Natural Recruitment (natural + recruitment)

Distribution by Scientific Domains

Selected Abstracts

Restoration of sturgeons: lessons from the Caspian Sea Sturgeon Ranching Programme

D.H. Secor
Depletion of sturgeon stocks world-wide has increased interest in aquaculture-based restoration programmes. The Caspian Sea Sturgeon Ranching Programme (SRP) of the former Soviet Union represents a unique opportunity to evaluate expense, benefits and potential ecological and genetic effects of such restoration programmes. The SRP was initiated in the 1950s to compensate for lost spawning habitat in the Volga River and elsewhere. After its completion in 1962, the Volgograd Dam reduced spawning grounds in the Volga River system, the principal spawning tributary of the Caspian Sea, by ,80%. For two of the three commercial sturgeon species (Russian sturgeon, Acipenser güldenstädti, and stellate sturgeon, A. stellatus), yields improved after the imposition of the 1962 moratorium on sturgeon harvests in the Caspian Sea. Volga River fisheries were managed for spawning escapement. Although imprecisely known, the contribution of the millions of stocked Russian and stellate juveniles during 1962,91 was most likely important to sustaining fisheries, although less so (contributing to <30% of the adult stock) than natural recruitment. Apparently, reduced spawning grounds, supplemented with artificial spawning reefs were sufficient to support reproduction and large fishery yields of Russian and stellate sturgeons. For beluga sturgeon, Huso huso, harvests in the Volga River were nearly all dependent upon hatchery stocking. Beluga sturgeon spawning grounds were mostly eliminated with the construction of the Volgograd Dam. Without the hatchery programme, beluga sturgeon in the Volga River and Caspian Sea would in all likelihood have been extirpated. Currently, sturgeons are severely depleted in the Volga River and Caspian Sea due to poaching and lack of co-operation between countries exploiting the species. Aquaculture-based restoration in Russia is now viewed a chief means of rebuilding stocks of Caspian Sea sturgeons. [source]

Evaluation of large-scale stocking of early stages of brown trout, Salmo trutta, to angler catches in the French,Swiss part of the River Doubs

A. Champigneulle
Abstract Around 500 000 brown trout, Salmo trutta L., alevins are stocked annually in the 24-km section of the River Doubs under study. All the alevins stocked in the period 1994,1996 were identifiable by fluoromarking their otoliths with tetracycline chlorhydrate. Anglers' catches, between June 1997 and September 1998, comprised trout aged 1+ to 7+ , but most (90% +) were 2+ to 3+ or 4+ , with the majority at 2+ and 3+. There was no significant difference in the size for a given age between marked and unmarked angled trout. The contribution of stocked fish in anglers' catches was around 22% for the 1995 cohort. The contribution of stocking (cohorts 1994 to 1995,1996) to the 1998 catches was around 23% (95% confidence limits: 19,27%). The estimated recapture rate was three to four trout per 1000 alevins stocked for the 1995 cohort. The major contribution (78%) of natural recruitment to anglers' catches suggests that the fishery management based on natural recruitment is still realistic in this part of River Doubs. [source]

Annual trend of fish assemblages associated with FADs in the southern Tyrrhenian Sea

F. Andaloro
Summary A study on fish assemblage associated with fish aggregating devices (FADs) in Sicily was carried out between January 2000 and January 2001. With a fortnightly periodicity, 156 experimental hauls were carried out by means of a purse seine in a FAD-containing marine section banned to commercial fishing. A total of 14 229 fish specimens belonging to six families and 10 species was found. These species were Balistes carolinensis, Caranx crysos, Naucrates ductor, Seriola dumerili, Seriola fasciata, Tachurus picturatus, Coryphaena hippurus, Schedophilus ovalis, Thunnus thynnus, Polyprion americanus and showed all young-of-the-year undergoing a rapid growth. The applied ordination technique highlighted the existence of four assemblage periods describing the annual trend. The results confirm that fish assemblages associated with FADs are related to season, following a fish colonization tied to natural recruitment. Comparison of the ecological indices across the four periods showed that the assemblages in the periods from summer to winter were more structured than those in spring. The quantity of individuals also showed a strong variation peaking in the summer period. The results of this study reveal that FADs represent a particular nursery area for the associated species that could influence their survival. [source]

Changes in the structure and composition of miombo woodlands mediated by elephants (Loxodonta africana) and fire over a 26-year period in north-western Zimbabwe

