Modelling Size-Dependent Cannibalism in Barramundi Lates calcarifer: Cannibalistic Polyphenism and Its Implication to Aquaculture

This study quantified size-dependent cannibalism in barramundi Lates calcarifer through coupling a range of prey-predator pairs in a different range of fish sizes. Predictive models were developed using morphological traits with the alterative assumption of cannibalistic polyphenism. Predictive models were validated with the data from trials where cannibals were challenged with progressing increments of prey sizes. The experimental observations showed that cannibals of 25–131 mm total length could ingest the conspecific prey of 78–72% cannibal length. In the validation test, all predictive models underestimate the maximum ingestible prey size for cannibals of a similar size range. However, the model based on the maximal mouth width at opening closely matched the empirical observations, suggesting a certain degree of phenotypic plasticity of mouth size among cannibalistic individuals. Mouth size showed allometric growth comparing with body depth, resulting in a decreasing trend on the maximum size of ingestible prey as cannibals grow larger, which in parts explains why cannibalism in barramundi is frequently observed in the early developmental stage. Any barramundi has the potential to become a cannibal when the initial prey size was <50% of the cannibal body length, but fish could never become a cannibal when prey were >58% of their size, suggesting that 50% of size difference can be the threshold to initiate intracohort cannibalism in a barramundi population. Cannibalistic polyphenism was likely to occur in barramundi that had a cannibalistic history. An experienced cannibal would have a greater ability to stretch its mouth size to capture a much larger prey than the models predict. The awareness of cannibalistic polyphenism has important application in fish farming management to reduce cannibalism.     

Sibling cannibalism in Koi carp, Cypvinus carpio L., larvae and juveniles reared under controlled conditions

The potential impact of sibling cannibalism on the size and population structure of captive populations of Koi carp, Cyprinus carpio L., which were fed only dry foods, was determined. The rate of cannibalism was found to be positively density-dependent, with the highest rate being recorded at initial stocking densities of 40 larvae 1 (33.3% of the fish had been eaten by cannibals by day 52). Two types of cannibalism could be distinguished: type I, where the head of the prey was rejected once the caudal region of the prey had been progressively ingested; type II, where the prey were caught either head-first or tail-first and were ingested completely. The first type was limited to the period from the ninth to the 15th day of rearing, and the second type started on the 18th day. Measurement of the mouth, head and tail allowed the calculation of predicted cannibal -prey relationships. The consequences and implications of coeval sibling cannibalism in culture systems are discussed.     

Cannibalism in pike fry, Esox lucius L.: some experiments with fry densities

Pike fry were kept in 0.054-m³ tanks at densities of 50, 100 and 150 (277, 555, 833 fry m⁻²) for seven weeks during which the development of social and feeding behaviour was observed. Zooplankton, macro-invertebrates and perch fry were provided sequentially as food; the pike fry were allowed to feed ad libitum.
Zooplanktivorous fry stopped growing at 22 mm whilst, in the presence of abundant suitable alternative prey, 1–4% turned cannibalistic at 5 weeks of age; cannibals subsequently grew rapidly (mean 1.88 mm day⁻¹). Cannibalism ensued in all tanks when the ratio of predator size: prey size was c .2:1.
Fry tended to space-out evenly in the tanks with no overt aggression or territoriality. Behaviour was typified by remaining still for long periods, particularly subsequent to the onset of cannibalism. Cannibals were attracted by fry movements which often initiated attacks.
Daily per capita mortality rates showed no crowding effects before cannibalism but significant density-dependent mortality due to cannibals. Consumption rates of cannibals varied between 0.63 and 6.0 fry per cannibal per day. Cannibals accounted for 54–96% of daily mortality in the experimental tanks. These results are discussed in relation to proposed mechanisms of pike population density regulation.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

Cannibalism and early instar survival in a larval damselfly

Cannibalism by larval damselflies late in larval development on larvae a few instars smaller has been widely documented. I examine here the survival of eggs oviposited near the end of the flight season of adult Enallagma boreale in the presence and absence of potential cannibals, individuals that hatched from eggs earlier in the season, over an extended part of the life-cycle. The role of competition as a modifier of cannibalism was examined by manipulating egg density, environmental productivity, and habitat complexity. Survival in the absence of potential cannibals ranged from 5% to nearly 50% but was only 0–3% in the presence of cannibals. Survival of small larvae was related to manipulations of habitat complexity but not initial density or resources. There were no significant interactions of the presence of large larvae with other experimental treatments on the survival of small larvae. The mean size of small larvae was greater in the presence of cannibals. This may be because the cannibalism treatment reduced the density of small larvae and reduced competition for resources, or that the cannibals preferentially fed on small larvae and only relatively large individuals remained. Fertilization of the habitat or manipulating the initial density of small larvae did not affect mass of small larvae at the end of the experiment, which would be expected if small larvae were affected by competition for resources. Potential cannibals, however, emerged at higher mass when small larvae were present at low density and when productivity of the habitat was increased. This suggests that the negative effect of competition by small larvae outweighs the positive effect of being potential prey for large larvae.     

