Eggsac development rates and phenology of agrobiont linyphiid spiders in relation to temperature

Spider densities are often low after winter in annual crops, and crop management decimates spider populations several times per year. Population recovery rates and phenology depend on reproductive and development rates, which in turn are driven largely by temperature. We aimed to quantify the relationships between eggsac development rates and temperature in order to understand the relative value of different linyphiid species for the biological control of agricultural pests. Female adults of nine linyphiid species were collected from winter wheat in the UK over 3 years; Bathyphantes gracilis (Blackwall), Erigone atra (Blackwall), Erigone dentipalpis (Wider), Erigone promiscua O.P.‐Cambridge), Tenuiphantes tenuis (Blackwall) [formerly Lepthyphantes tenuis (Blackwall)], Meioneta rurestris (C.L. Koch), Oedothorax apicatus (Blackwall), Oedothorax fuscus (Blackwall), and Oedothorax retusus (Westring). These are agrobiont species that are dominant in agroecosystems. We tested how well development in the field can be predicted on the basis of laboratory experiments. We also built a simple phenology simulation model to test whether spider phenology in the field can be predicted by a general knowledge of the relationship between temperature and development rate. The relationships between temperature and development rates of eggsacs were not linear as described by a day‐degree model, but exponential as described by a biophysical model. Duration of the eggsac development period in the field was predicted accurately from laboratory experiments. We only found minor differences between development thresholds of eggsacs at constant temperatures compared with fluctuating temperatures. The phenology model predicted the phenology of L. tenuis and E. atra well, but the number of generations predicted for O. fuscus was not realised in the field. This suggests that development of this species may be affected by factors other than temperature. The methods used here could also be applied to other natural enemies, to determine whether their thermal biology is conducive to a role as biocontrol agents in disturbed agricultural systems.     

Microcosm studies on intraguild predation between female erigonid spiders and lacewing larvae and influence of single versus multiple predators on cereal aphids

Abstract: Intraguild predation between female erigonid spiders [Erigone atra (Blackwall) and Oedothorax apicatus (Blackwall), Araneae, Erigonidae] and lacewing larvae (second instar larvae of Chrysoperla carnea (Stephens), Neuropt., Chrysopidae) and interaction effects of predator combinations on cereal aphids were investigated in a microcosm system under laboratory conditions. The microcosm experiments were run for 7 days and consisted of 15wheat seedlings, 15 Sitobion avenae (F) (Hom., Aphididae) as start population, plus a female spider or a lacewing larva or a combination of a spider plus a lacewing larva. The mortality rate of lacewing larvae was significantly increased by 44 and 31% due to intraguild predation by female spiders of E. atra and O. apicatus in comparison with lacewing larvae that were kept alone. The final aphid numbers in the microcosms were significantly reduced by all single predator treatments (spiders, lacewing larvae) and the predator combinations in comparison with controls without predators. The predation effect on aphid populations due to both spider species was similar and not statistically different. An additive effect of the predator combinations ‘spider plus surviving lacewing larva’ was found for both spider species resulting in reduced aphid numbers compared with the single predator treatments. When the lacewing larva was killed by an E. atra female the effects on aphids were non‐additive, but aphid numbers were not statistically increased compared with the lacewing larva treatment. When the lacewing larva was killed by an O. apicatus female, the effects of spider and C. carnea larva were additive on aphid numbers. In the presence of additional prey (fruit flies and Collembola) intraguild predation was not found and E. atra females had no significant effect on the survival of lacewing larvae. In addition, E. atra females had no significant effect on aphid numbers in the presence of fruit flies and Collembola, but in combination with a lacewing larva that survived, a significantly greater reduction of the aphid population was observed compared with the lacewing larva treatment. The body mass of lacewing larvae at the end of the experiment was not statistically influenced by the presence or absence of an E. atra female.     

