Contemporary status of the North American paddlefish, Polyodon spathula

North American paddlefish, Polyodon spathula, were once abundant in most large rivers and tributaries of the Mississippi River basin, but numbers have declined dramatically in most areas during the past 100 years. Habitat destruction and river modification are the most obvious changes affecting their distribution and abundance. Although peripheral range has dwindled, paddlefish still occur over most of their historic range and are still found in 22 states. Populations are currently increasing in 3 states, stable in 14, declining in 2, unknown in 3, and extirpated in 4. Sport harvests presently occur in 14 states, however two states with traditionally important sport fisheries report decreased recruitment into the population and are planning more restrictive regulations. Commercial fisheries are reported in only six states. During the past 10 years, five states have removed paddlefish from their commercial list primarily because of declines in adult stocks due to overfishing or illegal fishing. Ten states are currently stocking paddlefish to supplement existing populations or to recover paddlefish populations in the periphery of its native range.     

Fish recruitment in a large,temperate floodplain: the importance of annual flooding,temperature and habitat complexity

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young‐of‐the‐year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature.     

Fish, climate and the Gulf Stream: the influence of abiotic factors on the recruitment success of cyprinid fishes in lowland rivers

1. Climatic effects are increasingly being recognised as an important factor causing inter‐annual variability in organism abundances in aquatic and terrestrial ecosystems. This study investigated the relationships between water temperature (cumulative degree‐days >12 °C), river discharge (cumulative discharge‐days above basal discharge rate), the position of the North Wall of the Gulf Stream (NWGS), and the 0+ growth (September mean length) and recruitment success (year‐class strength, YCS) of three species of cyprinid fishes in two contrasting English lowland rivers, using a 21‐year dataset. 2. Contrary to the majority of studies on 0+ fishes, growth in the Yorkshire Ouse was most significantly correlated with river discharge, with water temperature of less importance. By contrast, temperature was more influential than discharge in the River Trent, possibly because of its regulated hydrological regime, although none of the relationships were statistically significant for this river. 3. Year‐class strength of roach (Rutilus rutilus) was positively correlated with the position of the NWGS, and there was evidence of synchrony in recruitment success between rivers, but the relationships were poorer for chub (Leuciscus cephalus) and dace (Leuciscus leuciscus). The strongest relationships between YCS and discharge during specific time periods were for when the fish were in their early (especially larval) developmental stages, although none of the relationships were statistically significant because of inter‐annual variations in river discharge relative to the timing of fish hatching. 4. Fishes are key predators in the majority of aquatic ecosystems and, as such, fluctuations in their abundances can have implications for ecosystem functioning as a whole. This study has demonstrated an underlying influence of broad‐scale climatic effects on the recruitment of riverine fishes, in spite of local variations in biotic and abiotic conditions. The relative importance of various abiotic factors on the recruitment success of riverine cyprinid populations varies spatially and temporally. For example, river discharge is likely to be of relatively greater importance in poorly‐structured rivers or those that are prone to large and rapid fluctuations in flow, while temporal variations occur because of inter‐annual differences in river discharge relative to the timing of fish hatching. Biotic factors may also be important determinants of fish recruitment success, especially in rivers with stable and predictable flow regimes.     

The Influence of Discharge on Duration,Ascent Distance,and Fidelity of the Spawning Migration for Paddlefish of the Yellowstone-Sakakawea Stock,Montana and North Dakota,USA

