Growth and mortality of the introduced Lake Tanganyika clupeid, Limnothrissa miodon, in Lake Kariba

The growth and mortality of the small clupeid fish Limnothrissa miodon were studied in Lake Kariba, a large, man-made lake in central Africa. Growth could be described by the von Bertalanffy equation l_t = 74.2 (1—e^{−0.254( t +0.07)}) where length ( l ) is expressed in millimetres and time ( t ) in months. The monthly rates of mortality were high and increasing under intensive fishing; in 1983 the total mortality rate Z =1.15 and natural mortality M =0.7. Limnothrissa appears to be stunted in the man-made lakes where it occurs (Kariba and Cahora Bassa) where it grows at a faster rate but to a smaller size than it does in natural lakes (Kivu and Tanganyika). Various hypotheses to account for this are discussed and it was concluded that this is an adaptation to the relatively unstable conditions found in the man-made lakes.     

Temperature-dependent energetics of Chaoborus populations: hypothesis for anomalous distributions in the great lakes of East Africa

Chaoborus, the phantom midge (Insecta, Diptera, Chaoboridae), has a widespread distribution, commonly occurring in lakes and ponds all over the world. In the great lakes region of East Africa Chaoborus is present in Lakes Victoria, Albert, Edward, Malawi and George, but absent from Lakes Tanganyika, Kivu and Turkana. Tropical lakes typically have water temperatures in the range of 22–26 °C year round. Lakes Tanganyika and Kivu have only 20% of their bottom sediments oxygenated during full circulation, contrary to 95–100% in all of the other lakes, excluding Lake Malawi (45%) (Hecky & Kling, 1987). Planktivorous fish are present in all lakes (Lehman, 1995). We hypothesized that the absence of Chaoborus larvae from some lakes of East Africa may be the result of interaction among high temperatures, low oxygen levels, and fish predation.We developed a model to estimate energetic costs for Chaoborus larvae at temperatures greater than 14 °C. We hoped to shed light on the bioenergetics of Chaoborus populations, and the possibility that extant distributions of Chaoborus larvae are the result of energetic constraints.We found that relative respiratory and growth costs of Chaoborus larvae are highest in the early stages of development. We estimated that non-feeding instar I larvae living in 25 °C water will starve to death in less than one day. It is possible that Chaoborus populations are prevented from establishing in certain areas because high energetic costs condemn young larvae to death by either predation or starvation.     

Horizontal variations in associations of zooplankton in Lake Kariba

The horizontal distribution of the zooplankton in Lake Kariba was sampled by means of vertical hauls using 55 and 250 μm meshed nets. Crustacea were sparse in the main lake, but increased dramatically in the very shallow water at the end of Crocodile Creek. The composition of the zooplankton is influenced by the presence in the main lake of the Tanganyika sardine, Limnothrissa miodon , and by the large size of the dominant diatom in the phytoplankton. In the creeks the distribution can be explained by the zooplankton occurring in open spaces into which planktivorous fish do not enter because of the possibility of predation by Tiger fish ( Hydrucyori ) and birds.
The systematic composition of the zooplankton in L. Kariba is reviewed and compared with two natural lakes and another man-made lake. It is concluded that the zooplankton and the fishes in L. Kariba have reached approximately the specific diversity that one would expect from a similar sized natural lake in the same geographical area.     

Morphological and genetic differentiation of the African clupeid Limnothrissa miodon 34 years after its introduction to Lake Kivu

The evolutionary consequences of an artificial introduction of the clupeid Limnothrissa miodon from Lake Tanganyika into Lake Kivu, East Africa were examined. In 1959, 57 400 fry (mixture of Limnothrissa and the related clupeid, Stolothrissa tanganicae ), were released into Lake Kivu to boost fisheries production. Comparisons were made between respective source and transplant populations 34 years later (1993) using morphometrics ('truss' method), allozymes (29 enzyme-coding loci) and mitochondrial (mt) DNA variation (RFLP analysis of PCR-amplified ND5/6 genes). Significant morphological and genetic differentiation between source and transplant samples was detected, with a distinct clustering of Kivu Limnothrissa on respective dendrograms, especially at the morphometric and mtDNA levels. Differentiation within Lake Tanganyika was, however, consistently higher than that between lakes. Allozymic diversity was similar in samples from both lakes (Lake Tanganyika: heterozygosity = 0.0658, mean number of alleles=1.44; Lake Kivu: heterozygosity = 0.0655; mean number of alleles = 1.48), however, a significantly lower mtDNA haplotype diversity was detected in Lake Kivu (Lake Tanganyika: 0.905; Lake Kivu: 0.755). Data suggest that high post-introduction mortality and various demographic factors reduced the effective population size of the introduced population to tens rather than thousands of individuals, resulting in a reduction in genetic diversity and founder effect.     

