrate was larger than the potential direct uptake of nitrogen from captured prey.     

A laboratory study of feeding and assimilation in Euchlanis dilatata lucksiana

Feeding of Euchlanis dilatata lucksiana, a brachionid rotifer isolated from the Loosdrecht Lakes (The Netherlands), was examined in the laboratory using ¹⁴C-labelled food. The gut-filling time at a food concentration of 9.6 µg C ml^–1 was about 15 minutes. Animals which were fed on 3 size fractions of the lake seston (< 7 µm, 7–15 µm, and 15–33 µm) showed a clear preference (Jacobs' selectivity index) for the largest size fraction. This fraction was composed predominantly of filamentous cyanobacteria. For animals weighing 0.37–0.49 µm C ind.^–1 the daily ration (daily food consumption per unit body weight × 100) ranged from 50 to 100% at food levels of 2 mg C l^–1 and below, but increased to 150–250% at food concentrations of 5 mg C l^–1 and above. The assimilation efficiency was 100% up to 5 mg C l^–1 of food, but decreased to about 80% at higher food levels.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) II. Experimental study

Interactions between the larvae of Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel, and their zooplankton prey were studied experimentally. Prey species preference and size selectivities were measured. Larvae were hatched in the lab from eggs collected in the field, and fed different food items in various concentrations. The food items included lake zooplankton, algae, and commercial pellets. It was shown that small, first feeding larvae (7–8.5 mm SL) prefer small bodied zooplankters (< 180 µ). The effect of these food sources on larval growth was measured. It was found that larval Mirogrex grew at a higher rate when fed zooplankton prey sized from 63 µ–250 µ. Food items smaller than 63 µ, larger than 250 µ and Scenedesmus sp., produced less than optimal growth rates. The importance of Mirogrex feeding habits and their potential influence on the Kinneret ecosystem is considered.     

Spatial and Temporal Variability in the Diet of Anadromous Arctic Charr, Salvelinus Alpinus, In Northern Labrador

Spatial and temporal variation in the diet of anadromous Arctic charr, Salvelinus alpinus, was determined from over 2500 prey-containing stomachs collected from three stock complex areas in northern Labrador. Based on the percent wet mass, sand lance, Ammodytes spp., capelin, Mallotus villosus, and sculpins, Triglops spp. and Myoxocephalus spp., were the most important components of the diet in the Voisey stock complex, with capelin, sand lance, and hyperiid amphipods, Parathemisto spp., dominating in the Nain region. In contrast, hyperiid amphipods and sculpins were the main prey items in the more northern Hebron–Saglek complex indicating that diet differs among stocks, and at times over limited spatial scales. The relative importance of prey varied with size of predator. Mean weight of charr has declined over time (p < 0.01), and for one stock complex, is highly correlated with the average quantity of capelin found in the stomachs (r = 0.66, p < 0.001). Dietary shifts observed in anadromous charr in some areas of northern Labrador could be associated, in part, with changes in the distribution of capelin in the northwest Atlantic Ocean, possibly as a result of anomalous ocean climate conditions.     

Field estimations of temperature-dependent processes: case growth of young burbot

In this study, the growth of young burbot (Lota lota) was temperature-dependent, but varied between age-groups. The growth of age-0 increased with temperatures 4–18°C. Growth of age-groups 1 and 2 was not significantly related to surface water temperatures and varied between the 2 years studied. This study was a re-evaluation of burbot sampled between 1978 and 1980 by plankton net, beach seine, and electric fishing in Lake Kuohijärvi, the Finnish Lake District. In the re-evaluation, growth was studied both as mm day^–1 and as absolute growth (mm), whereas the original study did not include absolute growth. Though the methods should be equivalent, they are not. In our example, time between measurements, temperature, and growth were negatively correlated. When analysing mm day^–1 the estimate variability, therefore, increased with temperature, whereas the variability was homogenously spread on the two axes, growth and degree-day, when analysing absolute growth. Field rate analyses contain an error risk that should be recognised when assessing the temperature-dependency of any processes, e.g. growth or mortality.     