Paddlefish, Polyodon spathula, of the Yellowstone-Sakakawea stock, Missouri and Yellowstone Rivers, Montana and North Dakota, were radio-tagged to assess the influence of spring discharge on duration of river residency, ascent distance, and site-fidelity during spawning migrations of 1999–2002. Contrary to expectations and reported results from other paddlefish populations, fish remained in the river for similar periods of time and ascended to similar reaches in years of higher, more sustained discharge and in years of lower, more fluctuating discharge. In all years, 65 of the 74 migrants (88%) restricted their ascent to reaches below Yellowstone River kilometer (YRkm) 55; only six migrants were found to further ascend to upriver reaches within 20 river kilometers (rkm) of the Intake Diversion Dam (YRkm 114). The lack of detectable annual differences in ascent distance over the study period despite annual differences in Yellowstone River spring flow regimes may have been partially attributed to the apparent site-fidelity demonstrated by the tagged fish over the study period. Ten of the 22 paddlefish contacted in more than one spring migration repeatedly limited their upriver movement to sites that were within 10 rkm of each other. In addition, similar to the reproductive homing tendencies documented in other large-river migratory fishes, site-fidelity occurred in different reaches of the river system. Results from this study suggest that, in years of moderate discharge, site-fidelity may be as influential as the spring flow regime in determining the reaches to which migratory paddlefish ascend. Further research is needed to investigate potential differential spawning success in fish that return to different reaches of the lower Yellowstone River.     

Characterization of Pallid Sturgeon (Scaphirhynchus albus) Spawning Habitat in the Lower Missouri River

Acipenseriformes (sturgeons and paddlefish) globally have declined throughout their range due to river fragmentation, habitat loss, overfishing, and degradation of water quality. In North America, pallid sturgeon (Scaphirhynchus albus) populations have experienced poor to no recruitment, or substantial levels of hybridization with the closely related shovelnose sturgeon (S. platorynchus). The Lower Missouri River is the only portion of the species’ range where successful reproduction and recruitment of genetically pure pallid sturgeon have been documented. This paper documents spawning habitat and behavior on the Lower Missouri River, which comprises over 1,300 km of unfragmented river habitat. The objective of this study was to determine spawning locations and describe habitat characteristics and environmental conditions (depth, water velocity, substrate, discharge, temperature, and turbidity) on the Lower Missouri River. We measured habitat characteristics for spawning events of ten telemetry-tagged female pallid sturgeon from 2008–2013 that occurred in discrete reaches distributed over hundreds of kilometers. These results show pallid sturgeon select deep and fast areas in or near the navigation channel along outside revetted banks for spawning. These habitats are deeper and faster than nearby river habitats within the surrounding river reach. Spawning patches have a mean depth of 6.6 m and a mean depth-averaged water-column velocity of 1.4 m per second. Substrates in spawning patches consist of coarse bank revetment, gravel, sand, and bedrock. Results indicate habitat used by pallid sturgeon for spawning is more common and widespread in the present-day channelized Lower Missouri River relative to the sparse and disperse coarse substrates available prior to channelization. Understanding the spawning habitats currently utilized on the Lower Missouri River and if they are functioning properly is important for improving habitat remediation measures aimed at increasing reproductive success. Recovery efforts for pallid sturgeon on the Missouri River, if successful, can provide guidance to sturgeon recovery on other river systems; particularly large, regulated, and channelized rivers.     

Biology, fisheries, and conservation of sturgeons and paddlefish in China

This paper reviews five of the eight species of acipenseriforms that occur in China, chiefly those of the Amur and Yangtze rivers. Kaluga Huso dauricus and Amur sturgeon Acipenser schrenckii are endemic to the Amur River. Both species still support fisheries, but stocks are declining due to overfishing. Acipenseriformes of the Yangtze River are primarily threatened by hydroelectric dams that block free passage to spawning and feeding areas. The Chinese paddlefish Psephurus gladius now is rare in the Yangtze River system, and its spawning activities were severely limited by completion of the Gezhouba Dam in 1981. Since 1988, only 3–10 adult paddlefishes per year have been found below the dam. Limited spawning still exists above the dam, but when the new Three Gorges Dam is complete, it will further threaten the paddlefish. Artificial propagation appears to be the only hope for preventing extinction of P. gladius, but it has yet to be successfully bred in captivity. Dabry's sturgeon A. dabryanus is a small, exclusively freshwater sturgeon found only in the Yangtze River system. It is concentrated today in reaches of the main stream above Gezhouba Dam. The fishery has been closed since 1983, but populations continue to decline. Acipenser dabryanus has been cultured since the 1970s, and holds promise for commercial aquaculture; availability of aquacultural methods offers hope for enhancing natural populations. The Chinese sturgeon A. sinensis occurs in the Yangtze and Pearl rivers and seas of east Asia. There is still disagreement about the taxonomy of the Pearl and Yangtze River populations. The Yangtze River population is anadromous. Adults begin spawning at about age 14 years (males) and 21 years (females), and adults spend over 15 months in the river for reproduction. Spawning sites of A. sinensis were found every year since 1982 below the Gezhouba Dam, but it seems that insufficient suitable ground is available for spawning. Since 1983, commercial fishing has been prohibited but more measures need to be taken such as establishing protected areas and characterizing critical spawning, summering and wintering habitats.     

