Migration of green sturgeon, Acipenser medirostris, in the Sacramento River

Adult green sturgeon, Acipenser medirostris, were collected in San Pablo Bay, California, and surgically implanted with ultrasonic acoustic tags from 2004 to 2006. An array of automated acoustic monitors was maintained in the Sacramento River to record movements of these fish. We presumed movements to known spawning areas (based on previous green sturgeon egg collections) or areas with potential spawning habitat (characterized by substrate, flow, and temperature criteria) represented a “spawning migration.” Three separate annual “spawning migrations” were recorded involving 15 individuals. The majority of the Sacramento River migrants entered the system in the months of March and April. Two different patterns of “spawning migration” and out-migration were observed. Six individuals potentially spawned, over-summered and moved out of the river with the first fall flow event. This is believed to be the common behavior of the green sturgeon. Alternatively, nine individuals promptly moved out of the Sacramento River before 1 September, and any known flow or temperature cue. Some green sturgeon appeared to be impeded on their upstream movement by the 15 May closure of the Red Bluff Diversion Dam, and at least five passed under the dam gates during downstream migration. A delay in the closure of the Red Bluff Diversion Dam would likely allow upstream passage of spawning green sturgeon, further, the potential mortality affects of downstream passage beneath the Red Bluff Diversion Dam should be assessed. Specific protection should be also given to the large aggregation of green sturgeon located in the reach of the Sacramento River adjacent to the Glen Colusa Irrigation District pumping facility.     

The acute temperature tolerance of green sturgeon (Acipenser medirostris) and the effect of environmental salinity

We investigated the effect of environmental salinity on the upper thermal tolerance of green sturgeon (Acipenser medirostris), a threatened species whose natural habitat is vulnerable to temperature and salinity variation as a result of global climate change. Freshwater (FW)-reared sturgeon were gradually acclimated to salinities representing FW, estuary water (EST), or San Francisco Bay water (BAY) at 18 degrees C, and their critical thermal maximum (CTMax) was measured by increasing temperature 0.3 degrees C/min until branchial ventilation ceased. CTMax was 34.2+/-0.09 degrees C in EST-acclimated fish, with FW- and BAY-acclimated fish CTMax at 33.7+/-0.08 and 33.7+/-0.1 degrees C, respectively. Despite the higher CTMax in EST-acclimated fish, FW-acclimated sturgeon ventilation rate reached a peak that was 2 degrees C higher than EST- and BAY-acclimated groups and had a greater range of temperatures within which they exhibited normal ventilatory function as assessed by Q(10) calculation. The osmoregulatory consequences of exposure to near-lethal temperatures were assessed by measuring plasma osmolality and hematocrit, as well as white muscle, brain, and heart tissue water contents. Hematocrit was increased following CTMax exposure, most likely owing to the elevated metabolic demands of temperature increase, and plasma osmolality was significantly increased in EST- and BAY-acclimated fish, which was likely the result of a greater osmotic gradient across the gill as metabolism increased. To our knowledge, this represents the first evidence for an effect of salinity on the upper thermal tolerance of sturgeon, as well as the first investigation of the osmoregulatory consequences of exposure to near-lethal temperatures. J. Exp. Zool. 309A:477-483, 2008. (c) 2008 Wiley-Liss, Inc.     

Movements of green sturgeon, Acipenser medirostris, in the San Francisco Bay estuary, California

