Taxonomy of the genus Megacyclops (Crustacea,Copepoda): morphometry and the use of enzyme electrophoresis

Morphometric investigations on the populations of Megacyclops in Lake Constance showed, that in a sublittoral region from about 20 to 50 m depth an intermediate type between the two species M. viridis (littoral) and M. gigas (benthal) could be distinguished. Using enzyme electrophoresis it was evident, that these specimens belonged to M. viridis.The annual range of water temperatures in the sub-littoral region corresponds to that found in the shallow areas of the lake or in ponds during winter and early spring, when the adults of M. viridis are markedly larger than in summer and fall (seasonal variation). Nevertheless the sublittoral population represents a different local type with special characteristics in morphometric details.     

The effects of depth and salinity on juvenile Chinook salmon Oncorhynchus tshawytscha (Walbaum) habitat choice in an artificial estuary

The energetic cost for juvenile Chinook salmon Oncorhynchus tshawytscha to forage in habitats of different salinity and depth was quantified using a behavioural titration based on ideal free distribution theory. When given a choice between freshwater habitats of different depths (>0·83 or <0·83 m), a greater proportion of fish used the deeper habitat. When the deeper habitat was saltwater, the proportion of fish using it increased. When food was added to both the shallow freshwater and deep saline habitats, however, fish distribution returned to that observed when both habitats were fresh water. This indicates that the preference for deep saline habitats during the stratified phase was driven by some benefit associated with residency in deeper water, rather than salinity. The low perceived cost of low salinity might be in part due to the fish's ability to minimize this cost by only making brief forays into the alternate freshwater habitat. When the food ration delivered to the more costly, shallow habitat was 50% greater than that delivered to the less costly, deep habitat, fish distributed themselves equally between the two habitats, presumably because of equal net benefits. This study demonstrates that juvenile Chinook salmon prefer deep saline habitat to shallow freshwater habitats but will make brief forays into the freshwater habitat if food availability is sufficiently high.     

Diel and seasonal distribution of perch in Lake Constance: a hydroacoustic study and in situ observations

A near-shore belt 50 km in length was surveyed parallel to the shoreline of Lake Constance, central Europe, with a single-beam echosounder five times between July 1993 and February 1994. The species and age composition of fish in the survey area was investigated by gillnet fishing and SCUBA-diving. In summer, the horizontal distribution of perch was patchy. Population density declined from east to west, and highest densities occurred in one shallow bay and close to ports and jetties at steeper shores. During daytime, perch stayed in the sublittoral zone between 3 and 15 m depth and between 2 and 6 m above the thermocline. Within this layer age classes were separated spatially: the relative number of young-of-the-year perch declined with depth whereas the relative number of adult perch (2+ and older) increased with depth. At dusk the fish migrated to the littoral zone, where they spent the night resting on the bottom. In winter, under almost homothermal conditions, perch of all ages were located between the 35 and 70 m depth contours, where they performed pronounced diel vertical migrations. They rested on, or close to, the bottom during daytime and ascended up to 20 m below the surface at night. During this season, horizontal distribution of perch was much more homogeneous than in summer.     

Depth distribution and biological characteristics of the European eel Anguilla anguilla in Lough Ennell, Ireland

Using a longline survey, a total of 196 European eels Anguilla anguilla were collected at different depths in Lough Ennell (maximum depth 30 m), central Ireland. The catch per unit of effort of A. anguilla that were caught from 1 to 25 m depths was lowest at 0·5–5·0 m and greatest at the deepest depth range (22·5–25·0 m). Sub-samples of A. anguilla from depths of <15 m showed little or no difference in size, sex ratio, age, growth rate, condition factor, length–mass relationship, gonado-somatic index, fin index or eye index with fish from depths of >15 m. All fish examined were female yellow-phase A. anguilla that had ages from 7 to 20 years (mean ± s . d . = 10·3 ± 2·9 years), with growth rates from 24·0–60·8 mm year⁻¹ (mean ± s . d . = 40·7 ± 8·5 mm year⁻¹). Variations in the growth rates were greater in the shallow group than that of the deep group. This study suggested that deeper regions are important feeding habitats for A. anguilla and that fish in this lake were growing moderately fast compared to similar habitats and areas in the species' range.     

