Studio Sul Ciclo Annuale del Fitoplancton del Lago di Piediluco (Umbria)

Abstract

Researches about the cycle of the phytoplankton in the lake of Piediluco (Umbria). — The Authors studied the phytoplanktonic productivity within an year (1960–1961) in the lake of Piediluco (Central Italy); they have tried also to bring into evidence eventual changes in the phytocoenosis of the lake, basing themselves on some phytoplanktonic analysis made by Pasquini (1924) and by Marchesoni (1940) on samples collected before that Nera River was put in the lake for hydroelectric purpose.

The lake of Piediluco, which is a relict of the ancient Pleistocenic lake (Velino lake), is prevàilingly alluvial in origin and it is the second lake of Umbria in extension, after Trasimeno lake, having a surface of Kmq. 1,52. It is situated in the plain of Rieti at 368 m. level s. l. m. and it is 19 m. deep (Fig. 1).

In 1932 this lake was exploited to have hydroelectric energy and the « Società Elettrica Terni » built the Stoney dike along Velino River and digged also a canal connecting the lake with the river; another derivation canal, which puts Nera River into the lake of Piediluco, was also digged.

Every month, from October 1960 to October 1961 in the eulimnic zone of the lake were collected qualitative (surface and vertical) and quantitative samples (at 1, 3, 5, 10, 15 and 19 m.) and some environmental factors as: temperature, contents in oxygen, in organic matter, in N-NH₄, in N-NO₃ and in P-PO₄ were studied.

After the putting of Nera River in the lake, the thermic regime changed: in summer there was a sinking and in winter a rise in temperature.

As we can see (Table I), the lake is of a tropical type and in winter it is in homothermal conditions; in summer there is a thermal stratification, which is of brief duration. The thermocline is evident only in June and July (Fig. 2).

The Table II, in which the quantities of oxygen dissolved in the water are reported, puts in evidence that the lake of Piediluco has an holomictic regime.

After the period of June, July and August, in which the oxygen contents of the deep waters is very low, there is in the late autumn the turnover which produces a complete mixing of superficial and deep waters (Fig. 3).

During the most part of the year phenomena of hypersaturation in O₂ take place in the surface (Table III): the highest amounts are reached in May, June, July and August. The highest percentage in O₂ is reached during the most part of the year in the surface waters (Fig. 4).

This high concentration is simply biologic in origin: it is due to the photosynthetic activity of phytoplankton which has numerically increased in this period.

Tables IV, V, VI and VII show the seasonal variations of the dissolved organic matter, of N-NH₄, of N-NO₃ and P-PO₄; they show also that the nutritive substances (mainly N-NO₃ and P-PO₄ which affect the productivity of the lake of Piediluco, undergo an annual cycle: a phase of consumption on the part of the algal flora living in the surface during spring and summer; a phase of production at the bottom during the summer-autumnal stratification and a winter phase of redistribution of these bottom reserves to all the mass of water owing to the late-autumnal turnover.

The qualitative (Table VIII and Table IX) and quantitative (Table 10/A and 10/B) analysis show an absolute prevalence of Diatoms during all the period in which the planktonic hauls have been carried out.

On a quantitative point of view they have a very large majority with regard to the other classes: Diatoms by themselves reaches 98%.

The Algae which tipically characterize the lake of Piediluco are: Cyclotella kuetzingiana, C. melosiroides, Fragilaria crotonensis, Synedra ulna, S. danica, S. acus, Asterionella formosa; these are followed by: Peridinium inconspicuum, P. cinctum, Dinobryon divergens, Rhizochrysis limnetica, Gloeococcus schroeteri and Closterium aciculare.

The prevalence of Diatoms during all the year is due not to one species, but to three species which characterize our lake (Table XI): in January and February there is a facies with Asterionella formosa, in April a facies with Synedra ulna and with Asterionella formosa, Cyclotella kuetzingiana, Synedra danica and Fragilaria crotonensis; in May and June a facies with Cyclotella kuetzingiana with Cyclotella melosiroides and Synedra ulna, while in August we have again a facies with Synedra ulna, and with Synedra acus and Cyclotella kuetzingiana. September and October are characterized by Synedra ulna, even though in a smaller amount.