Synopsis The green sturgeon, Acipenser medirostris, is a long-lived, iteroparous, anadromous acipenserid that is native to the San Francisco Bay Estuary, California. Sub-adult and adult fish are oceanic, but enter the estuary during the spring and remain through autumn. Little is known about green sturgeon distribution within the estuary or what, if any, physical parameters influence their movements. We report the results of a telemetry study conducted between September 2001 and November 2002. Five sub-adult and one adult green sturgeon were captured by trammel net in the San Pablo Bay region of the estuary. We implanted depth-sensing, ultrasonic transmitters within the peritoneum of individuals and tracked them from a boat for 1 – 15 h per day over periods ranging from 1 to 12 days. Salinity, temperature, and dissolved oxygen profiles of the water column were measured hourly. Observed movements were categorized as either non-directional or directional. Non-directional movements, accounting for 63.4% of observations, were closely associated with the bottom, with individuals moving slowly while making frequent changes in direction and swim speed, or not moving at all. Directional movements consisted of continuous swimming in the top 20% of the water column while holding a steady course for extended periods. Four of the five sub-adult fish remained within the confines of San Pablo Bay for the duration of their tracking period. The remaining sub-adult moved over 45 km up-river into Suisun Bay before contact was lost. The adult fish exited the bay and entered the ocean 6 h after release near Tiburon, CA, a movement of approximately 10 km. The sub-adult fish typically remained at the shallower depths (<10 m) of the estuary, but there were no apparent preferences for temperature, salinity, or dissolved oxygen, with the fish moving widely and rapidly across the range of these physical parameters. Activity is believed to be independent of light level with no discernable crepuscular, nocturnal, or diurnal peaks in activity.     

Evidence of spawning by green sturgeon, Acipenser medirostris, in the upper Sacramento River, California

This study reports the only direct evidence of spawning of green sturgeon, Acipenser medirostris, in the upper Sacramento River, CA. Two green sturgeon eggs were collected with substrate mats immediately below Red Bluff Diversion Dam. One green sturgeon larva was collected with a larval net at Bend Bridge. We concluded that green sturgeon spawn in the upper Sacramento River, both above and below RBDD. Temperature ranges in the study area (10–15°C) are similar to conditions used in successful artificial rearing of green sturgeon and do not appear to be a limiting factor to successful spawning of green sturgeon; however, suitable habitat upstream of RBDD is inaccessible when dam gates are lowered.     

Time of day and water temperature modify the physiological stress response in green sturgeon,Acipenser medirostris

The effects of time of day and water temperature on the acute physiological stress response were investigated in young-of-the-year green sturgeon (Acipenser medirostris). The response to a 1-min air-emersion stressor was assessed during the day (08.00 h) and at night (20.00 h), as well as after acclimation to either 11 degrees C or 19 degrees C. Blood samples were collected prior to stress and at several times after exposure to the stressor, and plasma concentrations of cortisol, lactate, and glucose were determined. The magnitudes of cortisol (19.1 ng ml(-1) vs. 4.9 ng ml(-1)) and lactate (190.6 mg l(-1) vs. 166.7 mg l(-1)) were significantly higher in fish stressed at night when compared with the day. There were no significant differences in glucose levels between time periods. Although, acclimation temperature did not affect peak cortisol concentrations (56.7 and 50.3 ng ml(-1) at 11 degrees C and 19 degrees C, respectively), the duration of the response was significantly extended at 11 degrees C. Post-stressor lactate increases were similar between temperature groups, but at 11 degrees C post-stressor glucose levels were significantly increased through 6 h, suggesting stressor-induced glycogenolysis and gluconeogenesis or decreased glucose utilization. These data demonstrate that the physiological stress response in green sturgeon is modified by both time of day and temperature.     

Mitochondrial DNA variation, effective female population size and population history of the endangered Chinese sturgeon, Acipenser sinensis

The anadromous Chinese sturgeon (Acipenser sinensis), mainly endemic to the Yangtze River in China, is an endangered fish species. The natural population has declined since the Gezhouba Dam blocked its migratory route to the spawning grounds in 1981. In the near future, the completion of the Three Gorges Dam, the world's largest hydroelectric project, may further impact this species by altering the water flow of the Yangtze River. Little is currently known about the population genetic structure of the Chinese sturgeon. In this study, DNA sequence data were determined from the control region (D-loop) of the mitochondrial genome of adult sturgeons (n = 106) that were collected between 1995–2000. The molecular data were used to investigate genetic variation, effective female population size and population history of the Chinese sturgeon in the Yangtze River. Our results indicate that the reduction in abundance did not change genetic variation of the Chinese sturgeon, and that the population underwent an expansion in the past. AMOVA analysis indicated that 98.7% of the genetic variability occurred within each year's spawning populations, the year of collection had little influence on the diversity of annual temporary samples. The relative large effective female population size (N _ef) indicates that good potential exists for the recovery of this species in the future. Strikingly, the ratio of N _ef to the census female population size (N _f) is unusually high (0.77–0.93). This may be the result of a current bottleneck in the population of the Chinese sturgeon that is likely caused by human intervention. This revised version was published online in July 2006 with corrections to the Cover Date.     