PCR gut analysis reveals that Tenuiphantes tenuis (Araneae: Linyphiidae) is a potentially significant predator of Argentine stem weevil,Listronotus bonariensis (Coleoptera: Curculionidae), in New Zealand pastures

Polymerase chain reaction (PCR) gut analysis was conducted on specimens of the introduced spider Tenuiphantes tenuis collected from dairy pasture in Canterbury, New Zealand. PCR primers were specifically designed to amplify a fragment of the mitochondrial gene cytochrome c oxidase subunit 1 (COI) from Listronotus bonariensis and revealed that this major pasture pest species is consumed in the field by T. tenuis. The field predation rate of L. bonariensis by T. tenuis was estimated from our PCR results together with published data on the degradation of DNA and the density of T. tenuis in Canterbury pastures. We found that T. tenuis is a potentially significant predator of L. bonariensis in New Zealand pastures.     

Microbial modification of host long-distance dispersal capacity

Background  

Dispersal plays a key role in shaping biological and ecological processes such as the distribution of spatially-structured populations or the pace and scale of invasion. Here we have studied the relationship between long-distance dispersal behaviour of a pest-controlling money spider,Erigone atra, and the distribution of maternally acquired endosymbionts within the wider meta-population. This spider persists in heterogeneous environments because of its ability to recolonise areas through active long-distance airborne dispersal using silk as a sail, in a process termed 'ballooning'.     

Side-effects of insecticides on two erigonid spider species

The current rearing technique forErigone atra (Blackwall) andOedothorax apicatus (Blackwall) (Araneae, Erigonidae) was improved. To reduce time spent rearing on live fruit flies the spiders were kept on a culture of the Collembola speciesLepidocyrtus lanuginosus (Gmelin) (Entomobryidae). Side-effects on spiders of two pyrethroid insecticides (fenvalerate and lambda-cyhalothrin) and one carbamate insecticide (pirimicarb) were tested. Sensitivity of adults of both sexes and juveniles to insecticides and their influence on the rate of emergence of spiderlings from cocoons were investigated using topical application, spraying or residual contact. LD₅₀ values for adults ranged from 0.49 to 2.52 ng a.i./spider for lambda-cyhalothrin and from 5.75 to 98.20 ng a.i./spider for fenvalerate. Topical application also resulted in up to a week's delay of web-building. A moving laboratory spraying equipment was used to spray spiders with different insecticide dosages and water volumes. Pyrethroids sprayed onto adults in webs had stronger effects than pyrethroids sprayed onto sitting or walking spiders on the soil surface. Residual contamination caused higher mortality of spiders after contact with lambda-cyhalothrin than fenvalerate. In all tests, males were more susceptible to pyrethroids than females; this difference was related to body weight. Mortality rate was higher forE. atra than forO. apicatus. Both pyrethroids were also toxic to spiderlings. Lambda-cyhalothrin inhibited emergence ofE. atra spiderlings from cocoons. Pirimicarb was harmless to both spider species.     

Evaluation of lycosid,micryphantid and linyphiid spiders as predators ofRhopalosiphum padi (Hom.: Aphididae) and their functional response to prey density-laboratory experiments

Laboratory experiments were performed to determine the potential of dominant spider species in winter wheat in Germany,Erigone atra (Blackwall),Lepthyphantes tenuis (Blackwall) andPardosa agrestis (Westring) adults and youngs, in suppressing the population ofRhopalosiphum padi (L.) on wheat plants and their functional response to different aphid densities. The presence of spiders significantly caused between 34 and 58% reduction in aphid population development on wheat plants compared to the aphid population in the absence of spiders. The functional response curves for these spiders as predators ofR. padi seem to descrive a typical type II functional response with the prey consumed increasing to a plateau as aphid densities increased. Prey killed without eating was linear on prey density.     

Host location by Gelis festinans, an eggsac parasitoid of the linyphiid spider Erigone atra

Interactions were investigated between the spider Erigone atra (Blackwall) (Araneae, Linyphiidae) and the insect parasitoid Gelis festinans (Fabricius) (Hymenoptera, Ichneumonidae), both found in an agricultural wheat field surrounded by grass edges. The searching behaviour of G. festinans was studied in laboratory experiments. Odours from detached wheat and grass leaves were preferred by male and female parasitoids when tested against clean air in Y-tube olfactometer experiments. Female parasitoids, collected from the field, and their lab-reared female offspring, preferred wheat odours when given the choice between wheat vs. grass odours. Male parasitoids showed no significant preference for wheat or grass leaves. Both field-captured and lab-reared female parasitoids responded to silk from the spider E. atra with an increased searching behaviour, while showing no response to silk from the spiders Lepthyphantes tenuis (Blackwall), Bathyphantes gracilis (Blackwall), Oedothorax retusus (Westring) and O. fuscus (Blackwall) (Araneae, Linyphiidae). Female parasitoids spent significantly less time on surfaces that were pre-searched by themselves or conspecific females than on unsearched areas, even in the presence of E. atra webbing. Parasitized eggsacs were distinguished from unparasitized ones; superparasitization was not observed.     