The analysis of two qualitative hauls of surface carried out in the zone with a vegetation mainly constituted by Arundo phragmites (Table XII) put in evidence that the littoral vegetation affects the phytoplanktonic composition because it is on a qualitative point of view more rich of algal species than the eulimnic zone.

Before that Nera River was put in the lake, the phytoplankton of this lake was characterized by Peridinium cinctum and Ceratium hirundinella; now these species reduced themselves very much.

The Authors, in order to explain this modification in the algological composition, attach importance to the changing of the thermic conditions and perhaps to the increased turbulence of waters caused by the putting in of Nera River (on an average 16–17 mc/sec.).

Valuing the degree of trophism of the lake of Piediluco with the Diatom quozient (Nygaard et al.) and the Compound index (Nygaard, 1949), the lake must be considered eutrophic, but if we consider some environmental factors and the presence of some Algae, we conclude that the lake has a character tending to eutrophy.     

Description of two new actinosporean types from a brook of Fuji Mountain,Honshu, and from Chitose River,Hokkaido, Japan

Actinospore infection of oligochaetes living in the mud of 3 freshwater biotopes in Japan was studied. Using the cell-well plate method, a new aurantiactinomyxon type was found in 0.77% of the examined Tubifex tubifex oligochaete specimens from a brook near Yamanashi Prefectural Fisheries Experimental Station on Fuji Mountain. In 0.14% of Lumbriculus variagetus collected from Chitose River, near Chitose Salmon Hatchery, a new siedleckiella type was found, while at the same time 8.1% of the Lumbriculus spp. oligochaetes released triactinomyxons of Myxobolus arcticus. Of the examined Rhyacodrilus komarovi oligochaetes collected from the Mena River system, Hokkaido, 0.2, 0.6, 0.5 and 0.8% were infected with echinactinomyxon, neoactinomyxum and 2 types of triactinomyxon spores, respectively, and described in our previous paper. The oligochaetes released actinospores for several weeks. Actinospore infection showed high intensity in positive oligochaetes in the case of all the actinosporean types. Two of the actinospore types (aurantiactinomyxon and siedleckiella) presented here have not been previously described.     

Whirling disease: host specificity and interaction between the actinosporean stage of Myxobolus cerebralis and rainbow trout Oncorhynchus mykiss

Scanning electron microscopic studies were conducted on rainbow trout Oncorhynchus mykiss in the first 60 min after their exposure to the triactinomyxon spores of Myxobolus cerebralis. The results demonstrated that as early as 1 min post exposure the whole process, from the attachment of the triactinomyxon spores to the complete penetration of their sporoplasm germs, had occurred. The triactinomyxon spores sought out the secretory openings of mucous cells of the epidermis, the respiratory epithelium and the buccal cavity of trout and used them as portals of entry. Exposure experiments of the triactinomyxon spores of M. cerebralis to non-salmonid fish, such as goldfish Carassius auratus, carp Cyprinus carpio, nose Chondrostoma nasus, medaka Oryzias latipes, guppy Poecilia reticulata and also the amphibian tadpole Rana pipiens as well as to rainbow trout fry indicated a specificity for salmonids. Attempts to activate the triactinomyxon spores by exposure to mucus prepared from cyprinid and salmonid fish showed no significant differences from those conducted in tap water. The results suggest that the simultaneous presence of both mechano- and chemotactic stimuli was required for finding the salmonid fish host.     

