Interactions between riparian shading and food supply: a seasonal comparison of effects on time budgets, space use and growth in Atlantic salmon Salmo salar

This study examines seasonal (winter v. summer) differences in space-time budgets, food intake and growth of Atlantic salmon Salmo salar parr in a controlled, large-scale stream environment, to examine the direction and magnitude of shifts in behaviour patterns as influenced by the availability of overhead cover and food supply. Salmo salar parr tested in the presence of overhead cover were significantly more nocturnal and occupied more peripheral positions than those tested in the absence of overhead cover. This increase in nocturnal activity was driven primarily by increased activity at night, accompanied by a reduction in daytime activity during winter. The presence of overhead cover had no effect on rates of food intake or growth for a given food supply in a given season. Growth rates were significantly higher for fish subjected to a high food supply than those subjected to a low food supply. Food supply did not affect the extent to which S. salar parr were nocturnal. These results were consistent between winter and summer. The use of riparian shading as a management technique to mitigate the effects of warming allows the adoption of more risk-averse foraging behaviour and may be particularly beneficial in circumstances where it serves also to increase the availability of food.     

Intraspecific competition and density dependence of food consumption and growth in Arctic charr

1. Intraspecific competition for restricted food resources is considered to play a fundamental part in density dependence of somatic growth and other population characteristics, but studies simultaneously addressing the interrelationships between population density, food acquisition and somatic growth have been missing. 2. We explored the food consumption and individual growth rates of Arctic charr Salvelinus alpinus in a long-term survey following a large-scale density manipulation experiment in a subarctic lake. 3. Prior to the initiation of the experiment, the population density was high and the somatic growth rates low, revealing a severely overcrowded and stunted fish population. 4. During the 6-year period of stock depletion the population density of Arctic charr was reduced with about 75%, resulting in an almost twofold increase in food consumption rates and enhanced individual growth rates of the fish. 5. Over the decade following the density manipulation experiment, the population density gradually rose to intermediate levels, accompanied by corresponding reductions in food consumption and somatic growth rates. 6. The study revealed negative relationships with population density for both food consumption and individual growth rates, reflecting a strong positive correlation between quantitative food intake and somatic growth rates. 7. Both the growth and consumption rate relationships with population density were well described by negative power curves, suggesting that large density perturbations are necessary to induce improved feeding conditions and growth rates in stunted fish populations. 8. The findings demonstrate that quantitative food consumption represents the connective link between population density and individual growth rates, apparently being highly influenced by intraspecific competition for limited resources.     

Consumption of Atlantic salmon smolts and parr by goosanders: estimates from doubly-labelled water measurements of captive birds released on two Scottish rivers

The field metabolic rates (FMRs) of nine captive goosanders, Mergus merganser , released on two Scottish rivers were estimated using the doubly-labelled water (DLW) technique. Mean (± S.E.) FMR was 2.322±0.239ml CO₂ g⁻¹ h⁻¹ and daily energy expenditure (DEE) 1939 ± 184 kJ per day. This was significantly greater (x 1.5) than previous estimates which assumed DEE to be three times the basal metabolic rate (BMR) based on regression equations predicting BMR from body mass. FMR of captives and dietary data from previous studies were used to estimate daily consumption of salmon, Salmo salar , smolts and parr by natural populations of these ducks on the river North Esk, north-east Scotland. Goosanders are likely to consume 480-522 g fish per day of which two-thirds are juvenile salmon; equivalent to a daily intake of 10-11 smolts and 48-52 parr. Annual predation of smolts by goosanders was estimated to be between 8000 and 15 000 or 3 and 16% of annual production.     

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.     