Intra‐cohort cannibalism and size bimodality: a balance between hatching synchrony and resource feedbacks

Cannibalistic interactions generally depend on the size relationship between cannibals and victims. In many populations, a large enough size variation to allow for cannibalism may not only develop among age‐cohorts but also within cohorts. We studied the implications of variation in hatching period length and initial cohort size for the emergence of cannibalism and bimodal size distributions within animal cohorts using a physiologically structured population model. We found that the development of size bimodality was critically dependent on hatching period length, victim density and the presence of a feedback via shared resources. Cannibals only gained enough energy from cannibalism to accelerate in growth when victim density was high relative to cannibal density at the onset of cannibalism. Furthermore, we found that the opportunity for early hatchers to initially feed on an unexploited resource increases the likelihood both for cannibalism to occur and size bimodality to develop. Once cannibals accelerated in growth relative to victims size bimodality, reduced victim numbers and relaxed resource competition resulted. Thus, in addition to that cannibals profited from cannibalism through energy extraction, their potential victims also benefited as the resource recovered due to cannibal thinning. To ensure recruitment success, it can be critical that a few individuals can accelerate in growth and reach a size large enough to escape size‐dependent predation and winter starvation. Hence, within‐cohort cannibalism may be a potentially important mechanism to explain recruitment variation especially for cannibalistic species in temperate climates with strong seasonality. However, the scope for size bimodality to develop as a result of cannibalism may be limited by low victim densities and size and food‐dependent growth rates.     

Pathogens as a factor limiting the spread of cannibalism in tiger salamanders

Summary Intraspecific predation is taxonomically widespread, but few species routinely prey on conspecifics. This is surprising as conspecifics could be a valuable resource for animals limited by food. A potential cost of cannibalism that has been largely unexplored is that it may enhance the risk of acquiring debilitating pathogens or toxins from conspecifics. We examined how pathogens affect variation in the incidence of cannibalism in tiger salamander larvae (Ambystoma tigrinum nebulosum), which occur as two environmentally-induced morphs, typicals and cannibals. Salamanders from one population were more likely than those in another to develop into cannibals, even when reared under identical conditions. Variation in the propensity to become a cannibal may be caused by variation in pathogen density. In the population with cannibals at low frequency, bacterial blooms in late summer correlated with massive die-offs of salamanders. The frequency of cannibals correlated significantly negatively with bacterial density in ten different natural lakes. In the laboratory, cannibals exposed to a diseased conspecific always preyed on the sick animal. As a result, cannibals wre more likely to acquire and die from disease than were typicals that were similarly exposed, or cannibals that were exposed to healthy conspecifics. Since conspecifics often share lethal pathogens, enhanced risk of disease may explain why cannibalism is generally infrequent. Pathogens may constrain not only the tendency to be behaviorally cannibalistic, but also the propensity to develop specialized cannibal morphologies.     

Cannibalistic Behavior and Functional Response in Chrysomya albiceps (Diptera: Calliphoridae)

Chrysomya albiceps is a facultative predator and cannibal species during the larval stage. Very little is known about cannibalism and prey size preference, especially in blowflies. The purpose of this investigation was to determine the influence of prey size and larval density on cannibalism by third-instar larvae of C. albiceps under laboratory conditions. Our results indicate that no cannibalism occurs by third-instar larvae on first- and second-instar larvae, but third-instar larvae do eat second-instar larvae. The functional response on second-instar larvae is consistent with Holling type II. The consequences of consuming second-, compared to first- or third-, instar larvae as well as the implications of cannibalism for the population dynamics of C. albiceps are discussed.     