Trout Salmo spp. complex in Serbia and adjacent regions of the western Balkans: reconstruction of evolutionary history from external morphology

The multivariate phenetic approach to the classification of Salmo spp. samples from Serbia and adjacent regions of western Balkans for 22 continuous external morphological characters suggests the occurrence of the following distinct stocks: West Danubian (Crno Osoje Stream and upper Zeta River) Salmo taleri , marble trout Salmo marmoratus (Trebuščica River), hatchery-reared Atlantic Salmo trutta , Mlava River drainage (Mlava and Krupaja rivers and Buk Stream) trout Salmo cf. trutta , Velika Morava River system (Godljevača, Bela and Resava rivers) trout S. cf. trutta , Ohrid Lake belvica Salmo ohridana and Aegean coastal drainage Salmo macedonicus (Božica River). In contrast to the phenetic similarity, the phylogenetic reconstruction places the Lake Ohrid belvica as part of an unresolved polytomy with other trout groups. Salmo cf. trutta in the Mlava River appears to form the basal group for the trout species in the region. The position of marble trout implies its independent and more recent origin from the West Danubian trout stock.     

Ultraviolet irradiation inactivates the waterborne infective stages of Myxobolus cerebralis: a treatment for hatchery water supplies

The effects of ultraviolet (UV) irradiation on the viability of the waterborne triactinomyxon stages of Myxobolus cerebralis were evaluated by vital staining and the infectivity for juvenile rainbow trout Oncorhynchus mykiss. A dose of 1300 mWs cm-2 was required to inactivate 100% of the triactinomyxons held under a static collimated beam of UV as determined by vital staining. Juvenile rainbow trout were protected from infections with M. cerebralis when exposed to 14,000 or 1400 triactinomyxon spores per fish that had been treated with the collimating beam apparatus (1300 mWs cm-2). Among all fish receiving UV-treated triactinomyxons, none had clinical signs of whirling disease, or evidence of microscopic lesions or spores of M. cerebralis after 5 mo at water temperatures of 15 degrees C. In contrast, 100% of the fish receiving the higher dose of untreated triactinomyxons developed clinical signs of whirling disease and both microscopic signs of infection and spores were detected in all of the high and low dose trout receiving untreated triactinomyxon exposures. Two additional trials evaluated the Cryptosporidium Inactivation Device (CID) for its ability to treat flow-through 15 degrees C well water to which triactinomyxons were added over a 2 wk period. CID treatments of a cumulative dose exceeding 64,000 triactinomyxons per fish protected juvenile rainbow from infections with M. cerebralis. Rainbow trout controls receiving the same number of untreated triactinomyxons developed both microscopic lesions and cranial spore concentrations up to 10(4.6) per 1/2 head, although no signs of clinical whirling disease were observed. UV (126 mWs cm-2, collimated beam apparatus) was also effective in killing Flavobacterium psychrophilum, the agent causing salmonid bacterial coldwater disease, as demonstrated by the inability of bacterial cells to grow on artificial media following UV treatment.     

Comparative susceptibility of Atlantic salmon, lake trout and rainbow trout to Myxobolus cerebralis in controlled laboratory exposures

The susceptibility of lake trout Salvelinus namaycush, rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar to Myxobolus cerebralis, the causative agent of whirling disease, was compared in controlled laboratory exposures. A total of 450 (225 for each dose) fry for each species were exposed to a low (200 spores per fish) or high (2000 spores per fish) dose of the infective triactinomyxon. At 22 wk post-exposure, 60 fish from each group, as well as controls for each species, were examined for clinical signs (whirling behavior, blacktail, deformed heads and skeletal deformities), microscopic lesions, and presence of spores. Rainbow trout were highly susceptible to infection, with 100% being positive for spores and with microscopic pathological changes in both exposure groups. Rainbow trout were the only species to show whirling behavior and blacktail. Atlantic salmon were less susceptible, with only 44 and 61% being positive for spores, respectively, in the low and high dose groups, while 68 and 75%, respectively, had microscopic pathology associated with cartilage damage. Rainbow trout heads contained mean spore concentrations of 2.2 (low dose) or 4.0 (high dose) x 10(6) spores g tissue(-1). The means for positive Atlantic salmon (not including zero values) were 1.7 (low) and 7.4 (high) x 10(4) spores g tissue(-1). Lake trout showed no clinical signs of infection, were negative for spores in both groups and showed no histopathological signs of M. cerebralis infection.     