Morphological differences in parr of Atlantic salmon Salmo salar from three regions in Norway

Morphological characters were compared in parr (total length 33-166 mm) of Atlantic salmon Salmo salar sampled from eight wild populations in three regions, three in northern, two in the middle and three in southern Norway, covering a distance of 1700 km (from 70° N to 58° N). On the basis of morphological characters 94·6% of the individuals were correctly classified into the three regions. Discrimination between populations within these three regions also had a high degree of correct classification (89·0-95·8%). Principle component analysis identified largest differences to be in head characters, notably eye diameter and jawbone, with the smallest diameter and head size among the northernmost populations. Fish from the southern rivers had a deeper body form whereas fish from the middle region had larger heads and pectoral fins. This illustrates that S. salar already in the early parr stage has morphological traits, which can be used in discrimination between regions and populations and that these differences are discernible in spite of the volume of escaped farmed fish spawning in Norwegian rivers during the past 30 years.     

Characteristics of mature male parr in the northernmost Atlantic salmon populations

The incidence, relative abundance and biological characteristics of mature male parr of Atlantic salmon Salmo salar were studied in two subarctic (69–70 N°) river systems in the northernmost part of Scandinavia. The proportion of mature males was low (0–25%) among ≥1 year‐old parr, and clearly lower than reported earlier in more southern rivers. The ages of mature males ranged from 1+ to 6 + year, the most abundant age group was 3 + year. Both age structure and size of the mature male parr varied between rivers and years. Mature male parr in the main stem were younger and also smaller than the parr of the same age in the tributaries.     

Maturation of male age-0 Atlantic salmon following a massive, localized flood

Maturation of male age-0 Atlantic salmon Salmo salar parr in New England, U.S.A. streams is rare (˜5%), but age-0 parr maturation was high (74%) by autumn in the Sawmill River following a massive, localized flood. Maturation was low in two other study streams (3, 7%) in the same year as the flood, and in the Sawmill River (6%) and the other rivers (5%) in the subsequent year, suggesting that high maturation rates were related to the flood. The high age-0 maturation rates appear to have been the result of greater growth opportunity following the flood. Masses of fish in October were two-fold greater in the Sawmill River (13·2 g) than in the other rivers (6·5, 6·9 g). Mechanisms contributing to the fast growth may include community reorganization following the flood and water temperature differences among rivers. The flood caused an age-0 year-class failure for brook trout Salvelinus fontinalis and brown trout Salmo trutta and a large reduction (69%) in the number of salmon compared to the other rivers, possibly reducing competition or agonistic interactions among remaining fish. Average water temperatures were slightly warmer in the Sawmill River (17·0° C) than in the other rivers (15·5, 14·9° C). By influencing community structure and growth of remaining fish, it appears that a strong environmental disturbance can also alter the direction and timing of life histories in Atlantic salmon.     

Food limitation of planktonic rotifers: field experiments in two mountain ponds

1. Resource competition is thought to be important in controlling zooplankton population dynamics and structuring zooplankton communities. Resource competition requires that resources are limiting. Ten field experiments were conducted to determine the presence and intensity of resource limitation of herbivorous planktonic rotifers in two mountain ponds. The intensity of food limitation was measured as Δ r , the difference between population growth rates in food supplemented versus control enclosures.
2. Rotifers were significantly food limited (Δ r  > 0) in most experiments. The intensity of food limitation varied between species. In Grady Lake, mean Δ r was 0.26 day⁻¹ for Polyarthra vulgaris and 0.43 day⁻¹ for an unidentified Synchaeta sp. In L1 Pond, mean Δ r was 0.07 day⁻¹ for Keratella cochlearis and 0.28 day⁻¹ for S. oblonga . The frequent and intense food limitation suggests that edible phytoplankton in both ponds were often present at low densities, were of low nutritional quality, or both. The intensity of food limitation often changed rapidly over time, indicating rapid temporal variation in resource availability.
3. For P. vulgaris , Δ r was not correlated with its population growth rate in Grady Lake. Thus, although food limitation was common, population dynamics may have been more strongly affected by other factors. For K. cochlearis , Δ r increased as population density increased, suggesting the existence of intraspecific resource competition that may regulate population dynamics.     

