Accumulation of plerocercoids of Triaenophorus crassus in the second intermediate host Coregonus lavaretus and their effect on growth of the host

Patterns of accumulation of Triaenophorus crassus in its second intermediate host whitefish Coregonus lavaretus s.l. were studied between 1991 and 1996 from two host populations in two separate areas of Lake Saimaa, Finland. Whitefish were infected commonly with several T. crassus plerocercoids and the parasites were aggregated into the oldest hosts. In one host population the annual parasite accumulation was 0·9 parasites in all host age groups between 3 and 8 years. In the other host population the annual accumulation was 1·6 parasites in 3–5-year-old fish, but increased up to 3 to 4 parasites per year in fish over 5 years old. The increase did not coincide with the period of maturation or any increase in whitefish growth, both of which could alter the food intake of the fish. The sharp increase in the annual accumulation suggests a threshold intensity above which the probability of acquiring further parasites increases. In spite of a heavy aggregation of parasites there was no evidence of parasite-induced host mortality. The annual increase in mean abundance was not correlated with the mean annual weight increase in 2–4-year-old fish within cohorts. However, evidence of a negative effect of parasites on whitefish growth was revealed by back-calculation of lengths of uninfected and infected whitefish and correlations between length or weight of fish and intensity of infection with fish age. Both analyses suggested that larger young fish harboured more parasites than the smaller ones while in older fish the reverse was true, a pattern that has not been shown earlier for parasitized fish.     

Ergasilid copepods as parasites of perch Perca fluviatilis and roach Rutilus rutilus in Central Finland: seasonality, maturity and environmental influence

A total of 1255 roach Rutilus rutilus (L.) and 866 perch Perca fluviatilis (L.) from four interconnected lakes in Central Finland differing in trophic status and pollution level were studied for parasitic ergasilid copepods between August 1985 and December 1988. In addition, 109 whitefish ( Coregonus sp.) were studied from one of the lakes.
Four ergasilid species were found: (the prevalence and intensity/fish, respectively, for the whole material are given in parentheses) Ergasilus briani (16·9%, 0·5), Neoergasilus japonicus (15·6%, 0·4) and Paraergasilus longidigitus (2·1%, 0·02) on the roach and Ergasilus sieboldi (9·9%, 0·1) and P. longidigitus (4·9%, 0·05) on the perch. Logit analysis was used to assess the significance of the year, the lake and the season for the prevalence of infection. Generally, the prevalence of E. briani on roach depended on the season only, with higher prevalences in summer. That of E. sieboldi on perch was mainly dependent on the lake, being most frequent in the oligotrophic lake. Both lake, year and season affected the prevalence of N. japonicus in roach. The more complicated system for N. japonicus was explained by the long duration of its free-living stage, its loose attachment and different location on the host, being found on the fins whereas E. briani and E. sieboldi were on the gills.
All species were found throughout the year, and age had no clear influence on infection. The occurrence of egg-sacs and length distributions of the copepods suggest that there are two generations a year, e.g. an overwintering E. briani generation which produces offspring in June to attack the fish in July, and a second generation which attacks the fish from August onwards. Whitefish were not infected by these parasites, only one fish being found which carried a single individual of E. sieboldi.     

Origin of the large burbot (Lota lota (L.)) caught in an Arctic ice-fishing competition: a case study

Recreational ice-fishing and ice-fishing competitions are popular in Finland. In recent years the prizes have become increasingly valuable. One such competition is held yearly at Lake Kilpisjärvi. During the 1998 ice-fishing competition, two participants brought two large burbot ( Lota lota (L.)) to the officials for weighing. These two burbot were much larger than the rest of the fish caught. The origin of these two burbot was questioned and subsequently studied by comparing their age-specific size, stomach contents and parasite fauna with those of the smaller burbot caught during the competition. Both the age-specific lengths and weights of the suspected burbot were higher than those of the smaller burbot, and the stomach analysis revealed that the large burbot had eaten roach ( Rutilus rutilus (L.)), which are not found in Lake Kilpisjärvi. Moreover, the parasite fauna in these two burbot was not typical of the lake. It was therefore concluded that the suspect burbot could not have been caught in Lake Kilpisjärvi, and the contestants were disqualified.     