The use of artificial spawning substrates as media to support the reproduction of Eurasian perch in Lake Piediluco

Lake Piediluco is a naturally regulated lake used for hydroelectric purposes. Management of the lake and the eutrophic conditions of its waters have had a negative impact on fish populations, particularly that of Eurasian perch (Perca fluviatilis L.). In order to increase the natural reproduction of this species, an experimental program was undertaken from 1997 to 2003 to facilitate reproduction by providing artificial spawning substrates. In Lake Piediluco perch spawn from March to May. Egg ribbons ranged in length from 30 to 180 cm and contained from 12,740 to 205,466 eggs. The fraction of degenerated eggs ranged from 0.20% to 63.30% (average 10.20%). The results demonstrate the effectiveness of artificial substrates as suitable substrates for the reproduction of perch. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Molecular faunistics of accidental infections of <Emphasis Type="Italic">Gyrodactylus</Emphasis> Nordmann, 1832 (Monogenea) parasitic on salmon <Emphasis Type="Italic">Salmo salar</Emphasis> L. and brown trout <Emphasis Type="Italic">Salmo trutta</Emphasis> L. in NW Russia

Salmon Salmo salar L. and brown trout S. trutta L. juveniles were examined for the presence of accidental monogenean ectoparasitic species of Gyrodactylus Nordmann, 1832 in the Baltic and White Sea basins of Russian Karelia in order to estimate the frequency of host-switching attempts on an ecological timescale. To collect phylogeographical information and for exact species identification, the parasites were characterised by nuclear internal transcribed spacer sequences of rDNA (ITS) and, for some species, also by their mitochondrial DNA (CO1 gene) sequences. Four accidental Gyrodactylus species were observed on salmon and brown trout. A few specimens of G. aphyae Malmberg, 1957, the normal host of which is the Eurasian minnow Phoxinus phoxinus (L.), were observed on lake salmon from the Rivers Kurzhma (Lake Kuito, White Sea basin) and Vidlitsa (Lake Ladoga, Baltic basin). G. lucii Kulakovskaya, 1952, a parasite of the northern pike Esox lucius L., was observed on salmon in the Kurzhma. In the River Vidlitsa, two specimens of G. papernai Ergens & Bychowsky, 1967, normally on stone loach Barbatula barbatula (L.), were found on salmon. On anadromous White Sea salmon in the River Pulonga in Chupa Bay, a few salmon parr carried small colonies of G. arcuatus Bychowsky, 1933, which were shown to have originated from the local three-spined stickleback Gasterosteus aculeatus L. consumed as prey. No specimens of Gyrodactylus salaris Malmberg, 1957 were observed, although the Pulonga is the nearest salmon spawning river to the River Keret', which is heavily infected with introduced G. salaris. In the River Satulinoja, Lake Ladoga, three specimens of G. lotae Gusev, 1953, from burbot Lota lota (L.), were collected from a single brown trout S. trutta. All nonspecific gyrodactylid infections on salmonids were judged to be temporary, because only a few specimens were observed on each of the small number of infected fishes. The prevalence of endemic G. salaris was also low, only 1% (Nfish = 296) in Lake Onega and 0.7% (Nfish = 255) in Lake Ladoga, while brown trout specific Gyrodactylus species were not observed on any of the 429 trout examined from the Ladoga basin. The host-specific and unspecific burden of Gyrodactylus spp. on these 'glacial relict' populations of salmon and brown trout was very low, suggesting a generalised resistance against the co-evolved freshwater parasite community, or some kind of 'vaccination' effect. These hypotheses deserve further testing.     