Reproduction, Growth, and Migration of Fishes in a Regulated Lowland Tributary: Potential Recruitment to the River Meuse

Many European rivers are characterized by a canalized main channel, steep stony embankments, the absence of aquatic vegetation, regulated flow dynamics, reduced hydrological connectivity to the floodplains and a lack of spawning and nursery areas for many fish species. In such regulated rivers, tributaries may be particularly important for recruitment of fish populations in the main channel. This paper describes the reproduction, growth and migration of fishes in the Everlose Beek, a regulated lowland tributary stream of the river Meuse (The Netherlands), using bi-weekly sampling from January to December 2002. A total of 8615 fishes were caught, belonging to 13 different species. The fish species were classified into three groups, viz., residents, migrants and transients, based on the presence of various life-stages in the tributary. Size-frequency data suggest that each group uses the Everlose Beek differently: (i) Stone loach (Barbatula barbatula), Gudgeon (Gobio gobio) and Three-spined stickleback (Gasterosteus aculeatus) were resident species using the tributary as a spawning, nursery and adult habitat; (ii) Bream (Abramis brama), Roach (Rutilus rutilus), Rudd (R. erythrophthalmus), Tench (Tinca tinca), and Pike (Esox lucius) were migratory species, using the tributary as a spawning area, as well as a nursery habitat during their first year of growth, but migrating towards the river Meuse typically at a length of 5–15 cm; and (iii) Bleak (Alburnus alburnus), Sunbleak (Leucaspius delineatus), Carp (Cyprinus carpio), Crucian carp (Carassius carassius), and Perch (Perca _,fluviatilis) were transient species, characterized by an absence of reproduction, ^.and the occurrence in very low densities of >age-1 juveniles and adults only. Lowland tributaries, such as the Everlose Beek, can contribute to the recruitment of particularly migrant species, hence contributing to fish populations of the regulated river Meuse.     

Prey size,prey abundance,and temperature as correlates of growth in stream populations of cutthroat trout

Growth and maximum size of stream fishes can be highly variable across populations. For salmonid fishes in streams, individuals from populations confined to headwater streams often exhibit small size at maturity in comparison to populations with access to main-stem rivers. Differences in prey size, prey availability, and metabolic constraints based on temperature may explain patterns of maximum size and growth. In this study, cutthroat trout from headwater stream populations that were isolated above a waterfall were compared to individuals from populations in similar sized streams without a movement barrier and from large main-stem rivers. Cutthroat trout from smaller streams with or without a movement barrier were significantly smaller at a given age than fish from main-stem rivers, where individuals were able to achieve a much larger maximum size. Comparisons of invertebrate drift abundance and size in the three types of streams revealed that drift size did not differ between stream categories, but was highest per volume of water in large main-stem rivers. Across all stream types, prey abundance declined from summer to fall. Temperature declined over the course of the season in a similar manner across all stream types, but remained relatively high later in the season in main-stem river habitats. Prey availability and temperature conditions in main-stem rivers may provide more optimal growing conditions for fish as individuals increase in size and become constrained by prey availability and temperature conditions in small streams. Maintaining connectivity between small spawning and rearing tributary streams and main-stem river habitats may be critical in maintaining large-bodied populations of stream salmonids.     