Diel,semi‐lunar and seasonal patterns in the fish community of an intertidal zone of the Yangtze estuary

The structure and temporal variations of the fish community in the intertidal estuarine zone of shallow mud areas have been poorly studied in China. This paper analyses the diel, semi‐lunar and seasonal patterns of fish assemblages in the Yangtze estuary in 2006. Fish were collected by consecutive day and night samplings using tide‐stow‐nets deployed parallel to each other in three stations. A total of 56 fish species belonging to 21 families was caught during the study period. The family Cyprinidae dominated with 25 species. Freshwater fish species were the important dominant commercial fishery species and well represented with five species (sharpbelly Hemiculter bleekeri, goldfish Carassius auratus, bream Parabramis pekinensis, likely‐bream Pseudobrama simony, and glossy yellow catfish Pelteobagrus nitidus) in the three stations. Juvenile fishes dominated the fish community, comprising 93.9% in station 1 and 96.6% in station 2 of the total abundance. The number of fish species in day tides was slightly lower than those in night tides in spring and summer, but the opposite in other seasons. In neap tides, the numbers and abundance of fish species were both lower than those in the spring tides. Fish abundance was lowest in winter, increasing during spring and summer (March–September) in both stations 1 and 2, with obviously large fluctuations in each season. The pattern of habitat selection of fishes could effectively decrease the food competition of intraspecies or interspecies and favour the growth and nursing of young fishes. These findings indicate that the intertidal zones in the estuary may serve as important nursery areas for fish communities.     

Temporal activity of spiders and earwigs during winter in apple trees under a Mediterranean climate

It is well known that spiders are present in high numbers in orchards and may contribute to biocontrol. Some recent studies in central Europe further showed that some spiders are active year-round and consume pests even in winter. Using cardboard traps laid every two weeks, we carried out a survey to determine which spider and earwig species are active from September to May in an experimental, pesticide-free, apple orchard under a Mediterranean climate. We observed that spider activity was never completely absent. The structure of the spider communities showed a marked seasonality in three periods (so-called ‘autumn’, ‘winter’ and ‘spring’). Only two spider genera, Philodromus and Trachelas, were highly active in winter (percentage of catches during this season above 40%) and six others (Lathys, Clubiona, Gnaphosa, Theridion, Phrurolithus) had moderate activity (between 20 and 40%). The two earwig species had different patterns of winter activity with Forficula auricularia almost absent whereas F. pubescens was moderately active on trees. Spider community abundance, diversity and evenness significantly decreased between autumn and winter and remained low in the following spring probably because the attractiveness of the traps is much lower at this time of year due to mild temperatures and the presence of leaves on the trees. Winter-active spiders could contribute to pest biocontrol during the cold season and we advocate that the use of broad-spectrum pesticides at the end of winter, as classically applied in orchards, may be counter-productive for pest control.     

Contrasting host: parasitoid synchrony drives differing levels of biocontrol by two introduced Microctonus spp. in northern New Zealand pastures

Two species from the genus Microctonus Wesmael (Hymenoptera: Braconidae) have been introduced into New Zealand as biocontrol agents of pest weevils in pasture. Both parasitoids have similar life cycles and co-exist in pasture along with their respective weevil hosts. However, winter parasitism rates by M. hyperodae Loan are low in comparison to the Irish biotype of M. aethiopoides’ Loan. Population studies at two Waikato sites over three consecutive seasons of parasitoid activity showed that M. aethiopoides recovered from near extinction each spring and built up to effective levels by winter because hosts were available continuously throughout summer and autumn. In contrast, M. hyperodae began each season at higher larval populations and parasitism levels than M. aethiopoides, but populations and parasitism levels declined during late summer and early autumn due to low host availability. The contrast between species is consistent with the high levels of endophyte-conferred pest-resistant grass in the pastures, which impacts strongly on M. hyperodae’s host weevil abundance during summer but has no effect on M. aethiopoides’ host weevils which feed only on clovers. It was accentuated by a warming climate with the now regular occurrence of a third host generation after most M. hyperodae adult activity had ceased.