The effects of predation on the survival and size-distribution of estuarine fishes: an experimental approach

Synopsis The effect of southern flounder,Paralichthys lethostigma, predation on the survival and size-distribution of spot,Leiostomus xanthurus, was investigated using feeding experiments in outdoor tanks and estuarine ponds. In the outdoor tank experiments, southern flounder, 143 mm average standard length (SL), fed for 48 h on spot (25–30 mm SL), over a range of densities (4–20 m^–2). In these trials, flounder showed a Type II functional response to spot density. In the estuarine pond experiment, flounder density (2 or 4 flounders 70 m^–2) contributed significantly to spot mortality relative to predator-free controls. The effect of flounder density on spot mortality was non-linear; in the 4-flounder treatment the daily instantaneous mortality due to flounder (0.0177) was 3.5 times that in the 2-flounder treatment (0.0051). The total spot mortality rate in the 4-flounder treatment (0.028) was similar to that observed from published field observations. Flounder also altered the size distribution of spot relative to predator-free controls. Predation by flounder resulted in fewer intermediate-sized spot at the end of the experiment when compared to predator-free controls. Flounder also significantly reduced survival and mean length of two killifishes,Cyprinodon variegatus andFundulus heteroclitus, that invaded the pond. Flounder predation was a significant size-structuring force on the prey fish assemblage in the pond. Results from both the outdoor tank experiment and the pond experiment indicate that flounder may represent a significant source of mortality for juvenile estuarine fishes.     

In situ predatory behavior ofMysis relicta in Lake Michigan

Selectivity coefficients (W) and predation rates on Lake Michigan zooplankton were determined forMysis relicta during spring through fall using anin situ method. W values indicated the following ranked order of prey preference: Cladocera > copepod copepodites and copepod nauplii > adult diaptomids and cyclopoids. With few exceptions, W values for different prey categories remained fairly constant despite greatly changing relative abundances of prey. Predation rates and prey selectivity were similar in most cases to those determined in laboratory studies. Ingestion rates (percent dry body weight · day^–1) were correlated to total prey biomass (r = 0.38) and to effective prey biomass (r = 0.85), where the weighting factors were overall mean selectivity coefficients for the different prey categories. This result suggested that seasonally varying composition of prey caused much of the variation in ingestion rates among experiments. Feeding trials performed at the same depth with daytime and nighttime assemblages of zooplankton indicated that Cladocera may escape heavyMysis predation at night by migrating from the metalimnetic-hypolimnetic interface into the epilimnion.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.Contribution 333 from the Great Lakes Research Division, The University of Michigan and contribution 287 from the Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration.     

Abundance and life history ofNeomysis intermedia Czerniawsky in Lake Kasumigaura

Weekly observations ofNeomysis intermedia in Lake Kasumigaura showed two major peaks in abundance during spring and autumn (more than 10⁴ individuals m^–2) and minimum levels in summer and winter (less than 10³ individuals m^–2). Their increase in abundance followed a high egg ratio, suggesting that the increase in abundance was caused by a high reproductive rate. Major contributors to mysid population decreases include fish predation and commercial fisheries, and possible horizontal migration of the mysids. N. intermedia showed two types of life history in the lake. One type (overwintering generation) has a life span of about 6–7 months and produces about 27 eggs per brood. Another, appearing from spring to autumn, matures in 3–6 weeks at a smaller size, and produces 12 eggs per brood. The reproductive season ofN. intermedia was continuous from March through November.     

