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.     

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.     

Seasonal variability of ichthyoneuston assemblage on the continental shelf and slope of the Southwest Atlantic Ocean,Brazil (20°–23°S)

Many marine organisms spend the early life history stages in neuston domain. Although the importance of ichthyoneuston, few studies were developed in the Southwest Atlantic Ocean. Therefore, this study aimed to improve the knowledge on ichthyoneuston of the Brazilian coast analyzing the vertical stratification and horizontal distribution of these organisms. Neuston samples were collected in daylight between 20°S and 23°S from February to April 2009 (late summer/early autumn; rainy season) and from August to September 2009 (late winter/early spring; dry season). Eggs of six taxa were identified: Anguilliformes, Engraulidae, Clupeidae, Synodontidae, Trichiuridae, and Maurolicus stehmanni (Sternoptychidae). The neustonic larval fish assemblage was composed by 40 families and 63 species. Mullidae and Myctophidae larvae were the most abundant in the rainy period while Mullidae and Mugilidae dominated in the dry season. Seasonal and spatial variation of larval fish assemblages in the neuston were structured by oceanographic features. The larval fish abundance on the outer slope stations may have been favored by the advection of an anticyclone that encompassed most of the study region during the cruise periods. In the rainy season, salinity and local depth structured the larval fish assemblage in a cross‐shelf gradient, while in the dry season the larval assemblage was structured around temperature and north‐south gradient. In the dry season, the cross‐shelf gradient was less pronounced mainly because of the low abundance and frequency of mesopelagic larvae. The low frequency and abundance of some species are probably related to the net avoidance of fish larvae during the day or dial vertical migration, as many species migrate to deeper layers during the day and ascend to neuston only at night. Nevertheless, the present study presented baseline information about the seasonal and spatial variation of the neustonic larval fish assemblage influenced by the oceanographic conditions in the Campos Basin. We recommend to additionally collect night samples to decrease larval escapement rates and to compare night versus day catches to further investigate the influence of daily migration in the neustonic larval fish in the area.     

Structure and diversity of fish assemblages on rocky and soft bottom shores on the Swedish west coast

Density and biomass of fishes, from shallow rocky and soft bottom habitats on the Swedish west coast, showed large seasonal variation with low values in winter and spring and with peaks in June. Season was also the most important factor determining the fish assemblage structure. Within season, however, there was a clear separation in assemblage structure between rocky-and soft-bottom habitats. There were significantly higher total fish abundances and biomasses during night compared with day catches. On soft bottoms density and biomass of fishes decreased with increasing depth, but no such pattern was seen in rocky habitats indicating that the distribution of fishes was related to vegetation cover. Altogether, 53 fish species were recorded of which 30 were common to both habitats. Species richness was similar on rocky and soft bottoms. Of the 10 most abundant species found in rocky habitats four belonged to the Labridae and three to the Gadidae. The fish assemblage on soft bottoms were of a more mixed nature with representatives among the 10 dominants from six families (Clupeidae, Cottidae, Gadidae, Gobiidae, Labridae and Pleuronectidae). When ranking the 10 dominant fish species on rocky bottoms according to biomass c . 50% of the mass was Labridae, 19% Gadidae and 13% Cottidae. In soft bottom habitats, fish biomass was mainly distributed between six families. Pleuronectidae and Gadidae were dominant and each made up 25% of the biomass, whereas Labridae only contributed 4% of the fish mass. It is concluded that the fish assemblage in rocky habitats is dominated by permanent non-commercial species (63% of biomass), whereas soft bottoms mainly function as nurseries for juvenile fishes and as feeding grounds for seasonal migrants of commercial species (80% of biomass).     

The effect of extreme northerly climatic conditions on the life history of the minnow, Phoxinus phoxinus (L.)

