Abundance and distribution of Diphyllobothrium ditremum Creplin (Cestoda: Pseudophyllidea) plerocercoids in benthic whitefish, in northern Finnish Lapland

The prevalence of Diphyllobothrium ditremum plerocercoids in whitefish Coregonus lavaretus ranged between 70 and 100% in Lake Kilpisjärvi and in three other lakes in Northern Lapland, Finland. The mean abundance in Lake Kilpisjärvi (age groups 1+-10+ years), ranged between 103·5± 71·3 in 1992–1993 to 110·9± 80·0 plerocercoids per fish in 1997. The asymptotic value of the infection levelled at 113 plerocercoids per host after age 3. No significant difference in abundance was detected between study years ( P >0·10). Abundances in other lakes ranged between 4·8±9·7 and 91·1±115·1. Two seasonal peaks of plerocercoid recruitment were observed in Lake Kilpisjärvi; between March and April ( P <0·002) and between September and October ( P =0·042). In autumn the numbers of larvae increased particularly in female fish. The invasion rate of the parasite was lower in other lakes studied, and the infection rate in whitefish was closely related to the copepod food eaten.     

Seasonal and ontogenetic dynamics in trophic transmission of parasites

Transmission rates from the intermediate (amphipods) to the definitive hosts (fish) were quantified for two helminth species ( Cyathocephalus truncatus , Cestoda, and Cystidicola farionis , Nematoda) both seasonally and through the ontogeny of the final hosts (arctic charr, Salvelinus alpinus , and brown trout, Salmo trutta ). Amphipods ( Gammarus lacustris ) were important prey for both fish species, especially in the autumn. Both parasite species had low infection levels in amphipods compared to high abundance in fish. The seasonal variations in transmission rate of C. truncatus procercoids from amphipods to fish were in accordance with the observed abundance in fish hosts, being highest in the autumn and lowest during late winter and early summer. During summer, however, the estimated monthly transmission rates of C. truncatus were higher than the observed infection levels in the fish, suggesting restricted establishment success and shorter development time and longevity of the cestode in fish at higher temperatures. The accumulated transmission of C. farionis over the ontogeny of arctic charr was similar to the observed age-specific infection levels, reflecting a high establishment success and longevity of this parasite in charr. In contrast, brown trout exhibited an infection level that was much lower than the estimated transmission rates, suggesting a high resistance against C. farionis in these fish. The magnitudes of the estimated transmission rates were sufficient to explain the paradoxical contrast between low infection levels in the intermediate and high in the final hosts.     

Food borne parasites as indicators of trophic segregation between Arctic charr and brown trout

The habitat and diet choice and the infection (prevalence and abundance) of trophically transmitted parasites were compared in Arctic charr and brown trout living sympatrically in two lakes in northern Norway. Arctic charr were found in all main lake habitats, whereas the brown trout were almost exclusively found in the littoral zone. In both lakes the parasite fauna reflected the niche segregation between trout and charr. Surface insects were most common in the diet of trout, but transmit few parasites, and accordingly the brown trout had a relatively low diversity and abundance of parasites. Parasites transmitted by benthic prey such as Gammarus and insect larva, were common in both salmonid host species. Copepod transmitted parasites were much more common in Arctic charr, as brown trout did not include zooplankton in their diets. Parasite species that may use small fish as transport hosts, were far more abundant in piscivorous fish, especially brown trout. The seasonal dynamics in parasite infection were also consistent with the developments in the diet throughout the year. The study demonstrates that the structure of parasite communities of charr and the trout is highly dependent on shifts in habitat and diet of their hosts both on an annual base and through the ontogeny, in addition to the observed niche segregation between the two salmonid species.     

The ecological interaction between arctic charr, Salvelinus alpinus (L.), and the plerocercoid stage of Diphyllobothrium ditremum

Six hundred and two arctic charr, Salvelinus alpinus , from a small lake (30 ha) were examined for infection with plerocercoids of Diphyllobothrium ditremum the only species of this genus present; 466 (77.4%) harboured the parasite in an overdispersed infection. The prevalence of infection increased with age and reached 100% at 7+ years for males and 8+ years for females. The rate of infection appeared to be related to the feeding biology of the charr which did not change within the age span studied. The rate of increase in density of infection (plerocercoids per fish) decreased with age as did the degree of overdispersion. It is concluded that the infection probably induces mortality in the fish population, and that such mortality is dependent on parasite density.     