Zooplankton of Lake Kivu, East Africa, half a century after the Tanganyika sardine introduction

The introduction into Lake Kivu of the planktivorous fish Limnothrissamiodon at the end of the 1950s raised major concern about thefate of mesozooplankton, but few data were available to assessthe impact of predation. In this study, we followed zooplanktonvariations in Lake Kivu for 3.5 years. Present Lake Kivu mesozooplanktonis dominated by cyclopoid copepods (Thermocyclops consimilis,Mesocyclops aequatorialis and Tropocyclops confinis), but cladoceransand rotifers are also present. Each year, total crustacean abundancein the plankton increased to a distinct seasonal maximum followinga rise of phytoplankton production associated with a deep epilimneticmixing in the dry season (August–September). This dependenceon phytoplankton resource suggests that mesozooplankton dynamicsin Lake Kivu is mainly bottom-up controlled, contrary to expectationsfrom the food web structure. However, measurements of body sizeindicate that sardine predation affects the cladoceran Diaphanosomaexcisum, whereas the larger copepods may efficiently escapepredation by migration in the deep mixolimnion at daytime. Totalbiomass of mesozooplankton in Lake Kivu is lower than in lakesTanganyika and Malawi. This may be related to the disappearanceof a large grazer that existed before the sardine introduction,whereas the large lakes of the same region have a more complexpelagic food web, with piscivorous fish, and with calanoid copepods,which can more efficiently exploit phytoplankton production.     

The influence of river flow on pelagic sardine catches in Lake Kariba

The nutrient level of Lake Kariba is governed by the Zambezi river which provides about 70% of its water supply. Other tributaries are richer in nutrients and have an appreciable local effect. This is shown by the catches of Limnothrissa miodon which increase after strong flows in the Sanyati river. This is in contrast to natural lakes such as Lake Tanganyika and may be an important management factor in Lake Kariba and other man-made lakes. Possible periods of poor rainfall could thus reduce the Limnothrissa yield in Lake Kariba.     

High predation is of key importance for dominance of small-bodied zooplankton in warm shallow lakes: evidence from lakes, fish exclosures and surface sediments

The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. Daphnia and Simocephalus) appeared in fishless mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of small-sized zooplankton in warm lakes.     

Relative significance of direct and indirect effects of predation by planktivorous fish on zooplankton

One of the most obvious features of tropical lakes and reservoirs is the small body size of their zooplankton taxa. It is believed that this is the result of high and persistent predation by abundant planktivorous fish, which select large-bodied zooplankton prey thus making them more vulnerable to extinction in tropical as compared to temperate habitats. Do these extinctions result directly from fish predation? Could the high predation-induced mortality alone be responsible for an extermination of the population from a habitat? Or could indirect effects of predation be responsible? Some important indirect effects can be seen at the demographic level; these include reduced reproduction in the population resulting from higher vulnerability of ovigerous females to predation by visually oriented planktivores. Other important indirect effects can be observed at the individual level; these include shifts in behavior (from foraging to predator avoidance) and adjustments in physiology (from high to low feeding rate) in those planktonic animals which detect danger from their predators by sensing either the ‘predator odor’ or an ‘alarm substance’ originating from injured conspecific prey. Although a zooplankton species density may mostly result from the brutal force of direct predator impact on the population (mortality), it is more likely that its distribution in time and space could be attributed to a combination of indirect effects of predation on individual behavior and physiology. An example of periodicity in density and depth distribution patterns of Cahora Bassa zooplankton species and their periodic exterminations seems to confirm the role of indirect effects of predation by planktivorous fish.     

Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

Effects of fish predation on larval chironomid (Diptera: Chironomidae) communities in an arctic ecosystem

Chironomid communities were analyzed in systems with three types of predator regimes to determine hierarchical effects of predation; ponds without fish present, lakes with slimy sculpin (Cottus cognatus) present, and lakes with slimy sculpin present along with burbot (Lota Iota) and lake trout (Salvelinus namaycush). Samples were collected by coring bare sediment habitats in 4 systems of each type near the Toolik Lake field station in northern Alaska. Lakes with burbot and lake trout present in addition to slimy sculpin displayed significantly higher (P<0.05) biomass, density, richness, and diversity. This is likely due to the increased complexity of the predator regime. Ponds without fish had a significantly greater percentage of predacious chironomids present.     

Human-induced changes in the composition of fish communities in the African Great Lakes

Summary Large natural African lakes contain unique and diverse fish faunas which have evolved within each lake in a comparatively short period of time. members of the family Cichlidae are particularly diverse, although there is strong evidence to show that the haplochromines in Lake Victoria, and possibly Lake Malawi, are monophyletic. The unique faunas in Lakes Victoria and Kyoga have been subject to perturbations from the introduction of exotic fish, and the faunas in these and other lakes have been disturbed by fishing activities and other human endeavours.Factors governing the establishment of exotic species are not clearly understood. The exotic fish must be physiologically adapted to their new environment, able to compete successfully both for habitat and for food at each stage of their life history, able to avoid predation and must have a suitable reproductive potential. Although about 50 species of fish have been introduced into African inland waters, including reservoirs, only comparatively few, in particular Nile perch (Lates niloticus), various cichlids (especially tilapias) and clupeids (Limnothrissa miodon), have been successful in establishing themselves. Those that have become established have had obvious but unquantifiable impacts on the indigenous faunas.It is difficult to differentiate between the effects of fishing and of the presence of alien fish on the fish species composition of the lakes (Witte et al., 1992). Many of the lakes were overfished before introductions were made, with a resultant decline in some species, especially the larger ones, and the virtual disappearance of others. Some lake fish faunas, such as those of Lakes Kyoga and Victoria, which have been subjected to the perturbations described above, continue to change rapidly (Ogutu-Owayo, 1990b).There is a fundamental need to collect biological information on the fish communities of African lakes for effective management, resulting not only in the conservation of unique fish faunas but also the production of sustainable fish yields for the people relying on this source of protein. This information is required before any more introductions of exotic fish are made.     

Unique qualities and special problems of the African Great Lakes

Synopsis The African Great Lakes consist of large, deep rift valley lakes (e.g. Malawi & Tanganyika) and shallower lakes between the Eastern and Western Rifts (e.g. Victoria). They are a group comparable in size to the North American Great Lakes, but are old. Most are seasonally thermally stratified, and wind is the decisive factor that determines the annual cycle of cooling and mixing. Lakes Tanganyika, Malawi and Kivu are meromictic, with deep relict hypolimnia. Large magnitudes and time scales of periodic internal motion, where these have been measured, appear unique among lakes. These lakes harbour the world's richest lacustrine fish faunas, and the family Cichlidae provides the supreme example of geographically circumscribed vertebrate evolution. The lakes provide a unique comparative series of natural laboratories for evolutionary studies. Primary production is generally high, but in the deeper lakes standing stocks of plankton and of small fish species are low. These pelagic populations are characterised by very high P:B ratios. The fisheries are productive and of socio-economic importance. Large-scale mechanised fishing is not compatible with the survival of the diverse fish communities. Cichlids appear especially vulnerable to unselective fishing. Aquatic reserves might offer a means of survival for at least some communities. Various pollution threats exist. Because water retention times are long, extremely long for some deep lakes, and flushing rates are low, the lakes are vulnerable to pollution which would be long-lasting. Introductions of alien fishes have mostly had undesirable or disastrous results. While the faunas are one of the significant natural heritages of mankind, their conservation must realistically be linked to the legitimate development of the lakes for the well-being of the people who live there. Scientific value alone will not protect the lakes. Just as survival of African terrestrial wildlife in extensive reserves depends heavily upon tourism, so also might the cichlid flocks in underwater reserves. Greater interest from the international scientific community is needed to further rational development and conservation of these great lakes.Invited Editorial     

