Deep-water fisheries along the British Isles continental slopes: status,ecosystem effects and future perspectives

In this paper, we revisit the state of deep-water fisheries to the west of the British Isles and aim to provide an overview on the key drivers behind community changes along continental margins. The deep-water fisheries to the west of the British Isles that extend from the shelf-slope break down to the lower slope and along banks and seamounts of the Rockall Basin, mainly target blue ling Molva dypterygia, roundnose grenadier Coryphaenoides rupestris, orange roughy Hoplostethus atlanticus, with by-catches of black scabbardfish Aphanopus carbo and tusk Brosme brosme. These fishing grounds experienced a long period of exhaustive exploitation until the early 2000s, but subsequently the implementation of management strategies has helped to relieve excessive fishing pressure. It is widely accepted that a better understanding of the long-term implications of disturbance is needed to understand patterns in deep-water communities and what sustainable use and exploitation of resources might look like in this context.     

African lakes and their fishes: conservation scenarios and suggestions

Synopsis Man has been associated with a variety of lakes throughout his evolutionary history in Africa. Lakeside dwellers have a close association with and understanding of these lakes. In the past four decades, however, overexploitation, introductions of alien fishes and the possibility of oil pollution present frightening prospects of irreversible losses and massive extinction. The development of scientific understanding of the life support processes, the ecosystems and the rich communities of these lakes has been so outpaced by exploitation and manipulations that accurate predictions regarding the future are impossible. Shallow lakes are more sensitive to physico-chemical changes caused by climatic as well as agricultural and industrial development than deep lakes, but their biotae are endowed with a resilience which facilitates recovery from major depletions to population size. In contrast, the speciose endemic cichlid faunae of deep lakes are sensitive to fishing pressure, are awkward to manage and should be regarded as representing a much smaller resource than initially imagined. The clupeids of Lake Tanganyika can sustain intensive fishing, but their introduction into other natural lakes is not recommended. Enormous changes to native faunae followed the introduction of Lates niloticus to lakes Kyoga and Victoria with dramatic consequences for the fisheries, for the socio-economic status of the region and for the maintenance of biotic diversity. Extinction of almost 300 fish species is a possible further consequence of L. niloticus predation. Intensive selective fishing for L. niloticus is being initiated, but is unlikely to resolve the problem. In the time necessary to substantially reduce L. niloticus, numerous endemic fishes may suffer extinction. A viable alternative to extinction of these species is captive propagation. Conservation of fishes by captive propagation is not a common or well documented practice. To encourage the acceptance of this conservation option a theoretical scenario in which the concepts and protocols are applied to the fishes of Lake Victoria is given. The possibility of returning rescued populations to the lake at a later date, assuming L. niloticus populations have been reduced, is also discussed. It is recommended that captive propagation should be practised to conserve species and to retain the option of returning rescued taxa. Scientists are urged to seek the funding to study tropical ecosystems so that conservation and rational development may acquire a sound foundation.     

Patterns of Spread of the Zebra Mussel (Dreissena polymorpha (Pallas)): the Continuing Invasion of Belarussian Lakes

The invasion of the freshwaters of Belarus by the zebra mussel, Dreissena polymorpha (Pallas), began at least 200 years ago by the opening of shipping canals linking the Black Sea and Baltic Sea drainage basins. However, zebra mussels have invaded only 93 (16.8%) of 553 studied lakes; at least 20 of these lakes were invaded within in the past 30 years. Zebra mussels were found disproportionately in lakes that were mesotrophic, larger, and had some commercial fishing. Although larger lakes have more intensive fisheries with larger catches, the intensity of the fishery and average catch did not affect the probability of zebra mussel invasion. Zebra mussels were not found in dystrophic lakes (10% of the lakes studied), probably due to their low pH and calcium content. Zebra mussels became locally extinct in one lake due to anthropogenic eutrophication and pollution. Many ecologically suitable lakes have yet to be invaded, which suggests that natural vectors of overland dispersal, e.g., waterfowl, have been ineffective in Belarus. Thus, future spread of this species will continue to depend on human activities such as commercial fishing.     