River otter population size estimation using noninvasive latrine surveys

Across much of North America, river otter (Lontra canadensis) populations were extirpated or greatly reduced by the early 20th century. More recently, reintroductions have resulted in restored populations and the recommencement of managed trapping. Perhaps the best example of these river otter reintroductions occurred in Missouri, regarded as one of the most successful carnivore recovery programs in history. However, abundance estimates for river otter populations are difficult to obtain and often contentious when used to underpin management activities. We assessed the value of latrine site monitoring as a mechanism for quantifying river otter abundance. Analyses of fecal DNA to identify individual animals may result in an improved population estimate and have been used for a variety of mammal species. We optimized laboratory protocols, redesigned existing microsatellite primers, and calculated genotyping error rates to enhance genotyping success for a large quantity of river otter scat samples. We also developed a method for molecular sexing. We then extracted DNA from 1,421 scat samples and anal sac secretions (anal jelly) collected during latrine site counts along 22–34-km stretches representing 8–77% of 8 rivers in southern Missouri in 2009. Error rates were low for the redesigned microsatellites. We obtained genotypes at 7–10 microsatellite loci for 24% of samples, observing highest success for anal jelly samples (71%) and lowest for fresh samples (collected within 1 day of defecation). We identified 63 otters (41 M, 22 F) in the 8 rivers, ranging from 2 to 14 otters per river. Analyses using program CAPWIRE resulted in population estimates similar to the minimum genotyping estimate. Density estimates averaged 0.24 otters/km. We used linear regression to develop and contrast models predicting population size based on latrine site and scat count indices, which are easily collected in the field. Population size was best predicted by a combination of scats per latrine and latrines per kilometer. Our results provide methodological approaches to guide wildlife managers seeking to initiate similar river otter fecal genotyping studies, as well as to estimate and monitor river otter population sizes. © 2011 The Wildlife Society.     

Diet overlap among two Asian carp and three native fishes in backwater lakes on the Illinois and Mississippi rivers

Bighead and silver carp are well established in the Mississippi River basin following their accidental introduction in the 1980s. Referred to collectively as Asian carp, these species are filter feeders consuming phytoplankton and zooplankton. We examined diet overlap and electivity of Asian carp and three native filter feeding fishes, bigmouth buffalo, gizzard shad, and paddlefish, in backwater lakes of the Illinois and Mississippi rivers. Rotifers, Keratella spp., Brachionus spp., and Trichocerca spp., were the most common prey items consumed by Asian carp and gizzard shad, whereas crustacean zooplankton were the preferred prey of paddlefish. Bigmouth buffalo diet was broad, including both rotifers and crustacean zooplankton. Dietary overlap with Asian carp was greatest for gizzard shad followed by bigmouth buffalo, but we found little diet overlap for paddlefish. Diet similarity based on taxonomy correlated strongly with diet similarity based on size suggesting filtration efficiency influenced the overlap patterns we observed. Although rotifers were the most common prey item consumed by both bighead and silver carp, we found a negative relation between silver carp CPUE and cladoceran density. The competitive effect of Asian carp on native fishes may be forestalled because of the high productivity of Illinois and Mississippi river habitats, yet the potential for negative consequences of Asian carp in less productive ecosystems, including Lake Michigan, should not be underestimated.     

Population,density estimates,and conservation of river dolphins (Inia and Sotalia) in the Amazon and Orinoco river basins

This study is part of an on‐going effort to evaluate and monitor river dolphin populations in South America. It comprises the largest initiative to estimate population size and densities of Inia and Sotalia dolphins using statistically robust and standardized methods. From May 2006 to August 2007, seven visual surveys were conducted in selected large rivers of Bolivia, Colombia, Brazil, Ecuador, Peru, and Venezuela in the Amazon and Orinoco river basins. Population sizes of Inia and Sotalia were estimated for different habitats (main river, tributary, lake, island, confluence, and channel). A total of 291 line and 890 strip transects were conducted, covering a distance of 2,704 linear kilometers. We observed 778 Inia geoffrensis, 1,323 Inia boliviensis, and 764 Sotalia fluviatilis. High‐density areas were identified (within 200 m from the river banks, confluences, and lakes) and we propose that these constitute critical habitat for river dolphins. High densities of river dolphins seem to coincide with well‐managed freshwater protected areas and should be considered as hot spots for river dolphins in South America.     