Development of Myxobolus portucalensis Saraiva & Molnár, 1990 (Myxosporea: Myxobolidae) in the oligochaete Tubifex tubifex (Müller)

The extrapiscine development of Myxobolus portucalensis, a myxosporean parasite of the subcutaneous connective tissue of eel Anguilla anguilla L. was studied in the experimentally infected oligochaete Tubifex tubifex. After infecting parasite-free tubificids with mature spores of M. portucalensis, development of actinosporean stages was first observed with a light microscope 26 days after infection. Triactinospores of M. portucalensis emerged from the worms after 160 days of intra-oligochaete development. It was observed in histological sections that early pansporocysts were located in the gut epithelium of the experimental oligochaetes. Mature pansporocysts, each containing eight triactinospores, appeared 149 days after infection. After the rupture of pansporocysts, free actinospores were found in the lumen of the oligochaete gut. Released actinospores were floating in the water and showed a typical triactinomyxon form. Each triactinospore had three pyriform polar capsules, a sporoplasm with 32 secondary cells, a moderately long style and three slightly curved, sharply pointed caudal processes. The prevalence of infection in T. tubifex proved to be 52.5% (n = 40). No infection was found in Limnodrilus hoffmeisteri or in control Tubifex specimens     

Effect of water temperature on the development, release and survival of the triactinomyxon stage of Myxobolus cerebralis in its oligochaete host.

The development of the triactinomyxon stage of Myxobolus cerebralis and release of mature spores from Tubifex tubifex were shown to be temperature dependent. In the present work, the effect of temperature over a range of 5-30 degrees C on the development and release of the triactinomyxon stages of M. cerebralis was studied. Infected T. tubifex stopped releasing triactinomyxon spores 4 days after transfer from 15 degrees C to 25 degrees C or 30 degrees C. Transmission electron microscopic examinations of the tubificids held at 25 degrees C and 30 degrees C for 3 days showed that all developmental stages degenerated and transformed to electron-dense clusters between the gut epithelial cells of T. tubifex. In contrast, tubificid worms held at 5 degrees C and 10 degrees C examined at the same time were heavily infected with many early developmental stages of triactinomyxon. At 15 degrees C, the optimal temperature for development, maturing and mature stages of the parasite were evident. Infected T. tubifex transferred from 15 degrees C to 20 degrees C stopped producing triactinomyxon spores after 15 days. However, 15 days at 20 degrees C was not sufficient to destroy all developmental stages of the parasite. When the tubificid worms were returned to 15 degrees C, the one-cell stages and the binucleate-cell stages resumed normal growth. It was also demonstrated that T. tubifex cured of infection by holding at 30 degrees C for 3 weeks and shifted to 15 degrees C could be re-infected with M. cerebralis spores. The waterborne triactinomyxon spores of M. cerebralis did not appear to be as short-lived as previously reported. More than 60% of experimentally produced waterborne triactinomyxon spores survived and maintained their infectivity for rainbow trout for 15 days at water temperatures up to 15 degrees C. In natural aquatic systems, the triactinomyxon spores may survive and keep their infectivity for periods even longer than 15 days.     

The contribution of newly established populations to the dynamics of range expansion in a one-dimensional fluvial-estuarine system: rainbow trout (Oncorhynchus mykiss) in Eastern Quebec