Age/size effects on juvenile green sturgeon, Acipenser medirostris, oxygen consumption, growth, and osmoregulation in saline environments

The green sturgeon, Acipenser medirostris, is an anadromous species that migrates from freshwater (FW) to seawater (SW) relatively early in its life history, although the ages and sizes of juveniles at SW entry are not known. Developmental constraints of osmoregulatory organs may either prohibit (i.e., due to salinity tolerance limits) or minimize (i.e., due to substantial osmoregulatory or ionoregulatory energetic costs) SW entry in small fish. Interestingly, larger green sturgeon are often encountered in brackish water (BW) estuaries, perhaps due to an energetic advantage in occupying these near-isosmotic environments. To test hypotheses concerning fish-size effects on the energetic costs of occupying habitats of different salinities, we measured oxygen consumption rates in green sturgeon representing three age groups (100, 170, and 533 days post hatch; dph), which were acclimated for 5 weeks to one of three salinities (FW, <3‰; BW, 10‰; or SW, 33‰). Also, after 7 weeks, final wet masses were compared and blood and muscle tissue samples were taken to assess osmoregulatory abilities. There were no differences in body-mass-adjusted oxygen consumption rates between any salinities or ages, indicating that the energetic costs were not prohibitively high to occupy any of these salinities. The only mortalities occurred in the 100 dph SW group, where 23% of the fish died, from apparent starvation. Final wet masses were comparable between FW and BW for each age group and with the 533 dph SW group, but were lower in SW groups at 100 and 170 dph. Similarly, osmoregulatory abilities, in terms of plasma osmolality, Na⁺, K⁺, lactate, and protein concentrations, and muscle water content, were comparable in FW and BW groups at all ages, and with the SW group at 533 dph. These results indicated an age/body size effect in hyperosmotic adaptability, and that juvenile green sturgeon may be found in FW or BW at any age, but only have the ability to enter SW by 1.5 years (75 cm, 1.5 kg) of age.     

Inheritance of microsatellite loci in the polyploid lake sturgeon (Acipenser fulvescens).

Inheritance in the expression of amplicons for four microsatellite primer pairs was determined using 10 families created from gametes of wild lake sturgeon (Acipenser fulvescens). Loci Afu34 and Afu68 expressed a maximum of two even-intensity bands per individual and had progeny genotype ratios that fit disomic inheritance (P > 0.05). Some variation exhibited at Afu34 and Afu68 was attributable to a null allele. Genotype expression at both loci also indicated that one female parent had transmitted unreduced gametes. Primer Afu39 amplified products that exhibited four gene doses, where genotype counts fit expected ratios for disomic inheritance (P > 0.05) indicating amplification of products from two disomic loci that share alleles. Meiotic drive was evident at the Afu39 loci based on a test for random segregation (P < 0.05). Only the expression of Afu19 gave evidence of tetrasomic inheritance based on a single progeny potentially produced by a double reduction gamete. No evidence for proposed octoploid inheritance was observed.     

Behavior, movements, and habitat use of adult green sturgeon, Acipenser medirostris, in the upper Sacramento River

We conducted the first continuous shipboard tracking of southern Distinct Population Segment green sturgeon, Acipenser medirostris, in the Sacramento River. Tracking of adult green sturgeon occurred between river kilometer (rkm) 434.8 and 511.6, a section of the putative spawning grounds located near Red Bluff, California. The recorded positions of acoustically tagged green sturgeon were analyzed using First Passage Time analysis to determine differences in habitat use between suitable and non-suitable habitats. Classification and Regression Tree modeling was used to determine explanatory inputs attributable to above average habitat use. Green sturgeon exhibited above average habitat use at five sites, identified as potential spawning aggregate sites. Three types of movements (holding, milling, and directed) could be categorized from tracks. Lastly, we show that green sturgeon while on the spawning grounds exhibit a high degree of mobility throughout the spawning grounds, often making large movements between specific habitat units. Our study illustrates how the application of shipboard tracking can be useful for describing movement, behavior and habitat utilization at a spatial scale not achieved by stationary acoustic monitors.     

