Åge Brabrand

Comparison of Three Methods for Studies of Stream Habitat Use by Young Brown Trout and Atlantic Salmon

Abstract Surface observation, diving, and electrofishing were compared as methods to study habitat use by young brown trout Salmo trutta and Atlantic salmon S. salar in a Norwegian river. These three methods often gave widely disparate information about habitat use by young of these two species. The probability of encountering individual fish varied when the same method was used in different habitats. Surface observation and diving produced similar results in habitats with low mean water velocities (<20 cm/s) and fine substrate (mean diameter, ≤64 mm), whereas electrofishing was more effective than the sighting methods in shallow areas with greater water velocities and larger substrate.

Habitat shift in roach (Rutilus rutilus) induced by pikeperch (Stizostedion lucioperca) introduction: predation risk versus pelagic behaviour

Changes in the fish community structure and habitat use were followed after the introduction of pikeperch (Stizostedion lucioperca) to the roach-dominated Lake Gjersjøen. Quantitative echosounding showed that the density of juvenile roach (Rutilus rutilus) was dramatically reduced in pelagic areas, from 12 000–15 000 fish/ha to 250 fish/ha, while total fish density remained unchanged in littoral areas. At the same time, the habitat segregation between different size groups of roach was altered as larger roach utilized the pelagic zone after pikeperch introduction. The loss of the pelagic refuge for juvenile roach increased the availability of juvenile roach to littoral predators, notably perch. In littoral areas, the fish community changed from one dominated by roach (> 95%) to one dominated by perch (> 50%).

Food of roach (Rutilus rutilus) and ide (Leusiscus idus): significance of diet shift for interspecific competition in omnivorous fishes

SummaryFeeding relationships between roach and ide from two sites in a mesotrophic lake SE Norway are presented and discussed. When animal food supply was scarce, both fish species increased their consumption of macrophytes; roach sevenfold, and ide threefold. Along a typical littoral vegetation gradient, ide fed among helophytes, while roach fed in the zone outside. This different habitat selection was reflected in the most important plants consumed (ide: Equisetum fluviatile, roach: Characeae), and confirmed by gill net catches. During the vegetative season, roach avoided areas with dense vegetation. In shallow areas beyond the littoral, the most important food plant for both fish species was Potamogeton perfoliatus, which constituted 80% of the total food consumed (dry weight basis) in the roach and 35% in the ide. The diet shift to plants seemed to be strongly influenced by the supply of animal food and the intensity of intra-and interspecific competition.

Predictability and possible mechanisms of plankton response to reduction of planktivorous fish

The predictability of plankton response to reductions of planktivorous fish was investigated by comparing the plankton community in three biomanipulated lakes and ten unmanipulated lakes differing in intensity of fish predation. Data collected on total phosphorus, phytoplankton and zooplankton biomass and share of cyanobacteria and large grazers, as well as specific growth rate of phytoplankton, were further used to test some of the proposed underlying response-mechanisms. In the biomanipulated lakes the algal biomass and share of cyanobacteria decreased, specific growth rate of phytoplankton increased, and zooplankton biomass and share of large grazers increased or remained unchanged. This pattern was largely reflected in the differences in food-chain structure between the unmanipulated lakes with highversus those with low fish predation. The qualitative response to planktivorous fish reduction thus seems largely predictable. The biomanipulated lakes differed, however, in magnitude of response: the smallest hypertrophic, rotenone-treated lake (Helgetjern) showed the most dramatic response, whereas the large, deep mesotrophic lake (Gjersjoen), which was stocked with piscivorous fish, showed more moderate response, probably approaching a new steady state. These differences in response magnitude may be related to different perturbation intensity (rotenone-treatmentversus stocking with piscivores), food-chain complexity and trophic state. Both decreased phosphorus concentration and increased zooplankton grazing are probably important mechanisms underlying plankton response to biomanipulation in many lakes. The results provide tentative support to the hypothesis that under conditions of phosphorus limitation, increased zooplankton grazing can decrease algal biomassvia two separate mechanisms: reduction of the phosphorus pool in the phytoplankton, and reduction of the internal C:P-ratio in the phytoplankton cells.