Seasonal dynamics and distribution of the digenean Phyllodistomum pawlovskii (Trematoda: Gorgoderidae) in the bullhead catfish, Pseudobagrus fulvidraco, in a lake of China

Seasonal population dynamics of the digenean Phyllodistomum pawlovskii in the urinary bladder of the bullhead catfish, Pseudobagrus fulvidraco, were investigated in Liangzi Lake in the flood plain of the Yangtze River in China from February 2001 to July 2002. The overall prevalence of the parasite was high, 41.5% (n = 1,476), while the mean abundance was relatively low, 1.24 +/- 2.11. The parasite exhibited evident seasonality in changes of prevalence and abundance. In brief, prevalence and abundance were very low in midwinter (January), but increased and remained relatively high in other seasons and months. The distribution pattern of this parasite in the fish was overdispersed, with a variance to mean ratio > 1, but its frequency distribution could not be described by the negative binomial model. There were positive correlations between the number of the parasites per fish and the age and length of the fish; a peaked age-parasite abundance curve was not detected in the parasite-host association. It is suggested that the parasite P. pawlovskii has little effect on the population structure of the bullhead catfish.     

Overwinter body condition,mortality and parasite infection in two size classes of 0+ year juvenile European bitterling Rhodeus amarus

Body condition and parasite abundance were examined in two size classes of European bitterling Rhodeus amarus during the first overwintering period in two seasons (2007–2008 and 2009–2010). Body condition of large fish did not change during winter, and increased significantly in March. From November to February, small fish showed a decreasing trend in condition. Despite a significant increase in March condition of small fish only reached the same level as before winter. Total parasite abundance increased significantly in winter in both fish size classes, reflecting a seasonal increase in monogenean infection. Large fish were parasitized significantly more than small fish during winter, but only in small fish was a negative correlation between parasite infection and condition found and a significant decrease in parasite abundance recorded after wintering, indicating mortality of heavily infected individuals with low condition during the winter. A trend for higher overwinter mortality in small fish was found under semi‐experimental conditions. The decrease in condition during the winter period in small fish may reflect faster energy depletion generally expected in smaller individuals. The results indicate that parasite infection may contribute to the overwinter mortality of 0+ year R. amarus, with a stronger effect in smaller individuals.     

Copepod ectoparasites of Merlangius merlangus and Platichthys flesus

Seasonal changes in the levels of copepod ectoparasitic infections from Merlangius merlangus and Platichthys flesus were examined and related to the annual migrations of young fish into an estuary. Lernaeocera branchialis (L.) was the only parasite common to both fish species; its life-cycle stages infected the branchial chambers of 96·7% flounders ( P. flesus ) and 40·4% whiting ( M. merlangus ). Infection in the two fish species exhibited similar seasonal fluctuations and a temporal relationship between the parasite's occurrence on its two hosts was apparent. Whiting were also parasitized by Clavella uncinata (Milller); 26·9 % fish were affected. The copepods' distribution was highly localized with the majority restricted to the long gill rakers of the first gill arch. Infection was related to size of whiting up to 12·5 cm; there was little increase beyond this length. In addition to L. branchialis , flounder was also host to Acanthochondria depressa (T. Scott) and Lepeophtheirus pectoralis (Müller). A. depressa showed a preference for the ocular side of the body with its developing stages attached mainly to the second gill arch and the adults located in the posterior pocket of the branchial chamber. L. pectoralis was associated with the paired fins. Factors which could result in such localized distributions are discussed.     

