Competition in Umbra-Perca fish assemblages: experimental and field evidence

Summary One potentially important effect of interspecific competition in freshwater fish communities is to increase predation intensity from gape-limited piscivores by lowering growth rates of prey species. We investigated the operation and consequences of competition between central mudminnows (Umbra limi) and yearling yellow perch (Perca flavescens) in a system where size-limited predation on mudminnows by larger perch is a principle structuring mechanism. During laboratory experiments in which mudminnows foraged for patchily-presented food in the presence and absence of yearling perch, the food intake of mudminnows decreased at both the population and individual-fish levels when perch were present. Mudminnows were neither less active nor did they occupy lower-quality food patches in the presence of perch; exploitation competition, rather than interference or predator avoidance, appeared responsible for reduced feeding success. To assess effects of competition in the field, we examined size distributions and condition factors of yearling mudminnows and perch in small Wisconsin lakes having mudminnow-only and mudminnow-perch assemblages. Yearling mudminnows were smaller and in poorer condition in an assemblage composed (by mass) of 45% yearling perch than in two mudminnow-only assemblages. Conversely, yearling perch were larger and in better condition when the mudminnow-perch assemblage contained only 45% perch then when it contained 92% perch. Our experimental and field results indicated that an asymmetrical competitive relationship could contribute directly to the interspecific population dominance of perch over mudminnows in Umbra-Perca assemblages through reduced food intake, growth, and condition of mudminnows, and indirectly through increased vulnerability of mudminnows to size-limited predation.     

The spring development of phytoplankton in Lake Erken: species composition,biomass, primary production and nutrient conditions — a review

Relative abundance and within-lake distributions of three fishes, northern redbelly dace (Phoxinus eos), finescale dace (Phoxinus neogaeus), and central mudminnow (Umbra limi), were examined using minnow traps in Tuesday Lake, a small bog lake in the Upper Peninsula, Michigan. For these species, catches in minnow traps placed at the perimeter of the lake were 21 to 52 times higher than catches in midlake traps. Variance: mean ratios of perimeter trap catches indicated that both dace species were highly aggregated while the distribution of mudminnows was less aggregated or random. Over an 11 day period during which all fish caught were removed from the lake, catch per unit effort (CPUE) of both dace species declined in response to fish removal. In contrast, CPUE for mudminnows was low initially, increased to an asymptote and then declined only in the last 5 days of the fish removal. The patterns of CPUE for mudminnows indicated that mudminnow trapability and/or activity was reduced in the presence of high densities of dace. The low abundance of dace in traps with many mudminnows suggested mudminnows avoided traps already containing dace. Throughout the removal period, CPUE provided an accurate index of dace abundance, whereas this was true for mudmnnows only after dace populations had been reduced drastically. Therefore, in any use of minnow traps to estimate populations, both spatial distributions and relative species abundance of small fishes must be taken into account.     

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.     

The foraging behavior of the central mudminnow and yellow perch: the influence of foraging site,intraspecific and interspecific competition

Summary The central mudminnow (Umbra limi) and yellow perch (Perca flavescens) are two species of fishes that commonly co-occur in small bog lakes in the Great Lakes region of North America. Both species are dietary generalists with a high degree of dietary overlap, and perch populations have been shown to have a strong negative effect on mudminnow populations. I compared the foraging rates of the two species in a series of laboratory experiments in which four foraging sites were simulated (open bottom substrate, water column, submerged aquatic macrophytes, and water surface). Fish were observed as they foraged singly, in monospecific pairs, and in mixed species pairs to evaluate the effects of intra- and interspecific competition on foraging success and the potential for resource partitioning based on foraging site. Single species trials showed that each species had similar foraging rates at each of the four sites. Across all sites combined, interspecific competition had a greater negative effect on the foraging success of mudminnows than did intraspecific competition. Conversely, intraspecific competition had a greater negative effect on the foraging success of perch than did interspecific competition. In mixed species trials perch took more food items from each of the four foraging sites than did mudminnows and caused shifts in site-use patterns of mudminnows. Differences in the foraging success of the two species reflected the superior ability of perch to discover the presence of food and to search rapidly for additional items, but were unrelated to handling times. These differences were linked to the species' foraging modes. The competitive superiority of perch, demonstrated in these experiments, could contribute to the negative impact of perch on mudminnow populations under natural conditions.     