Lunar periodicity and the timing of river entry in Atlantic salmon Salmo salar

Historical catch records of Atlantic salmon Salmo salar from three rivers discharging to the Baltic Sea in an area free from tides and from strong effects of the moon on illumination were analysed to investigate whether timing of S. salar river entry was associated with lunar cycles directly. Although a significant effect of lunar phase on river entry was detected, with more fish entering rivers around the full moon than other phases, the effect of the lunar cycle was very small compared with other sources of variation. Hence, the biological role of lunar cycle as a determinant of the timing of S. salar runs in the investigated populations was negligible, suggesting that lunar cycle per se does not play a role in the timing of S. salar river entry.     

Interspecific competition and coexistence of the two chrysomelids,Gastrophysa atrocyanea motschulsky andGalerucella vittaticollis baly (Coleoptera: Chrysomelidae), under limited food resource conditions

Field experiments were conducted in order to investigate the mode of exploitation of food resources and the mechanism of coexistence of mixed larval populations of the two chrysomelids,Gastrophysa atrocyanea andGalerucella vittaticollis, under limited food resource conditions. The larval survival rates seemed high enough to assure coexistence when hatchlings of the two species were released in 1∶1 and 1∶3 ratios on a host plant. However, the survival rate became almost nil for both species when a 3∶1 ratio was employed, suggesting asymmetrical interspecific competition. Wasted food consumption was much higher inG. atrocyanea larvae. The population ofG. atrocyanea seemed to be regulated more by intraspecific competition, while on the other hand, the population ofG. vittaticollis was considered to be more likely affected by the interspecific competition withG. atrocyanea, depending on the initial ratio and density of the two species.     

Inter- and intra-population morphological differences between wild and farmed Atlantic salmon juveniles

Whether population-specific morphological differences were detectable in small (26–52 mm) Atlantic salmon Salmo salar parr reared under similar conditions was tested. Discrimination based on morphological characters was total (100%) between the fish of farmed origin (from AquaGen) and four wild fish populations. Between the four wild populations the corresponding discrimination was 59·8–86·3%. The inter-population variation in morphological characters was larger than the intra-population variation. The fish originating from the local populations at Driva and Innfjord were narrower in body form, whereas fish from the AquaGen and Innfjord populations had smaller and less pointed heads with smaller eyes. The Driva population fish had the smallest mouth while the longest pectoral fin was found in the Bjoreio population, the river that also has the largest fall gradient. Population-specific morphological characters were thus detectable among Atlantic salmon parr relatively rapidly after yolk absorption.     

Three-dimensional microhabitat use by young pool-dwelling Atlantic salmon and brown trout

This study, conducted in deep pools in three rivers, is the first to show a clear three-dimensional habitat segregation in size groups (equivalent to age groups) of juvenile Atlantic salmon, Salmo salar, and brown trout, S. trutta. Young-of-the-year (YOY) held position near the river bed and the river bank; height above bottom and distance from river bank increased significantly with fish size. Brown trout held position significantly further from the substratum, and were on average closer to the river bank, than salmon. The vertical segregation of young salmonids was most evident among young trout, with YOY being closest to the bottom. This size-dependent segregation is probably a result of different outcomes of the trade-off between the conflicting interests of higher food availability and greater predation risk in the upper part of the water column. We suggest that intercohort predation and competitive interactions were the main reasons why YOY of both species and salmon yearlings held positions close to the river bed. We found no evidence of salmon and trout parr preferring particular water depths, as studies in shallow parts of rivers have suggested, as the correspondence of use and availability of microhabitats at different water depths was high in the pools. Copyright 1999 The Association for the Study of Animal Behaviour.     

Genetic divergence and interactions in the wild among native, farmed and hybrid Atlantic salmon

There is concern that the progeny resulting from the spawnings of escaped farmed Atlantic salmon may compete with and disrupt native salmon populations. This study compared, both in the hatchery and in the wild, fitness-related traits and examined interactions among farmed, native and hybrid 0+ parr derived from controlled crosses and reared under common conditions. The farmed salmon were seventh-generation fish from the principal commercial strain in Norway and native salmon were from the rivers Imsa and Lone, Norway. In the hatchery, farmed salmon were more aggressive than both native populations and tended to dominate them in pairwise contests. Farmed salmon were also more prone to risk, leaving cover sooner after a simulated predator attack, and had higher growth rates than native fish. Interbreeding between farmed and native fish generally resulted in intermediate expression of the above traits. There was, however, evidence of hybrid vigour in Lone/farmed crosses which were able to dominate both pure Lone and farmed parr in pairwise contests. In the wild, observations of habitat use and diet suggested that the populations compete for territory and food, and both farmed fish and hybrids expressed higher growth rates than native fish. Our results suggest that these innate differences in behaviour and growth, that probably are linked closely to fitness, will threaten native populations through competition and disruption of local adaptations.     