     

Effects of grazing by fish and waterfowl on the biomass and species composition of submerged macrophytes

Biomanipulation improved water transparency of Lake Zwemlust (The Netherlands) drastically. Before biomanipulation no submerged vegetation was present in the lake, but in summer 1987, directly after the measure, submerged macrophyte stands developed following a clear-water phase caused by high zooplankton grazing in spring. During the summers of 1988 and 1989 Elodea nuttallii was the most dominant species and reached a high biomass, but in the summers of 1990 and 1991 Ceratophyllum demersum became dominant. The total macrophyte biomass decreased in 1990 and 1991. In 1992 and 1993 C. demersum and E. nuttallii were nearly absent and Potamogeton berchtholdii became the dominant species, declining to very low abundance during late summer. Successively algal blooms appeared in autumn of those years reaching chlorophyll-a concentrations between 60–130 µg l^–1. However, in experimental cages placed on the lake bottom, serving as exclosures for larger fish and birds, E. nuttallii still reached a high abundance during 1992 and 1993. Herbivory by coots (Fulica atra) in autumn/winter, and by rudd (Scardinius erythrophthalmus) in summer, most probably caused the decrease in total abundance of macrophytes and the shift in species composition.     

Influence of habitat modification on the intestinal helminth community ecology of cottontail rabbit populations

The influence of five brush management treatments using the herbicides tebuthiuron and triclopyr, with or without prescribed burning, on the intestinal helminth community of cottontail rabbits (Sylvilagus floridanus) was studied in 1987 on the Cross Timbers Experimental Range in Payne County, Oklahoma (USA). Six helminth species were found (Dermatoxys veligera, Trichostrongylus calcaratus, Passalurus nonanulatus, Wellcomia longejector, Taenia pisiformis cystercercus, and Mosgovoyia pectinata americana) in 102 rabbits (88 adult and 14 juveniles) collected over two seasons (winter and summer). Prevalence of M. pectinata americana in cottontail rabbits was significantly greater in untreated control pastures than herbicide treated pastures in winter, while prevalence of T. pisiformis was significantly greater in burned than unburned pastures. Abundances of helminth species in the intestinal tract of cottontail rabbits were unaffected by brush treatments. Mosgovoyia pectinata americana abundance demonstrated a highly significant increase from winter to summer; conversely, abundance of all oxyurid pinworms combined (D. veligera, P. nonanulatus, W. longejector) was significantly higher in winter than summer. Helminth community dynamics were significantly influenced by season, but were unaffected by brush treatments. Habitat modification could have influenced cestode transmission by altering the ecology of invertebrate and vertebrate hosts.     

Effects of grazing on the abundance and diversity of annual plants in Chihuahuan desert scrub habitat

We assess the impact of release from cattle grazing on the abundance and diversity of both winter and summer annual plant communities at an upper Chihuahuan Desert scrub site in south-eastern Arizona. In contrast to previous studies, we found that removal of herbivores (cattle) had little impact on ephemeral plant assemblages at our site. The total number of summer annual individuals per quadrat did not differ significantly, but there were significantly more winter annual plants on ungrazed quadrats. The number of species per quadrat, however, did not differ significantly between sites exposed to, or protected from, grazing in either season. Of 79 annual species recorded (34 in winter, 45 in summer), only 2 species, 1 in each season, responded significantly to the removal of cattle: Stephanomeria exigua and Polygala tweedyi were more abundant on ungrazed plots. Three additional species, Eriastrum diffusum and Cryptantha micrantha in winter, and Mollugo cerviana (summer), approached statistical significance. Differences in the effect of cattle grazing on annual plants between our results and those at other sites in the arid southwest most likely reflect differences in the speed of response by annuals in different areas. Comparisons of this with other studies underscores recent calls for studies at broader spatial and greater temporal scales.     

