Colin R. Townsend

Aggregation, interference and foraging by larvae of Plectrocnemia conspersa (Trichoptera: Polycentropodidae)

The predatory net-spinning caddis larva Plectrocnemia conspersa aggregates in patches of high density during most of the year. The behavioural basis of the aggregation depends on (a) the initial tendency to spin a net where prey have been captured and (b) the abandonment of sites where prey are not captured within a threshold time after a previous meal. There is a powerful behavioural mutual interference effect in laboratory experiments between larvae at densities similar to those in the field. Larvae contest ownership of nets and the outcome is determined mainly by body size, usually without injury to either contestant. The lack of an aggressive response in August may be due to this mutual interference or to aspects of environmental instability which make an optimal allocation of foraging effort between patches difficult to attain.

The cost of copepod reproduction: increased susceptibility to fish predation

Summary1.We describe a laboratory investigation to determine how the possession of egg sacs by a freshwater copepod influences the likelihood of its capture by both efficient (bream) and inefficient (roach) zooplanktonivorous fish.2.For both predators the reaction distance was greater for the larger, more visible ovigerous prey than for nonovigerous copepods.3.Copepods spent more time stationary, in contact with the substrate, when a fish was present. The more susceptible ovigerous individuals were generally less active than nonovigerous individuals even in the absence of predators. The likely adaptive significance of this behaviour in the natural environment is discussed.4.The inefficient zooplanktonivorous fish had a significantly increased attack efficiency on ovigerous prey because their egg sacs reduced acceleration and manoeuvrability. Attack efficiency was only marginally enhanced in the case of the more efficient predator.5.The energy value of ovigerous prey, expressed in terms of biomass consumed per unit handling time, was greater than for non-ovigerous individuals.

The predatory Chironomidae of an iron-rich stream: feeding ecology and food web structure

Abstract. 1. Three species of Tanypodinae (Chironomidae) were found in an acid and iron‐rich stream in southern England. Maximum abundance was achieved in summer and they were sparse at other times. Individuals were aggregated on the stream bed and were overrepresented in accumulations of leaf litter.

Stream flow and predation effects on the spatial dynamics of benthic invertebrates

Field experiments were carried out to determine the influence of predation and prey movements on the accumulation of prey in enclosures. Experimental enclosures permitted exchange of prey with the benthos, but not of the large, predatory larvae of the caddisfly, Plectrocnemia conspersa (Curtis). Unseasonally heavy rainfalls during the experiment resulted in high flows and enabled us to examine the effects of a major, abiotic disturbance on invertebrate spatial dynamics. Prey colonization rates of cages without predators were determined in nine 24 h periods. Colonization rates increased exponentially with flow and were species-specific, depending on dispersal behaviour. Prey accumulation and predator impacts were measured in cages, with and without P. conspersa larvae, placed in the stream for 1, 2 or 3 weeks. Prey densities in cages increased with exposure time, but increases were not gradual and depended on flow regime. Flow was reduced within cages and they accumulated large numbers of invertebrates during high discharge. Analogous, naturally occurring refugia in the stream channel could be important for the recovery of lotic communities after major disturbances. Overall, prey densities were lowest in cages with predators. For fast colonizers, predation effects were detectable early in the experiment, but quickly obscured thereafter by continuous exchange of prey. For slow colonists, predation effects were detectable later, but persisted longer. Consumption rates for P. conspersa varied with prey density and flow regime. We suggest that the spatial dynamics of benthic invertebrates, especially as they are influenced by stochastic events, are important in understanding and detecting predation effects in stream communities.

The influence of light level on the functional response of a zooplanktonivorous fish

SummaryLight and vision are clearly of significance in foraging behaviour by underyearling common bream [Abramis brama (L.)]. These fish are effective predators at 1.25 Lux but they were also shown to be capable of taking prey, at a reduced rate, at a much lower light intensity (less than 5x10-3 Lux). In the latter case they may have been using sensory modes other than vision, perhaps involving tactile and/or olfactory stimuli.We investigated the influence of light level on the functional response of bream to Daphnia magna prey. At 1.25 Lux the predator showed a typical type II response. However, the relatively unfavourable conditions in the lower light intensity appear to have been responsible for generating a sigmoid type III functional response. Observations, using infra-red sensitive equipment, suggested a behavioural basis for this result. Thus, the predators attack rate was not constant, but increased with prey density. The significance of the type III functional response is discussed, both in terms of predator energetics and predator-prey population stability.

