Does the trace element composition of brown trout Salmo trutta eggs remain unchanged in spawning redds?

The temporal stability of trace element concentrations in fertilized, artificially incubated anadromous brown trout Salmo trutta eggs and newly hatched fry was investigated. The anadromous status of the parental fish was confirmed using strontium isotopic analysis of otoliths. Whilst manganese concentrations in eggs varied over time, concentrations of aluminium, potassium, magnesium, strontium, barium and calcium were all unchanged 1 week and 6 weeks post‐fertilization as well as in recently hatched larvae. The results clearly suggest that the distinctive trace element signature present in the eggs and newly hatched larvae of anadromous S. trutta (typically characterized by high strontium, low barium) is stable over time. Therefore analysis of the trace element composition of eggs is concluded to be a cost‐effective and reliable method for determining the spatial and temporal extent of upstream spawning migration by anadromous salmonids. The temporal variability of at least one element in this study suggests the stability of untested multi‐element signatures cannot automatically be assumed.     

Utilization of brown trout Salmo trutta by Acanthocephalus clavula in an Irish lake: is this evidence of a host shift?

The population biology of the fish acanthocephalan Acanthocephalus clavula was described from 161 wild brown trout, Salmo trutta sampled over a two-year period in Clogher Lake in the west of Ireland. Overall prevalence of the parasite was 86% and the mean abundance was 53 worms per fish. Despite the presence of large numbers of worms in the trout very few females (2%) attained full reproductive maturity. This suggests that trout is an accidental host. A sample of yellow eels, Anguilla anguilla was examined at a different time from the same lake. The prevalence of A. clavula was 97% and the average abundance was 8 worms per fish. In contrast to the situation in trout, the proportion of female worms attaining full reproductive maturity was 61% fulfilling the expected characteristic of a preferred definitive host. The possible explanations for the very high abundance of A. clavula in trout are discussed and include the influence of fluctuations in host populations, host diet and the absence of a potential competitor.     

Seasonal and spatial variation in the diet of brown trout (Salmo trutta L.) in the subarctic River Laxá, north-east Iceland

This study examines seasonal and spatial patterns in the diet composition, stomach fullness and condition of a landlocked population of brown trout (2.5–61.3 cm) in the lake-fed River Laxá in NE-Iceland. The stomach contents consisted predominantly of benthic invertebrates, such as blackfly larvae Simulium vittatum (58%), chironomids (24%) and the freshwater snail Radix peregra (7%). The seasonal and spatial patterns of S. vittatum in the stomach contents were consistent with published studies on the life cycle and production of S. vittatum in the River Laxá. Close to the Lake Myvatn outlet (0–16 km), S. vittatum constituted 63% of the stomach content volume, and exhibited two peaks (i.e., generations) in its seasonal contribution to the stomach contents, whereas further downstream (16–35 km) S. vittatum constituted 45% of the volume and showed only clear evidence of one generation. Seasonal fluctuations in S. vittatum availability appear to affect the stomach fullness and the condition factor of the trout, especially close to the lake outlet where the fluctuations are pronounced.     

Invasive brown trout Salmo trutta induce differential growth strategies in the native snow trout Schizothorax richardsonii of Himalaya: Are natives in unaltered rivers better at picking the gauntlet of invasion?

Brown trout Salmo trutta is a potent global invader and its establishments have progressively altered physiologies, life-histories and niche-availabilities for native fish species. River impoundments further escalate its invasion potential. The Himalayan rivers however, stay uncharted for the effects of brown trout interactions with the native fish fauna. Snow trout Schizothorax richardsonii a Himalayan cold-water native, concerningly overlaps its range with brown trout. To understand its responses to invasion pressures, we investigated brown trout effects on the age and growth of snow trout populations in three rivers with varying levels of perturbation: (a) a dammed and (b) an undammed river with the invasive brown trout in comparison to (c) an undammed river without invasion pressures. We found sympatric snow trout in the undammed river to respond to brown trout invasion with fast life history responses, showing an early age-at-maturity (A₅₀ = 1.2 years) and fast growth with a higher growth constant (K = 0.40 yr⁻¹) and specific rate of linear growth across life. On the contrary, sympatric snow trout in the dammed river showed an explicitly slow life-history by maturing at a higher age (A₅₀ = 2.9 years) and a slow growth, with a lower growth constant (K = 0.26 yr⁻¹) and specific linear growth rates. Our findings suggest that, the snow trout appear to present stronger response to brown trout invasions when the river is unaltered and free from hydropower operations and damming. Further research is strongly warranted from other high-altitude Himalayan basins to delineate the variation in growth strategies exhibited by snow trout in sympatry with the invasives.     

Analysis of the reproductive cycle of female wild marble trout <Emphasis Type="Italic">Salmo marmoratus</Emphasis> in a prealpine brook of the Soča River basin (Northeast Italy)

The reproductive cycle of female wild marble trout Salmo marmoratus was studied in a prealpine watercourse (Northeast Italy). Gonadosomatic index, oocyte growth and plasma levels of 17β-oestradiol and testosterone were bimonthly measured during one year to obtain a detailed dataset and to gain useful tools for the identification of the reproductive stage without impact for the species. Monitored features showed significant variations during the study period: gonadosomatic index and oocyte size increased slightly but significantly during the first part of the cycle (from February to June), while steroid levels remained quite constant; for all parameters, major increases were highlighted between August and the spawning season (which occurred from the middle of November to the middle of December). Ranges and trends observed for gonadosomatic index (from 0.87 ± 0.41% to 10.91 ± 3.37%) and oocyte diameter (from 0.692 ± 0.031 mm to 4.624 ± 0.208 mm) were generally in agreement with literature regarding salmonids, while plasma levels of 17β-oestradiol (from 0.214 ± 0.015 ng ml^?1 to 78.090 ± 23.882 ng ml^?1) and testosterone (from 0.327 ± 0.086 ng ml^?1 to 71.800 ± 29.406 ng ml^?1) showed wider ranges. A strong non-linear relationship was found between oocyte size and 17β-oestradiol plasma concentration (r ² = 0.890) and especially between oocyte size and testosterone plasma concentration (r ² = 0.947). This last relationship could likely be used to obtain information relative to the gonadal development, especially in the case of an endangered species like Salmo marmoratus, which needs non-invasive tools for management.     

Identification of follicular vitellogenesis stage by monitoring of plasma calcium and estradiol-17β concentrations in the cultured Caspian brown trout, Salmo trutta caspius Kessler, 1877

To identify a simple indicator for detection of follicular vitellogenesis stage of cultured Caspian brown trout, the changes of plasma calcium and estradiol-17β along with ovarian histology were investigated over a reproductive cycle. For this purpose, the blood and gonad samples were taken monthly over a six months period. According to results, the concentrations of plasma calcium increased significantly during October and November but then declined steadily until the end of experiment. Estradiol-17β concentrations were low throughout the experiment, a clear peak was observed in October. Histological observations showed that all examined females were in vitellogenesis when the peak of plasma calcium and estradiol-17β were observed. Also, significant positive relationship was found between plasma calcium and estradiol. The good fitness of plasma calcium and estradiol-17β peak with histological observations suggests that plasma calcium could be a good and simple indicator for detection of follicular vitellogenesis stage in the cultured Caspian brown trout.