Detecting seasonal variation in composition of adult Coccinellidae communities

1. Following its recent global spread, Harmonia axyridis (Coleoptera: Coccinellidae), an aphid predator of East Palearctic origin, dominated local coccinellid communities. While long‐term trends in this change are well documented, little is known about seasonal changes. 2. Seasonal changes in communities of adult Coccinellidae were investigated in central Europe (Prague), from April to November in 2010–2014. The beetles were collected at 2‐weekly intervals by net‐sweeping particular stands of trees (Tilia spp., Acer spp. and Betula sp.). The numbers of beetles caught were recalculated per unit effort, and average species abundance and community richness, dominance and diversity were calculated for each 2‐week period. 3. Frequencies of 21 established species varied throughout the vegetative season and general trends in this variation were similar in each of the 5 years of this study. In May–June the abundant species was Adalia decempunctata, which was later replaced by Harmonia axyridis as the most abundant species in late summer. Coccinella septempunctata colonized trees occasionally in summer. This sequence of species substitution occurred every year and was reset during winter. Species richness decreased and dominance increased during the course of each vegetative season. The diversity of the coccinellid community (Shannon index H′) significantly decreased during the course of the vegetative season. This was caused by a gradual increase in the dominance of H. axyridis, the abundance of which explained 88% of variation in H′. 4. The existence of a significant seasonal difference indicates that it is important to take this into consideration when comparing coccinellid communities.     

Fluctuations in fruit fly abundance and infestation in sweet gourd fields in relation to varied meteorological factors

The study was conducted with the aim of furthering our understanding of seasonality in the population dynamics and infestation rates of the fruit fly Bactrocera spp. in sweet gourd (Cucurbita moschata) during winter and summer in 2017. We also investigated the effects of using methyl eugenol traps on fly abundance and infestation. Two fruit fly species, namely, B. cucurbitae and B. dorsalis, were present in the sweet gourd field, and we observed fluctuations in their abundance. Compared to B. dorsalis, B. cucurbitae was significantly more abundant in both winter and summer. Infestation level was found to be the highest in fields lacking methyl eugenol traps in both seasons. Fruit fly larval population per infested fruit was higher in summer than in winter. Fly abundance was significantly and positively correlated with mean temperature and rainfall but significantly and negatively correlated with light intensity. Relative humidity was insignificantly but positively correlated with fly abundance. The temperature, light intensity, relative humidity, and rainfall individually explained 48.9, 24.1, 0.8, and 1.6% of variation in fruit fly abundance, respectively. The combined effect of the weather parameters on fruit fly abundance was 75.4% and was significant predictor of fruit fly abundance.     

Diel and Seasonal Changes in the Structure of a Decapod (Crustacea: Decapoda) Community of Cymodocea nodosa from Southeastern Spain (West Mediterranean Sea)

The study of a decapod community in a Cymodocea nodosa meadow from Southeastern Spain (Western Mediterranean Sea) showed a stable structure, in which the families Hippolytidae, Processidae, Majidae and Portunidae were the most abundant and the species Hippolyte niezabitowskii dominated. The animal community was more numerous and diverse during the night, showing the existence of nychthemeral movements, which are essentially related to the trophic behaviour and shelter. In this way, many species increased their abundance as a result of an increasing activity and, also, of an influx of other species and specimens from adjacent sandy bottoms, such as Processa spp. (mainly P. modica) Sicyonia carinata, Liocarcinus spp. (mainly juveniles) and several species of hermit crabs, which were rare or absent during the day. All these changes produced modifications in the dominance curves and in the values of all ecological indices (richness, diversity and evenness). Monthly samples were grouped and ordered (MDS) by the factor “day–night”, which showed slight qualitative and quantitative differences (SIMPER, dissimilarity average of the factor day–night = 61.67). On the other hand, no global seasonal differences have been found (one way ANOSIM), but there was a significant level of similarity between winter and spring, while the summer samples were the most different. The differentiation of the summer 1999 can be attributed to a decrease in species abundance and richness, probably due to the dynamics of the decapod populations and the balance with predators (fishes), while that of the summer 2000, to an anomalous event: the massive proliferation of filamentous algae, mainly Ectocarpus s.l., which modified the environmental conditions.     