Feeding,development, and growth of juvenile perch <Emphasis Type="Italic">perca fluviatilis</Emphasis> in mesocosms in the presence of filter-feeding zebra mussel <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> pallas

Quantitative and qualitative changes in the feeding spectra and growth patterns are studied in the larvae and juveniles of perch in artificial water ecosystems (mesocosms) in the presence of a filter-feeding zebra mussel. At a stocking density of 0.75 kg/m², the presence of a zebra mussel leads to a change in the feeding conditions of zooplankton, to a decrease in its abundance in regards to critical values for fish feeding, to an increase in the abundance of organisms of macrobenthos in the food, and to the rapid transition of the fish to feeding on chironomids. As a result of these changes, the growth rate of perch larvae decreases, their development at step D ₁ is delayed, the differentiation of the juveniles by size is accelerated, their size and weight variability increases, and individual predators (cannibals) appear.     

Eat or be eaten: prevalence and impact of egg cannibalism on two-spot ladybirds, Adalia bipunctata

Ladybirds commonly engage in cannibalistic behaviour. Egg cannibalism by first instars is considered advantageous to the cannibal, because it not only results in direct metabolic gain but also a reduction in potential competitors. In this study, we quantified the effect of cannibalism on the development rate and survival of Adalia bipunctata (L.) (Coleoptera: Coccinellidae) larvae through development to the adult stage. We also assessed the synchrony of egg‐hatching in relation to laying order and compared the proportion of eggs cannibalized in egg batches laid as clusters or linearly. Larvae that had consumed a conspecific egg after hatching reached the adult stage 1.65 days earlier than those larvae that had not. Larval and pupal mortality was lower for cannibals compared to non‐cannibals; only 46% of non‐cannibalistic individuals reached the adult stage whereas 81% of cannibals pupated successfully. Egg cannibalism is undoubtedly advantageous to A. bipunctata larvae both in terms of faster development and increased survival. There is a positive correlation between laying and hatching order for eggs laid linearly or in a cluster. There was no significant difference in the proportion of eggs hatching in clusters or in a line (80 and 77%, respectively). The remaining eggs were either cannibalized or did not hatch. The ecological implications of these results are discussed with particular reference to trophic egg plasticity.     

High stocking density during larviculture and effect of size and diet on production of juvenile Lophiosilurus alexandri Steindachner, 1876 (Siluriformes: Pseudopimelodidae)

Different stocking densities were investigated in larviculture and feeding of Lophiosilurus alexandri, as well as analyses of the effects on juveniles of two size‐classes and two different commercial formulated diets. The first experiment was two‐phased: (a) larvae stocked at densities of 60, 120, 180, 240, and 300 larvae L⁻¹ fed with Artemia nauplii and reared for 15 days; (b) in phase 2, densities of 5, 10, 15, 30, and 40 juveniles L⁻¹ were evaluated during feed training (20 days). Mean water temperature in both phases was 28°C. In the first phase of experiment 1, the different stocking densities did not affect fish growth or survival. In phase 2, growth was similar in all densities; however, survival was lower at higher densities. The increased density provided a rise in biomass and number of individuals produced in both phases. In the second experiment, two size‐classes of feed‐trained juveniles (30.22 ± 1.84 and 34.66 ± 2.41 mm) were given pellets of two different diameters (1.2 and 2.6 mm) for 20 days. The largest juveniles fed the 1.2 mm inert diet had higher final weights and lengths. Larviculture and feed training of L. alexandri can thus be performed successfully at high stocking densities of 300 larvae L⁻¹ during the first 15 days of feeding, and at densities of up to 40 juveniles L⁻¹ during the 20 days of feed training, respectively.     

Cannibalistic interactions in two co-occurring decapod species: Effects of density, food, alternative prey and habitat

Cannibalism is a potentially important factor in the regulation of populations in a range of habitats. The intensity of this biotic factor may be determined by both intra and interspecific interactions. Cancer pagurus and Porcellana platycheles are two co-occurring decapods on Atlantic rocky shores. In laboratory mesocosms, we investigated intra and intersize class cannibalistic and interspecific predatory behaviours in those species. We addressed the effects of prey and predator densities, food, starvation, alternative prey and habitat type. No agonistic behaviour was noted in P. platycheles, suggesting a non-aggressive co-existence between gregarious individuals. Predation of C. pagurus on P. platycheles was intense, possibly accounting for the spatial segregation observed in the natural environment. Cannibalism among C. pagurus juveniles was low and only on vulnerable prey (i.e. at moulting), suggesting a non-aggressive co-existence among juveniles. However, intersize class cannibalism in C. pagurus was intense (ontogenetic shift), possibly reflecting the juvenile-adult segregation in the natural environment. Prey and predator densities, food and habitat type strongly influenced this behaviour. Possible interference among cannibals was noted, with lower prey consumption at high predator density. Food supply alone had more effect on cannibalistic rate than did alternative prey (P. platycheles) and predator starvation. Structurally complex habitats (small pebble and Fucus serratus habitats) yielded higher prey survival than the sandy habitat, and the behaviours of both prey and cannibals reflected the small-scale spatial distribution of individuals in the wild. Intersize class cannibalism and interspecific agonistic relationships may account for the intertidal distribution of crab species at low tide.     