The effects of substrate composition on foraging behavior and growth rate of larval green sturgeon, Acipenser medirostris

We investigated the effects of substrate composition on foraging behavior and growth rate of␣larval green sturgeon, Acipenser medirostris, in the␣laboratory at 20±1°C over a period of 5 weeks. Larval groups (n = 100) with mean wet weight (0.72 ± 0.01 g) at 50 days post-hatch were reared on slate-rocks, cobble, sand or glass. Typically, fish were negatively rheotactic and exhibited dispersed skimming behaviors on provided substrates during pre-feeding and feeding, respectively, but were all positively rheotactic during feeding. Fish reared on slate-rock substrates were negatively phototactic, remained benthic, and aggregated underneath the substrates. In all substrates except slate-rocks, fish displayed frequent episodes of burst and glide swimming activity, tank wall skimming and vertical swimming behaviors, however these behaviors ceased immediately during feeding and reappeared at the end of the feeding period. Substrate composition led to variable foraging effectiveness and likely contributed to significant differences in specific growth rates (2.28, 1.14, 1.77, and 2.27% body weight per day) and mortality (7%, 40%, 11%, 0%) among the treatment groups; slate-rocks, cobble, sand, and glass, respectively. There were no significant differences in morphometrics, somatotopic indices, and whole-body lipid content among treatment groups at the end of the experiment. The present findings indicate that certain substrates in artificial/natural habitats may negatively affect larval growth and may lead to decreased recruitment of juvenile green sturgeon in the wild.     

Microsatellites assessment of Chinese sturgeon (Acipenser sinensis Gray) genetic variability

Four microsatellites were used to examine the genetic variability of the spawning stocks of Chinese sturgeon, Acipenser sinensis, from the Yangtze River sampled over a 3‐year period (1999–2001). Within 60 individuals, a total of 28 alleles were detected over four polymorphic microsatellite loci. The number of alleles per locus ranged from 4 to 15, with an average allele number of 7. The number of genotypes per locus ranged from 6 to 41. The genetic diversity of four microsatellite loci varied from 0.34 to 0.67, with an average value of 0.54. For the four microsatellite loci, the deviation from the Hardy–Weinberg equilibrium was mainly due to null alleles. The mean number of alleles per locus and the mean heterozygosity were lower than the average values known for anadromous fishes. Fish were clustered according to their microsatellite characteristics using an unsupervised ‘Artificial Neural Networks’ method entitled ‘Self‐organizing Map’. The results revealed no significant genetic differentiation considering genetic distance among samples collected during different years. Lack of heterogeneity among different annual groups of spawning stocks was explained by the complex age structure (from 8 to 27 years for males and 12 to 35 years for females) of Chinese sturgeon, leading to formulate an hypothesis about the maintenance of genetic diversity and stability in long‐lived animals.     

Short-term genetic changes: evaluating effective population size estimates in a comprehensively described brown trout (Salmo trutta) population

The effective population size (N(e)) is notoriously difficult to accurately estimate in wild populations as it is influenced by a number of parameters that are difficult to delineate in natural systems. The different methods that are used to estimate N(e) are affected variously by different processes at the population level, such as the life-history characteristics of the organism, gene flow, and population substructure, as well as by the frequency patterns of genetic markers used and the sampling design. Here, we compare N(e) estimates obtained by different genetic methods and from demographic data and elucidate how the estimates are affected by various factors in an exhaustively sampled and comprehensively described natural brown trout (Salmo trutta) system. In general, the methods yielded rather congruent estimates, and we ascribe that to the adequate genotyping and exhaustive sampling. Effects of violating the assumptions of the different methods were nevertheless apparent. In accordance with theoretical studies, skewed allele frequencies would underestimate temporal allele frequency changes and thereby upwardly bias N(e) if not accounted for. Overlapping generations and iteroparity would also upwardly bias N(e) when applied to temporal samples taken over short time spans. Gene flow from a genetically not very dissimilar source population decreases temporal allele frequency changes and thereby acts to increase estimates of N(e). Our study reiterates the importance of adequate sampling, quantification of life-history parameters and gene flow, and incorporating these data into the N(e) estimation.     