Aim Rainbow trout (Oncorhynchus mykiss, Walbaum 1792) is an exotic salmonid invading eastern Canada. First introduced for recreational fishing in Ontario, Quebec and the Maritime provinces, the species is now spreading in salmon rivers located in Eastern Quebec, where its stocking is strictly forbidden. Newly established populations have been found along the St Lawrence Estuary. To effectively mitigate the potential threat the invasion poses to native salmonids, we aimed to document the invasion’s origin and progress in the St Lawrence River and estuary. We first determined genetic origins among several potential wild and cultured source populations, found at the upstream and downstream extremities of the St Lawrence system. Thereafter, we studied the range expansion, predicting that the invasion process conforms to a one‐dimensional stepping‐stone dispersion model. Location Recently invaded salmon rivers that flow into the Estuary and Gulf of St Lawrence in Quebec, and watercourses supporting naturalized populations (Lake Ontario, Lake Memphremagog and Prince‐Edward‐Island rivers). Methods Rainbow trout from 10 potential source populations (wild and domestic strains) and 243 specimens captured in salmon rivers were genotyped at 10 microsatellite loci. Genetic origins of specimens and relationship between colonies were assessed using assignment analyses based on individuals and clusters. Results Invasion of rainbow trout in Eastern Quebec is directed downstream, driven by migrants from upstream naturalized populations, found in the Ganaraska River (Lake Ontario), and, to a lesser extent, in Lake Memphremagog. Populations from the Maritime provinces and domestic strains do not contribute to the colonisation process. A recently established population in Charlevoix (Eastern Quebec) supplies other downstream colonies. Main conclusions Rainbow trout is spreading from Lake Ontario downstream to Eastern Quebec using the St Lawrence River system as an invasion corridor. Range expansion in a downstream direction is driven by a more complex stepping‐stone dispersion model than predicted. Management options to protect native salmonids include reducing the effective size of the Charlevoix population, impeding reproduction in recently colonized rivers, halting the upstream migration of anadromous spawners, and curtailing stocking events inside the stocking area.     

Fatty Acid Status of Freshwater Resident and Anadromous Forms of Young Brown Trout (<Emphasis Type="Italic">Salmo trutta</Emphasis> L.)

No qualitative or quantitative differences in the profiles of fatty acids (FAs) belonging to the main classes have been found between freshwater resident (Orzega River, Onega Lake Basin) and anadromous (Indera River, White Sea Basin) forms of young brown trout. Differences have been registered in the conversion rate of the food-derived 18:2ω-6 and 18:3ω-3 FAs into long-chain and physiologically active FAs: 20:4ω-6 (arachidonic) and 22:6ω-3 (docosahexaenoic). Freshwater residential form of young brown trout (parr, at 3+-age) from the Orzega River are distinguished by a higher index of 20:4ω-6/18:2ω-6. Anadromous form of young of brown trout (smolts, at 4+-age) from the Indera River have a higher ratio of essential FAs 22:6ω-3/18:3ω-3 and ω-3/ω-6 polyunsaturated FAs; the last index correlates with higher weight–length characteristics of the studied individuals.     

Evaluation of a range of doses of ultraviolet irradiation to inactivate waterborne actinospore stages of Myxobolus cerebralis

The ability of a range of doses of ultraviolet irradiation (UV) to inactivate the waterborne actinospore or triactinomyxon stages (TAMs) of Myxobolus cerebralis was evaluated by infectivity for juvenile rainbow trout Oncorhynchus mykiss. TAMs were UV-irradiated using a low pressure mercury vapour lamp collimated beam apparatus. All doses 40, 80, 120 and 160 mJ cm(-2) were found to completely inactivate the TAMs as demonstrated by the absence of microscopic lesions, myxospores and parasite DNA detected by quantitative PCR (qPCR) among rainbow trout 5 mo post-exposure. In contrast, rainbow trout receiving the same concentrations of untreated TAMs (1000 fish(-1)) developed clinical signs of whirling disease at 2 mo post-exposure and had severe microscopic lesions, high myxospore counts and high qPCR values when examined at 5 mo following exposure to the parasite.     

Fragmentation of riverine systems: the genetic effects of dams on bull trout (Salvelinus confluentus) in the Clark Fork River system

Migratory bull trout (Salvelinus confluentus) historically spawned in tributaries of the Clark Fork River, Montana and inhabited Lake Pend Oreille as subadult and adult fish. However, in 1952 Cabinet Gorge Dam was constructed without fish passage facilities disrupting the connectivity of this system. Since the construction of this dam, bull trout populations in upstream tributaries have been in decline. Each year adult bull trout return to the base of Cabinet Gorge Dam when most migratory bull trout begin their spawning migration. However, the origin of these fish is uncertain. We used eight microsatellite loci to compare bull trout collected at the base of Cabinet Gorge Dam to fish sampled from both above and further downstream from the dam. Our data indicate that Cabinet Gorge bull trout are most likely individuals that hatched in above-dam tributaries, reared in Lake Pend Oreille, and could not return to their natal tributaries to spawn. This suggests that the risk of outbreeding depression associated with passing adults over dams in the Clark Fork system is minimal compared to the potential genetic and demographic benefits to populations located above the dams.     