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     

Landscape variables affecting fishery yield in lake systems of the Central Amazon region, Brazil

According to fisheries data, lakes are important systems for fish production in the Amazon basin. However, there is no information about the relationship between landscape variables and fishing yield that allows foresight into potential resource exploitation in this environment. The present study aims to evaluate this relationship with the hypothesis: lakes of different shapes give the same fishery yield in the Amazon, after considering the effects of lake size, distance to the river, fishing effort, fuel and ice used. Fishery data from 1994 to 1996 were analyzed with regard to 3228 trips on 50 lakes of the main white water tributaries of the Amazon basin. Analysis of covariance was applied to test this hypothesis. With variables such as fishing grounds access, fishing effort and lake shape the model explained a significant 72% of variabilities in the fisheries yield. Fishing yields among lake systems were different, thus the null hypothesis was rejected (P < 0.05). Results indicate that dendritic lakes far distant from the main river have greater productivity than floodplain lakes because there are more habitats of fish refuge for reproduction and feed available to the fish; there are also more limitations to access by predators.     

Sensitivity and responses of diatoms to climate warming in lakes heavily influenced by humans

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Land use and ostracod community structure

Ostracods are important members of the benthos and littoral communities of lake ecosystems. Ostracods respond to hydrochemistry (water chemistry) which is influenced by climatic factors such as water balance, temperature, and chemicals in rainfall runoff from the land. Thus, at local scales, environmental preferences of ostracods and characteristics of lakes are used to infer changes in climate, hydrology, and erosion of lake catchments. This study addresses potential drivers of ostracod community structure and biodiversity at multiple spatial scales using NMS, CART^?, and multiple regression models. We identified 23 ostracod species from 12 lake sites. Lake area, maximum depth, spring conductivity, chlorophyll a, pH, dissolved oxygen, sedimentary carbonate, and organic matter all influence ostracod community structure based on our NMS. Based on regression analysis, lake depth, chlorophyll a, and total dissolved solids best explained ostracod richness and abundance. Land uses are also important community structuring elements that varied with scale; locally and regionally agriculture, wetlands, and grasslands were important. Nationally, using regression tree analysis of lakes sites in the North American Non-marine ostracod database (NANODe), row-crop agriculture was the most important predictor of biodiversity. Low agriculture corresponded to low species richness but greater landscape heterogeneity produced sites of high ostracod richness.     

Macroinvertebrates as indicators of fish absence in naturally fishless lakes

1. Little is known about native communities in naturally fishless lakes in eastern North America, a region where fish stocking has led to a decline in these habitats. 2. Our study objectives were to: (i) characterise and compare macroinvertebrate communities in fishless lakes found in two biophysical regions of Maine (U.S.A.): kettle lakes in the eastern lowlands and foothills and headwater lakes in the central and western mountains; (ii) identify unique attributes of fishless lake macroinvertebrate communities compared to lakes with fish and (iii) develop a method to efficiently identify fishless lakes when thorough fish surveys are not possible. 3. We quantified macroinvertebrate community structure in the two physiographic fishless lake types (n = 8 kettle lakes; n = 8 headwater lakes) with submerged light traps and sweep nets. We also compared fishless lake macroinvertebrate communities to those in fish‐containing lakes (n = 18) of similar size, location and maximum depth. We used non‐metric multidimensional scaling to assess differences in community structure and t‐tests for taxon‐specific comparisons between lakes. 4. Few differences in macroinvertebrate communities between the two physiographic fishless lake types were apparent. Fishless and fish‐containing lakes had numerous differences in macroinvertebrate community structure, abundance, taxonomic composition and species richness. Fish presence or absence was a stronger determinant of community structure in our study than differences in physical conditions relating to lake origin and physiography. 5. Communities in fishless lakes were more speciose and abundant than in fish‐containing lakes, especially taxa that are large, active and free‐swimming. Families differing in abundance and taxonomic composition included Notonectidae, Corixidae, Gyrinidae, Dytiscidae, Aeshnidae, Libellulidae and Chaoboridae. 6. We identified six taxa unique to fishless lakes that are robust indicators of fish absence: Graphoderus liberus, Hesperocorixa spp., Dineutus spp., Chaoborus americanus, Notonecta insulata and Callicorixa spp. These taxa are collected most effectively with submerged light traps. 7. Naturally fishless lakes warrant conservation, because they provide habitat for a unique suite of organisms that thrive in the absence of fish predation.     