Over its exceptionally wide geographical range the minnow displays a remarkable variability in its life history. In southern England many fish mature after one year and few fish survive to their third birthday. Very few fish spawned for more than one season but a 65-mm female is estimated to spawn up to 3172 eggs over the extended spawning season. In the River Utsjoki in Finnish Lapland maturity was strongly size-dependent and delayed until the fish reached 5, 6 or even 7 years of age, with a maximum age of 13 years at a length of only 75 mm. Unlike some other small-sized species in unproductive environments, individual females continued to spawn successive clutches of eggs though over a much shorter spawning season. The maximum estimated egg production was 824 for a 65-mm fish. Temperature had a strong direct or indirect effect on growth, with significantly lower growth increments in cold summers. At a second Arctic site in eastern Lapland growth rates were higher and maturity earlier, yet clutch size was greatly reduced with a seasonal fecundity estimate of only 314. There were only small differences in size of eggs or larvae between the populations. Evidence from other species indicates that most of the observed changes over the range of the minnow are phenotypic responses to the contrasting environments. However, the different strategies displayed at the two Arctic sites could represent the results of selection for differences in pre- and post-reproductive survival.     

Do SASS5 scores vary with season in the South African highveld? A case study on the Skeerpoort River,North West province,South Africa

Macroinvertebrate assemblages can vary with season as a result of changing environmental conditions and phenology of individual species. The objective of this study was to determine whether results from the South African Scoring System version 5 (SASS5) are affected by season. The standard SASS5 protocol was used to sample three sites on the Skeerpoort River, South Africa, in autumn, winter and spring 2013, and summer 2014. At each site, stream morphology was characterised and physico-chemical variables were measured. Two-way analysis of variance (ANOVA) was used to compare SASS indices in relation to site and season, and a one-way ANOVA was used to test the effect of season using both current and historical data. Macroinvertebrate assemblage similarity of sites was determined using non-metric multidimensional scaling ordination, followed by analysis of similarity. Whilst macroinvertebrate assemblages differed between seasons and sites, there was no seasonal variation in the values of SASS indices, but they did differ between sites. SASS5 appears to be robust to seasonal variation in macroinvertebrate assemblage and seems suitable for use throughout the year in a perennial river draining the South African highveld region.     

Predation pressure induced by seasonal fishing moratorium changes the dynamics of subtropical Cladocera populations

Seasonal fishing moratorium with selective fish harvesting is a common means of increasing fish production in Asia. Such manipulation might be expected to lead to a seasonal variation in abundance and composition of fish and predation pressure on zooplankton, and consequently a seasonal change in the Cladocera populations. In this study, we examined the seasonal dynamics of cladocerans in a subtropical plateau lake in southwestern China, Lake Erh, in which a fishing moratorium was implemented in spring and early summer. Cladocerans showed a bimodal seasonal distribution linked to variations in predation pressure induced by the seasonal fishing moratorium. When predation was low in winter because of cool water temperatures and decreasing fish density, Daphnia galeata appeared in the water column and increased towards spring. The cladoceran peaked in the middle of the fishing moratorium, coinciding with a clear water phase. The feeding rate increased with temperature in the late moratorium, allowing planktivorous fish to strongly suppress large daphnids. Small cladocerans benefited from the reduced competition with large daphnids by increasing their numbers rapidly, which resulted in a second peak of cladocerans in the fishing season.     

Seasonal response of fish assemblages to habitat fragmentation caused by an impoundment in a Neotropical river

Changes in fish assemblages between the zones above and below Funil dam in Southeastern Brazil were investigated to evaluate the possible impacts of this impoundment in two contrasting seasons: summer/wet and winter/dry. We expect differences in fish assemblage structure and in environmental conditions between seasons and between the reservoir and the zone downriver of the dam. A total of 3,579 individuals comprising 38 species, including six non-natives, were collected. As expected, the comparatively high habitat complexity and water flow regime of the downriver zone favored a richer and more abundant fish assemblage compared with the reservoir, especially in the wet season. In this period, water covers part of the riparian vegetation, increasing habitat availability and nutrient input. Additionally, the dam prevents upriver migration of rheophilics fish species such as the Characiformes Prochilodus lineatus and Leporinus copelandii, and the Siluriformes Pimelodus fur and Pimelodus maculatus, thus increasing shoals below the dam. Although the reservoir represents a simplified ecosystem highly influenced by non-native top predator species (e.g. the Perciformes Cichla kelberi and Plagioscion squamosissimus), seasonal processes (e.g. water level fluctuations and flood pulses) seem to play a role in structuring of the fish assemblage. Environmental variables, mainly turbidity, temperature, and conductivity were significantly associated to spatial-temporal patterns of fish assemblage. In this freshwater tropical reservoir, the spatial scale, rather than the seasonal changes in environmental variables, was the dominant factor structuring fish assemblage in the reservoir and in the zone downriver of the dam.     