Is coarse woody debris in lakes a refuge or a trap for benthic invertebrates exposed to fish predation?

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Ecosystem development in different types of littoral enclosures

Macrophyte growth was studied in two enclosure types (gauze and polythene) in a homogeneousPotamogeton pectinatus bed in Lake Veluwe (The Netherlands). The gauze was expected to allow for sufficient exchange with the lake to maintain similar seston densities, the polythene was expected to exclude fish activity and most water exchange. Polythene enclosures held higher totalP. pectinatus biomass (ash-free dry weight, AFDW) than the lake, gauze enclosures were intermediate. The enclosures had a higher abundance of other macrophyte species (Chara sp.,Potamogeton pusillus) than the lake. Seston ash content was not but seston AFDW, periphyton ash content and AFDW were lower in polythene than in gauze enclosures. The difference in plant biomass between gauze and polythene may be attributed to a difference in periphyton density and in seston AFDW due to zooplankton grazing (Rotatoria andDaphnia densities were higher in polythene enclosures). Since seston and periphyton AFDW and ash content were similar in lake and gauze enclosures, the intermediate macrophyte biomass in the gauze enclosures may be explained by reduced wave action and mechanical stress. Alternatively, phytoplankton inhibition by allelopathic excretions from the macrophytes may have caused the high macrophyte biomass in the polythene, and an absence of sediment-disturbing fish the intermediate biomass in the gauze enclosures. Creation of sheltered areas may favour macrophyte growth through both mechanisms and we conclude that this can be an important tool in littoral biomanipulation.     

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade

Summary We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m² enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m³), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.     

Impact of the Nile perch on the fisheries of Lakes Victoria and Kyoga

The combined effects of lack of effective management, over-exploitation with destructive fishing gear and interspecific competition, particularly among tilapiines have had profound effects on the fish stocks of lakes Victoria and Kyoga. It has been proposed that these have been more important in the decline of the indigenous fisheries than predation or competition from Nile perch.     

Space use and feeding patterns of an offshore fish assemblage in a shallow mesotrophic lake

Synopsis Diel and spring/summer space-use and feeding patterns were investigated in an assemblage dominated by five fish species occupying the offshore waters of Lake Opinicon, a shallow mesotrophic lake in southeastern Ontario. We assessed fish distribution and diel movement in May and July through the use of gill nets set at various depths in 1.5–7.0 m depth contour zones, supplemented by observations of fish reaction to the nets. Golden shiners and alewives occupied the upper part of the water column, with the former concentrated at the littoral zone-open water interface, and the latter in the open water. Yellow perch occupied the lower part of the water column in all depth contours. Bluegills were abundant in the upper to midwater depths in all contour zones; black crappies were concentrated in the 2.5–3.5 m zones. All of these species showed either a diel or a spring-summer change in distribution pattern. Bluegills were more abundant in offshore locations in July, whereas golden shiners and yellow perch were more abundant onshore in May. Alewives and black crappies showed distinct diel movements in July, as they were largely absent from the study area during the day, but returned at night to feed. In general, there was more spatial separation among the five species in July than in May.Patterns of spatial distribution among the species generally corresponded with the type and variety of prey consumed, and with diel movement of prey in the case of water column feeders. Other factors that apparently affected spatial distribution and seasonal shifts in this assemblage were risk of predation (golden shiner), spawning activity (alewife), and a decline in prey abundance from spring to summer (bluegill and yellow perch).     