Mercury accumulation in five species of freshwater fish in Lake Tyrifjorden,south-east Norway,with emphasis on their suitability as test organisms

Synopsis Mercury concentration in axial muscle of brown trout, Salmo trutta, whitefish, Coregonus lavaretus, Arctic charr, Salvelinus alpinus, smelt, Osmerus eperlanus and pike, Esox lucius, were studied in Lake Tyrifjorden during 1978–1982. Our data demonstrate that older and bigger fish on an average have higher mercury concentration than smaller and younger. Further, complex life histories as in brown trout influence the pattern of mercury accumulation. During young stages accumulation in brown trout is moderate, while accumulation in older stages is highly correlated to lake residency time. Based on our data we suggest the following requirements for a test organism and the collecting procedure; (1) life history should be simple with small sexual differences, (2) ageing should be easy and reliable, and (3) large representative samples should be easily obtained during (4) a fixed biological period i.e. the spawning period. We consider smelt as an appropriate test organism based on these criteria.     

Stylochus tauricus,a predator of the barnacle Balanus improvisus in the Black Sea

The flatworm Stylochus tauricus Jacubova has been found associated with the barnacle Balanus improvisus Darwin, on which it feeds. The predation rate (the number of barnacles eaten by one polyclad in a month) ranges between 5–10. Inside the empty shells of B. improvisus some egg-plates of S. tauricus were observed. Pelagic Götte's larvae aged 2–3 days possess 4 lobes while those aged 7–8 days have 5 lobes. Flatworms can prey on the young of another species Balanus eburneus Gould, whereas predation on the mussels Mytilus galloprovincialis Lam. is rare. There is a direct correlation between predator abundance and prey ingested.     

Use of enclosures to detect the contribution of particular zooplankton to growth of young-of-the-year yellow perch (Perca flavescens Mitchell)

Summary Densities of the cladoceran, Holopedium gibberum, were manipulated in 18 enclosures containing juvenile (age 0+) yellow perch (Perca flavescens) and mean-lake densities of other zooplankton. In enclosures, where nearlake densities of all zooplankton species including Holopedium were maintained, young-of-the-year perch grew significantly heavier and longer than in experimental enclosures where Holopedium was excluded. Holopedium comprised between 15–45% of the diet (wet weight) of perch in the first 2 weeks of July in the control treatment (Holopedium at or near ambient lake densities) and only 3–7% of total biomass ingested in the experimental treatment (Holopedium density selectively reduced). Predation on Holopedium decreased dramatically after the 2nd week of July in the control treatment after which Chaoborus, chironomids, and Sida became dominant prey items (by weight) of juvenile perch. These findings suggest that growth and survivorship of age 0+ perch in Precambrian Shield lakes may be coupled to Holopedium abundance. Thus, utilization of Holopedium by young-of-the-year yellow perch may affect recruitment of this species since overwintering survivorship, range of accessible prey sizes or species, and vulnerability of juvenile perch to predation by larger fish depend on body size, which is reduced when Holopedium is excluded from the diet.     

Macrolophus caliginosus: Functional response to whiteflies and preference and switching capacity between whiteflies and spider mites

Laboratory experiments were performed with adult female Macrolophus caliginosus Wagner (Heteroptera: Miridae) at 22ºC on bean plants to determine the functional response towards whiteflies, as well as the preference and switching capacity between the two prey species: whiteflies and spider mites. Predation of females presented with first instars of Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) was of a Type III functional response. The observed maximum predation was approximately 75 first instars at high prey densities within a 24-h period. The preference of M. caliginosus females between eggs of T. vaporariorum and Tetranychus urticaeKoch (Acarina: Tetranychidae) changed with the ratio of offered prey. The preference for T. vaporariorum eggs increased non-linearly with increasing proportions of this prey type. The average maximum predation of whitefly and spider mite eggs were approximately 166 and 111 eggs per day, respectively, at the highest ratio of the two preys. The proportion of M. caliginosus females found on the test plants at the end of the experiment increased with prey density suggesting that this mirid spends more time in areas with high prey density. Macrolophus caliginosus females are voracious predators of eggs and first instars of T. vaporariorum as well as of spider mite eggs and may thus be a valuable addition to existing methods of biological control of T. vaporariorum and T. urticae.