Seasonal changes in phagocytic capacity and superoxide anion production of blood phagocytes from tench (Tinca tinca,L.)

Seasonal variations in the ex vivo phagocytic function of blood cells from tench, including ingestion capacity of inert particles and its destruction (microbicide capacity) assessed by measurement of superoxide anion production, were studied. Tench were maintained under natural conditions throughout the year, and the different assays of samples taken during each season were initially performed in vitro at 22°C and the results compared. Subsequently, assays were performed at the same temperature as that of the water ponds in which the fish were kept (“seasonal temperature”: 12°C in winter, 22°C in spring and autumn and 30°C in summer) and the results compared seasonally. The results at 22°C showed that phagocytic capacity was greatest in spring and summer and lowest in winter. However, when phagocytic capacity was measured at seasonal temperature, highest values appeared in winter and lowest in summer and autumn. Nitroblue tetrazolium reduction by tench phagocytes after phagocytosing latex beads demonstrated a similar seasonal behaviour at both 22°C in each season and at seasonal temperature. The highest values appeared in summer, which suggests a better microbicide capacity in this season. The results obtained in this study suggest that for a correct interpretation of ex vivo phagocytic capacity of fish through the year it is necessary to use the same assay temperature as that of the water in which the fish is kept.     

Biology of the minnow Phoxinus phoxinus and its interactions with brown trout Salmo trutta in Øvre Heimdalsvatn, Norway

Minnows have recently been introduced into the mountain lakes of southern Norway. In Øvre Heimdalsvatn, an intensively studied lake, the minnow population has increased rapidly during the last decade. Studies were performed on feeding habits, habitat preferences, age, growth, energy content, densities, parasites, predation and spawning activities. Several aspects were found to be different from other studies, e.g. higher age of sexual maturation, lower growth rate, no feeding on plants, and low infection of parasites. The interaction with the only other fish, the brown trout, is discussed in relation to the minnow population. In 1978 the minnows constituted little more than 10% of production and food consumption of the trout population, and only limited food competition and predation was documented. Increasing competition is likely with a greater minnow population.     

Variability of the lysozyme content in bream from the Rybinsk Reservoir in different seasons of the annual cycle

The seasonal variability of the lysozyme content has been detected in the liver, kidney, and spleen of bream. It is manifested in a higher lysozyme content in cold winter months and lower values in other seasons. It is shown that the enzyme activity is absent in blood serum of the studied fish regardless of its amount in immunocompetent organs. A high content of lysozyme in organs in the cold season of the year is not the result of the immunomodulatory effect of water temperature on this parameter but can be determined by physiological factors that do not depend on seasonal variations of the environmental temperature. It has been established that the blood serum in fish does not indicate the entire dynamics of the enzyme in the organism.     

Surviving winter hypoxia: behavioral adaptations of fishes in a northern Wisconsin winterkill lake

Synopsis Winterkill lakes often have a characteristic fish community, presumably composed of species able to survive winter hypoxia. Our research on a small winterkill lake in northern Wisconsin indicates that fishes common in winterkill lakes have behavioral adaptations for tolerating or avoiding winter hypoxia. We examined the distribution of fishes within the lake during one winter (December through May), and fish migrations into and out of the lake for two consecutive years. As DO within the lake declined in late fall, adult-sized fishes of four species, brook stickleback, finescale dace, redbelly dace, and fathead minnow, moved to the ice-water interface where DO levels were highest. Stickleback, and to a lesser extent, fathead minnows, also moved toward the more highly oxygenated water near the inlet. During the first year, young-of-the-year fishes of blacknose shiner, Iowa darter, redbelly dace, and fathead minnow, avoided hypoxic conditions by emigrating from the lake via the outlet stream in late fall and early winter while DO within the lake was still relatively high. Blacknose shiner, redbelly dace, and fathead minnow returned to the lake in spring. Almost no fishes were trapped leaving the lake in the second fall-winter season. Central mudminnows neither moved to the ice-water interface nor emigrated from the lake as DO dropped. Mudminnows survive winter hypoxia by breathing oxygen-containing bubbles trapped beneath the ice. These relatively simple behavioral adaptations allow fishes to survive or avoid hypoxic conditions lethal to other species and may help explain the consistency in fish communities of winterkill lakes.     

