Temperature preference in two populations of juvenile coho salmon,oncorhynchus kisutch

Synopsis Two groups of coho salmon,Oncorhynchus kisutch, were raised under identical regimes to test the hypothesis that the group from a stream with lower and less variable temperatures would have a lower and less variable preferred temperature than would the group from a stream with warmer and more variable temperatures. The preferred (modal) temperatures in an electronic shuttlebox of coho salmon young from a relatively cool, groundwater-fed stream were slightly lower and less variable than those of young from a warmer and more heterothermal stream (mean = 9.6° C, range: 6–16° C vs. mean = 11.6° C, range: 7–21° C). However, there was a great deal of variation within and among individual fish. While some genetic variation in thermal preference may exist, the species seems best characterized as tolerant of relatively large temperature fluctuations.     

Elevated spring temperature stimulates growth, but not smolt development, in anadromous Arctic charr

Parr–smolt transformation and growth were studied in captive offspring of anadromous Arctic charr (Salvelinus alpinus) from the Hals watercourse in northern Norway (70°N), held either at a natural temperature (< 1 °C until May) or at a temperature elevated to 6 °C in late March. In mid-May, 5 weeks after the increase in photoperiod from 8:16 h light:dark to continuous light, gill Na⁺, K⁺-ATPase activity started to increase in both temperature groups, concurrent with the final development of full seawater tolerance. Temperature had no effect on the development of gill Na⁺, K⁺-ATPase activity, or on hypoosmoregulatory ability. The fish in both treatments resumed growth in mid-May, but from then on growth was faster in the elevated than in the ambient temperature group. In the former group, fish mass doubled in 6 weeks (from 65 to 137 g), and growth ceased at the time when the fish were about to complete their parr–smolt transformation. These findings show that an early vernal temperature increase advances the seasonal growth cycle, but not the parr–smolt transformation, in anadromous Arctic charr.     

Weir removal in salmonid streams: implications, challenges and practicalities

Low-head dams and weirs can greatly limit the distribution and abundance of Atlantic salmon and other migratory salmonids in streams. Weirs can significantly increase the vulnerability of migratory fish to anglers, alter natural migration patterns, and exacerbate the effects of opportunistic predators. Overcrowding of fish at downstream pools can also facilitate the spread of parasites and infectious diseases, magnify the impact of pollution incidents, and increase the risk of mass mortalities, particularly at low flows. Not surprisingly, augmenting the accessible stream area constitutes one of the best ways to restore depleted salmonid populations. In this context, the removal of unused or illegal weirs can be an efficient, cheap solution to increase stream accessibility. Here, I examine some impacts of weirs on Atlantic salmon populations, and document with case studies the removal and breaching of weirs in several Iberian streams. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Habitat effects on invertebrate drift in a small trout stream: implications for prey availability to drift-feeding fish

In this study, we focused on the drivers of micro- and mesohabitat variation of drift in a small trout stream with the goal of understanding the factors that influence the abundance of prey for drift-feeding fish. We hypothesized that there would be a positive relationship between velocity and drift abundance (biomass concentration, mg/m³) across multiple spatial scales, and compared seasonal variation in abundance of drifting terrestrial and aquatic invertebrates in habitats that represent the fundamental constituents of stream channels (pools, glides, runs, and riffles). We also examined how drift abundance varied spatially within the water column. We found no relationship between drift concentration and velocity at the microhabitat scale within individual pools or riffles, suggesting that turbulence and short distances between high- and low-velocity microhabitats minimize changes in drift concentration through settlement in slower velocity microhabitats. There were also minimal differences in summer low-flow drift abundance at the mesohabitat scale, although drift concentration was highest in riffle habitats. Similarly, there was no differentiation of drifting invertebrate community structure among summer samples collected from pools, glides, runs, and riffles. Drift concentration was significantly higher in winter than in summer, and variation in drift within individual mesohabitat types (e.g., pools or riffles) was lower during winter high flows. As expected, summer surface samples also had a significantly higher proportion of terrestrial invertebrates and higher overall biomass than samples collected from within the water column. Our results suggest that turbulence and the short length of different habitat types in small streams tend to homogenize drift concentration, and that spatial variation in drift concentrations may be affected as much by fish predation as by entrainment rates from the benthos. Handling editor: Robert Bailey     

Ontogenetic variation in density‐dependent growth of brown trout through habitat competition

1. Density‐dependent growth has been widely reported in freshwater fishes, but the ontogenetic evolution of competition and its subsequent effects on growth through a life span remains unclear. 2. Patterns of competition can be described by integrating population abundance data with habitat‐modelling results. Weighted usable area (WUA; m² WUA ha^?1) curves are obtained for each flow value and are then coupled with demographic data to obtain the occupancy rates (trout m^?2 WUA, the density of a given age class related to its suitable habitat) of the WUA for every age class, year and site. 3. We examined a long‐term data series searching for temporal variation in the influence of habitat occupancy rate on the growth of brown trout Salmo trutta. We tested whether (i) mean cohort mass (mean mass of the cohort during the first 3 years of life) is affected by the occupancy rate experienced across a life span; and (ii) the occupancy rate experienced at different ages influenced mean body size. 4. We observed a consistent negative power relationship between average cohort mass and mean occupancy rate through a life span, indicating that stronger cohorts were related to a reduced growth, with likely consequences for individual fitness. 5. The effects of occupancy rate on size‐at‐age were mainly detected in the size attained at the second year of life, but they were because of the competition at different times. Thus, the level of competition varied through ontogeny, in some of the rivers affecting growth since the first year of life, whereas in most of the rivers the main effects on body size resulted from the competition during the second year of life. 6. Occupancy rate appears more appropriate than density for assessing the occurrence of habitat competition in freshwater fishes, since it encompasses the differences in quantity and quality of suitable habitat for each age class. 7. Our study highlights the importance of density‐dependent growth as a key process in the dynamics of brown trout populations, its temporal variation depending on the temporal changes of density and the variation of competition associated with the habitat capacity for each life stage.     