The growth of whitefish in relation to water quality and fish species composition

Water quality in 16 Finnish lakes did not affect directly densely gill-rakered whitefish growth, except possibly in an acid (pH 4·9) lake, Iso Lehmaälampi, where acidity may have retarded the growth of whitefish. The density of roach affected whitefish growth in the second year of life: highest growth rates were in lakes without a roach population and lowest growth rates in lakes having strong roach populations. Competition by vendace also retarded the growth of young whitefish. The efficient mass removal of roach from a eutrophic lake was considered to have increased the growth rate of young whitefish. It is suggested that an examination of the fish species composition and relative abundance, as well as the growth of whitefish, can be used as an aid in predicting the success of stocking with whitefish.     

Morphological variation of perch Perca fluviatilis in humic lakes: the effect of predator density,competition and prey abundance

Between and within‐lake variations in morphology of perch Perca fluviatilis were studied in four humic lakes in eastern Finland. Perca fluviatilis were more streamlined and smaller headed in a lake with the highest abundance of cyprinids, but lowest abundance of predators (Lake Tuopanjärvi), indicating adaptation to planktivorous feeding and low predator density. Highest bodied fish were found from a lake with the lowest cyprinid but highest predator abundance (Lake Koppelojärvi), which conversely indicates adaptation to more effective predator avoidance. Furthermore, the length of the paired fins was longest in Lake Kinnasjärvi and Lake Tuopanjärvi, where the abundance of benthic macroinvertebrates was lowest, suggesting selection for more effective benthivory. Clear morphological differences of P. fluviatilis between habitats were found only in Lake Kinnasjärvi, whereas in Lake Koppelojärvi and Lake Tuopanjärvi only the length of the paired fins differed and in Lake Harkkojärvi no differences were found. Taken together, these results suggest that inter and intrapopulation morphological differences are probably highly dependent on different biotic factors (i.e. predation risk, resource availability and competition). Spatial and temporal variations in these factors may have a great effect on body morphology of P. fluviatilis.     

Energy Density of Lake Whitefish Coregonus clupeaformis in Lakes Huron and Michigan

We collected lake whitefish Coregonus clupeaformis off Alpena and Tawas City, Michigan, USA in Lake Huron and off Muskegon, Michigan USA in Lake Michigan during 2002–2004. We determined energy density and percent dry weight for lake whitefish from both lakes and lipid content for Lake Michigan fish. Energy density increased with increasing fish weight up to 800 g, and then remained relatively constant with further increases in fish weight. Energy density, adjusted for weight, was lower in Lake Huron than in Lake Michigan for both small (≤800 g) and large fish (>800 g). Energy density did not differ seasonally for small or large lake whitefish or between adult male and female fish. Energy density was strongly correlated with percent dry weight and percent lipid content. Based on data from commercially caught lake whitefish, body condition was lower in Lake Huron than Lake Michigan during 1981–2003, indicating that the dissimilarity in body condition between the lakes could be long standing. Energy density and lipid content in 2002–2004 in Lake Michigan were lower than data for comparable sized fish collected in 1969–1971. Differences in energy density between lakes were attributed to variation in diet and prey energy content as well as factors that affect feeding rates such as lake whitefish density and prey abundance.     

Diet, growth and competitive abilities of sympatric whitefish forms in a dense introduced population: results of a stocking experiment

The food of five whitefish stocks Coregonus lavaretus planted in Lake Vuokalanjärvi consisted mainly of zooplankton and insects taken from the water surface. The amount of benthic food in the diet of the two sparsely gill-rakered (23 and 28 rakers) stocks increased from the age of 2 years onwards. The growth of whitefish was slow in all stocks. The densely gill-rakered stock (50–51 rakers) originating from the same lake system as Lake Vuokalanjärvi grew faster than the other stocks, and the food consumption of 4- and 5-year-old fish belonging to this stock was nearly double compared with the other stocks. The habitat choice of the whitefish differed between the stocks even in the densely-rakered stocks classified as the same form. A stock with flexible habitat use and feeding habits is most likely to succeed. Introducing whitefish stocks from distant areas is not advisable.     