The influence of light on the diel vertical migration of young-of-the-year burbot Lota lota in Lake Constance

The diel vertical distribution of young-of-the-year (YOY) burbot Lota lota in the pelagic zone of Lake Constance was compared to light intensity at the surface and to the light intensity at their mean depth. Lota lota larvae inhabited the pelagic zone of Lake Constance from the beginning of May until the end of August. From early June, after the stratification of the water column, fish performed diel vertical migrations (DVM) between the hypolimnion and epilimnion. The amplitude of DVM increased constantly during the summer and reached 70 m by the end of August. Lota lota started their ascent to the surface after sunset and descended into the hypolimnion after sunrise. As the YOY fish grew from May to August, they experienced decreasing diel maximum light intensities: in May and early June L. lota spent the day at light intensities >40 W m⁻², but they never experienced light intensities >0·1 W m⁻² after the end of June. From this time, L. lota experienced the brightest light intensities during dusk and dawn, suggesting feeding opportunities at crepuscular hours. The present study implies, that YOY L. lota in the pelagic zone of Lake Constance increased their DVM amplitude during the summer to counteract a perceived predation risk related to body size and pigmentation.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

Direct observations on the sublittoral marine algae of Argyll,Scotland

Summary The distribution with depth of sublittoral marine algae has been investigated at 11 sites on the west coast of Scotland. More than half of the species found were confined to the sublittoral zone. The greatest variety of species was found in shallow water at sites sheltered from excessive turbulence. With increasing depth the number of species found steadily decreased. There was no evidence of a specifically distinct algal flora confined to deeper water. No algae were found deeper than 36 m below ELWS at any site, but the lower limit of algal growth was reduced to 9 m in a turbid water loch and to 3 m at a site where the herbivorous echinoderm Ophiocomina nigra was abundant. Fairly distinct algal communities were found on different substrates. The major communities recognised underwater were algae attached to stable substrata, algae attached to unstable substrata and epiphytic on other algae, especially on the stipes of Laminaria hyperborea. On stable rock in areas subject to water movement L. hyperborea forest was the dominant vegetation whereas on unstable substrata and in sheltered localities L. saccharina was dominant.     

Life history patterns of parental and hybrid Daphnia differ between lakes

1. We investigated whether Daphnia galeata × hyalina hybrids of Lake Constance and Lake Greifensee show the same pattern of life history parameters as previously reported for D. galeata × cucullata hybrids and whether such a pattern is consistent between Daphnia populations from those two lakes. 2. Hybrids in Lake Constance were intermediate in size compared with the parental species. Hybrids in Lake Greifensee were smaller than D. galeata. The intrinsic growth rate (r) of hybrids from Lake Constance was not significantly different from the faster growing parental taxon D. galeata. However, r of hybrids from Lake Greifensee was significantly lower than that of D. galeata. 3. The observed juvenile body length differences between the taxa varied with the clutch number. The first clutch juvenile lengths of the three taxa did not differ for Lake Constance. First clutch juveniles of Lake Greifensee D. galeata were smaller than hybrid first clutch juveniles. The third clutch juvenile length did not differ between taxa from Lake Greifensee, but D. galeata juveniles from Lake Constance were bigger than those of D. hyalina. 4. The life history pattern found in Lake Constance corresponds to previous findings from other studies. The hybrids in this lake combine the faster population growth of one parental species with a relatively small size. In the case of Lake Greifensee hybrids, the relatively large size of first clutch juveniles and the small size of the adults could be interpreted as dual adaptations to invertebrate and fish predation. We speculate that the lower population growth rate of the hybrids is a trade‐off for this twofold protection.     

Three different patterns of how low-intensity waves can affect the energy budget of littoral fish: a mesocosm study

In a mesocosm study, somatic and otolith growth of six types of juvenile cyprinids differing in body size and body shape were studied in a low-intensity wave treatment and a no-wave control. Depending on fish type, somatic growth was either reduced by up to 60% or increased by up to 50% following exposure to the wave treatment. Somatic growth and otolith daily increment width (ODIW), the latter being used as a proxy for the fish energy turnover, were compared to reveal the effects of waves on the energy budget of the fish. Three different reaction types to waves, which correlated to the body morphology of the six fish groups, could be distinguished. Small and fusiform fish benefitted from low-intensity waves and showed higher somatic growth rates and greater ODIW in the wave treatment. In small, deep-bodied fish, growth and ODIW were reduced by waves. Finally, in larger fish with either a fusiform or deep-bodied shape, ODIW was decoupled from somatic growth, with larger ODIW in waves, but reduced somatic growth. These results show that low-intensity hydrodynamic stress is a much more important and complex habitat factor than previously assumed. It is concluded that hydrodynamic stress by waves should be accounted for in bioenergetic models and studies on habitat choice in littoral fish species.     