Seasonal dietary shifts enhance parasite transmission to lake salmonids during ice cover

Changes in abiotic and biotic factors between seasons in subarctic lake systems are often profound, potentially affecting the community structure and population dynamics of parasites over the annual cycle. However, few winter studies exist and interactions between fish hosts and their parasites are typically confined to snapshot studies restricted to the summer season whereas host‐parasite dynamics during the ice‐covered period rarely have been explored. The present study addresses seasonal patterns in the infections of intestinal parasites and their association with the diet of sympatric living Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in Lake Takvatn, a subarctic lake in northern Norway. In total, 354 Arctic charr and 203 brown trout were sampled from the littoral habitat between June 2017 and May 2018. Six trophically transmitted intestinal parasite taxa were identified and quantified, and their seasonal variations were contrasted with dietary information from both stomachs and intestines of the fish. The winter period proved to be an important transmission window for parasites, with increased prevalence and intensity of amphipod‐transmitted parasites in Arctic charr and parasites transmitted through fish prey in brown trout. In Arctic charr, seasonal patterns in parasite infections resulted mainly from temporal changes in diet toward amphipods, whereas host body size and the utilization of fish prey were the main drivers in brown trout. The overall dynamics in the community structure of parasites chiefly mirrored the seasonal dietary shifts of their fish hosts.     

Parasite-induced Host Mortality: Indirect Evidence From a Long-term Study

A long-term field study of a perturbed host–helminth system provides indirect evidence that a long-lived swimbladder nematode, Cystidicola farionis, induces mortality of Arctic charr, Salvelinus alpinus. The prevalence and abundance of this parasite has changed little over the period from 1987 to 1999. The cumulative numbers of L₃-stage larvae steadily increased with increasing host age, indicating a continuous exposure to infection throughout the life of the target fish host. Indirect methods, which used data pooled over years and long-term cohort analyses, indicate that parasite-induced host mortality (PIHM) occurs in hosts older than 10 years. Furthermore, using a short-term cohort method adjusted for worm recruitment, we found indications of PIHM occurrence even in younger age groups. These patterns do not seem to be caused by high parasite mortality rates since dead worms are rarely observed inside the swimbladder. Age-related changes in infection rates or in resistance to infection seem to play only a minor role as there were only slight changes in the preference of charr for feeding on amphipods (which are intermediate hosts) and in the acquisition rate of L₃ larvae in older hosts. Mortality of the most heavily infected hosts is the most probable explanation for the observed patterns.     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

The effect of Schistocephalus solidus infection on meal size of three‐spined stickleback

The effect of infection with the pseudophyllidean cestode Schistocephalus solidus on the meal size of individually housed three‐spined stickleback Gasterosteus aculeatus was quantified. Infected fish harboured plerocercoid loads that contributed from 1·1 to 33·9% of their total mass. Across this range of infection levels, the presence of S. solidus infection had no significant directional effect on standard length ( L _S) corrected meal size of host three‐spined sticklebacks. Amongst multiply infected fish there was a significant negative relationship between L _S‐corrected meal size and the proportion of host mass contributed by S. solidus parasites. This relationship, however, did not hold for singly‐infected fish. Furthermore, the data suggest that multiply‐infected fish that harbour a combined mass of parasites contributing < c . 15% to host body mass might exhibit meal sizes that exceed those of length‐matched uninfected fish. The results suggest that although heavy infections can significantly reduce the meal size of heavily infected three‐spined sticklebacks, in the early stages of multiple S. solidus infections host food intake may increase. The probable causes of these differential effects on meal size and their consequences for the host‐parasite system are discussed.     

Comparison of growth, diet and food consumption of sea-run and lake-dwelling Arctic charr

Sea-run post-smolt Arctic charr Salvelinus alpinus , (15–26 cm) from Storvatn, northern Norway (70°39'48"N) had significantly higher average specific growth rates in two years (1·64 and 1·66) than the corresponding lake-dwelling charr (0·53 and 1·20). The post-smolts displayed fast compensatory growth in the first 2–3 weeks of their sea residency, but then almost stopped growing prior to their return to fresh water. Lake-dwelling charr grew more evenly during the same time period. Thus, the anadromous charr may return to the lake after only 5–6 weeks in the sea, because the potential to maintain a high growth rate in the sea is reduced. The marine diet consisted mainly of the two crustacean plankton species Calanus finmarchicus , and Thysanoëssa , sp. (88%), and less of fish (6%), insects (4%) and benthos (2%). The diet of lake-dwelling charr consisted mainly of insects (58%, mostly chironomid pupae) and zoobenthos (29%), and less of zooplankton (13%) during the same time period. Although post-smolts had the highest growth rates, they had significantly lower food consumption rates and higher frequencies of empty stomachs than the corresponding lake-dwelling fish. Possible explanations for this paradox are discussed in relation to stomach evacuation rates, water temperature, feeding behaviour and the energy content of the food in the two environments.     