Top-down effects of underyearling fish on a phytoplankton community

1. The influence of underyearling (0+) fish on planktonic algal biomass and composition was examined during June–August in eight large enclosures (40 m³). Averaged for the entire period, Daphnia densities, transparency and phosphate concentrations were significantly higher ( P < 0.05) and chlorophyll a values significantly lower in fishless controls than in enclosures with fish (bleak ( Alburnus alburnus ), roach ( Rutilus rutilus ) or perch ( Perca fluviatilis )).
2. Enclosures without fish demonstrated a more heterogeneous algal community structure than enclosures with fish. Desmidiales and dinoflagellates were more abundant in controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing-resistant algae occurred most frequently in enclosures without fish.
3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic conditions. The strongest effects on Daphnia densities, phytoplankton biomass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top-down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator in structuring the plankton community in lakes.     

Top-down effects of underyearling fish on a phytoplankton community

1. The influence of underyearling (0+) fish on planktonic algal biomass and composition was examined during June–August in eight large enclosures (40 m³). Averaged for the entire period, Daphnia densities, transparency and phosphate concentrations were significantly higher ( P < 0.05) and chlorophyll a values significantly lower in fishless controls than in enclosures with fish (bleak ( Alburnus alburnus ), roach ( Rutilus rutilus ) or perch ( Perca fluviatilis )).
2. Enclosures without fish demonstrated a more heterogeneous algal community structure than enclosures with fish. Desmidiales and dinoflagellates were more abundant in controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing-resistant algae occurred most frequently in enclosures without fish.
3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic conditions. The strongest effects on Daphnia densities, phytoplankton biomass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top-down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator in structuring the plankton community in lakes.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Long-term stability in the population levels of the eyefluke Tylodelphys podicipina (Digenea: Diplostomatidae) in perch

Annual changes in the population size of the metacercariae of Tylodelphys podicipina in the eyes of perch in a small eutrophic lake were studied over ten years by following changes in prevalence, abundance and overdispersion of parasites throughout the life of each year class of fish. The population increased rapidly in the first two years after its introduction to the lake, but for the next six years fluctuated within very narrowly constrained limits before declining as a result of a catastrophic decline in the perch population. No evidence of parasite-induced host mortality was found: the decline in parasite abundance throughout the life of each year class was due to density-independent parasite mortality within the fish as a result of natural variation in the life span of the metacercariae. Levels of infection in the first year of a year class were determined principally by transmission processes, as transmission could only occur over a very brief period when release of cercariae from parent generation snails coincided with the presence of perch fry. Transmission appeared to be density-independent, and infection levels in perch to be determined by the dimensions of the transmission' window' and subsequently modified by parasite mortality. No evidence of host responses or any other regulatory factor of fish origin was found, and the infrapopulations in perch appear to be non-equilibrial and unstable. Although the possibility of regulatory processes acting on infrapopulations of the parasite in other hosts remains, the importance of transmission windows in determining infrapopulation levels in fish is emphasized.     

Temporal distribution of perch (Perca fluviatilis L.) in a lake-reservoir (Moselle,France): analysis of catches with vertical gill nets

Four fishing periods within a yearly cycle between 1990 and 1991 were carried out in the lake-reservoir of Mirgenbach, lake of the nuclear power station in Cattenom (Moselle, France). The catches of perch (Perca fluviatilis L.) were analyzed.In all, about 630 perch were caught in 8 sampling stations. The analysis of seasonal catches of this carnivorous fish was made according to size and mesh; a simple correspondence factor analysis enabled us to establish that this species lived in different areas depending on the season.     