Some observations on the fish nurseries along the lake Kinneret shores: the interrelationships between species, and the influence of environmental factors

Observations were made of characteristic nursery areas along the southern and western coasts of Lake Kinneret, Israel, during the years 1982–84. Cyprinid and cichlid fingerlings were found to have different specific habitats. Most cyprinid species are of palaearctic origin, and prefer low water temperatures. Fingerlings of some species of cyprinids have exclusive habitats. A common characteristic of all the species is the direct relationship between body size and water depth. The presence of cichlid fingerlings in the nursery areas during the day is directly related to the occurrence of sunlight and to water temperature. An important finding was the time of dispersion of the cichlid fingerlings, which is when they attain a size ? 5 cm and weight of ? 3 g. Stocking of the lake with cichlid fingerlings below 3 g is not recommended.     

Strong impact of wintering waterbirds on zebra mussel (Dreissena polymorpha) populations at Lake Constance, Germany

1. Since zebra mussel invaded Lake Constance in the 1960s the number of wintering waterbirds increased fourfold. We studied the impact of predation by waterbirds (tufted duck Aythya fuligula, pochard Aythya ferina and coot Fulica atra) on the population of Dreissena polymorpha in winter 2001/2002. These three species, with monthly peak numbers of approximately 230 000 individuals, currently comprise up to 80% of the waterbird population wintering at Lake Constance. 2. Four different study sites and four depths, that represent typical and characteristic habitats of mussels in Upper Lake Constance, were chosen. 3. Zebra mussels were sampled before, during and after predation by waterbirds. Their biomass in shallow areas decreased by >90%; the biomass reduction in deeper areas was highly variable and dependent on the substratum. With one exception, no changes could be detected at the greatest depth (11 m). 4. Concurrent exclosure studies revealed that the decrease in zebra mussels was caused by waterbird predation. A GIS‐based approximation revealed that in an area of 1 km² a total of approximately 750 t mussel fresh mass was removed by birds, which is equivalent to 1390 g mussels per bird per day. 5. Wintering waterbirds have a strong structural impact on the littoral community of Lake Constance and could be the key predator of zebra mussels.     

Habitat selection and juvenile persistence control the distribution of two closely related Caribbean damselfishes

Summary On many Caribbean fringing coral reefs, two closely related and ecologically similar damselfishes, the beaugregory (Stegastes leucostictus Müller and Troschel) and the cocoa damselfish (S. variabilis Castelnau), occupy nonoverlapping vertical distributions. In St. Croix (USVI), beaugregory are very abundant in shallow water back reef habitats (1–2 m depth) while cocoa damselfish are restricted to the base of the forereef (10–15 m depth).In this study, the roles of habitat selection at settlement and juvenile persistence were investigated to determine their influence on this pattern of zonation. Settlement events observed at intervals over a two-year period revealed that habitat selection occurred at settlement and was confined to habitats occupied by adults. In addition, differences in juvenile persistence (due to mortality and/or emigration) were found when species were translocated between depths. Over a period of 100-days, juvenile beaugregory moved from 1 m to 12 m depth suffered four-fold greater losses at the deeper sites than shallow water controls, while translocated cocoa damselfish suffered twice as many losses in shallow water than controls at 12 m depth. Despite these differences in persistence, growth rates of the two species were similar and independent of depth. These results indicate that preferential habitat selection at settlement, perhaps an evolutionary response to differential juvenile mortality, may play a deciding role in determining distributions of ecologically similar species of coral reef fishes.     

Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa

Synopsis Lake Malawi/Niassa is a permanently stratified lake, consisting of an oxygenated layer of 230 m depth that overlies an anoxic zone that extends to the lake bottom at a maximum depth of over 700 m. Some fish are found throughout the oxygenated zone of the offshore waters, and lead a pelagic existence, although many of the species form part of the demersal community in shallower waters where the lake-bed is oxygenated. This paper reports on the distribution of cyprinids and catfish at six offshore locations, sampled with a mid-water trawl and gillnets, along the north-south axis of the lake. These two groups formed, respectively, 5% and 13% of the offshore fish biomass in the trawl catches. The offshore cyprinids comprised the small lake-spawningEngraulicypris sardella, the only species to have pelagic larvae, and the larger river-spawningOpsaridium microcephalum andO. microlepis. These cyprinids occurred mainly in the upper 100 m of the water column. The biomass of both larvae and adults ofE. sardella showed considerable fluctuations over the two years of study. The spawning site ofE. sardella remains a mystery, but is no longer believed to be in the offshore pelagic zone. The most abundant catfish was the mochokidSynodontis njassae, that exhibited diurnal movements from just above the oxic-anoxic boundary layer during the day to the surface layers at night. The clariid catfish of the genusBathyclarias were caught infrequently offshore, but owing to their large size, they could form a significant part of the biomass. Their abundance was difficult to quantify, as it was believed that the majority are active in the near-surface layers and were under-sampled by the trawl and gillnets. Information on some aspects of the breeding biology is included in the paper, although our knowledge in this area is very incomplete.     

Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south‐west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length ( L _F) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day⁻¹, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day⁻¹). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day⁻¹ in 2003 but sockeye salmon grew only 0·63 mm day⁻¹(average specific growth rates were 3·0 and 1·8% day⁻¹, respectively). Pink salmon increased from c . 32 mm L _F and 0·2 g at emergence to 78 mm L _F and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.     

Distribution,growth, production,and ecological significance of the clam Unio terminalis in Lake Kinneret,Israel

The distribution, body composition, growth rate, and population structure of Unio terminalis were measured at different sites of Lake Kinneret (Israel). Maximum clam density was found on the muddy sand between 0.3–6 m depth. Clams were most abundant in the River Jordan inlet zone, where they showed the highest growth rate. This was probably related to both highest food availability and the highest density of fish hosting Unio glochidia in this area. U. terminalis in Lake Kinneret has a more massive shell and ash content as compared with the European Unio species. The annual P/B ratios of U. terminalis populations at different sites were similar and ranged within 0.17–0.18. The computated filtration capacity and energetic budget permit the assumption that the U. terminalis population plays a substantial role in removal of organic particles from the water in the Kinneret shallow inshore zone (up to 15 m depth), and in nutrient recycling.     

Spatial Structure of the Insect Community of a Small Dimictic Lake in the Laurentians (Québec)

The distribution of merolimnic benthic insects was studied in Lake Cromwell, a small (9 ha) and shallow (max. depth 9 m) dimictic lake with a summer hypolimnetic warming on the Laurentian highlands of Québec. Communities were described from 22 sets of emergence data. Two hundred and twelve species were recognized of which 153 were Chironomidae (Diptera). Through clustering and ordination techniques, five communities were recognized: one in the flooded shrub zone along the shore, two littoral, one sublittoral and one profundal. Differences between the communities were more quantitative than qualitative. The majority of species inhabited the shore and littoral stations, and only the hardiest reached the deepest zone. There were few species characteristic of the profundal, none of them abundant. Discriminating, dominant, and characteristic species were determined for each community, as were the ubiquitous species. There is a general decrease in species richness, species diversity and population density, and a change in community structure along the depth gradient. A discontinuity at 1–2 m is related to a sharp increase in Chaoboridae in the deeper waters.     

Seasonal and habitat-related distribution pattern of Synechococcus genotypes in Lake Constance

The abundance and distribution of Synechococcus spp. in the autotrophic picoplankton of Lake Constance, were followed in the pelagic and littoral habitat by qPCR over 2 years. One genotype, represented by isolated phycoerythrin-rich strain BO 8807, showed a seasonal distribution pattern in both habitats. Before a stable thermal stratification, the maximum of both the Synechococcus population and genotype BO 8807 occurred at 15 or 20 m water depth in the pelagic habitat. During the summer stratification, when the absolute abundance of all Synechococcus spp. was highest above 15 m, the absolute and relative abundance of genotype BO 8807 was maximal at 20 m. These results indicate that Synechococcus spp. or single genotypes are present in deep maxima in Lake Constance. The in situ dynamics of genotype BO 8807 is consistent with the observation that isolated strain BO 8807 requires higher phosphate concentrations for maximum growth rates than a strain from the same phylogenetic cluster that dominates the pelagic summer population. In contrast to these findings, low genome numbers of phycocyanin-rich genotype BO 8805 were found temporarily only in both the littoral and pelagic plankton. Microscopy revealed that PC-rich cells in general occurred preferentially in the littoral habitat. We discuss our results with respect to the versatility of picocyanobacteria of the evolutionary lineage VI of cyanobacteria, and a habitat-related distribution pattern of Synechococcus genotypes.     