Fitness consequences of cannibalism in the fall armyworm, Spodoptera frugiperda

We investigated the consequences of cannibalism for some correlatesof fitness in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae).The benefits gained by cannibals were ascertained by comparing survival,development rate, and pupal weight of larvae that had the opportunityto cannibalize with those that did not, at two levels of food availability.Larvae in the cannibalism treatments were provided with a conspecificone instar younger than themselves on five dates throughoutlarval development. Cannibalism was frequent; given the opportunityall larvae predated at least one younger conspecific. The frequencyof cannibalism was not affected by the sex of the cannibal orby the availability of alternative food. However, cannibalssuffered a significant reduction in survival at both high andlow food availability. Furthermore, cannibals had a lower pupal weightin the high food treatment and a reduced development rate inthe low food treatment. The only detectable fitness benefitassociated with cannibalism was a positive correlation betweenthe number of victims consumed and development rate under conditionsof low food availability. On balance, cannibalism appears tobe costly; alternative explanations for its occurrence in thisspecies are discussed.     

Sibling cannibalism among juvenile vundu under controlled conditions. II. Effect of body weight and environmental variables on the periodicity and intensity of type II cannibalism

Cannibalism among starved groups of juvenile (19–48 days old) vundu catfish Heterobranchus longifilis was 66·5% nocturnal, and its impact under modified day length was proportional to the duration of the dark phase. Shallow depth and high population density decreased the intensity of cannibalism, whereas low density and deeper environments had an opposite effect. The presence of refuges had no significant effect on cannibalism. The maintenance ( R _maint) and maximum ( R _max) daily food rations (% day⁻¹) of cannibals feeding on live prey were modelled as R _maint=3·899 W _C^0·327 ( r ²=0·684; d.f.=31), and R _max=49.545 W _C^0·321 ( r ²=0·999; d.f.=5), where W _C was the body weight of the cannibal (g). The latter model indicated that the impact of a cannibal on a population decreased by a 20% margin each time the cannibal doubled its body weight, and suggested that cannibalism among vundu would become insignificant for cannibals heavier than 30 g. The significance of these findings is discussed within the contexts of vundu aquaculture and of general, conceptual models of the dynamics of cannibalism among fishes.     

Recruitment in the swash zone – temporal variations in juvenile recruitment of an exposed sand beach surf clam

This study examined factors affecting juvenile recruitment in the tuatua Paphies donacinaon an exposed surf beach in Pegasus Bay, South Island New Zealand between October 1994 and April 1997. Tuatua recruitment followed female gonad maturation with a delay of approximately 2 months. Juveniles (1–10 mm shell length) occurred throughout the study except during June and July 1995 (winter). Average densities (m^–2 ) were 28 in winter, 230 in spring, 174 in summer and 108 in autumn. There was high juvenile mortality and/or emigration and the maximum average density of (741m^–2 ) was recorded in November 1996. Recruit densities were similar between years except during winter. Adult tuatua populations were dominated by modal shell lengths greater than 70 mm.There was no significant relationship between adult and juvenile recruit length. Recruit length decreased with recruit density suggesting density dependent competition for space or food resources. Juvenile density increased following increases in food availability (chlorophyll a concentration) but average length of juveniles did not correlate directly with any of the environmental variables. Recruit density decreased as the modal adult length increased and recruit length reduced when adult populations had a high condition index. Because larvae, juveniles and adult are mobile the beach population is characteristic of an open system with adults and juveniles from the resident population affecting post-settlement density- dependent processes.     