Thermal tolerance of a northern population of striped bass Morone saxatilis

Thermal tolerance of age 0+ year Shubenacadie River (Nova Scotia, Canada) striped bass Morone saxatilis juveniles (mean ± s . e . fork length, L _F, 19·2 ± 0·2 cm) acclimated in fresh water to six temperatures from 5 to 30° C was measured by both the incipient lethal technique (72 h assay), and the critical thermal method ( C _m). The lower incipient lethal temperature ranged from 2·4 to 11·3° C, and the upper incipient lethal temperature ( I _U) from 24·4 to 33·9° C. The area of thermal tolerance was 618° C². In a separate experiment, the I _U of large age 2+ year fish (34·4 ± 0·5 cm L _F) was 1·2 and 0·6° C lower ( P < 0·01) than smaller age 1+ year fish (21·8 ± 0·5 cm L _F) at acclimation temperatures of 16 and 23° C. Using the C _m, loss of equilibrium occurred at 27·4–37·7° C, loss of righting response at 28·1–38·4° C and onset of spasms at 28·5–38·8° C, depending on acclimation temperature. The linear regression slopes for these three responses were statistically similar (0·41; P > 0·05), but the intercepts differed (25·3, 26·0 and 26·5° C; P < 0·01). The thermal tolerance of this northern population appears to be broader than southern populations.     

Differences in susceptibility to sea lice infection between a sea run and a freshwater resident population of brown trout

Two experimental groups comprising mixed Norwegian sea run and freshwater resident brown trout Salmo trutta were infected with sea lice Lepeophtheirus salmonis in replicate tanks. Comparison of mean abundance and louse development between the sea run and resident groups revealed highly significant differences in lice abundance. The resident trout had an average abundance ± of 6·3±0·37 and 6·6±0·43 lice whilst the sea trout had an average abundance of 3·5 ±0·25 and 3·3 ±0·28 lice 29 days post infection at 9° C. No differences in development of lice, of either sex, were detected between the groups. As host groups were naive to sea lice at the start of the experiment, this suggests that there was a significant difference in susceptibility to sea lice infection between them, which may be genetically determined.     

Morphological differentiation and resource polymorphism in three sympatric whitefish Coregonus lavaretus(L.) forms in a subarctic lake

Three field‐identified whitefish Coregonus lavaretus forms in Lake Muddusjärvi, Finland, were compared in morphology, diet and prey size. First, these forms were studied with univariate and multivariate analysis to assess morphological divergence at a higher resolution level than in the field. Second, stomach contents were analysed to estimate diet‐overlap among forms. Finally, the relationship between prey size and morphology was examined. The whitefish were assigned to the initial field‐classification with 99·2% and 98·8% accuracy for morphologic and meristic traits, respectively. The small sparsely‐rakered form (SSR) had the shortest rakers and largest gillraker space, followed by the large sparsely‐rakered form (LSR) with intermediate gillraker length and gillraker space, while densely‐rakered whitefish (DR) had the longest rakers and smallest gillraker space. The two sparsely‐rakered whitefish forms (LSR and SSR), consumed mainly benthic macroinvertebrates, while densely‐rakered whitefish (DR), utilized pelagic food items. Average diet‐overlaps between whitefish forms were low in June‐September (Schoener's α = 0·02 − 0·23). Gillraker number and length were negatively correlated to prey length in the diet ( r  = −0·73, and r  = −0·60), while gillraker space was positively correlated with prey length ( r  = 0·81). The fact that these whitefish forms were morphologically and ecologically segregated, and that gillraker traits probably have a functional value in food selection, further suggests that natural selection has been important in structuring life‐history trajectories into divergent niche use.     