Phenology of recruitment and infection patterns of Ascocotyle pachycystis, a digenean parasite in the sheepshead minnow, Cyprinodon variegatus

In northwest Florida, Cyprinodon variegatus are parasitized by Ascocotyle pachycystis, a digenean parasite that forms metacercarial cysts in the lumen of the bulbus arteriosus. Field experiments revealed that fish accumulated parasites at an uneven rate within the highly seasonal trematode recruitment period. Older (= larger) fish had higher rates of parasite recruitment and higher parasite prevalences and densities (numbers of metacercariae per individual fish) than did younger (= smaller) fish. Nearly all adults were parasitized (prevalence range 70-100%), and parasite densities ranged from zero to 6800 per fish. Parasite distributions were clumped (= aggregated) in fish of all age classes but were less heterogeneous in early juveniles and adults than they were in late juveniles. Parasites affected the population dynamics of sheepshead minnows by causing reduced winter survivorship, as evidenced by an increase in the average size of fish and a decrease in the average infection intensity over the winter. This revised version was published online in July 2006 with corrections to the Cover Date.     

The life cycle of myxosporidians in fish of the Kola Peninsula

Monthly studies on the seasonal parasite fauna dynamics of pike, minnow and perch were carried out from May, 1974 to September, 1977. It was found out that the myxosporidian Henneguya creplini has a one-year life cycle. The infection of perch with this parasite takes place at a water temperature not exceeding 13 degrees. In other species (Chloromyxum esocinum, Ch. mitenevi, Myxobolus cybinae) the life cycle is shorter, in consequence of which fishes can be infected with these parasites several times a year and in different seasons. The nature of seasonal changes in abundance of myxosporidians depends to a great extent on weather conditions of each concrete year.     

Regulation and stability in fish assemblages of neotropical floodplain lakes

Neotropical floodplain lakes provide an excellent opportunity to examine the regulation and stability of fish assemblages. At low water, when lakes are separated, fish are concentrated in the lakes and are presumably subject to strong interspecific interactions that can shape assemblage structure. At high water, when the lakes and river channels become broadly interconnected, ample potential exists for alteration of assemblage structure because eggs, larvae, and older fish may undergo spatial reshuffling among waterbodies. Twenty lakes of the Orinoco River floodplain in Venezuela were surveyed in the early and late dry seasons of two consecutive years. Marked differences in assemblage structure among lakes were established in the early dry season. Changes in assemblage structure during the dry season were large and detectable even at the ordinal level; they reflected a strong reduction of visually oriented fish relative to fish with adaptations to low light. Changes were similar in the two years and were apparently due to species-specific differences in mortality, which was high over the dry season. The annual flood is a strong natural fluctuation that greatly modifies assemblage properties during the wet season. Nevertheless, the potential for alteration of assemblage structure by reshuffling was not realized: assemblage properties early and late in the dry season were similar in the two years, indicating a regular and predictable annual cycle of change in assemblage structure. This regularity can be explained by mechanisms of regulation which appear to be linked to piscivory and the optical environment. In contrast with the prevailing views on neotropical fish assemblages, there seems to be a strong deterministic component to assemblage structure and dynamics in Orinoco floodplain lakes.     

Investigations into the soil mycoecology of Chambal ravines of India

Summary The present investigation deals with the seasonal succession of fungal communities in wheat fields. The studies were performed during the year 1975–76 and involved four study seasons,viz. winter (1975), summer (1975), rainy (1975) and winter (1976). The observations during two winter seasons were done to find out whether or not fungal communities exhibit definite cyclic pattern. Of the 69 isolated species, 25 showed wider ecological amplitude whereas some species were specific of a particular season. Each season was marked by different fungal mycoflora. The population ofAspergillus terreus, a dominant member of the community, along with many other autochthonous species, exhibited a definite periodicity during various seasons. Seasonal variations in the fungal communities, variations in the extent of similarity among the fungal communities during various seasons and the impact of climatic conditions on the components of fungal communities are discussed.     