Dynamics of artisanal fisheries in two Brazilian Amazonian reserves: implications to co-management

In this study we compare the dynamics of artisanal fishery in two adjacent reserves located in the Brazilian Amazon, Mamirauá (being managed for more than 12 years) and Amanã (initiating a management process), through the record of 485 fish landings in one fishing community in each reserve during high and low water seasons in 2003. Our goals were, first, to make a rapid and comparative assessment of some main aspects of fisheries in these two communities (fish species caught, CPUE, fishing gear and habitats exploited). Second, we used such data to evaluate if management strategies already in place in Mamirauá would be also valid for Amanã. Third, we compared fishing CPUE between the two communities, in order to check if co-management measures have contributed, at least partially, to preclude over-fishing, maintaining a higher fishing reward in Mamirauá reserve. We analyzed fisheries directed to the two most important marketable fishes in the region: the pirarucu (Arapaimas gigas) and the tambaqui (Colossoma macropomum), besides those fisheries aimed to subsistence and lower valued fishes. Our results indicated that the tambaqui was intensively fished year-round in Mamirauá, while Amanã fishers caught a higher variety of fishes, including catfishes and migratory scale fishes. Such differences might reflect differences in gear used and habitat exploited by fishers during the high water season. Mamirauá fishers caught a higher fish biomass considering both marketable and all fishes. Differences in gear used, habitats exploited and fishes caught during high water season indicate that distinct management initiatives might apply for each reserve. Notwithstanding their differences, both communities exploited the commercial fishes (tambaqui and pirarucu) in a similar way during the low water season. Therefore, the higher mean fishing yield (CPUE) observed in Mamirauá may be partially attributable to co-management measures, considering that Mamirauá has possibly been experiencing a higher fishing intensity than Amanã. Fishing related data are seldom available in Brazil and other tropical developing countries. We thus provided a framework of fast assessment of fishing dynamics, which may represent a first and useful step for management initiatives in the absence of more detailed data.     

Fish condition in introduced tilapias of Ugandan crater lakes in relation to deforestation and fishing pressure

This study identifies environmental predictors of the condition of two introduced tilapia species (Oreochromis leucostictus and Tilapia zillii) that are known to have divergent trophic niches (planktivore and herbivore, respectively) in 17 crater lakes in western Uganda. We asked whether fish condition differs among lakes characterized by differences in fishing pressure and catchment deforestation; and we related relative condition factor to gradients of environmental variation across lakes. Lakes characterized by severe catchment deforestation tended to be lakes with high fishing pressure, so it was difficult to explore independent and interactive effects. However, mean relative condition factor was higher in populations with high fishing pressure compared to populations with low fishing pressure for both O. leucostictus and T. zillii. The condition of O. leucostictus populations was higher in lakes with severely deforested catchments; but mean relative condition factor of T. zillii did not differ between deforestation categories. Principal components analysis (PCA) was used to describe the major environmental gradients of variation among the lakes; and PCA factor scores were regressed against relative fish condition. The association between fish condition and environmental gradients was stronger for O. leucostictus than for T. zillii. For O. leucostictus, fish condition was related to PC1 (43% of the variance) and factors that loaded most heavily included Chl-a, water transparency, lake area and depth, suggesting higher condition in lakes characterized by higher primary productivity and smaller size. For T. zillii, PC3 (11%) was the only axis related to fish condition; and factors that loaded most heavily included lake area (positive), and conductivity and total nitrogen (negative). Some of the larger lakes are characterized by higher availability of macrophytes that may positively affect the food base for T. zillii.     