The ecology of Saint Paul's Rocks (Equatorial Atlantic)

St Paul's Rocks are a remote group of islets, barely 400 m across, in the equatorial mid-Atlantic (o°55'N 29°21'W). The results of a primarily zoological survey of the Rocks, carried out in September 1979 by the Cambridge Expedition to Saint Paul's Rocks, are reported. In addition, earlier studies of the Rocks' biology are discussed in the light of recent research. The terrestrial fauna is dominated by three species of sea-bird and a land crab. Also reported from on land are a few species of insects, ticks and spiders, an endemic pseudoscorpion, and a centipede; the majority of these species are associated with the seabirds.
The islets are devoid of tracheophytes, bryophytes and lichens.
The marine ecology of the Rocks is discussed with respect to habitat. Five habitats are distinguished in the supralittoral and littoral zones; the fauna and to a limited extent the flora of each is described. Sublittorally the Rocks consist almost entirely of steep escarpments extending to 60 m depth and beyond. The shallow sublittoral (< 60 m deep) is divided into four habitats: (1) Palythoa zone; (2) Caulerpa zone; (3) sub- Cauferpa zone, and (4) areas of unstable substrates. The Palythoa zone occupies a band extending from the base of the littoral to a depth of c . 5m and is dominated by a mat-forming hexacoral, Palythoa caribaeorum. Below this zone to a depth of c . 33 m the rock faces are thickly invested by a species of green-alga, Caulerpa racemosa. At depth this Caulerpa dominated zone is succeeded by an invertebrate dominated community. Unstable substrates are represented by a few pockets of coarse sand and some patches of rubble. The communities in each of these habitats are described. A primarily taxonomic review of the deep-water fauna is also included. Finally, the influence of ocean currents on the zoogeographical relationships of the Rocks' fauna is briefly examined.     

Submersed macrophytes as a food source for wintering waterbirds at Lake Constance

We investigated the relationship between Charophytes and herbivorous waterbirds in the littoral zone of Lake Constance by quantifying their seasonal dynamics. The waterfowl extensively consumed Charophyte vegetation in shallower areas (<1 m water depth) at the beginning of the winter season, while deeper regions were only used in late winter. By the end of winter, the waterfowl had almost completely depleted the available Charophyte biomass down to a water depth of approximately 2 m (relative to average mean water level (AML)). Enclosure experiments revealed that senescence processes had a negligible influence on Charophytes biomass loss until early February. Coot, Red-crested Pochard and Pochard are the main avian Chara consumers in winter. Despite their great influence on biomass in winter, their influence on subsequent Charophyte regeneration is probably limited in Lake Constance, since Charophytes as typical pioneer species produce innumerable diaspores forming dense Charophyte meadows at a depth of between 1 and 4 m every year.     

Seasonal and daily movements of the bluntnose sixgill shark (<Emphasis Type="Italic">Hexanchus griseus</Emphasis>) in the strait of Georgia from satellite tag data

The bluntnose sixgill shark (Hexanchus griseus Bonnaterre 1788) is a large, deepwater apex predator that, in a few unique locations such as the Strait of Georgia, Canada, occupies shallow, inshore waters as juveniles. The occurrence of pregnant females in the Strait of Georgia suggests that this is an important area for parturition and juvenile rearing. Bluntnose sixgill sharks are listed under the Canadian Species at Risk Act, and an understanding of their behaviour in habitat selection during important life stages is an important component for conservation measures. We employed satellite tags to identify the daily and seasonal depth and thermal habitats of juvenile bluntnose sixgill sharks in the Strait of Georgia. Tag release and depth information suggests that juvenile bluntnose sixgill sharks within the Strait of Georgia remain there until they mature and migrate to offshore deepwaters. The dominant depth habitat utilized by the juvenile sharks in our study was deepwater (>200 m), with only occasional occupation of depths <100 m, indicating that even within coastal areas, juveniles still prefer deepwater. Overall the sharks occupied shallower depths at night than during the day with increased vertical activity in both the day and night compared to crepuscular periods. Seasonally, the sharks occupied shallower depths in the summer than in winter, but inconsistency in the temperatures with which those depths were associated suggests that their vertical behaviour is influenced by local foraging opportunities and not by thermoregulation.