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Population Size Structure (population + size_structure)
Selected AbstractsEffects of stocked trout on native fish communities in boreal foothills lakesECOLOGY OF FRESHWATER FISH, Issue 2 2010Leslie E. Nasmith Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes. Ecology of Freshwater Fish 2010: 19: 279,289. © 2010 John Wiley & Sons A/S Abstract,,, Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005,2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size-limited predation. Dace were also more crepuscular and concentrated on the lake-bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population-level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes. [source] Aseasonality in the abundance and life history of an ecologically dominant freshwater crab in the Rift Valley, KenyaFRESHWATER BIOLOGY, Issue 2 2007MICHAEL DOBSON Summary 1. Freshwater crabs appear to show at least two alternative life history patterns, which differ in the timing of seasonal reproduction. Reproduction occurs during low flow among temperate lotic species, but during high water levels among wetland species. Crab biomass is often very high and both strategies would lead to spatial and temporal pulses in density and biomass. The life history and reproductive strategy adopted by tropical lotic species is poorly known, however, despite their importance in community and ecosystem dynamics. 2. In this study, we determined annual patterns of life history, density and biomass of a lotic freshwater crab in a small headwater stream in the East African highlands where it maintains high biomass. This crab is an as yet undescribed species of Potamonautes, here referred to as the Chinga crab. 3. Crabs were sampled non-destructively for 15 months using baited traps and benthic sampling with a Surber sampler. At the end of the study, an intensive hand search was carried out. Each method was biased towards different size classes of crabs and the efficiency of both long-term methods varied according to water levels in the stream. The intensive search was more effective than benthic sampling, but failed to record the large individuals caught by baited traps. 4. Population density and biomass remained constantly high throughout the study period. Reproduction, as evidenced by the presence of ovigerous females and small free-living juveniles, also showed no seasonality. As a consequence, the population size structure (size-frequency distribution) of crabs remained constant throughout the year. 5. The Chinga crab illustrates a third life history pattern, with no clear breeding season, and this may be common among tropical species. This is probably a consequence of the non-seasonal nature of its habitat: temperature varied little throughout the year and rainfall fluctuations were relatively small. This strategy allows the species to maintain high biomass without seasonal pulses and, perhaps, to dominate community and ecosystem processes. [source] Increased growth and recruitment of piscivorous perch, Perca fluviatilis, during a transient phase of expanding submerged vegetation in a shallow lakeFRESHWATER BIOLOGY, Issue 12 2005ANDERS HARGEBY Summary 1. In this study, we examine how a 7-year period of expanding submerged stonewort (Chara spp.) vegetation during a shift from turbid to clear water in a shallow lake influenced individual growth and population size structure of perch (Perca fluviatilis). We expected that a shift from phytoplankton to macrophyte dominance and clear water would improve feeding conditions for perch during a critical benthivorous ontogenetic stage, and enhance the recruitment of piscivorous perch. 2. Growth analysis based on opercula showed that growth during the second year of life was significantly higher in years with abundant vegetation than in years with turbid water and sparse vegetation. Growth was not affected during the first, third and fourth year of life. Stable isotope analyses on opercula from 2-year-old perch showed that the increase in growth coincided with a change in carbon source in the diet. Stable nitrogen ratio did not change, indicating that the increased growth was not an effect of any change in trophic position. 3. Following the expansion of submerged vegetation, perch size range and abundance of piscivorous perch increased in central, unvegetated areas of the lake. In stands of stoneworts, however, mainly benthivorous perch were caught, and size range did not change with time. 4. Our findings provide empirical support for the notion that establishment of submerged vegetation may lead to increased recruitment of piscivorous perch, because of improved competitive conditions for perch during the benthivorous stage. This is likely to constitute a benthic-pelagic feedback coupling, in which submerged vegetation and clear water promote the recruitment of piscivorous perch, which, in turn, may increase water clarity through top-down effects in the pelagic. [source] Recruitment and growth of two small-bodied resident fish species (Gobiidae and Atherinidae) in oligohaline, seasonally open lagoonsJOURNAL OF FISH BIOLOGY, Issue 6 2010P. G. Close Spatio-temporal recruitment patterns, growth and survival of the Swan River goby Pseudogobius olorum and western hardyhead Leptatherina wallacei are described from two small, coastal lagoons on the south coast of Western Australia. In these lagoons, estuarine salinity dynamics were relatively stable over much of the autumn,spring period when freshwater inputs from rivers were reduced and there was no oceanic connection. Preflexion