Luciolates stapperssi in Lake Tanganyika. Demographical status and possible recent variations assessed by length frequency distributions

Over two years, monthly length frequency distributions of Luciolates stapperssi have been obtained from commercial fisheries of Lake Tanganyika using purse seines. Due to low selectivity of this fishing method, the length frequency distributions have been used to get estimates of growth parameters, breeding season and total mortality. Information is provided for the years 1972, 1982, 1983, in the northern part of the lake (Burundi Sector). Growth parameters remain more or less constant, but total mortality between two and six years displays important variations.     

Zooplankton seasonal dynamics in a recently filled mine pit lake: the effect of non-indigenous <Emphasis Type="Italic">Daphnia</Emphasis> establishment

We examined the temporal and vertical dynamics of zooplankton in Weavers Lake, New Zealand, between October 2004 and October 2005, at a time when it was colonised by a non-indigenous Daphnia species. Zooplankton community composition changed during the study from one of rotifer dominance (e.g. Asplanchna, Polyarthra, Brachionus and Keratella species) to cladoceran (Daphnia dentifera) dominance. Temporal changes in zooplankton community composition were strongly associated with a gradual increase in lake water clarity, and were attributable to the highly efficient filter feeding of D. dentifera. The corresponding reduction in rotifer densities may have resulted from the superior competitive abilities of the newly established Daphnia. As Daphnia were rare inhabitants of New Zealand lakes before 1990, the arrival and rapid spread of the non-indigenous D. dentifera has lead to widespread changes in both water clarity and zooplankton community composition. An apparent lack of mixing in the lake was facilitated by the lake’s extremely small surface area:depth ratio. However, we conclude that physical features of the lake had minimal influence on water clarity relative to the invasion of D. dentifera.     

<Emphasis Type="Italic">Bythotrephes</Emphasis> invasion elevates trophic position of zooplankton and fish: implications for contaminant biomagnification

We estimated the effects of Bythotrephes longimanus invasion on the trophic position (TP) of zooplankton communities and lake herring, Coregonus artedi. Temporal changes in lacustrine zooplankton communities following Bythotrephes invasion were contrasted with non-invaded reference lakes, and along with published information on zooplankton and herring diets, formed the basis of estimated changes in TP. The TP of zooplankton communities and lake herring increased significantly following the invasion of Bythotrephes, whereas TP in reference lakes decreased (zooplankton) or did not change significantly (lake herring) over a similar time frame. Elevated TP following Bythotrephes invasion was most prominent in lakes that also supported the glacial relict, Mysis diluvania, suggesting a possible synergistic interaction between these two species on zooplankton community composition. Our analysis indicated that elevated TPs of zooplankton communities and lake herring are not simply due to the presence of Bythotrephes, but rather reflect changes in the zooplankton community induced by Bythotrephes; namely, a major reduction in the proportion of herbivorous cladoceran biomass and a concomitant increase in the proportion of omnivorous and/or predatory copepod biomass in invaded lakes. We demonstrated that increases in TP of the magnitude reported here can lead to substantial increases in fish contaminant concentrations. In light of these results, we discuss potential mechanisms that may be responsible for the disconnect between empirical and theoretical evidence that mid-trophic level species invasions (e.g., Bythotrephes) elevate contaminant burdens of consumer species, and provide testable hypotheses to evaluate these mechanisms.     