Effects of surgically implanted transmitters on swimming performance, food consumption and growth of wild Atlantic salmon parr

Experiments were conducted on wild Atlantic salmon Salmo salar parr to determine the effect of surgically implanted dummy transmitters on swimming performance, food consumption and growth. Swimming performance of tagged fish (tag 1·7–3·7% of fish mass) was similar to that of control fish 1, 5 and 10 days after surgery. Negative effects on growth, however, were found up to day 36 of a 45 day experiment (tag 0·9–2·6% of fish mass). Consumption rates were similar between tagged and control fish and did not explain differences in growth.     

Genetic changes in an Atlantic salmon population resulting from escaped juvenile farm salmon

The study was undertaken on three adjacent rivers in NW Ireland, on one of which an Atlantic salmon Salmo salar freshwater juvenile rearing unit is situated. Two markers which distinguished farm and wild populations were used. An Ava II-B RFLP in the ND1 region of mtDNA was at a frequency of 0.58 in the farm strain but absent in the wild populations. Allele E at minisatellite locus Ssa- A45/2/l was at a frequency of 0.91 in farm samples, but at a maximum of 0.41 in the populations in the two rivers adjacent to the one with the juvenile rearing unit. The farm strain showed a significant reduction in mean heterozygosity (0.281 ± 0.057), over three minisatellite loci examined, compared to wild samples (0.532 ± 0.063). The occurrence of farm genotypes and the independent occurrence of mtDNA and minisatellite markers in several parr samples from the river indicated that escaped juvenile salmon completed their life cycle, bred and interbred with native fish, upon their return to the river. Escaped fish homed accurately, as adults, to the site of escape, i.e. the area adjacent to the hatchery outflow in the upstream part of the river. Breeding of males in the lower part of the river was also indicated but this could have been due to mature male parr which had moved downstream. The return of adults of farm origin to the river to breed was indicated by the presence of the Ava II-B haplotype in adults netted in the estuary.     

Size variability of juvenile Atlantic salmon: links to environmental conditions

Climate change models predict a 2 to 6° C increase in air temperature within the next 100 years in the Maritime Provinces of eastern Canada. Higher air temperatures are expected to contribute to increased water temperatures, alterations in stream flow conditions, and ultimately reductions in fish growth. Mean annual size-at-age of juvenile Atlantic salmon Salmo salar decreased in the Northwest Miramichi and Southwest Miramichi Rivers between 1971–1999. Lengths-at-age of juveniles were significantly correlated between the two rivers. For Atlantic salmon parr, stronger associations between inter-cohort fork length ( L _F) than intra-cohort L _F were observed, suggesting that environmental conditions in the current year of growth have the more significant effects on size of age 2 year parr than conditions encountered the previous year by age 1 year parr of the same cohort. Fork lengths of parr were significantly and negatively associated with spring air and water temperatures. In the Miramichi River, increases in air and water temperature as predicted from climate change models may adversely affect growth of juvenile Atlantic salmon parr, reducing the overall productivity of the Atlantic salmon populations in this region.     

Effect of water temperature and density of juvenile salmonids on growth of young-of-the-year Atlantic salmon Salmo salar

A von Bertalanffy growth model for young-of the-year Atlantic salmon Salmo salar in a small French coastal stream was fitted using water temperatures and densities of juvenile salmonids (S. salar and brown trout Salmo trutta) as covariates influencing daily growth rate. The Bayesian framework was used as a template to integrate prior information from external data sets. The relative influence of the covariates on parr growth was quantified and results showed that growth of S. salar juveniles depended on both water temperatures and densities, but that most of the spatiotemporal variability of growth resulted from local spatiotemporal variations of 0+ age salmonid (S. salar and S. trutta) densities. Further analysis revealed that the fluctuations in young-of-the-year salmonid densities are likely to dominate the effects of potential future warming of water temperature due to climate change. It is concluded that factors that could affect salmonid densities might well have a greater effect on S. salar population dynamics than factors influencing water temperatures.     