Rainbow trout (Salmo gairdneri) stocking and Contracaecum spp

A stocking program with rainbow trout (Salmo gairdneri) at High Rock Lake, Manitoba failed due to infections with large numbers of Contracaecum spp. larvae. Nematode larvae in the intestinal tract, body cavity and musculature made the fish unmarketable. A combination of experimental infections of rainbow trout and pelicans (Pelecanus erythrorhynchos), observations on the behavior of fish-eating birds, and numbers of larval Contracaecum spp. in minnow species led to the following conclusions. The introduction of rainbow trout attracted large numbers of fish-eating birds, particularly pelicans. Concurrent predation by rainbow trout on fathead minnows (Pimephales promelas), five-spined sticklebacks (Culaea inconstans), and nine-spined sticklebacks (Pungitius pungitius), concentrated the parasites. The combined increase in densities of the introduced fish host and fish-eating birds, and the short life cycle of the parasite, increased the numbers of parasites in rainbow trout over a season and in the indigenous minnow species between years. Numbers of larvae in the indigenous minnow species declined when stocking of rainbow trout was stopped and use of the lake by fish-eating birds, particularly pelicans, returned to normal levels.     

小鞘指环虫种群的季节动态

作者在１９９２－１９９４年期间对鲢指环虫病原小鞘指环虫种群的季节动态进行了研究。小鞘指环虫种群从每年的冬季开始出现,到次年春末（４月）,其染率与丰度均匀发展至高峰。此 度随着水温的上升而迅速下降,到６月上旬,种群消失,在感染强度低于２００虫／鱼的情况下,小鞘指环虫对宿主肥满度没有显著影响。     

Seasonal dynamics of the component community structure of parasites of the minnow Phoxinus phoxinus (L.)

The material represented by 75 specimens of minnow of the age 2-2(+) was collected according to the standard technique in the Chovju River (tributary of the Vychegda River, a region of the settlement Nizniy Chov, district of Syktyvkar town) during the period June-September 2000. The quantitative estimation of the structure of the component parasite communities was performed by calculating errors of the equation of regression for each species group separately, with subsequent summarizing of means of errors by all parasite groups comprising the community (Dorovskikh, 2001 6; 2002 B). In order to have a possibility to get data complementing each other, the calculation of variety indexes and other indexes have been made for metazoan parasites only and for the whole community, including the protozoan parasites. In both cases, i.e. considering only metazoan parasites and the whole composition of parasite species, three states of the component parasite community have been recognised: the formed community (June), the community in destroying (July and August), the community in the process of formation (September). In the course of working on the total species composition we recorded the beginning of community destroying on 30th of June, and the beginning of community formation in August. Considering only the metazoan parasites, the community is defined (after: Pugachev, 1999) as the mature (balanced) one in June, while in July, August and September, it was unmature (off-balance) by its characteristics. Regarding the protozoan parasites, the community in June, August and September was characterized as the mature one based on indices of parasite biomass, and as unripe one based on the number of parasite individuals; however in July, both groups of indices allowed to refer it to the unripe state. However, these unripe states are essentially different. In July, it is the result of dieing out the parasites of the past generation; in August and September, it is the result of the appearances of new generations. Therefore we recognise three states of parasite community named above. Considering the whole species composition of parasites we noted the greater difference of index values based on the parasite specimen numbers and their conventional biomass, that was in the case of the metazoan parasites only. It is particularly noticeable in the middle of the June, in the period of the formed community. This fact, together with high errors of the equations of regression and the presence of the high number of Apiosoma, points to the disturbance in the structure of component parasite community in the minnow from the Chovju River. This is easily explicable, because the Chovju River is the polluted reservoir, and pollution comes from agricultural fields, Verhny-Chov settlement, pigsties and cow-sheds. The pollution is a seasonal factor here. The most powerful pollution was noted at May-June, and then it decreased along the beginning of rains in the end of August; in September it increased again. It is important to point out that the monitoring of the metazoan parasites only allows to reveal the general dynamics of the community during the period of observation, but does not allow to catch the beginning of its destroying and developing and to notice possible disturbances in the community structure caused by pollution of reservoirs, particularly, if this pollution is a seasonal factor as in the Chovju River. Three named states of the component community of the fish parasites take place in other periods of year than this observed in the intestional parasite communities of fish helminths of the temperate climate zone. The developing of parasite communities of the intestinal helminths of the Anguilla anguilla in England (Kennedy, 1997) and of Leuciscus idus from the Rybinsk reservoir (Zhohov, 2003) starts in the beginning of summer. In May, their species diversity is minimal and in August is maximal. In conditions of the middle stream of the Vychegda River, the species diversity of parasite community associated with the minnow is maximal in June and minimal in August, when it only begins developing.