Effects of resuspension and eutrophication level on summer phytoplankton dynamics in two hypertrophic areas of Lake Taihu,China

The composition of summer phytoplankton communities in two highly eutrophic areas of Lake Taihu, a large, shallow water body in China (area 2,338 km²; mean depth 2.0 m), was studied in relation to selected environmental variables. Both areas have multiple uses, including drinking water supply, fisheries, and recreation. The first area, Meiliang Bay, is much closer to central lake and subjected to wind-driven sediment resuspension, while the second, at Wuli Lake, is less affected by wave action but is considered more nutrient-rich due to its proximity to human inhabitation. By comparing two lake areas, we attempt to advance the understanding and management of different regions of shallow lakes, mainly differing in resuspension levels and eutrophication levels. A comparison of these two lake areas indicated a higher standing crop of phytoplankton in Wuli Lake than in Meiliang Bay. Canonic Correspondence Analysis indicated that nutrient status and sediment resuspension were the principal factors influencing phytoplankton composition in both areas. The dominant algal group in Meiliang Bay comprised immobile R-type strategists (Reynolds in Growth and reproductive strategies of freshwater phytoplankton, 1988), including filamentous diatoms and green algae (Aulacoseira granulate and Planctonema lauterbornii), and recruitment-beneficial RS-type strategists, mainly the colonial cyanobacteria (Microcystis spp.). The community of Wuli Lake was dominated by S-strategists, principally motile flagellates (Cryptomonas spp.) and C-strategists including small nonmotile centric diatoms and chlorococcal green algae.     

Different Predation Impacts of Two Cyclopoid Species on a Small-sized Zooplankton Community: An Experimental Analysis with Mesocosms

We used mesocosms to analyze predation impacts on the prey populations and prey community structures by two cyclopoid copepod species, the larger Mesocyclops pehpeiensis and the smaller Thermocyclops taihokuensis, who coexist with small-sized herbivorous zooplankton species in a fish-abundant lake. The overall predation impact on the prey populations was stronger for Mesocyclops than for Thermocyclops. Mesocyclops had a strong and less selective impact on the rotifer community but a selective impact on the crustaceans. In contrast, Thermocyclops had a selective predation impact on rotifers but a weak and less selective impact on the crustacean community. As a result, the former predator reduced the diversity of the crustacean community but not the rotifer community, while the latter had an opposite impact on the diversities of the two communities. It has been suggested that fish induce development of a zooplankton community dominated by the small-sized zooplankton species in fish-abundant lakes. Our results demonstrated that cyclopoid copepods altered species composition and diversity of the small-sized zooplankton community in such lakes. Thus, the results have given an important suggestion on the role of the invertebrate predator cyclopoid copepods, which often coexist with fish, that they determine population dynamics and community structures of small-sized zooplankton in fish-abundant lakes.     

Comparative analysis of rotifer community structure in five subtropical shallow lakes in East China: role of physical and chemical conditions

Worldwide, there have been few comparative studies on rotifer communities in subtropical lakes. We studied changes in rotifer community structure over 1 year and its relationship to several physicochemical variables in five subtropical shallow lakes in East China, covering a nutrient gradient from mesotrophy to moderate eutrophy. In these lakes, the genera Brachionus, Lecane, and Trichocerca dominated the rotifer species composition, and Polyarthra dolichoptera, Keratella cochlearis, Filinia longiseta, T. pusilla, and Anuraeopsis fissa were the dominant species. With increased nutrient loading, total rotifer abundance and species dominance increased, indicating that rotifer abundance might be a more sensitive indicator of trophic state than species composition. Comparative analyses of the six rotifer community indices calculated in this study and redundancy analysis (RDA) revealed that the two slightly eutrophic lakes and the other two moderately eutrophic lakes exhibited a high degree similarity in community structure. This suggests that the trophic state of a lake determines the rotifer community structure. In contrast, in the two moderately eutrophic lakes, the mass ratios of TN:TP and the contents of TP suggested N-limitation and cyanobacteria dominance in phytoplankton communities might be possible. In these lakes TN played a more important role in shaping the rotifer community according to stepwise multiple regression and RDA. RDA analysis also suggested that rotifer species distribution was strongly associated with trophic state and water temperature, with water temperature being the most important factor in determining seasonality.     

