Interhabitat migration of juvenile Atlantic salmon in a Newfoundland river system, Canada

Migrations of juvenile Atlantic salmon between habitats (both fluvial and lacustrine) were studied in a Newfoundland river system from May through September. Salmon parr showed poor site fidelity in fluvial habitats, and high rates of migration, especially in spring-early summer. Most habitat shifts were upstream from fluvial habitats to a lake at the head of a small tributary. In September, 10% of the salmon parr caught in the lake ( n =275) had been marked in the main stem of the river or in the tributary stream (total n =641). The abundance of parr in the lake increased in May-June, then declined gradually to September. Most of the lacustrine immigrants were 1+ parr. Mature males were found amongst the autumnal emigrants. Salmon parr in the lake grew larger than those in the river, and lacustrine smolts captured in the lake were larger than those caught at the mouth of the river.     

Juvenile Anadromous Atlantic Salmon of Three Lakes of Newfoundland

Anadromous Atlantic salmon parr (Salmo salar) were captured from three Newfoundland lakes. Yearling and 2 year parr were captured most often in shallow water <2 m deep around lake perimeters and were not abundant at greater depths. The 3+ and 4+ parr age groups were rarely found inshore (<2 m) but were captured in deeper offshore areas. Though the parr population in one deep lake was too small to be estimated by mark-recapture, estimates of the yearlings plus 2+ age groups in two shallower lakes were 55 and 63 parr ha⁻¹. respectively. No under yearlings were found in lake habitats. Parr density in the three study lakes varied inversely with mean lake depth. Lacustrine parr growth rate was greater than or equal to that of stream dwelling parr. It is concluded that some shallow lakes of Newfoundland provide major rearing areas for juvenile anadromous Atlantic salmon.     

The relationships between anadromy, sex ratio and parr growth of Arctic charr in a lake in North Norway

Relationships between the degree of anadromy, sex ratio and parr growth of Arctic charr Salvelinus alpinus were investigated in Lake Storvatn in Hammerfest town, by estimating the densities of resident and anadromous charr in the lake, and by comparing the smolt characteristics of fish captured in the outlet river with those of fish of the same age in the lake. About two-thirds of the charr older than 5 years were anadromous. The sex ratio of smolts was approximately 1 : 1 and the frequency of mature male parr in the lake was low. As females tended to dominate the mature portion of the anadromous population, there was probably a relatively higher mortality among sea-run male smolts. The youngest smolts (3+ and 4+) were probably recruited from fast-growing parr in the littoral zone, while older smolts (>4+) may have been recruited from all habitats in the lake.     

Relative production of Atlantic salmon from fluvial and lacustrine habitats estimated from analyses of scale characteristics

Empirical and back-calculated growth of Atlantic salmon parr were compared between fish reared in fluvial and lacustrine habitats of Conne River, Newfoundland. Length at age was significantly higher for lacustrine parr. Various classification and maximum likelihood estimators indicated that 75% or more of the fish used lakes for rearing. Lacustrine use is another aspect of the inherent variability and plasticity of Atlantic salmon life-history traits. As most Newfoundland river systems include lakes, estimates of regional spawning targets and potential smolt production will need to take lake habitat into account.     

Growth of Atlantic Salmon parr in fluvial and lacustrine habits

The growth of naturally recruited Atlantic salmon Salmo salar parr in fluvial and lacustrine habitats was investigated in three northern Norwegian watercourses. Salmon parr were sampled in autumn with small mesh gillnets in lakes, and by electrofishing in streams. In three northern Norwegian watercourses, lacustrine Atlantic salmon Salmo salar parr were larger in length and weight at age than fluvial parr. Back-calculated lengths indicated that lacustrine salmon parr were recruited from the fastest growing part of the populations and they had larger growth (length) increments between age intervals than fluvial parr. There was no consistent pattern in growth comparisons between parr from the inlet and outlet streams. Temperature regime alone could not explain the growth difference observed between parr from the different habitats.     