Isaac Mapaure
Abstract Changes in structure and composition of miombo woodlands mediated by elephants and fire were studied in 26-year-old permanent transects established in 1972 in north-western Zimbabwe. Elephants caused 48% decline in proportions of large trees (>11 cm diameter), significant reductions (30.9,90.9%) in tree heights, reductions in stem areas (43.5%) and densities (2.5%) of all trees. There were increases in proportions of small trees (64.8%), shrub canopy volumes (271%) and shrub densities (172%). These increases are attributed to natural recruitment because of longer fire-free periods and reduction of tree suppression effects on lower strata as a result of elephant-induced tree declines. Frequencies of occurrence of most species dropped by 28,89.6%. Brachystegia boehmii was replaced by Pseudolachnostylis maprouneifolia as the most dominant tree, largely because of high elephant preference for Brachystegia boehmii. A new suite of species, dominated by Combretaceae, increased in dominance resulting in local floristic changes. Reductions in old elephant (33.4%), old unknown (89.9%) and new elephant (13.7%) damage suggest that elephant occupancy of miombo woodlands has declined, possibly because of limited availability of preferred browse species. This study clearly shows that elephants and fire have contributed significantly to the changes in miombo woodlands in the area. Résumé Les changements de la structure et de la composition des forêts de miombo dus aux éléphants et aux feux ont étéétudiés sur des transects permanents établis en 1972 dans le nord-ouest du Zimbabwe. Les éléphants ont causé un déclin de 48% de la proportion de grands arbres (>11 cm de diamètre), des réductions significatives (30,9,90,9%) de la hauteur des arbres, des réductions de la surface des tiges (43,5%) et de la densité (2,5%) de tous les arbres. Il y avait des augmentations de la proportion de petits arbres (64,8%), du volume des buissons (271%) et de leur densité (172%). Ces augmentations sont attribuées au recrutement naturel dûà de plus longues périodes sans feux, et à la réduction des effets suppressifs des arbres sur les couches inférieures due au déclin des arbres induit par les éléphants. La fréquence de la plupart des espèces a chuté de 28%à 89,6%. Brachystegia boehmi a été remplacé par Pseudolachnostylis maprouneifolia comme arbre dominant, en grande partie à cause de la forte préférence marquée par les éléphants pour Brachystegia boehmi. Une nouvelle série d'espèces, dominée par des Combrétacées, a accru sa dominance et entraîné des changements floristiques locaux. La réduction des dommages « anciens dus aux éléphants » (33,4%), « anciens d'origine inconnue » (,9,9%) et « nouveaux dus aux éléphants » (13,7%) suggère que l'occupation des forêts de miombo par les éléphants a diminué, peut-être à cause de la disponibilité limitée des espèces fourragères qu'ils préfèrent. Cette étude montre clairement que les éléphants et les feux ont contribué significativement aux changements survenus dans les forêts de miombo de la région. [source]

Genetic signatures in an invasive parasite of Anguilla anguilla correlate with differential stock management

S. Wielgoss
In this article, it is shown that available genetic tools for the omnipresent parasite Anguillicoloides crassus in European eels Anguilla anguilla are sensitive to different immigration rates into local A. anguilla stocks for two separated river systems. Relying on four highly polymorphic microsatellite markers, it was inferred that under natural recruitment, nematode samples meet Hardy,Weinberg expectations for a single panmictic population, while genetic signals show signs for a strong Wahlund effect most likely due to very recent population mixing under frequent restocking of young A. anguilla. This was indicated by a low but significant FST value among within-host populations (infrapopulations) along with high inbreeding indices FIS consistent over all loci. The latter signal is shown to stem from high levels of admixture and the presence of first-generation migrants, and alternative explanations such as marker- and sex-specific biases in the nematode populations could be dismissed. Moreover, the slightly increased degree of relatedness within infrapopulations in the stocked river system cannot explain the excessive inbreeding values found and are most likely a direct consequence of recent influx of already infected fish harbouring parasites with different genetic signatures. Applying a simulation approach using known variables from the nematode's invasion history, only the artificial introduction of a Wahlund effect leads to a close match between simulated and real data, which is a strong argument for using the parasite as a biological tag for detecting and characterizing fish translocation. [source]

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

Peter J. Clarke
Abstract Field experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface-sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment. [source]