Effect of main-stem dams on zooplankton communities of the Missouri River (USA)

The persistence of plankton in flowing water presents an enigma, i.e., how can populations be sustained while constantly losing individuals downriver? We examined the distribution and abundance of zooplankton from 146 sites on the Missouri River (USA) and found large shifts in the dominance of major taxa between management zones of this regulated river. Crustacean zooplankton were dominant in the inter-reservoir zone of the river, and their taxonomic composition was similar to regional lakes and reservoirs. The exponential decline of cladocerans and copepods with distance from main-stem dams suggests that conditions within the river are adverse to population growth and that reservoirs are the main source of these crustaceans in the river. Rotifers dominated in the channelized zone of the river. High algal biomass and rapid population growth rates likely allow persistence of rotifers in segments of the river that do not receive direct reservoir inputs. Rotifers were less abundant in the inter-reservoir zone, suggesting that their numbers are limited by internal processes, such as food or predators. Since zooplankton are known to be an important food for larval fishes in rivers, this shift of major taxa in regulated rivers has implications for river food webs.     

Feeding chronology of Atlantic salmon parr in subarctic rivers: consistency of nocturnal feeding

During the ice-free season in three subarctic rivers in northern Norway and Finland, Atlantic salmon parr Salmo salar exhibited significant diel fluctuations in stomach content weights in August and September, the largest weights being recorded consistently at 0300 hours. Accordingly, the feeding rate of the parr was highest during the night and lowest in daytime, whereas during the other periods of the ice-free season, no pronounced diel fluctuations in food intake occurred. The consistent nocturnal feeding periodicity in early autumn in subarctic rivers may be associated with benthic feeding and an increased availability of benthic prey at night.     

Dietary opportunism,resource partitioning,and consumption of coffee berry borers by five species of migratory wood warblers (Parulidae) wintering in Jamaican shade coffee plantations

Diets reflect important ecological interactions, but are challenging to quantify for foliage‐gleaning birds. We used regurgitated stomach samples from five primarily insectivorous species of long‐distance migrant warblers (Parulidae) wintering in two moderate‐elevation shade coffee farms in Jamaica to assess both foraging opportunism and prey resource partitioning. Our results, based primarily on 6120 prey items in 80 stomach samples collected during a one‐week period in March 2000, confirm opportunism. The diets of all five warblers, including American Redstarts (Setophaga ruticilla), Black‐and‐White Warblers (Mniotilta varia), Black‐throated Blue Warblers (S. caerulescens), Northern Parulas (S. americana), and Prairie Warblers (S. discolor), overlapped strongly based on consumption of the same prey types, even many of the same prey species (4 of 10 interspecific overlaps >0.9, range = 0.74–0.97). Moreover, all five species fed on similarly small, often patchily distributed prey, including coffee berry borers (Hypothenemus hampei; Coleoptera, Curculionidae). Nonetheless, permutational multivariate analysis of variance also revealed that the diets of these species differed significantly, primarily with respect to prey mobility (winged vs. sessile); American Redstarts fed on the most mobile prey, and Northern Parulas on the least mobile prey and a relatively restricted set of prey taxa compared to the other four species of warblers. Overall, our results suggest both dietary opportunism consistent with a migratory life‐history, and interspecific resource partitioning consistent with differences in morphology and foraging behavior during a food‐limited season. Having provided evidence of the three necessary conditions, namely intraspecific competition, resource limitation, and interspecific overlap in resource use, the results of our study, in combination with those of other studies, also provide evidence of interspecific competition among wintering migrant insectivores. We thus argue that diffuse interspecific exploitative food competition may be more important than previously recognized.