Predation risk suppresses the positive feedback between size structure and cannibalism

1.?Cannibalism can play a prominent role in the structuring and dynamics of ecological communities. Previous studies have emphasized the importance of size structure and density of cannibalistic species in shaping short- and long-term cannibalism dynamics, but our understanding of how predators influence cannibalism dynamics is limited. This is despite widespread evidence that many prey species exhibit behavioural and morphological adaptations in response to predation risk. 2.?This study examined how the presence and absence of predation risk from larval dragonflies Aeshna nigroflava affected cannibalism dynamics in its prey larval salamanders Hynobius retardatus. 3.?We found that feedback dynamics between size structure and cannibalism depended on whether dragonfly predation risk was present. In the absence of dragonfly risk cues, a positive feedback between salamander size structure and cannibalism through time occurred because most of the replicates in this treatment contained at least one salamander larvae having an enlarged gape (i.e. cannibal). In contrast, this feedback and the emergence of cannibalism were rarely observed in the presence of the dragonfly risk cues. Once salamander size divergence occurred, experimental reversals of the presence or absence of dragonfly risk cues did not alter existing cannibalism dynamics as the experiment progressed. Thus, the effects of risk on the mechanisms driving cannibalism dynamics likely operated during the early developmental period of the salamander larvae. 4.?The effects of dragonfly predation risk on behavioural aspects of cannibalistic interactions among hatchlings may prohibit the initiation of dynamics between size structure and cannibalism. Our predation trials clearly showed that encounter rates among hatchlings and biting and ingestion rates of prospective prey by prospective cannibals were significantly lower in the presence vs. absence of dragonfly predation risk even though the size asymmetry between cannibals and victims was similar in both risk treatments. These results suggest that dragonfly risk cues first suppress cannibalism among hatchlings and then prevent size variation from increasing through time. 5.?We suggest that the positive feedback dynamics between size structure and cannibalism and their modification by predation risk may also operate in other systems to shape the population dynamics of cannibalistic prey species as well as overall community dynamics.     

Growth of Odontesthes bonariiensis (Atherinidae) larvae feeding on suboptimal zooplankton densities

Synopsis In this study we investigate the effect of food availability (zooplankton biomass) on the growth of Odontesthes bonariensis (Atherinidae) larvae. The larvae were stocked in four 45 m² outdoor tanks at relatively high densities (100 and 200 larvae m^–2). Because of the high stocking densities, the zooplankton biomass was depleted in all tanks. However, the patterns of food limitation, and particularly periods of severe food shortage, differed in tanks stocked at different densities. We could therefore, observe the effect of food limitation in larvae that differed in weight and age. The effects of variables suspected to influence O. bonariensis growth rates (age and weight of larvae, available zooplankton biomass, mean individual weight of available preys, total ingested prey weight, and mean weight of ingested preys) were investigated using standard multiple regression methods, and a model assuming: (1) an allometric relationship between maximum growth rates and weight of larvae, and (2) an inverse relationship between growth depression and the available zooplankton biomass. Both methods were consistent in showing that only the weight of larvae, and the availability of zooplankton prey had significant effects on the growth of O. bonariensis. The model's results additionally suggest that, if the observed growth rates are scaled by the maximum growth rate corresponding to the larva weight, the effect of zooplankton biomass is largely independent of age and weight of larvae.     

Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators

Occurrence of cannibalism and inferior competitive ability of predators compared to their prey have been suggested to promote coexistence in size-structured intraguild predation (IGP) systems. The intrinsic size-structure of fish provides the necessary prerequisites to test whether the above mechanisms are general features of species interactions in fish communities where IGP is common. We first experimentally tested whether Arctic char (Salvelinus alpinus) were more efficient as a cannibal than as an interspecific predator on the prey fish ninespine stickleback (Pungitius pungitius) and whether ninespine stickleback were a more efficient competitor on the shared zooplankton prey than its predator, Arctic char. Secondly, we performed a literature survey to evaluate if piscivores in general are more efficient as cannibals than as interspecific predators and whether piscivores are inferior competitors on shared resources compared to their prey fish species. Both controlled pool experiments and outdoor pond experiments showed that char imposed a higher mortality on YOY char than on ninespine sticklebacks, suggesting that piscivorous char is a more efficient cannibal than interspecific predator. Estimates of size dependent attack rates on zooplankton further showed a consistently higher attack rate of ninespine sticklebacks compared to similar sized char on zooplankton, suggesting that ninespine stickleback is a more efficient competitor than char on zooplankton resources. The literature survey showed that piscivorous top consumers generally selected conspecifics over interspecific prey, and that prey species are competitively superior compared to juvenile piscivorous species in the zooplankton niche. We suggest that the observed selectivity for cannibal prey over interspecific prey and the competitive advantage of prey species over juvenile piscivores are common features in fish communities and that the observed selectivity for cannibalism over interspecific prey has the potential to mediate coexistence in size structured intraguild predation systems.     