Mature males of Arctic charr,Salvelinus alpinus,release F-type prostaglandins to attract conspecific mature females and stimulate their spawning behaviour

Synopsis We examined the behavioural and biological functions of prostaglandins (PG) of F-type as a chemosensory signal during spawning in Arctic charr. Immunoassay revealed F-type PGs in water inhabited by ripe charr of both sexes performing spawning-related acts. High concentrations of immunoreactive PGFs seemed to coincide with male spawning activity. Ripe isolated males were more odorous to other charr (as assayed by EOG) than either females or immature charr. When these males received chemical cues from ovulated females, they increased their release of odorants, including F-type PGs, into the water. PGF_2α at concentrations of 0.1 nM attracted ovulated females and ripe males. It also stimulated digging behaviour in mature females. It is concluded that F-type PGs or their derivatives are released by ripe males on the spawning grounds to attract females and elicit their spawning behaviour.     

Selectivity of fly-fishing,spinning and gillnet for brook charr and ouananiche in a large northern Quebec river

Synopsis A comparison between fly-fishing, spinning and short 51 mm and 25 mm gill nets showed that fly-fishing was the least selective method of sampling brook chary,Salvelinus fontinalis, and ouananiche,Salmo salar, in rapid sections of the Caniapiscau River, north central Quebec. Angling techniques had the added advantage of mobility which allowed rapids to be more evenly covered during sampling. Selectivity curves for the capture of brook chary by fly-fishing, spinning and a 51 mm mesh gill net are presented.     

Density,length distribution,and diet of age-0 arctic charrSalvelinus alpinus in the surf zone of Thingvallavatn,Iceland

Synopsis Population densities of age-0 arctic chaff in the surf zone averaged 1.83 and 4.70 fish m^-2 in August 1984 and June 1985, respectively. Length variation of the littoral fish was low in early summer, increasing in late summer and autumn. Newly emerged charr, ∼ 20 mm long, appeared in the stony shallow water zone during both May and June. From length variation and variation in mouth position of the young charr, it is concluded that at least two of the four chaff morphs in the lake are present in the surf zone during spring and summer. In August, some of the larger age-0 charr had moved out from the surf zone, into the pelagic and the deeper epibenthic waters. The food of young littoral charr was dominated by large chironomid larvae (instar 3 and 4) and pupae Contribution from the Thingvallavatn project     

Variation in salmonid mitochondrial DNA: Evolutioinary constraints and mechanisms of substitution

Summary Sequence comparisons were made from 2214 bp of mitochondrial DNA cloned from six Pacific salmonid species. These sequences include the genes for ATPase subunit 6, cytochrome oxidase subunit 3, NADH dehydrogenase subunit 3, NADH dehydrogenase subunit 4L, tRNA^GLY, and tRNA^ARG. Variation is found at 338 silent and 12 nonsilent positions of protein coding genes and 10 positions in the two tRNA sequences. A single 3-bp length difference was also detected. In all pairwise comparisons the sequence divergence observed in the fragment was higher than that previously predicted by restriction enzyme analysis of the entire molecule. The inferred evolutionary relationship of these species is consistent between methods. The distribution of silent variation shows a complex pattern with greatly reduced variation at the junctions of genes. The variation in the tRNA sequences is concentrated in the DHU loop. The close relationship of these species and extensive sequence analyzed allows for an analysis of the spectrum of substitutions that includes the frequencies of all 12 possible substitutions. The observed spectrum of substitutions is related to potential pathways of spontaneous substitution. The salmonid sequences show an extremely high ratio of silent to replacement substitutions. In addition the amino acid sequences of the four proteins coded in this fragment show a consistently high level of identity with theXenopus sequences. Taken together these data are consistent with a slower rate of amino acid substitution among the cold-blooded vertebrates when compared to mammals.     

Trophic state and its implications for natural reproduction of salmonid fish

Natural propagation of all fish species native to a particular lake is an ecological requirement of prime importance. Salmonid fish are particularly vulnerable to environmental degradation since egg development takes place on or in the bottom substrate. Dissolved oxygen concentration on the spawning site is commonly considered the main factor limiting egg development. In order to assess the relationship between trophic state, hypolimnetic oxygen concentration, and salmonid egg development, whitefish (Coregonus sp.) eggs were sampled just prior to hatching in late winter. Eggs were collected in 13 Swiss lakes of different trophic state, using a sled-dredge. The proportion of viable eggs among the total number of eggs found was taken as a measure of reproductive success. The results indicate that successful in situ embryogenesis is not primarily governed by hypolimnetic oxygen concentration which tends to be high anyway during egg incubation, but by trophic state in terms of total phosphorus concentration in spring. The findings are explained by the production-related oxygen uptake of the sediment, creating an oxygen gradient of varying steepness in the diffusive boundary layer. Implications for the restoration of lacustrine ecosystems are discussed.