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.     

The association between parasite infection and growth rates in Arctic charr: do fast growing fish have more parasites?

Trophically transmitted parasites are known to impair fish growth in experimental studies, but this is not well documented in natural populations. For Arctic charr [Salvelinus alpinus (L.)], individual growth is positively correlated with food consumption. However, increased food consumption will increase the exposure to trophically transmitted parasites. Using a correlative approach, we explore the association between parasite abundance and the individual growth of Arctic charr from five lakes within the same watercourse. The studied parasite species differ in their life cycles and cost to the host. We predicted a positive association between parasite abundance and fish growth for parasites of low pathogenicity reflecting high consumption rates, and a negative association at higher parasite abundances for more costly parasites. We found no direct negative associations between parasite abundance and fish growth. The relationship between parasite abundance and growth was linearly positive for the low costly Crepidostomum sp. and concave for the more costly Eubothrium salvelini. In natural fish populations, the negative effects of parasites on fish growth might be outweighed by the energy assimilated from feeding on the intermediate host. However, experimental studies with varying food consumption regimes are needed to determine the mechanisms underlying our observations.

     

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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Spring cannibalism on 1 year walleye pollock in the Doto area,northern Japan: is it density dependent?

Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island, Japan was important only during spring (April to June), and its importance increased from 0% in dry mass for <200 mm L _S fish to 48·9% for >400 mm L _S fish. Most of the prey was represented by age 1 year fish, showing a unimodal body size distribution with a mode at 121–130 mm. Although cannibal body size was larger in deeper (>150 m) water, there was no difference in prey size by depth, suggesting impingement of the predators inhabiting deeper water into the shallow areas to cannibalize 1 year fish. The minimum ratio cannibal: prey size was 1·74. There was a positive but non-significant correlation between the contribution of cannibalism to a potential predator's (>300 mm) diet and an estimate of the previous year's recruitment. This was due to an extremely high contribution of cannibalism during 1992, when a distinctly larger size of predators seemed to bias the contribution. When the 1992 data were removed from the analysis, a significant correlation was obtained ( r ²=0·77, P <0·01), showing that Pollock cannibalism is rather density dependent. Based on the results, it is hypothesized that the'overflow' of 1 year fish from the shelf waters due to their high abundance and the weak stratification in the spring water column results in increased co-occurrence with adult fish and consequent cannibalism.     

The role of food particle size in the growth of juvenile Atlantic salmon (Salmo salar L.)

The effect of particle size of a commercial pelleted feed on the growth of Atlantic salmon from first feeding alevins to first and second year smolts was investigated using 20-day feeding experiments. Ten experiments were performed over a two year period, each comprising six groups of fish separately selected from stock populations. Each group was presented with one of six sizes of food particle ranging from larger than the respective mean mouth breadth (100% feed size), through 50%, 25%, 12·5%, 6·25% to 3·125%. Experiments were performed in six radial flow/circumferential drain tanks under ambient photoperiod and water temperature. Growth rate was found to be closely related to feed size. Maximum growth in each case was shown only on one size of particle; larger and smaller sizes resulted in reduced growth. The particle size for maximum growth increased in direct proportion to fish length. Fish from 4·2 to 20·3 cm in length showed maximum growth on particle diameters 0·022 to 0·026 × fish fork length (PFR). First feeding alevins were found to show comparable growth rate on particle diameters 0·0115 to 0·090 PFR. Some seasonal variation in growth response was indicated. The results are discussed in relation to developmental and seasonal effects. Possible factors affecting the energetics of prey size related differential growth are discussed.     