Physiological impact of sea lice on swimming performance of Atlantic salmon

Atlantic salmon Salmo salar were infected with two levels of sea lice Lepeophtheirus salmonis (0·13 ± 0·02 and 0·02 ± 0·00 sea lice g⁻¹). Once sea lice became adults, the ventral aorta of each fish was fitted with a Doppler cuff to measure cardiac output ( ̇ ), heart rate ( f _H) and stroke volume ( V _S) during swimming. Critical swimming speeds ( U _crit) of fish with higher sea lice numbers [2·1 ± 0·1 BL (body lengths) s⁻¹] were significantly lower ( P  < 0·05) than fish with lower numbers (2·4 ± 0·1 BL s⁻¹) and controls (sham infected, 2·6 ± 0·1 BL s⁻¹). After swimming, chloride levels in fish with higher sea lice numbers (184·4 ± 11·3 mmol l⁻¹) increased significantly (54%) from levels at rest and were significantly higher than fish with fewer lice (142·0 ± 3·7 mmol l⁻¹) or control fish (159·5 ± 3·5 mmol l⁻¹). The f _H of fish with more lice was 9% slower than the other two groups at U _crit. This decrease resulted in ̇ not increasing from resting levels. Sublethal infection by sea lice compromised the overall fitness of Atlantic salmon. The level of sea lice infection used in the present study was lower than has previously been reported to be detrimental to wild Atlantic salmon.     

Piscivory and trophic position of Anguilla anguilla in two lakes: importance of macrozoobenthos density

The feeding habits of the European eel Anguilla anguilla (>300 mm total length, L _T) were compared in two lakes of different environmental state: Lake Großer Vätersee (LGV), Germany (clear water, mesotrophic and submerged macrophytes), and Lake Vallum (LV), Denmark (turbid, eutrophic and no submerged macrophytes). The density of macrozoobenthos was higher in LV (3500 individuals m⁻²) than in LGV (1500 individuals m⁻²). The abundance of small prey fishes (40–99 mm L _T) was highest in LV. In LV, A. anguilla fed on macrozoobenthos, in particular, chironomid larvae. In LGV, A. anguilla used fishes as the main food component. Stable isotope analyses confirmed the stomach contents dietary results. The estimated mean ± s.d . trophic positions of A. anguilla in LGV (3·7 ± 0·2) was one level higher than those of fish in LV (2·7 ± 0·2). Based on these results, it is concluded that piscivory among A. anguilla was generally controlled by the density of macrozoobenthos. Stable isotope analysis further indicated that A. anguilla may act as integrators between benthic and pelagic food webs when density of insect larvae is low.     

The "crowding effect" in the cestode Schistocephalus solidus: density-dependent effects on plerocercoid size and infectivity

The occurrence of the crowding effect was demonstrated in plerocercoids of the cestode Schistocephalus solidus infecting threespine stickleback Gasterosteus aculeatus from Walby and Scout lakes, Alaska. Contrary to an earlier report, relatively large numbers of parasites (>3-4 plerocercoids) were observed to grow large enough in an intermediate host fish to become competent to infect and to mature in the definitive host under any of 3 assumed threshold values and 1 scenario of graded sizes for parasite competency. In Walby Lake, intensity and host body mass were significant predictors of mean plerocercoid mass per host, whereas intensity, host body mass, and combined parasite index were significant predictors in Scout Lake. Slopes of equations expressing the relationship between mean parasite mass and intensity for both lakes were less than 1, implying that processes other than or in combination with simple resource limitation might be producing the observed crowding effect. The causal mechanism for the crowding effect could include exploitative competition, interference competition, and host immune response. There were significant differences in infection between the two lakes, including different distributions of parasite intensities among hosts and different expressions of the crowding effect; however, an explanation of the differences awaits further investigation.     

Post-stocking perturbations in a population of subarctic whitefish,Coregonus lavaretus (L.): effects on growth,condition and cestode infection