and flexion stages of both fish species contributed strongly to population size structure in downstream reaches, whereas upstream reaches were dominated by postflexion larvae and juvenile stages. Spawning of both species was protracted and largely asynchronous, although the episodic presence of stronger preflexion and flexion cohorts suggested some synchronized spawning had occurred. Comparison with estuarine conditions over this period provided evidence that synchronized spawning may be related to temperature and salinity variations from a combination of freshwater inputs and periods of marine exchange. Uninterrupted growth and the progression of cohorts through to juvenile stages were consistent with the generally stable estuarine conditions. Larval and juvenile stages of both species were also tolerant of abrupt changes in salinity and temperature, which occurred due to a non-seasonal oceanic connection. These findings were consistent with the euryhaline nature of adults of both species. [source] Response of a gorgonian (Paramuricea clavata) population to mortality events: recovery or loss?AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 6 2008R. Cupido Abstract 1.During late summer 1999 and 2003 two mass mortality events affected the population of the slow growing, long-lived Mediterranean gorgonian Paramuricea clavata living in the Gulf of La Spezia (Italy). 2.The population was monitored for three years after the mortality events. Availability of pre-event data (1998) allowed comparison of population density and population size structure of the healthy population with those recorded in the three years following the mortality events. 3.In 1998, before the two mass mortality events, mean colony density was 33.3,±,3.7 colonies m,2 and had fallen to 6.7,±,1.9 colonies m,2 in 2004. 4.In the post-event period the population size structure changed and the modal class of colonies shifted from 16,21 cm to 6,15 cm height. 5.In 2004 mortality affected 75,±,6.4% of colonies. A significant, positive correlation between the extent of damage and colony size was found throughout the monitoring period. 6.In the three years following the two mortality events, a small increase in density of recruits and of older undamaged colonies was recorded suggesting that the population was slowly recovering. 7.The bathymetric distribution of P. clavata straddles the summer thermocline making this population particularly sensitive to temperature increases. The lack of deeper colonies (less exposed to warming) and the geographical isolation of this population is likely to prevent any substantial external larval supply. 8.An increased frequency of mass mortality events associated with ever increasingly high temperature events represent a considerable threat to the persistence of a P. clavata population in the Gulf of La Spezia. Copyright © 2007 John Wiley & Sons, Ltd. [source] Rhodolith bed diversity in the Gulf of California: the importance of rhodolith structure and consequences of disturbanceAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue S1 2003D.L. Steller Abstract 1.Rhodolith beds, unattached coralline reefs, support a high diversity and abundance of marine species from both hard and soft benthos. We used surveys in multiple shallow (3,20 m) beds in the Gulf of California to (1) examine seasonal patterns in associated floral and faunal diversity and abundance, (2) compare differences in faunal associations between rhodolith beds and adjacent sedimentary habitats, (3) examine the importance of complexity of rhodolith structure to community structure, and (4) estimate the impact of anthropogenic disturbance on rhodoliths and associated species. 2.Macroalgal richness was seasonal, and beds supported higher richness in winter (to 36 species) than summer (6,7 species), primarily due to foliose red algae. Strong seasonal variation in the abundance of dominant cover organisms was due to a shift from macroalgae and mat-forming colonial invertebrate species to microalgae. 3.The community in a rhodolith bed of high-density thalli (El Coyote average ,11000 thalli/ m,2) had higher richness (52 versus 30 species) and abundance of epibenthic and crypto- and in-faunal species compared with an adjacent sand community. Species diversity and abundance was particularly high for unique cryptofaunal organisms associated with rhodolith interstices. Cryptofauna reached average densities of 14.4 organisms/ cm,3 rhodolith, the majority of which were crustaceans, polychaetes and cnidarians along with rhodolith-specific chitons. 4.Results from sampling across a range of rhodolith morphs in the El Requeson bed (with lower average cryptofaunal densities of 2.3 organisms/ cm,3) revealed that the total organisms supported by a rhodolith significantly increased with both complexity (branching density) and space available (thallus volume). These data suggest that reducing the population size structure, structural complexity and cover of living rhodoliths could decrease species richness and abundance. 5.While disturbance is a natural feature of these free-living beds, increased anthropogenic disturbance from trawling, anchoring and changes in water quality can directly impact the bed community through substrate alteration. Commercial fishing threatens rhodolith beds in the Gulf of California by decreasing rhodolith size and increasing sedimentation and burial rates. In addition to reducing direct destruction, conservation efforts should also focus on decreasing practices that breakdown thalli. Copyright © 2003 John Wiley & Sons, Ltd. [source] | ||||||||||