Energy acquisition and retention by age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to size,growth, and rearing conditions in two Great Plains reservoirs and hatchery ponds

The objective of this study was to investigate lipid accumulation and storage in age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to age, stock, year, and growth. Juvenile paddlefish were collected from three locations in North Dakota and Montana, USA, during July and August of 2011 and 2012 and proximate analysis was used to determine lipid content. RNA/DNA ratios were used as an index of growth rates. Differences in age‐based lipid accumulation and storage in juvenile paddlefish suggest a split allocation between growth and lipid storage, with growth being the highest initial priority and emphasis on energy storage occurring at a larger size, later in life. Differences in lipid allocation between stocks indicate that allocation is influenced by hatchery/wild rearing conditions. Differences within and between year‐classes are consistent with field evidence observed in 2012 of a strong 2011 year‐class, and indicate that during productive times, paddlefish may allocate energy to both body growth and lipid reserves, and that allocation differs among years. The lack of a relationship between RNA/DNA ratio and lipid does not support a physiologically exclusive allocation strategy between growth and lipid. Evidence from this and other studies suggests rather that an emphasis on growth, some energy storage, and a large rostrum size in relation to overall fish length in age‐0 and age‐1 fish, may be adaptive in avoiding predation while accruing necessary energy reserves for overwintering. Although this study also provides reference information regarding proximate composition of wild and hatchery origin juvenile paddlefish, much more study is needed into the relationships among growth, low and high lipid groups, lipid allocation in juvenile paddlefish as well as the existence and timing of allocation changes between growth and storage. To aid in understanding paddlefish survival and year‐class strengths, these relationships also need to be linked to inter‐annual differences in early rearing environments for age‐0 and age‐1 fish.     

Impact of habitat fragmentation on genetic population structure of roach, Rutilus rutilus, in a riparian ecosystem

During the last 200 years, the riparianecosystem along major rivers has been reducedto a few scattered remnants. Important elementsof the riparian ecosystem are water bodieswhich were originally connected to the mainriver channel by annual floodings. Due to riverregulations many of these remnants are nowvirtually isolated. In an allozyme analysisusing roach, Rutilus rutilus, as a studyspecies we demonstrate that the geneticdiversity (number of alleles per locus,expected heterozygosity) of populations livingwithin floodplain water bodies is not severelyimpoverished compared to the genetic diversitywithin the main river channel. However, wefound slight differences in the allelefrequencies of flood plain water bodies and themain river channel. Nevertheless, fishpopulations in floodplain water bodies mayserve as reservoirs of autochthonous geneticmaterial for restoration of fish populations inthe main river channel after populationextinction due to catastrophic accidents (e.g.industrial pollution).     

Restoration of the Kissimmee River: A Conceptual Model of Past and Present Fish Communities and Its Consequences for Evaluating Restoration Success

Efforts to restore fish communities of the Kissimmee River will require carefully defined criteria for assessing success. A goal of regaining communities mirroring those in the historical river may not be an appropriate target because the ecological conditions of the river before channelization are poorly known. The Kissimmee River is in a biogeographic region historically low in fish diversity, and no comparable rivers in that region remain substantially unaltered by human activity to permit their use as reference sites indicative of conditions in the Kissimmee before channelization. I propose alternative criteria for assessing restoration success emphasizing expectations for ecosystem function in similar floodplain rivers. Assessing ecosystem function will be less simple than assessing criteria such as fish condition or density of selected species. But criteria based solely on fish-population characteristics cannot be justified quantitatively. Information integrated from several levels of biotic organization (individuals, populations, communities, and systems) should be drawn upon in making conclusions about restoration success. I develop a conceptual model to outline aspects of ecosystem function that could serve as a basis for evaluation of the restoration of fish communities of the Kissimmee River. The model focuses on the dynamics of the flux of floodplain-channel nutrients and the movement of larvae, juvenile, and adult fishes and macroinvertebrates. The present community may be dominated more by species tolerant of low-oxygen conditions, such as gar and bowfin, than the restored community will be. I propose that nest sites may be the limiting recruitment success of substrate spawning species in the channelized river and that these species, including sunfish and large-mouth bass, will increase in abundance after restoration. Also, species relying on floodplain habitats, including sun-fish species, darters, and some minnows, may also increase in frequency with restoration of floodplain-channel hydro-logical conditions and habitats. The observation that no species are known to have disappeared from the Kissimmee River, and its relatively simple community structure compared to rivers of comparable size elsewhere, are encouraging for prospects of successful restoration.     