Parasites of coral reef fish larvae: its role in the pelagic larval stage

The pelagic larval stage is a critical component of the life cycle of most coral reef fishes, but the adaptive significance of this stage remains controversial. One hypothesis is that migrating through the pelagic environment reduces the risk a larval fish has of being parasitised. Most organisms interact with parasites, often with significant, detrimental consequences for the hosts. However, little is known about the parasites that larval fish have upon settlement, and the factors that affect the levels of parasitism. At settlement, coral reef fishes vary greatly in size and age (pelagic larval duration), which may influence the degree of parasitism. We identified and quantified the parasites of pre-settlement larvae from 44 species of coral reef fishes from the Great Barrier Reef and explored their relationship with host size and age at settlement, and phylogeny. Overall, less than 50% of the larval fishes were infected with parasites, and over 99% of these were endoparasites. A Bayesian phylogenetic regression was used to analyse host-parasite (presence and intensity) associations. The analysis showed parasite presence was not significantly related to fish size, and parasite intensity was not significantly related to fish age. A phylogenetic signal was detected for both parasite presence and intensity, indicating that, overall, closely related fish species were likely to have more similar susceptibility to parasites and similar levels of parasitism when compared to more distantly related species. The low prevalence of infection with any parasite type and the striking rarity of ectoparasites is consistent with the ‘parasite avoidance hypothesis’, which proposes that the pelagic phase of coral reef fishes results in reduced levels of parasitism.

     

Melatonin implantation during spring and summer does not affect the seasonal rhythm of feeding in anadromous Arctic charr (Salvelinus alpinus)

Plasma melatonin levels in the high-latitude teleost Arctic charr (Salvelinus alpinus) are constantly low during summer when feeding activity is high, and high during the dark winter when they eat little and loose weight. The question arises if melatonin is involved in the phase-setting of annual rhythms of feeding and growth and if low summer melatonin production is permissive for high summer growth in this species. The present study was therefore set out to compare the seasonal appetite and growth rhythms in Arctic charr with constantly high plasma melatonin levels from February throughout the Arctic summer (melatonin implanted, average mid-day plasma melatonin levels 1,106 ± 147 pg/ml) with those of fish with natural plasma melatonin levels (vehicle implanted and untreated fish with average mid-day plasma melatonin levels of 94 ± 13 and 58 ± 6 pg/ml, respectively). Feed intake, body mass or body length, as well as the timing of the seasonal growth rhythm, were not affected by the high summer plasma melatonin level. Further, Arctic charr fasted for 3 months had a 24 h plasma profile of melatonin which was consistently higher throughout the scotophase compared to fed charr. Although the daily melatonin production seems to be affected by the energy status of the fish, melatonin does not seem to be directly involved in regulation of the seasonal feeding and growth rhythm in the high-latitude, anadromous Arctic charr.     

A comparison of growth patterns between a stunted and two large predatory Arctic charr populations under identical hatchery conditions

Arctic charr are characterized by an extensive variability in growth and body size in natural waters. Although growth traits may involve a significant heritable component, most of this intraspecific variation presumably is environmentally induced and thus attributable to phenotypic plasticity. In the present study, size-at-age and length–weight relationship (body condition) were assessed for three Finnish Arctic charr populations of different geographical origins and extreme size forms (a stunted vs. two large-growing, predatory charr) held under standardized rearing conditions for 3 years (up to 37 months after hatching). In particular, our interest was to investigate whether the differences in growth between the large and the stunted charr as observed in the wild populations would diminish when the fish are offered suitable food in abundance. Population-specific mean body size and condition differed significantly in 0+, 1+, 2+ and 3+ fish. However, the identical rearing conditions resulted in the originally stunted charr reaching a comparable final mean size (317 mm/427 g) as the large charr populations (343 mm/510 g and 359 mm/497 g). Some individuals were of the same size as their parents at spawning already at the age of 0+ years. Furthermore, length–weight regression residuals of the stunted charr developed to a notably high level, indicating the largest final condition mean. The increase of size variation (CV of weight) in stunted charr lasted for over two growth seasons, whereas in large charr it remained stable since the end of the first summer. Variations in mortality and sexual maturation at age 2 seemed to be less relevant factors affecting overall growth performance. The study demonstrates an example of the high plasticity involved in the growth of fish: the stunted charr possess a tremendous capacity for growth in a benign environment, virtually corresponding to that observed in the large predatory populations.