PREDATION AND ITS EFFECTS ON MIGRATION AND SPECIATION IN AFRICAN FISHES: A COMMENT

Contrary to a recently expressed view very little factual evidence has been brought forward to support the idea that predatory fishes, and especially Lates and Hydrocynus , have had restrictive effects on speciation among other African freshwater fishes. A considerable body of data supports the view that the effects of these fishes do not differ from those of other predators and that predation has facilitated speciation. Further reasons why Lates and Hydrocynus cannot be considered as having effects different from those of other predators are given.
The situation in lakes containing these predators is reconsidered. There is no need to attempt to minimize their effects in Lake Tanganyika where their presence and effects are fully compatible with the existence of a rich endemic fauna. The situation in Lakes Albert and Rudolf is more satisfactorily explained by the brief duration of the existence of the present-day lakes and by their recent invasion by a fully differentiated Nilotic fauna, than by imagining that Lates and Hydrocynus have restricted speciation.
The suggestion that Lates and Hydrocynus were responsible for the inception of the habit of upstream migration for purposes of spawning is discredited, and what seem to be more probable reasons for the phenomenon are given.     

Snail populations in arctic lakes: competition mediated by predation?

Summary For 2 species of snails in arctic Alaskan lakes, I studied the patterns of snail distribution with respect to habitat, distribution of predatory fish, and the potential for interspecific competition. The snails Lymnaea elodes and Valvata lewisi co-exist in these arctic lakes, either in the presence of lake trout, Salvelinus namaycush, or in the absence of predation. Intensive sediment core sampling of Toolik Lake and Lake N-2, with trout and lacking trout, respectively, showed that the smaller snail, Valvata, was abundant in Toolik but ocurred at very low densities in Lake N-2. On the open sediments of lakes containing trout, diver surveys revealed very low densities of adult Lymnaea (0.12±0.12/m²), but similar surveys in lakes without trout revealed much higher densities of adult Lymnaea (7.1±1.8/m²). A survey of 14 lakes indicated that adult Lymnaea grew to a smaller mean size in lakes with trout than in lakes which lacked trout.In laboratory and field experiments, the presence of Lymnaea lowered the fecundity of Valvata. Laboratory experiments also showed that Lymnaea fecundity was enhanced by the presence of Valvata. Enhancement was not due to predation by Lymnaea on Valvata eggs or young. The observed patterns of distribution and abundance in the absence of trout, combined with results from laboratory experiments, are consistent with the hypothesis that competitive and facilitative interactions control the population dynamics of the two snails. The distribution and abundance patterns of snails where trout are present suggest that trout predation rather than competition controls snail population dynamics in lakes containing trout.     

Shift in predation regime mediates diversification of foraging behaviour in a dragonfly genus

1. Behavioural adaptations to avoid and evade predators are common. Many studies have investigated population divergence in response to changes in predation regime within species, but studies exploring interspecific patterns are scant. Studies on interspecific divergence can infer common outcomes from evolutionary processes and highlight the role of environmental constraints in shaping species traits. 2. Species of the dragonfly genus Leucorrhinia underwent well‐studied shifts from habitats being dominated by predatory fish (fish lakes) to habitat being dominated by predatory invertebrates (dragonfly lakes). This change in top predators resulted in a set of adaptive trait modifications in response to the different hunting styles of both predator types: whereas predatory fish actively search and pursue prey, invertebrate predator follow a sit‐and‐wait strategy, not pursuing prey. 3. Here it is shown that the habitat shift‐related change in selection regime on larval Leucorrhinia caused species in dragonfly lakes to evolve increased larval foraging and activity, and results suggest that they lost the ability to recognise predatory fish. 4. The results of the present study highlight the impact of predators on behavioural trait diversification with habitat‐specific predation regimes selecting for distinct behavioural expression.     

The distribution of calanoid copepods in the plankton of Wisconsin Lakes

Zooplankton communities from 499 lakes were examined for calanoid copepod species. Limnocalanus macrurus and Senecella calaloides are confined to Lakes Michigan, Superior and one inland lake each. Eurytemora affinis has recently become established in the coastal waters of Lakes Michigan and Superior. Epischura lacustris is present in the summer plankton of a wide variety of lake types. Aglaodiaptomus leptopus is sensitive to fish predation and confined to small lakes without fish. Leptodiaptomus ashlandi is restricted to Lakes Michigan and Superior, but L. sicilis is also found in some inland lakes. Leptodiaptomus minutus is a boreal species found mostly in lakes of the far north. Skistodiaptomus oregonensis is the most commonly occurring calanoid and seems well adapted to lakes of the meso-to low eutrophic types. Skistodiaptomus pallidus is also generally distributed throughout the state, but less common than oregonensis and adapted to more eutrophic lakes. Leptodiaptomus siciloides and A. clavipes are western species, adapted to eutrophic lakes, which are expanding their ranges eastward, aided by cultural eutrophication.     