The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria

The piscivorous Nile perch was introduced into Lake Victoria some 30 years ago, since when it has completely transformed the fishing industry and the species composition of the fish fauna of the lake. The original multispecies fishery, based mostly on cichlids (haplochromines, tilapias), cyprinids ( Barbus, Labeo, Rastrineobola ) and siluroids ( Bagrus, Clarias, Synodontis, Schilbe ), has changed dramatically to one based on three species: the introduced Nile perch, the cyprinids, Rastrineobola argenrea (Pellegrin), and the introduced Nile tilapia, Oreochromis niloticus (Linnaeus).
Within 25 years of its introduction the Nile perch became ubiquitous and now occurs in virtually every habitat with the exception of swamps and affluent rivers. It has preyed on all other species with profound effects, especially on the stocks of haplochromines. These originally comprised 80% of the total fish biomass in Lake Victoria, but have now decreased to less than 1% offish catches from the Kenyan waters of the lake. The fishermen of Lake Victoria have adjusted to this ecological crisis by using large-meshed nets to catch Nile perch, which has become the most important commercial species. For the first time in the history of Lake Victoria, fish fillets are now being exported to several overseas countries: the fillets are all from Nile perch.     

Surviving winter hypoxia: behavioral adaptations of fishes in a northern Wisconsin winterkill lake

Synopsis Winterkill lakes often have a characteristic fish community, presumably composed of species able to survive winter hypoxia. Our research on a small winterkill lake in northern Wisconsin indicates that fishes common in winterkill lakes have behavioral adaptations for tolerating or avoiding winter hypoxia. We examined the distribution of fishes within the lake during one winter (December through May), and fish migrations into and out of the lake for two consecutive years. As DO within the lake declined in late fall, adult-sized fishes of four species, brook stickleback, finescale dace, redbelly dace, and fathead minnow, moved to the ice-water interface where DO levels were highest. Stickleback, and to a lesser extent, fathead minnows, also moved toward the more highly oxygenated water near the inlet. During the first year, young-of-the-year fishes of blacknose shiner, Iowa darter, redbelly dace, and fathead minnow, avoided hypoxic conditions by emigrating from the lake via the outlet stream in late fall and early winter while DO within the lake was still relatively high. Blacknose shiner, redbelly dace, and fathead minnow returned to the lake in spring. Almost no fishes were trapped leaving the lake in the second fall-winter season. Central mudminnows neither moved to the ice-water interface nor emigrated from the lake as DO dropped. Mudminnows survive winter hypoxia by breathing oxygen-containing bubbles trapped beneath the ice. These relatively simple behavioral adaptations allow fishes to survive or avoid hypoxic conditions lethal to other species and may help explain the consistency in fish communities of winterkill lakes.     

Acid stress in respect to calcium and magnesium concentrations in the plasma of perch during maturation and spawning

Synopsis Plasma calcium and magnesium concentrations in perch,Perca fluviatilis, were monitored in an acid lake, Lake Iso Tenhetty (pH ∼5, Al concentrations 0.3–0.5 mg l^-1) compared with a nearby reference lake, Lake P?ij?nne, in Southern Finland. The effect of acid stress could be noticed as 1/3 lower fecundity of perch in the acid lake. Plasma Ca concentrations were temporarily lower in the middle of the winter in the fish of the acid lake, and in April–May samples the situation was opposite. Plasma Mg concentration was also lower in the perch from the acid lake in midwinter months, when compared with the double as high values (∼3 mmol l^-1) from the reference lake. Elevated level of plasma Mg have been reported for fish in cold acclimation especially in winter. Thus overwintering may be specially stressing for some specimens in acid waters. Also ova did contain clearly less magnesium in the midwinter sample from the acid lake perch compared with the reference lake sample. This difference, however, was no more to be found later in spring. Mg content in the ova of perch was in both cases high, which may be necessary to the breeding in comparable acid waters. According to the literature fish cannot increase their magnesium content directly from the water. Mortality for the reference lake embryos was lower than those of the acid lake when incubated in the acid lake. No physiological acclimation inside the fish could be demonstrated in this case.     