Spatial and temporal variability in intertidal assemblages dominated by the mussel Brachidontes rodriguezii (d’Orbigny, 1846)

The aim of this study is to examine the impact of man’s interventions on the present day distribution of fish in a mountain area in southern Norway, the Atna river system. River Atna originates in the Rondane mountains at altitudes of nearly 1600 m a.s.l., and joins River Glomma at 300 m a.s.l. There are 98 lakes in the watercourse (701–1565 m a.s.l.). Lake Atnsjøen is the largest lake (5.0 km²). Data on the occurrence, origin and status of fish in lakes were obtained by means of interviews, questionnaires and written sources. Occurrence in rivers and streams was surveyed by electrofishing. While the lower reaches of River Glomma contain most of the freshwater fish species found in Norway, the Atna watercourse has a poor fish fauna. Physical conditions, e.g. steep river gradients and several impassable waterfalls have prevented fish from reaching most lakes and river stretches after the deglaciation some 6000 years ago. Five species of fish are regarded as native to the area; brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus), grayling (Thymallus thymallus), Siberian sculpin (Cottus poecilopus) and European minnow (Phoxinus phoxinus). Although only native species are found in the area, the present distribution of these species within the watercourse is to a very large extent a result of human interventions during the past 100–130 years. Brown trout were originally found in the main branch of the river, including Lake Atnsjøen and a few small lakes (n<5) further upstream, but it is now found in 65 lakes. Arctic charr were native only to Lake Atnsjøen, but now inhabit 20 lakes. Grayling remains restricted to River Atna below the waterfall at the outlet of Lake Atnsjøen. The natural distribution of Siberian sculpin is restricted to the main river below Liafossen waterfall (14 km above Lake Atnsjøen). During the 1890s, this species was accidentally introduced to Lake Setningsjøen, and subsequently spread to another three lakes further upstream. The European minnow was native only to the River Atna below Lake Atnsjøen, but was accidentally introduced to seven lakes in the course of this century. There are 24 fishless lakes in the watershed, mainly mountain lakes between 1033 and 1587 m a.s.l.     

Cost of predation avoidance in young-of-year lake trout (Salvelinus namaycush): growth differential in sub-optimal habitats

Selection of habitat to avoid predation may affect the diet of young-of-year (YOY) lake trout (Salvelinus namaycush). YOY lake trout may use inshore habitat to avoid predation; this habitat may be sub-optimal for growth. To test this, YOY lake trout were penned in nearshore and offshore pelagic areas of two arctic lakes. Toolik Lake had a lake trout population, the other lake, S6, did not. YOY lake trout in Toolik Lake lost weight, but those offshore lost less weight. The YOY lake trout in Lake S6 gained weight and those offshore gained more weight. The primary diet item of the YOY lake trout in both lakes during this experiment was the zooplankter Diaptomis probilofensis; it was also one of the most abundant species. However, its density inshore in Lake S6 was similar to inshore and offshore densities in Toolik Lake. The increased availability of alternative zooplankton prey in Lake S6 may account for the growth differential of YOY lake trout in Lake S6 relative to Toolik Lake. Bioenergetic modeling of YOY lake trout suggests that growth similar to that in the offshore of Lake S6 would be necessary for successful recruitment. If the reduced zooplankton availability in Toolik Lake leads to the reduced growth of YOY in the inshore and offshore pelagic areas, then these fish will be more susceptable to winter predation/starvation. For YOY lake trout to survive in Toolik Lake they most likely shift to feeding on benthic prey before the end of their first summer. Dept. of Chemical Engineering