The evaluation of the stocking of pike fingerlings

Summary In order to evaluate the stocking of artificially propagated northern pike (Esox lucius L.) fingerlings (4–6 cm), of which 1–1,5 million/year are produced by the Organization for the Improvement of Inland Fisheries (O.V.B.), the composition and abundance of northern pike populations in four shallow waters were monitored during a 5–8 year period. The specimens stocked were marked by amputation of a ventral fin. At the end of their first growing season the numbers of these and of those that were naturally reproduced (O⁺ pike) as well as the abundance of larger specimens were estimated using mark-recapture methods. Sampling was executed by electrofishing, seining and (incidentally) trawling. The O⁺ pike was caught quantitively by electrofishing.Based on the distribution pattern observed during the sampling period, four age/length classes were distinguished. (1) O⁺ pike, caught almost without exeption within emerged or submerged vegetation; (2) O⁺<pike<41 cm (forklength) found within submerged, floating and ingrowing vegetation; (3) 41 cm<pike<54 cm caught within or in the vicinity of vegetated areas; (4) pike>54 cm, the majority of which — in two experimental waters — were found outside the vegetation. Within the length range of 0–54 cm, the biomass of O⁺ pike and of 0–41 cm pike appeared to be negatively correlated with those of larger pike. The standing stock<54 cm pike was found to be determined by the amount of aquatic vegetation. This phenomenon was still more pronounced when the different habitat preferences of the various length classes were taken into account (GRIMM, 1981).Based on the negative correlations it is concluded that the biomass of 0–41 cm pike/ha preferred habitat is determined by the biomass of larger northern pike and that the stocking of fingerlings does not influence this relationship.In one of the experimental waters a high mortality occurred due to the severe winter of 1978/–1979, causing a decline of at least 50% of the abundance of legal-sized (48 cm) northern pike. In the following year (1979) the numbers and biomass of the O⁺ class amounted to 10 and 5 times the previously recorded highest values, respectively. In 1980 the O⁺ class was virtually absent. As a result of these extreme values the negative correlation was found to be exponential, indicating a density-dependent relationship between smaller (0–41 cm) and larger (41–54 cm) pike. It was shown that, in the 3 years fingerlings were stocked, the abundance of 0–41 cm pike (kg/ha preferred habitat) relative to the abundance of larger pike, did not differ from that in the 4 years in which fingerlings were not introduced.In the two years that pike-fingerlings were introduced in the beginning of May in this water, the frequency of occurrence of O⁺ pike, originally stocked as fingerlings, amounted to ca. 80%. In the year stocking took place at the end of May, introduced pike constituted only ca. 6% of the O⁺ population. These high and low frequencies are explained by the fact that the first and last introduced specimens are either ahead of or behind the ecological time table: they are on the average larger or smaller then the naturally reproduced specimens.Intraspecific predation within the O-class can be a factor of importance (GRIMM, 1981). Therefore, it is supposed that the frequency of occurrence reflects the number of naturally reproduced pike that were replaced by the stocked ones.     

Precopulatory Sexual Cannibalism Causes Increase Egg Case Production,Hatching Success,and Female Attractiveness to Males

Precopulatory sexual cannibalism is an extreme form of sexual conflict that can entail significant costs to the cannibalized individual and a variety of costs and benefits to the cannibal itself. Characterizing these costs and benefits is fundamental to our understanding of how this behavior evolves. Using the spider Agelenopsis pennsylvanica, we tested the reproductive consequences of precopulatory sexual cannibalism by staging cannibalization events and comparing the performance of experimental cannibals against natural cannibals (i.e., those that cannibalized on their own) and non‐cannibals. We found two performance benefits associated with precopulatory sexual cannibalism: first, experimental cannibals were more likely to produce egg cases than non‐cannibals, and second, egg cases from experimental cannibals and natural cannibals were significantly more likely to hatch than those produced by non‐cannibals. We then tested whether males were more likely to approach the webs of experimental cannibals vs. non‐cannibalistic control females. Our data demonstrate that sexual cannibalism increases female attractiveness to males. Although this result seems counterintuitive, in fact, rates of precopulatory sexual cannibalism were much lower in females that had already cannibalized their first male: 38% of sexually naïve females engaged in precopulatory sexual cannibalism, whereas only 5% of females engaged in cannibalism a second time. Thus, males that approach cannibals receive two benefits: they are less likely to be cannibalized precopula, and they have the possibility of mating with females that have a higher probability of producing viable egg cases. Taken together, our data suggest that precopulatory sexual cannibalism affords females numerous benefits and may have a hand in shaping male mate choice decisions.