Low river flow alters the biomass and population structure of a riparian predatory invertebrate

1. Low flows in rivers are predicted to increase in extent and severity in many areas in the future, yet the consequent impacts of river drying on terrestrial communities via (i) changes to riparian microclimatic conditions and (ii) the identity and abundance of emerging aquatic insects available to riparian predators have not been quantified. 2. We investigated the influence of low river flow on a riparian fishing spider, Dolomedes aquaticus, in five New Zealand rivers containing permanently flowing and drying reaches and, in one river, along a longitudinal drying gradient. 3. The biomass of aquatic insects, potential prey for D. aquaticus, declined with low river flows while the abundance of potential terrestrial prey remained similar at all sites. In the replicate rivers, and along the longitudinal drying gradient, spider biomass was lower, and size classes were skewed towards more small and fewer large spiders, in drying sites. A desiccation experiment in the laboratory indicated high sensitivity of the spiders, with prey presence increasing spider survival. 4. Differences in the spatial distribution, biomass and population size structure of spiders were observed along the longitudinal drying gradient and disappeared within 16 days of the water returning to all sites. 5. In total, low river flow affected the biomass of D. aquaticus, as well as their size class structure and spatial distribution. This indicates that low river flows have the potential to affect adjacent terrestrial ecosystems.     

Ecology and biology of the lake sturgeon: a synthesis of current knowledge of a threatened North American <Emphasis Type="Italic">Acipenseridae</Emphasis>

The lake sturgeon is one of the largest North American freshwater fish and was once common in most inland rivers and lakes of the US and Canadian Midwest. World demand for caviar and sturgeon meat led to a dramatic decline of lake sturgeon populations throughout much of its range. Along with overfishing, lake sturgeon populations have been negatively affected by habitat degradation. Recruitment factors and early life history are poorly understood. Today, renewed interest in lake sturgeon restoration has led to numerous state and federally-funded research activities. Research has focused on identifying and assessing the size structure of remnant stocks, the availability of spawning habitat, and factors affecting reproductive success. Additional studies are needed to improve hatchery techniques, to better understand recruitment mechanisms, and how genetic diversity among and within meta-populations may affect long-term recovery of depleted populations.     

An approach for assessing paddlefish Polyodon spathula (Walbaum, 1792) populations using mark‐recapture information

Historically, management of fish populations has been achieved through the use of age‐derived estimates of growth and mortality. For long‐lived species such as the paddlefish, Polyodon spathula, the validation of calcified structures is necessary to correct for the presence of false annuli or the absence of growth rings. Regardless, numerous studies on paddlefish populations throughout their range have continued the use of un‐validated age estimates to evaluate dynamic rate functions. The use of mark‐recapture studies has been applied widely to evaluate growth of short‐lived fishes, but only recently to a few long‐lived freshwater fishes (i.e. white sturgeon, shovelnose sturgeon, and pallid sturgeon). This study provides the first simultaneous evaluation of both mark‐recapture and age‐estimate information in determining population characteristics for paddlefish. In doing so, this study has determined that the P. spathula population in the Black River below Clearwater Dam, Missouri is sustainable. Additionally, mark‐recapture information is sufficient to produce accurate and reliable assessments of paddlefish populations in lieu of validated aging structures; future management should be centered on accurate scientific methods, which is not the case when using un‐validated aging structures (e.g. scales, otoliths, fin rays, dentary bones) to determine population parameters. Mark‐recapture information can provide an accurate, alternative source of growth and mortality information for use in evaluating and managing paddlefish populations throughout their range.     