Interhabitat and instream movements of large Atlantic salmon parr in a Newfoundland watershed in winter

Radio‐telemetry was used to investigate movement of large, mainly mature male (80%) Atlantic salmon Salmo salar parr in Stoney River, Newfoundland during early winter (November; water temperature 6·0 ± 0·1° C) and mid‐winter (January to February; 0·8 ± 0·0° C). Site fidelity of parr in early winter was low. Parr moved between fluvial and lacustrine habitats and were active throughout the diel cycle. Parr caught in fluvial habitats in mid‐winter were smaller and younger than parr caught in early winter. Site fidelity of parr in mid‐winter was greater than in early winter. Parr in mid‐winter moved between fluvial and adjacent small lacustrine habitats, but avoided a larger pond inhabited by large piscivorous fishes. Instream movement rates in mid‐winter were lower than in early winter and occurred primarily during hours of darkness (dawn, dusk and night). Fluvial habitats were relatively stable and ice‐free throughout the study periods. These results suggested that large Atlantic salmon parr utilize a variety of habitats and remain active throughout the winter, even under stable environmental conditions.     

Use of space and food by resident and migrant brown trout,Salmo trutta

Synopsis Parr and resident forms of brown trout,Salmo trutta, from Vangsvatnet Lake, Norway live in freshwater, while migrant forms live in coastal waters during summer and in freshwater during winter. About 80% of parr and residents live at depths <5 m, smolts and migrants are more confined to near-surface water. Brown trout partly segregate by size, age and sex from spring through autumn. More than 90% of parr and residents in the tributaries are 0–2 years old, 2–14 cm in length, in the littoral zone 0–3 years old, 7–24 cm in length, and in the pelagic zone 2–6 years old, 18–32 cm in length. The mean body length of equal-aged fish increases from tributaries through littoral to pelagic zones in the lake. Smolts (2–7 years, 14–29 cm) leave the lake from April through August and return during September–October. Migrants (2–11 years, 23–67 cm) leave the take in April–May and return during August–September; sexually mature fish return earlier than immatures. Female brown trout are less stream-dwelling, but more migrant and pelagic than males. Most individuals in the lake population spend the winter in the littoral zone. In the tributaries, diet differs significantly between age-groups of parr; young fish feed on smaller food items than do older fish. In the lake, parr and residents living in the same habitats feed on the same food items. Littoral brown trout feed mainly on insect larvae and chironomid pupae, pelagic brown trout feed on zooplankton and surface insects. Migrants feed little while staying in freshwater, except for matures which feed on young salmonids and surface arthropods during the 2 first months after they had returned from coastal waters. The results are discussed in relation to growth possibilities and mortality risks of the different habitats.Reprint request to B. Jonsson.     

Species-specific effects of gastropods on leaf litter processing in pond mesocosms

A study of the diet of native brown trout (Salmo trutta) parr and introduced European minnow (Phoxinus phoxinus) in the subalpine lake, Øvre Heimdalsvatn, showed that the two species had considerable dietary overlap, both in the littoral zone and in the outlet of the lake. Chironomidae constituted a substantial proportion of the diet of the two species in both habitats. The results indicated that both zooplankton (Cladocera) and large macroinvertebrates (EPT-species) made up a higher proportion of the minnow diet in the early phase of the minnow establishment (1975–1977) than later, and that the significance of small macroinvertebrates (Chironomidae) as prey has increased during the same period. Dietary analysis of the sympatric brown trout and minnow population in Øvre Heimdalsvatn does not provide a definitive conclusion about the degree of competition between the two species. However, together with the findings in other studies in Øvre Heimdalsvatn that have documented reduced recruitment and individual growth of brown trout, decreased individual growth of minnows and a marked decline in the density of large crustaceans (Lepidurus arcticus and Gammarus lacustris) in the shallow littoral of the lake during the last decades indicates that competitive interactions between the two species are likely. This is probably an example of the competition between two fish species with a high degree of dietary overlap when living in sympatry, most likely caused by absence of alternative prey and alternative habitats due to the high predation risk for both brown trout parr and minnows in deeper parts and in the open waters of the lake.     