Fish size distribution and total fish biomass estimated by hydroacoustical methods: a statistical approach

Abstract Ten data sets of fish catches and echosounding recordings from two small Norwegian lakes were used to define the relationship between target strength (TS) and fish length for roach (Rutilus rutilus) and to estimate statistical errors. A maximum likelihood procedure was used, both separately for each data set, and jointly for all data, each with three alternatives regarding degrees of freedom. Results of the six different approaches have been compared with each other and to earlier estimates. Within estimated statistical errors our data confirmed an earlier estimate in the literature, TS = 20 log10L-68, (where L = length) for a community of whitefish (Coregunus lavaretus) and smelt (Osmerus eperlanus). Two different parameter sets in the TS-L relationship were selected for use in biomass calculations. Results showed that fish biomass of pelagic areas in the two lakes varied considerably, depending on pelagic behaviour of roach. The two different parameter sets gave high estimates from 200 to 400 kg ha−1 in Lake Gjersjoen, with standard deviation factors of 1.2 and 1.6 respectively. For Lake Arungen, high estimates were 300–500 and 900–2300 kg ha−1. The main statistical errors seemed to be errors in the parameters of the TS-L relationship, errors due to the TS scale resolution of 2 dB, and errors in measured fish density in different TS classes.

Dynamic micro-geographic and temporal genetic diversity in vertebrates: the case of lake-spawning populations of brown trout (Salmo trutta)

Conservation of species should be based on knowledge of effective population sizes and understanding of how breeding tactics and selection of recruitment habitats lead to genetic structuring. In the stream‐spawning and genetically diverse brown trout, spawning and rearing areas may be restricted source habitats. Spatio–temporal genetic variability patterns were studied in brown trout occupying three lakes characterized by restricted stream habitat but high recruitment levels. This suggested non‐typical lake‐spawning, potentially representing additional spatio–temporal genetic variation in continuous habitats. Three years of sampling documented presence of young‐of‐the‐year cohorts in littoral lake areas with groundwater inflow, confirming lake‐spawning trout in all three lakes. Nine microsatellite markers assayed across 901 young‐of‐the‐year individuals indicated overall substantial genetic differentiation in space and time. Nested gene diversity analyses revealed highly significant (≤P = 0.002) differentiation on all hierarchical levels, represented by regional lakes (FLT = 0.281), stream vs. lake habitat within regional lakes (FHL = 0.045), sample site within habitats (FSH = 0.010), and cohorts within sample sites (FCS = 0.016). Genetic structuring was, however, different among lakes. It was more pronounced in a natural lake, which exhibited temporally stable structuring both between two lake‐spawning populations and between lake‐ and stream spawners. Hence, it is demonstrated that lake‐spawning brown trout form genetically distinct populations and may significantly contribute to genetic diversity. In another lake, differentiation was substantial between stream‐ and lake‐spawning populations but not within habitat. In the third lake, there was less apparent spatial or temporal genetic structuring. Calculation of effective population sizes suggested small spawning populations in general, both within streams and lakes, and indicates that the presence of lake‐spawning populations tended to reduce genetic drift in the total (meta‐) population of the lake.

Biomanipulation and food-web dynamics : the importance of seasonal stability

Responses of phytoplankton biomass were monitored in pelagic enclosures subjected to manipulations with nutrients (+N/P), planktivore roach (Rutilus rutilus) and large grazers (Daphnia) in 18 bags during spring, summer and autumn in mesotrophic Lake Gjersjøen. In general, the seasonal effects on phytoplankton biomass were more marked than the effects of biomanipulation. Primary top-down effects of fish on zooplankton were conspicuous in all bags, whereas control of phytoplankton growth by grazing was observed only in the nutrient-limited summer situation. The effect of nutrient additions was pronounced in summer, less in spring and autumn; additions of fish gave the most pronounced effect in spring. The phytoplankton/zooplankton biomass ratio remained high (10–100) in bags with fish, with the highest ratios in combination with fertilization. The ratio decreased in bags without fish to<2 in most bags, but a real grazing control was only observed in bags with addition ofDaphnia. No direct grazing effects could be observed on the absolute or relative biomass of cyanobacteria (mainlyOscillatoria agardhii). The share of cyanobacteria in total phytoplankton biomass was lowest in summer (7–26%), higher in spring (39–63%) and more than 90% in the autumn experiment. The development of the cyanobacterial biomass was rather synchronous in all bags in all the three experiments. A high biomass ofDaphnia gave no increase in the pool of dissolved nutrients in spring, a slight increase in summer and a pronounced increase in autumn. While a strong decrease in the P/C-cell quota of the phytoplankton was observed from spring to autumn, no effect of grazing or nutrient release could be related to this P/C-status. The experiments indicate that such systems, with high and stable densities of inedible cyanobacteria, are rather insensitive to short-term (3–4 weeks) biomanipulation efforts. This is supported by observations on the long-term development of the lake.