Changes in Aquatic Macrophytes after Liming Thrush Lake, Minnesota

Thrush Lake, Minnesota, was treated with limestone in 1988 to evaluate the efficacy of protective base addition against the loss of sport fisheries in a sensitive, mildly acidic lake. Prior to treatment, the lake was stressed (pH 6.46, ANC 64 μeq/L) but not severely degraded by acidic deposition and had a macrophyte community typical of lakes in northeastern Minnesota with low acid-neutralizing capacity (ANC). This paper describes the changes observed in aquatic plant communities during the 5 years after treatment, as pH and ANC slowly returned to pretreatment levels. Sphagnum platyphyllum, intolerant of non-acid conditions, was completely eliminated from the lake. The charo-phyte, Nitella, that originally shared dominance in the deep littoral zone with S. platyphyllum, decreased in importance during the first 2 years after treatment. Two vascular plants, Potamogeton pusillus and Najas flexilis, were first found in the lake the year after treatment and were abundant for 2 years after liming, probably in response to a combination of more neutral pH and reduced cover of Nitella. As the ANC and pH slowly returned to pretreatment conditions, Nitella again increased in coverage and depth range, with a concomitant decrease in P. pusillus and N. flexilis. The moss, S. platyphyllum, had not reinvaded the lake by 1993, 2 years after its dramatic decline.     

Size and structure of crayfish (Astacus astacus) populations on different habitats in Finland

The many inland waters in Finland make crayfish production an important potential resource. The rational utilization and management of this resource requires knowledge of the size and structure of the crayfish populations. The difficulties often encountered in catching crayfish complicate population studies. Mark-recapture and electric fishing have been used in the studies. The number of adult crayfish measuring more than 70 mm in a 4-ha lake was estimated at 620, and the number in a 13-ha lake at 3 480. In the lakes, the density of adult crayfish was around 0.6–1.4 m^–2 and in one stream studied about 2.5 m^–2 rising to several individuals per m² in the best biotopes.     

Assessing lake eutrophication using chironomids: understanding the nature of community response in different lake types

1. Total phosphorus (TP) and chlorophyll a (Chl a) chironomid inference models ( Brodersen & Lindegaard, 1999 ; Brooks, Bennion & Birks, 2001 ) were used in an attempt to reconstruct changes in nutrients from three very different lake types. Both training sets were expanded, particularly at the low end of the nutrient gradient, using contemporary chironomid assemblages and environmental parameters from 12 British lakes, although this had little improvement on the model performances. 2. Dissimilarity analyses showed that the historic chironomid assemblages did not have good analogues in the original calibration or extended datasets. However, since the transfer functions are based on weighted averages of the trophic optima for the taxa present and not on community similarities, reasonable downcore inferences were produced. Ordination analyses also showed that the lakes retain their ‘identity’ over time, as the sample dissimilarities within lakes were less than the dissimilarities between lakes. 3. Analysis of the three historic lake profiles showed a range of chironomid community responses to lake development. Chironomids from a shallow lake, Slapton Ley, responded indirectly to nutrient enrichment (TP), probably through altered substrate, macrophyte and fish conditions, rather than directly to primary productivity (Chl a). A stratified lake, Old Mill Reservoir, showed a loss of the profundal chironomid fauna due to increasing primary productivity (Chl a) coupled with increasing hypoxia. A response to nutrients (TP or total nitrogen (TN)) at this site is also indirect, and the TP reconstruction therefore cannot be reliably interpreted. The third lake, March Ghyll Reservoir has little change in historic chironomid communities, suggesting that this well mixed, relatively unproductive lake has changed less than the other lakes. 4. Using chironomids to reconstruct nutrient histories does not follow a simple scheme. The response to changes in nutrients may be direct, but mediated through other ecosystem components. As alternative stable states are possible at a given level of TP it is also likely that alternative chironomid communities exist under similar nutrient conditions. Changes in biological communities can thus occur over thresholds, and it is only biological proxies that can reflect such ecosystem switches within palaeoenvironmental investigations.