Comparisons of habitat availability and habitat use by an allopatric cohort of juvenile Atlantic salmon Salmo salar under conditions of low competition in a Norwegian stream

Allopatric Atlantic salmon parr occupied a wider and lower range of mean water velocities than have been previously reported for parr in sympatry. With regard to depth, substrate and cover no substantial differences were found. Different fish sizes did not affect habitat use by parr significantly, neither did habitat use change during the season. Habitat availability strongly influenced observed habitat use thus, invalidating use of habitat suitability curves based on observations of habitat occupancy by fish only. The allopatric parr consistently selected habitats different from the available habitat, i.e. had preferences also in the absence of intraspecific interactions between age classes. Habitat conditions not used by the fish were more readily identifiable than habitats used. The parr rarely used shallow (≤ 10 cm) and deep (> 60 cm) stream areas, low mean water velocities (< 10 cm s^-1), fine substrate (sand and finer) and stream areas without cover. However, the calculated habitat preferences were also affected by habitat availability. Multivariate analysis did not identify any of the habitat variables as substantially more important than the others, but suggested that the importance of variables may vary with type of habitat studied.     

Seasonal variation in density of juvenile Atlantic salmon in fluvial shoreline habitats of a large subarctic river

The densities of Atlantic salmon fry (0+ years) and parr (1+ years and older) in shoreline habitats of the large River Teno watercourse and its tributary, the River Utsjoki generally fluctuated considerably, showing an increase from early summer towards late August and a subsequent decline towards autumn. The seasonal pattern of variation in density was more distinct for parr than for fry. In the period between late July and early September, parr density followed a sinusoidal curve, being highest in late August and lowest in early August and in September. Fry density had a weaker seasonal profile than parr, being highest in late August and in early September. Frequency distributions of the parr age groups (1+, 2+ and 3+ years) were mainly independent of the sampling month.     

The morphological plastic response to water current velocity varies with age and sexual state in juvenile Atlantic salmon, Salmo salar

1. Salmonids, like many other fish species, exhibit morphological plasticity to variations in water current velocity. However, little is known about how this response varies with age and alternative sexual tactics that usually coexist in the same area. We therefore sampled immature 1- and 2-year-old and sexually mature Salmo salar parr to determine how the morphological response to slow and rapid water currents varies across these groups.
2. Both 1- and 2-year-old immature parr in rapid habitats can be distinguished from individuals in slow habitats using a combination of fin measurements. In contrast, body shape measurements were useful only to distinguish 2-year-old individuals in the different habitat types. We also showed that mature parr are notably robust, irrespective of habitat type. For these individuals, only their body length differed between slow and rapid water currents, being bigger in slow water currents.
3. Our results imply that fins are the first structures to respond to water current velocity, followed by changes in body shape as individuals grow bigger. The robust phenotype observed for mature parr is likely to pose extra limitations on movement due to an increase in drag forces, thus contributing to their smaller size in rapid water currents.     

Movements and habitat use by PIT-tagged Atlantic salmon parr in early winter: the influence of anchor ice

1. Movements and habitat use by Atlantic salmon parr in Catamaran Brook, New Brunswick, were studied using Passive Integrated Transponder technology. The fish were tagged in the summer of 1999, and a portable reading system was used to collect data on individual positions within a riffle‐pool sequence in the early winter of 1999. Two major freezing events occurred on November 11–12 (Ice 1) and November 18–19 (Ice 2) that generated significant accumulations of anchor ice in the riffle. 2. Individually tagged parr (fork length 8.4–12.6 cm, n = 15) were tracked from 8 to 24 November 1999. Over this period, emigration (40%) was higher from the pool than from the riffle. Of the nine parr that were consistently located, seven parr moved <5 m up‐ or downstream, and two parr moved more than 10 m (maximum 23 m). Parr moved significantly more by night than by day, and diel habitat shifts were more pronounced in the pool with some of the fish moving closer to the bank at night. 3. During Ice 2, there was relatively little movement by most of the parr in the riffle beneath anchor ice up to 10 cm in thickness. Water temperature was 0.16 °C above the freezing point beneath anchor ice, suggesting the existence of suitable habitats where salmon parr can avoid supercooling conditions and where they can have access to low velocity shelters. To our knowledge, these are the first data on habitat use by Atlantic salmon parr under anchor ice.     