Survival of eggs of Atlantic salmon (Salmo salar) in a drawdown zone of a regulated river influenced by groundwater

Groundwater may create refuges for Atlantic salmon egg survival during low flows in regulated rivers and thus play an important role for survival during winter. To investigate the links between the survival of salmon embryos and hyporheic hydrological processes during permanent winter drawdown, a 100-m-long and 50-m-wide gravel area in a regulated river, the River Suldalslågen, was used for an experimental study. Surface and subsurface water levels were monitored with 2-min time resolution by means of water pressure sensors placed in pipes. Temperature, conductivity and dissolved oxygen were also measured. Eight cylindrical boxes, each with two compartments (at 10- and 30-cm depth, respectively) containing 50 Atlantic salmon eggs, were placed in the river bed substrate of both the drawdown zone and the permanently wetted area as a reference. They were regularly checked for survival during winter from January to May, coinciding with egg development period for this river. Survival rates in boxes in the dewatered river bed were between 8 and 78% during winter, compared to 80 to 99% in the reference wetted area. The main driver for egg survival in the dewatered area was groundwater with sufficient oxygen levels.

Effects of siltation on resource utilization and dynamics of allopatric brown trout,Salmo trulta, in a reservoir

SynopsisThe water level of the old hydroelectric Ringedal Reservoir in western Norway was unusually low during 1985, resulting in severe erosion and siltation. The secchi disc transparency was 18 m in July 1984, decreased to 0.3 m in July 1985, and increased to 13 m in July 1986 after a rise in the water level. The abiotic changes induced major effects in the zooplankton community, with a strong reduction in occurrence of Cladocera (Holopedium gibberum andBosmina longispina). The allopatric population of brown trout,Salmo trutta, which mainly fed on zooplankton before the siltation, fed predominantly on surface insects during the summer of high siltation. The food consumption was lower than in the pre- and post-siltation years, resulting in reduced k-values, reduced number of spawners, and increased mortality of mature fish.

The ectoparasite Ichthyophthirius multifiliis and the abundance of roach (Rutilus rutilus): larval fish epidemics in relation to host behaviour

Abstract Natural epizootics caused by the ectoparasite Ichthyophthirius multifiliis were followed in a 0+ roach ( Rutilus rutilus ) cohort in a biomanipulated lake, stocked with the predatory pike-perch ( Stizostedion lucioperca ). The parasite was not observed in three neighbouring lakes. Infection was first observed about 1 month after hatching of roach larvae, simultaneously with the spring increase in water temperature. Within 2 weeks the parasite had spread to almost all roach larvae observed. In the beginning of July, the abundance of roach larvae in the lake declined dramatically. I. multifiliis was not observed on 0+ of rudd ( Scardinius erythrophthalmus ), bream ( Abramis brama ) or perch ( Perca fluviatilis ), even though these species were numerous in the same habitats as roach. The importance of I. multifiliis as a factor in roach mortality was examined experimentally by comparing the mortality of uninfected and artificially infected roach during a laboratory epidemic, and by comparing the mortality of uninfected and naturally infected roach under laboratory conditions. Both the naturally infected roach larvae and the roach larvae artificially exposed to I. multifiliis died within a few days and before uninfected roach. The shift in juvenile roach distribution to a more permanent littoral habitat induced by pike-perch introduction is suggested as an important cause of I. multifiliis epidemics, since the encystment of the free-living stage needs a substrate. In addition, the high roach fry abundance and high temperature in shallow waters will promote optimal conditions for outbreak of the infection.