The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria

Synopsis The Lake Victoria fish fauna included an endemic cichlid flock of more than 300 species. To boost fisheries, Nile perch (Lates sp.) was introduced into the lake in the 1950s. In the early 1980s an explosive increase of this predator was observed. Simultaneously, catches of haplochromines decreased. This paper describes the species composition of haplochromines in a research area in the Mwanza Gulf of Lake Victoria prior to the Nile perch upsurge. The decline of the haplochromines as a group and the decline of the number of species in various habitats in the Mwanza Gulf was monitored between 1979 and 1990. Of the 123+ species originally caught at a series of sampling stations ca. 80 had disappeared from the catches after 1986. In deepwater regions and in sub-littoral regions haplochromine catches decreased to virtually zero after the Nile perch boom. Haplochromines were still caught in the littoral regions where Nile perch densities were lower. However, a considerable decrease of species occurred in these regions too. It is expected that a remnant of the original haplochromine fauna will survive in the littoral region of the lake. Extrapolation of the data of the Mwanza Gulf to the entire lake would imply that approximately 200 of the 300+ endemic haplochromine species have already disappeared, or are threatened with extinction. Although fishing had an impact on the haplochromine stocks, the main cause of their decline was predation by Nile perch. The speed of decline differed between species and appeared to depend on their abundance and size, and on the degree of habitat overlap with Nile perch. Since the Nile perch upsurge, the food web of Lake Victoria has changed considerably and the total yield of the fishery has increased three to four times. Dramatic declines of native species have also been observed in other lakes as a result of the introduction of alien predators. However, such data concern less speciose communities and, in most cases, the actual process of extinction has not been monitored.     

The influence of submerged pulpwood on feeding and condition of fishes in a reservoir

One hundred and forty years of log driving in the Kennebec River, Maine, left an estimated one to two million cords of submerged pulpwood logs, with major accumulations in reservoirs such as Wyman lake. Commercial salvaging of these logs began in 1978, which prompted a study to assess the importance of these logs to fishes and the potential effects of log salvaging. Vertical gill nets were set in Wyman Lake, in areas with and without logs, to measure fish densities and to obtain specimens for analysis of food habits and body conditions. Yellow perch (Perca flavescens) were more abundant in sites without logs (P < .05), but suckers (Catostomus spp.) were more abundant in sites with logs (P < 0.05). Yellow perch preyed primarily on cladocerans in sites without logs and on larger invertebrates in sites with logs, implying a difference in prey availability between study sites. Although the yellow perch and common shiners captured in non-log areas were longer than those in log areas (P < 0.05), their body conditions were not significantly different. Moderate log salvage operations would probably have little effect on the body conditions and abundances of fishes in Wyman Lake, if precautions were taken to avoid disruption of spawning and nursery areas at critical times of the year; impacts would vary in other lakes with differing topography and species compositions.     

High prevalence of biofilm synergy among bacterial soil isolates in cocultures indicates bacterial interspecific cooperation

Biofilms that form on roots, litter and soil particles typically contain multiple bacterial species. Currently, little is known about multispecies biofilm interactions and few studies have been based on environmental isolates. Here, the prevalence of synergistic effects in biofilm formation among seven different soil isolates, cocultured in combinations of four species, was investigated. We observed greater biofilm biomass production in 63% of the four-species culture combinations tested than in biofilm formed by single-species cultures, demonstrating a high prevalence of synergism in multispecies biofilm formation. One four-species consortium, composed of Stenotrophomonas rhizophila, Xanthomonas retroflexus, Microbacterium oxydans and Paenibacillus amylolyticus, exhibited strong synergy in biofilm formation and was selected for further study. Of the four strains, X. retroflexus was the only one capable of forming abundant biofilm in isolation, under the in vitro conditions investigated. In accordance, strain-specific quantitative PCR revealed that X. retroflexus was predominant within the four-species consortium (>97% of total biofilm cell number). Despite low relative abundance of all the remaining strains, all were indispensable for the strong synergistic effect to occur within the four-species biofilm. Moreover, absolute individual strain cell numbers were significantly enhanced when compared with those of single-species biofilms, indicating that all the individual strains benefit from inclusion in the multispecies community. Our results show a high prevalence of synergy in biofilm formation in multispecies consortia isolated from a natural bacterial habitat and suggest that interspecific cooperation occurs.