Variations in juvenile growth, energy allocation and life-history strategies of two populations of Arctic charr in North Norway

Within the populations of Arctic charr Salvelinus alpinus in Storvatn and Rungavatn, Norway, fish that attained the largest size in the late parr stage migrated as 4+ smolts, fish that attained a medium size became 5+ smolts, and the smallest parr became lake residents. Within the last 2 years of the parr stage, those that became anadromous had a lower growth rate during the winter and early summer than those that became resident, and vice versa in late summer. Thus, anadromous fish grew faster in the parr stage, but in years prior to migration, their growth pattern seemed to differ from that of parr that became resident. Due to early size differences between parr that became anadromous or resident, it is suggested that some of the basis for the decision to smoltify or not in charr depends on the growth rate through the whole parr stage, but with significant modification by the local environment. Rungavatn parr had a significantly higher population density (5 ×), higher lipid content and mortality, an earlier maturation, a lower rate of growth, and only one-third the degree of anadromy than those from Storvatn. It is suggested that the two charr populations have evolved different life-history adaptations due to unequal growth, energy allocation and competition opportunities in the lakes.     

Malaria vectors in Lake Victoria and adjacent habitats in western Kenya

The prevalence of malaria among the residents of the Lake Victoria basin remains high. The environment associated with the lake may maintain a high number of malaria vectors. Lake habitats including water hyacinths have been suspected to be the source of vectors. This study investigated whether malaria vectors breed in the lake habitats and adjacent backwater pools. Anopheline larvae were collected within the littoral zone of the lake and adjacent pools located along approximately 24.3 km of the lakeshore in western Kenya, and their breeding sites characterized. Three primary vector species, Anopheles arabiensis, Anopheles gambiae s.s. and Anopheles funestus s.s., and three potential vectors, were found in the lake habitats. Unexpectedly, An. arabiensis was the most dominant vector species in the lake sampling sites. Its habitats were uncovered or covered with short grass. A potential secondary malaria vector, Anopheles rivulorum, dominated the water hyacinths in the lake. Most breeding sites in the lake were limited to areas that were surrounded by tall emergent plants, including trees, and those not exposed to waves. Nearly half of adjacent habitats were lagoons that were separated from the lake by sand bars. Lagoons contained a variety of microhabitats. Anopheles arabiensis dominated open habitats, whereas An. funestus s.s. was found mainly in vegetated habitats in lagoons. The current study confirmed that several breeding sites are associated with Lake Victoria. Given that Lake Victoria is the second largest lake in the world, the lake related habitats must be extensive; therefore, making targeted vector control difficult. Further exploration is necessary to estimate the effects of lake associated habitats on malaria transmission so as to inform a rational decision-making process for vector control.     

Comparative proximate body composition of Atlantic salmon with emphasis on parr from fluvial and lacustrine habitats

The nutritional status of Atlantic salmon Salmo salar parr was assessed by examining how proximate body composition and energy content (kJ g⁻¹) were associated with the habitat rearing origin of parr utilizing fluvial or lacustrine areas. Comparisons were made among sampling sites within the two habitats, as well as between the two major habitat types applying both fixed effect and random effect models. Mean protein concentration varied from 13·87 to 15·67% among fluvial sites, and from 14·96 to 16·38% among lacustrine sampling locations. Mean fat concentration ranged from 1·71 to 3·32% and from 2·22 to 4·29% among fluvial and lacustrine locations, respectively, with energy concentration, on average, c . 11% higher in lacustrine parr. Interpretation of results of comparisons between habitats was dependent upon the statistical model used. Benefits associated with lacustrine rearing, such as greater body size and nutritional status, have implications for overwintering survival in fresh water as well as subsequent survival during initial sea entrance as smolts.     