High prevalence of non-synonymous substitutions in mtDNA of cichlid fishes from Lake Victoria

When a population size is reduced, genetic drift may fix slightly deleterious mutations, and an increase in nonsynonymous substitution is expected. It has been suggested that past aridity has seriously affected and decreased the populations of cichlid fishes in Lake Victoria, while geographical studies have shown that the water levels in Lake Tanganyika and Lake Malawi have remained fairly constant. The comparably stable environments in the latter two lakes might have kept the populations of cichlid fishes large enough to remove slightly deleterious mutations. The difference in the stability of cichlid fish population sizes between Lake Victoria and the Lakes Tanganyika and Malawi is expected to have caused differences in the nonsynonymous/synonymous ratio, ω (= d_N/d_S), of the evolutionary rate. Here, we estimated ω and compared it between the cichlids of the three lakes for 13 mitochondrial protein-coding genes using maximum likelihood methods. We found that the lineages of the cichlids in Lake Victoria had a significantly higher ω for several mitochondrial loci. Moreover, positive selection was indicated for several codons in the mtDNA of the Lake Victoria cichlid lineage. Our results indicate that both adaptive and slightly deleterious molecular evolution has taken place in the Lake Victoria cichlids' mtDNA genes, whose nonsynonymous sites are generally conserved.     

August Thienemann and Loch Lomond — an approach to the design of a system for monitoring the state of north-temperate standing waters

Limnocnida and Craspedacusta are the two main genera of Cnidaria from continental waters which have a free-swimming medusa in their life cycle. Both originated in tropical-subtropical Asia, but Limnocnida is also found in Africa, with one species (L. tanganjicae) perhaps endemic to lake Tanganyika. Halmomises from Trinidad, and two genera (Mansariella, Keralica) from India have limited ranges. No freshwater medusae evolved in Europe and the Americas (aside from Trinidad), but Craspedacusta sowerbii, by virtue of its drought resistant stages, has managed to colonise all continents, except Antarctica, in the course of the twentieth century. In all, there are about 10–14 freshwater species of medusae. An additional 4–5 species of Moerisia, Australomedusa, and Craspedacusta are sequestered in continental salt-lakes, while a few genera occur in brackish-water seas and estuaries (e.g. Moerisia, Odessia, Ostroumovia). This reduced diversity is probably due to (1) a rarity of drought resistant stages, adjusted to upstream dispersal, in the life cycle, and (2) difficulties to adapt the osmoregulatory system to a hypotonic environment.The feeding ecology of the freshwater medusae is also examined. Like all Cnidaria, they are opportunistic predators. However, fish egg predation might be their major means of subsistence, other types of food being taken only occasionally, or when fish eggs and larvae are scarce. Their impact on the true zooplankton might therefore be limited to short pulses of planktivory. Whether they are themselves limited by predation remains to be studied; many fish in the invasive part of their range avoid them, but some macrocrustaceans readily consume them. It is hypothesized that this might explain their success in Lake Tanganyika, and their absence from lake Baical. Finally, the absence of endemic species from South America could relate to the great diversity bf small-sized predatory aquatic reptiles on this subcontinent.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

The relationship between fecundity and adult body weight in Homeotherms

Summary Bythotrephes cederstroemii Schoedler, a predatory freshwater zooplankter (Crustacea: Cladocera), was first found in the Laurentian Great Lakes in December 1984. The first individuals were from Lake Huron, followed in 1985 with records from Lakes Erie and Ontario. By late August, 1986 the species had spread to southern Lake Michigan (43°N). Bythotrephes has not previously been reported from North America, but has been restricted to a northern and central Palearctic distribution. Its dramatic and widespread rise in abundance in Lake Michigan was greatest in offshore regions. Bythotrephes appears to be invading aggressively, but avoiding habitats presently occupied by glacio-marine relict species that became established in deep oligotrophic North American lakes after the Wisconsin glaciation. Because it is a voracious predator its invasion may lead to alterations in the native zooplankton fauna of the Great Lakes. It offers the chance to study how invading plankton species join an existing community. Judging from its persistence and success in deep European lakes, Bythotrephes may now become a permanent member of zooplankton communities in the Nearctic.