Lake Kilpisjärvi was stocked with a total number of 440 000 migratory and benthic whitefish fry during the period of 1959–1964. Thereafter, the Diphyllobothrium ditremum plerocercoid infection became a problem decreasing the feasibility and commercial exploitation of the whitefish stock. Since dense fish populations may cause stunting, all stockings in the study area were stopped in an attempt to enhance the fisheries. The aim of the present study was to explore the effect of the cessation of the stocking on the growth, condition, catches and D. ditremum infection of the whitefish population. A total of 1594 whitefish were sampled with benthic gill nets. Growth was estimated from length-at-age and mass–length relationship was estimated with a non-linear regression in 1982–1983, 1992–1993 and 1997. Parasite infection was studied in 1992–1993 and 1997. Growth and condition were poor in the beginning of the 1980s. In 1992–1993, the growth and condition of the fish improved as the numerical catch per unit effort decreased from 1982 to 1992. The mean catches of other fish species were low. There were no significant changes in whitefish growth or condition from 1992–1993 to 1997. From the 1970s to 1982–1983, during slow growth and poor condition of the fish, the median gillraker count was decreasing to 19 (min–max 14–26). In 1992–1993, the gillraker count had increased significantly to 23 (min–max 19–26). However, the improved growth and condition did not cause decrease in the D. ditremum burden. On the contrary, based on our own results and literature notes, the parasite abundance seems to increase during the observation period of 1964–1997.     

Metechinorhynchus salmonis infection and diet in the river-spawning whitefish of the Bothnian Bay

A total of 206 river-spawning whitefish Coregonus lavaretus (L.) from the Bothnian Bay was investigated for food objects and acanthocephalans in the years 1975–78. The only acanthocephalan species found was Metechinorhynchus salmonis (Müller 1780), the total number of which was 1295.
The whitefish is shown to be highly susceptible to infection by M. salmonis , with as many as 88·9% of specimens infected in a sample in June 1978, although the mean intensity was only 8·9 parasites per host. The river-spawning whitefish does not favour the intermediate host, Pontoporeia affinis , in its summer diet in the Bothnian Bay preferring to feed on eggs and molluscs, so that the incidence of M. salmonis infection decreases to 33·3% by the middle of summer. This is further reduced in the late summer by the fast associated with the migration to their upriver spawning grounds. The fact that the other acanthocephalan inhabiting the river-spawning whitefish in the southern Gulf of Bothnia, Echinorhynchus gadi Müller, 1776 is not to be found in the Bothnian Bay population suggests that the fish captured in spring have remained in the Bothnian Bay throughout the winter.     

Temporal stability of sea louse Lepeophtheirus salmonis Krøyer populations on Atlantic salmon Salmo salar L. of wild, farm and hybrid parentage

Atlantic salmon salmo salar smolts of wild, hybrid and farmed parentage were individually tagged then reared in a sea cage for 8 months. The fish were sampled three times during this period. On all occasions, farmed Atlantic salmon displayed the highest abundance and density of sea lice Lepeophtheirus salmonis , whilst no significant differences were observed between hybrid and wild Atlantic salmon. Percentage variation between the lowest and highest infected groups was as high as 175 and 144% for L. salmonis abundance and density respectively (sample 2). The temporal stability of interindividual sea lice infection levels was investigated pair‐wise between samples using correlation (sample 1 v . 2, 1 v . 3 and 2 v . 3). When calculated using sea louse abundance, correlations ranged from r ² = 0·11, P  < 0·01 to r ² = 0·39, P  < 0·001, but, when the effects of fish size were controlled for by converting abundance to density, all correlations were <  r ² = 0·1. Therefore, these data indicate that a fish's relative infection level in one sample was a weak predictor of its relative infection level in another sample. This suggests that identification of individual Atlantic salmon that display reduced susceptibility to sea lice, may be problematic.     