Effect of habitat types on survival, spatial distribution and production of an allopatric cohort of Atlantic salmon, Salmo salar L., under conditions of low competition

An allopatric cohort of Atlantic salmon, Salmo salar L., introduced to a small previously fishless stream was studied from parr to the smolt stages. In May 3900 0+ parr (mean total length 30mm) were planted at three different densities in habitats with slow, intermediate and fast water velocities. During the first year, high mortality occurred during the first 7 weeks after planting in May, and in September–October. Survival from May 1985 to April 1986, before the smolt emigration, was24.8%. The smolt yield 1 year after planting was 15.5%. It is suggested that the high survival was caused by low competition. Most of the redistribution of the fish took place during the first months. Type of planting habitat affected the timing of redistribution. The parr left slow-flowing, deep habitats with fine substrate soon after planting, while redistribution was slowest in the fastest flowing habitats with coarse substrate. The observed avoidance of slow, deep habitat types in the absence of interspecific competition, suggests that this may be a fixed behavioural response, and not due to competition. Long movements, up to 800 m, were recorded only within the first 7 weeks after planting. The effect of planting densities on population density was most pronounced immediately after planting in the fast and also intermediate habitats. Planting density effects declined and were not detectable after 1 year. The effect of habitat type on fish numbers and biomass was pronounced irrespective of planting densities. Growth was fastest in the intermediate habitat, and at the lower planting densities. Production was 7.2 g m⁻² the first summer-autumn. Due to smolt emigration, few fish remained in the stream the second summer-autumn, and the production was 1.0 g m⁻².     

Evidence for an autumn downstream migration and the subsequent estuarine residence of 0+ year juvenile Atlantic salmon Salmo salar L., in England

The use of passive integrated transponder technology facilitated the first observations of the autumn migration of 0+ year Atlantic salmon Salmo salar parr into tidal rearing habitats in the U.K. The quality of these habitats in relation to smolt production and the ecological significance of this alternative life-history strategy are presently not understood.     

Individual growth and phase differentiation of lacustrine masu salmon, Oncorhynchus masou, under artificial rearing conditions

 In lacustrine masu salmon, Oncorhynchus masou, originated from anadromous fish and inhabiting an artificial lake (Shumarinai Lake), we examined the relationship between individual growth during the juvenile stage and phase differentiation under artificial rearing conditions. In females, the mean fork length of potential (subsequent) 1+ smolts and potential 1+ parr (1+ parr show fish that neither smolted nor matured until 1+ autumn) always differed after their first summer (0+ summer), the former being larger. In males, the juveniles that grew faster during their first spring became 0+ mature male parr. After their first summer, potential 1+ smolts were always larger in body length than potential 1+ mature parr and potential 1+ parr. These results were similar to previous reports of anadromous masu salmon, suggesting that the lacustrine masu salmon studied seems to have maintained the phase differentiation as in the original fish, likely because only 60 years have passed since the formation of the lacustrine population.     

Spatial and temporal distributions of salmonids in two ponds in Newfoundland, Canada

Spatial and temporal distributions of salmonids were examined in Junction Pond, Northeast River, Placentia and Conne Pond, Conne River, Newfoundland using Lundgren multiple-mesh experimental gillnets. Both ponds contain populations of Atlantic salmon and brook trout with Junction Pond also possessing brown trout and resident Arctic charr. For salmon parr there was a significant month effect in distribution of catch rates in both ponds, and in Junction Pond, there was a significant diel effect. There was also significant variation in catch rates by lentic zone. For brook trout, there was a significant lentic zone × month interaction in Junction Pond; in Conne Pond, the main effects lentic zone and month were significant. Highest benthic catch rates of Atlantic salmon parr occurred in the littoral zone of both ponds. Most captures of brook trout also occurred benthically in both ponds; similar to salmon parr, littoral zone catch rates were higher than those of the deeper benthic area in Conne Pond but the reverse was true for Junction Pond. For both salmon parr and brook trout, the deeper benthic area and the pelagic area were relatively important rearing habitats in each pond. The distribution of catches for brown trout (few in number relative to the other species) in Junction Pond was similar to that of brook trout while Arctic charr were found mainly pelagically. Within the benthic area, most Arctic charr were caught at depths beyond the littoral zone. There was a tendency for Altantic salmon parr and brook trout found in the deeper benthic area and the pelagic area to be significantly larger and older than those in littoral zone in each pond. Sizes of Arctic charr did not differ significantly among lentic zones.     

Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year‐to‐year basis the juvenile Atlantic salmon population was found to exhibit an 'Ideal Free Distribution', which resulted in greatest variation in optimal habitats with year‐to‐year changes in population abundance.