Vital statistics of the baobab (Adansonia digitata)

Five populations of baobab trees in Zambia, Sudan, Mali, Kenya and Tanzania were measured to establish distribution of girth size classes. Absolute growth rates from rings or from empirical observations in each area were used to convert size into age classes. Baobab populations appear to be much younger than has generally been believed and only very few trees live to ages in excess of 400 years. Instantaneous mortality rates vary from 1 1 to 3–7% per year in the different areas. A lack of recruitment to the youngest age classes is caused by elephants in some areas but land-use changes due to increasing human populations and increased domestic animal numbers may also be responsible for low recruitment rates in other areas.     

Adult paddlefish migrations in relation to spring river conditions of the Yellowstone and Missouri rivers, Montana and North Dakota, USA

Migrations and movements of paddlefish (Polyodon spathula) into the regulated Missouri River and the largely unregulated Yellowstone River, North Dakota and Montana, USA were monitored with radio‐telemetry to assess if (i) differential discharge between the Yellowstone River and the Missouri River influenced river selection, (ii) river conditions influenced directional movements in the Yellowstone River and (iii) inter‐annual and sex‐related migration patterns existed. In 2003 and 2004, telemetered upriver migrants selected the Yellowstone River rather than the Missouri River above its confluence with the Yellowstone River 34 of 54 times (63%). Fish typically ascended the river with flows that were increasing at a greater rate or decreasing at a lesser rate than the other river. Most (70%) upriver movements occurred when discharge and suspended sediment were increasing. Most (80%) downriver movements occurred when flows and suspended sediment were decreasing. Total migration distance was greater in 2004 than in 2003 for both sexes despite greater peak discharge in 2003. Movement of pre‐spawn paddlefish into the Missouri River above its confluence with the Yellowstone River in response to attraction flows may have implications towards this stock’s reproductive success. Accordingly, these implications should be taken into account when regulating spring Missouri River discharge levels upriver at Fort Peck Dam.     

Endangered ecosystems: a review of the conservation status of tropical Asian rivers

Tropical Asian rivers are characterized by their flow seasonality. One (sometimes two) peaks in discharge cause temporary declines in phytoplankton, zooplankton and zoobenthos biomass, but lead to inundation of river floodplains and significant land-water interactions. Fishes undertake lateral or longitudinal breeding migrations within the river system during the flood season, which is marked also by intensive feeding upon allochthonous inputs.Among the diverse human influences upon tropical Asian rivers, three threats stand out. Firstly, degradation of drainage basins (particularly through deforestation and overgrazing) leads to increased suspended sediment loads and extensive flooding. Excessive floodplain siltation alters habitats causing species decline or disappearance. The second threat — river regulation and control — has been practised widely in the region for centuries but, with the planned development of massive projects on the Yangtze and Mekong Rivers, the potential for environmental damage has increased. Flow regulation reduces flood-season peaks, changing the magnitude and extent of floodplain inundation and land-water interactions. Fish breeding migrations may be disrupted, because dams block migration routes or changed flow regimes fail to stimulate reproduction. River pollution is pervasive throughout the region, and constitutes the third threat. Untreated sewage is a particular problem in densely-populated areas, and pollution by industrial effluents and mining wastes is becoming more important. The effects of pollution in tropical Asian rivers are essentially the same as those recorded in north-temperate regions. However, biological understanding has yet to be matched by an ability to halt or limit river degradation.Together, the three threats have led to declines and range constrictions of aquatic animals and those terrestrial species associated with riparian corridors and floodplains. River dolphins and certain crocodilians are particularly threatened, but declines in species of waterfowl, floodplain deer, a host of fishes, macrophytes, and invertebrates have been documented. Reversing the trend is difficult as pollution, flow regulation, and drainage-basin degradation have non-additive detrimental effects on river ecosystems, and enhance the success of exotic invasive species. Moreover, our ability to predict the outcome of man-made changes is hampered by a lack of knowledge of species' life histories and a paucity of data on the trophic basis of production. Despite a lack of detailed information, conservation of tropical Asian rivers will be effected only if limnologists move beyond the bailiwick of science. Ecologically viable management strategies for tropical Asian rivers will succeed only if the socioeconomic context of development plans is taken into account. A failure to rise this challenge will result in the further degradation of these endangered ecosystems.