Snow cover and extreme winter warming events control flower abundance of some,but not all species in high arctic Svalbard

The High Arctic winter is expected to be altered through ongoing and future climate change. Winter precipitation and snow depth are projected to increase and melt out dates change accordingly. Also, snow cover and depth will play an important role in protecting plant canopy from increasingly more frequent extreme winter warming events. Flower production of many Arctic plants is dependent on melt out timing, since season length determines resource availability for flower preformation. We erected snow fences to increase snow depth and shorten growing season, and counted flowers of six species over 5 years, during which we experienced two extreme winter warming events. Most species were resistant to snow cover increase, but two species reduced flower abundance due to shortened growing seasons. Cassiope tetragona responded strongly with fewer flowers in deep snow regimes during years without extreme events, while Stellaria crassipes responded partly. Snow pack thickness determined whether winter warming events had an effect on flower abundance of some species. Warming events clearly reduced flower abundance in shallow but not in deep snow regimes of Cassiope tetragona, but only marginally for Dryas octopetala. However, the affected species were resilient and individuals did not experience any long term effects. In the case of short or cold summers, a subset of species suffered reduced reproductive success, which may affect future plant composition through possible cascading competition effects. Extreme winter warming events were shown to expose the canopy to cold winter air. The following summer most of the overwintering flower buds could not produce flowers. Thus reproductive success is reduced if this occurs in subsequent years. We conclude that snow depth influences flower abundance by altering season length and by protecting or exposing flower buds to cold winter air, but most species studied are resistant to changes.     

Effects of fire on central Australian rangelands. II Changes in tree and shrub populations

The effect of a summer fire or a winter fire on tree and shrub populations in two rangeland vegetation types in central Australia was investigated. Changes in population structure of two major species occurred regardless of the season of burn. The decrease in overall density of Acacia aneura, Eremophila gilesii and Cassia spp. was similar for summer and winter fire treatments: greater reduction of E. latrobei followed the summer burn. Higher fire line intensity caused greater mortality in all species but enhanced germination in A. aneura, a potentially ‘weedy’ species. Winter fires appear to be preferable for management of woody plants in the particular vegetation types studied.     

The life history and abundance of the introduced earthworms Aporrectodea trapezoides and A. caliginosa (Annelida: Lumbricidae) in pasture soils in the Mount Lofty Ranges,South Australia

Abstract The introduced earthworms, Aporrectodea trapezoides and Aporrectodea caliginosa, are common in agricultural soils in southern Australia. The seasonal abundances of these species were monitored in five pastures in the Mount Lofty Ranges, South Australia. Highest numbers occurred from winter to early spring: densities of up to 407 and 446 m^?2 were recorded for A. trapezoides and A. caliginosa, respectively. Adults were restricted to winter and spring, when breeding occurred. Subadults and juveniles survived summer. Aporrectodea trapezoides and A. caliginosa were abundant in the surface layers of the soil (0–10 cm depth) for between 3 and 7 months (autumn to spring), when soils were moistest. During summer, most individuals were found > 20 cm below the surface where they were inactive, coiled tightly within sperical chambers. The abundance of A. trapezoides and A. caliginosa varied within and between pastures. Numbers were lowest where the clay content of the soil was low. A dipteran parasite attacked a small proportion of A. trapezoides at one site in spring.     

Winter ecology of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a subarctic lake,Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature <1°C) and poor light conditions, both fish species fed continuously during the ice-covered period, although at a much lower rate than during the summer season. No somatic growth could be detected during the ice-covered winter period and the condition factor of both species significantly declined, suggesting that the winter feeding rates were similar to or below the maintenance requirements. Also, the species richness and diversity of ingested prey largely decreased from summer to winter for both fish species. The winter diet of Arctic charr <20 cm was dominated by benthic insect larvae, chironomids in particular, and Gammarus lacustris, but zooplankton was also important in December. G. lacustris was the dominant prey of charr >20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.