Factors affecting prey selection by young bream Abramis brama and roach Rutilus rutilus: insights provided by parallel studies in laboratory and field

SynopsisPrey selection by underyearling bream and roach was studied in both the laboratory and field. When presented with cladoceran and a more elusive copepod prey both fish species select against copepods, regardless of the relative prey sizes. However, the field diet of bream, but not roach, consistently includes a large proportion of copepods. The explanation for this discrepancy lies in the timing and location of foraging in the field. Bream foraging, unlike that of roach, is largely restricted to the hours of darkness and to the lowest stratum of the lake. The Microcrustacea of this stratum is depleted of cladocerans at night, because of vertical migration, and is relatively rich in copepods.

Foraging in a patchy environment by a predatory net-spinning caddis larva: A test of optimal foraging theory

SummaryThe predatory larvae of the caddis Plectrocnemia conspersa (Curtis) cause significant prey depletion in a habitat in which prey are patchily distributed. Optimal foraging theory predicts that under these circumstances a predator should stay in any given patch until the prey capture rate there drops to a value equal to the average for the habitat as a whole. This was tested using a combination of field and laboratory data and the results were in broad agreement with the prediction. A second prediction is that the marginal capture rate should be higher in a habitat richer in prey and this was not supported. It is argued that by using a simple rule-of-thumb (constant giving-up-time) P. conspersa is able to approach the optimal solution for much of the time.

Detritivorous Stoneflies of an Iron-Rich Stream: Food and Feeding

1. n nThe food of two stoneflies in an acid, iron-rich stream, consisted overwhelmingly of numerous small detritus particles and a few large leaf fragments. Iron bacteria, algae, fungi and pollen were present but normally sparse in gut contents and their nutritional significance is unknown. n n n n n2. n nIn gut contents preparations, detrital food weighed 13.4 ± 0.89. 10-4 mg mm-2, and leafy food 8.3 ± 2.03. 10-4 mg mm-2. n n n n n3. n nGut contents of Leuctra nigra were replaced 7.3, 10.7 and 18.5 times per day and of Nemurella picteti 10.4, 16.0 and 24.0 times per day at 5, 10 and 13.5°C, respectively. n n n n n4. n nThese data on feeding, and quantitative estimates of population densities, enabled calculations of daily and annual population food intake to be made. Nemurella picteti consumed 30.8 gm-2y-1 of fine detritus and 34.9 g m-2 y-1 leaves; Leuctra nigra consumed 94.7 g m-2 y-1 fine detritus and 49.1 g m-2 y-1 leaves. Stoneflies thus consumed over 80 g leaf litter m-2 y-1, comfortably exceeding the mean standing crop.

The dynamics of a population of roach (Rutilus rutilus (L.)) in a shallow lake: is there a 2-year cycle in recruitment ?

Recruitment success of roach varied dramatically between 1978 and 1985 in Alderfen Broad, a small lake in eastern England. All size classes of roach feed to a significant extent upon zooplankton, but the underyearling fish have the greatest effects upon the abundance, species composition and mean size of zooplankton. During years of good recruitment (1979, 1981, 1983 and 1985) when the 0 + age group was abundant, they showed poor growth as a result of the depression of their prey populations. Older fish also tended to grow poorly in these years and may have been less fecund the following year. In years of poor recruitment (1980, 1982 and 1984), with the release of the depressive effect upon the zooplankton exerted by underyearling fish, the older size classes tended to grow well with higher fecundity the following season, giving rise to good recruitment of underyearling fish, even when the number of spawners was low. The evidence indicates that there is a 2-year cycle of roach recruitment in Alderfen and this will be described.

The sublittoral macrobenthic community composition of Lough Hyne, Ireland

Abstract This paper describes a survey conducted across the floor of the South basin of Lough Hyne, a small sea-lough in south-west Ireland. To test for any pattern in macrobenthic community structure in relation to prevailing hydrographic conditions five stations at approximately 20 m depth were sampled on four occasions over a 14 month period, by SCUBA diving. Measurements of sediment pH, redox potential, sulphide potential, grain size, organic matter content and macrobenthic community structure allow two distinct habitats to be defined; coarse gravelly substratum adjacent to the inflow area and fine silty substratum over the remainder of the basin floor. A correlation of physicochemical parameters with axis loadings from an ordination of species composition at each station on each sampling occasion failed to reveal a continuum of change in relation to prevailing hydrographic conditions. Rather, sediment physicochemistry, the abundance of common macrobenthic species, number of species, number of individuals, variations in the numerical dominance hierarchy and station position in the ordination space fluctuated in a haphazard manner. Various localized disturbances, observed while conducting this survey, are considered to contribute to this pattern. The disturbing agents include smothering of areas of sediment by anoxic water, deposition of accumulations of detached seaweed and sediment excavations by Cancer pagurus .