Succession and secondary production of gastropods in the Ebro Delta ricefields

The effects of water level regulation on the dynamics of littoral vegetation were studied in regulated Lake Ontojärvi and unregulated Lake Lentua in northern Finland by using permanent plots. The study was carried out during 1984– 1988 and the abundance of plant species was measured yearly. The annual changes were measured by comparing the mean dissimilarities (D_m) in species abundance and the diversity (mean number of species, N _sm) between the different research years in the same square. The mean dissimilarities in Lake Ontojärvi and Lake Lentua were 0.238 and 0.297, respectively. The difference between the lakes was not significant. The effect of the increased ecological stress was observed on the littoral zone; the number of species was lower in Lake Ontojärvi than in Lake Lentua. In Lake Ontojärvi, the area of almost permanent submersion (sublittoral) showed higher dissimilarity (D_m) and lower diversity (N _sm) values compared to Lake Lentua. The vegetation in both lakes was well adapted to the disturbance caused by waves and penetrating ice. The diversity and D_m values were lower on the exposed shores in both lakes compared to sheltered shores. The peaty bottoms were the most stable environment, whereas the muddy bottoms were unstable in our research lakes. Generally speaking the vegetation in Lake Ontojärvi is equally stable as in Lake Lentua. Both diversity and dissimilarity values were slightly higher in Lake Lentua. The vegetation in regulated Lake Ontojärvi is well adapted to the ecological disturbance caused by the fluctuating water level.     

Influence of different rations and water temperatures on the growth rates of shortfinned eels and longfinned eels

Juvenile (12–152 g) shortfinned eels Anguilla australis and longfinned eels A. dieffenbachia caught in New Zealand streams were fed squid mantle Nototodarus spp. 4 days per week in laboratory experiments. A linear multiple regression equation showed the amount of food eaten (0–2·7% w day⁻¹) explained 77·7% of the variation in specific growth rates (–0·60 to +1·07% w day⁻¹) among individual eels, while previous growth rates, water temperature (10·0–20·6°C), and eel weight (12–152 g) explained a further 5·6, 1·4 and 0·8%, respectively. Growth in length ranged from –0·3 to +0·9 mm day⁻¹. Eels which were starved and then given high rations grew substantially faster than expected. Once growth rates were adjusted for differences in ration and other factors, there were no significant differences in growth rates between species or individual fish. Growth of shortfinned eels fed maximum rations of commercial eel food depended on fish size and water temperatures and ceased below 9·0°C. Growth rates in the wild were substantially less than the maximum possible, after seasonal changes in water temperatures were taken into account, indicating that food supplies and not low water temperatures were controlling growth rates in the wild.     

Prey consumption rates and growth of piscivorous brown trout in a subarctic watercourse

Prey consumption rates of piscivorous brown trout Salmo trutta were studied in the Pasvik watercourse, which forms the border between Norway and Russia. Estimates of food consumption in the field were similar to or slightly less than maximum values from a bioenergetic model. The piscivore diet consisted mainly of vendace Coregonus albula with a smaller number of whitefish Coregonus lavaretus . Individual brown trout had an estimated mean daily intake of c . 1·5 vendace and 0·4 whitefish, and a rapid annual growth increment of 7–8 cm year⁻¹. The total population of brown trout >25 cm total length was estimated as 8445 individuals (0·6 individuals ha⁻¹), giving a mean ±  s . e . annual consumption of 1553880 ± 405360 vendace and 439140 ± 287130 whitefish for the whole watercourse. The rapid growth in summer of brown trout >25 cm indicated a high prey consumption rate.     

Nitrogen and phosphorus content of some temperate and tropical freshwater fishes

The nitrogen and phosphorus content of two temperate fishes, Rutilus rutilus and Perca fluviatilis , and six tropical fishes, Oreochromis niloticus , Cichla monoculus , Serrassalmus rhombeus , Plagioscion squamosissimus , Prochilodus brevis and Hoplias malabaricus , were investigated to test the hypothesis that variation in body P content and N:P ratio is related to body size. Regressions of %P and N:P ratios against fish size (length and mass) confirmed the hypothesis for P. fluviatilis and P. squamosissimus , suggesting that body size is an important factor driving body P content and N:P ratios in some fishes. Moreover, significant increases in %N and N:P ratio with body size was found for H. malabaricus , a common piscivorous fish of the Neotropics. Interspecific variation in %P and N:P ranged two-fold and significant differences ( P < 0·05) were found among the tested species. The mean ± s . d . elemental content across all fishes ( n = 170) was 10·35 ± 1·29% for N and 3·05 ± 0·82% for P, while the N:P ratio was 8·00 ± 2·14. Data on fish body nutrient content and ratio will improve parameterization of bioenergetics and mass balance models and help clarify the role of fishes in nutrient cycles in both temperate and tropical freshwaters.