Migration and inbreeding: the importance of recipient population size for genetic management

Sewall Wright demonstrated 70 years ago thatthe number of migrants required to maintainspecified levels of gene flow (i.e. avoidexcessive inbreeding) is virtually independentof the size of the recipient population. According to conventional wisdom, this idea isvalid provided population size exceeds 20. Itis well known that this independence implicitlyassumes that a population's effective size(N _e) is equal to its census size(N). However, it is not obvious whetherindependence between the required number ofmigrants (to avoid excessive inbreeding) andpopulation size constitutes a reasonableassumption for real populations of conservationconcern. Relying on empirical data, wedemonstrate that for real populations, theassumption (i.e. N _e = N) isroutinely violated to a degree such that therequired number of migrants is stronglydependent on the size of the recipientpopulation. Because a population's effectivesize (N _e) is typically much smallerthan its census size (N), the number ofmigrants required to avoid inbreeding isactually dependent on N even when it isconsiderably greater than 20. For example,when N _e/N = 0.1, the number ofmigrants required to maintain the inbreedingcoefficient (F) at 0.2 doubles (from 4 to8) as N increases from 9 to 60. Similarly, when N _e/N = 0.05, thenumber of migrants required increases by 50%as N increases from 18 to 45, andincreases again by 50% as N increasesfrom 45 to 260. Thus, for populations muchlarger than 20, the required number of migrantsincreases asymptotically with N, anddramatically so when N _e/N1. Simple conventions regarding the requisitenumber of migrants may not apply to manypopulations of conservation concern. Geneticmanagement should routinely rely on models thatexplicitly account for this and other recentconsiderations. Failure to do so mayjeopardize the viability of populations thatare sensitive to altered levels of inbreeding.     

The ups and downs of genome size evolution in polyploid species of Nicotiana (Solanaceae)

BACKGROUND: In studies looking at individual polyploid species, the most common patterns of genomic change are that either genome size in the polyploid is additive (i.e. the sum of parental genome donors) or there is evidence of genome downsizing. Reports showing an increase in genome size are rare. In a large-scale analysis of 3008 species, genome downsizing was shown to be a widespread biological response to polyploidy. Polyploidy in the genus Nicotiana (Solanaceae) is common with approx. 40 % of the approx. 75 species being allotetraploid. Recent advances in understanding phylogenetic relationships of Nicotiana species and dating polyploid formation enable a temporal dimension to be added to the analysis of genome size evolution in these polyploids. METHODS: Genome sizes were measured in 18 species of Nicotiana (nine diploids and nine polyploids) ranging in age from <200,000 years to approx. 4.5 Myr old, to determine the direction and extent of genome size change following polyploidy. These data were combined with data from genomic in situ hybridization and increasing amounts of information on sequence composition in Nicotiana to provide insights into the molecular basis of genome size changes. KEY RESULTS AND CONCLUSIONS: By comparing the expected genome size of the polyploid (based on summing the genome size of species identified as either a parent or most closely related to the diploid progenitors) with the observed genome size, four polyploids showed genome downsizing and five showed increases. There was no discernable pattern in the direction of genome size change with age of polyploids, although with increasing age the amount of genome size change increased. In older polyploids (approx. 4.5 million years old) the increase in genome size was associated with loss of detectable genomic in situ hybridization signal, whereas some hybridization signal was still detected in species exhibiting genome downsizing. The possible significance of these results is discussed.     

Microsatellite DNA variation in Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) and cross-species amplification in the Acipenseridae

Overharvest and habitat alteration have led toa collapse of most commercial Atlantic sturgeon(Acipenser oxyrinchus oxyrinchus)fisheries while pushing the species to rarityor extirpation in most of its historical range. A biologically sound conservation program forthis species requires knowledge of its geneticdiversity and of the evolutionary relationshipsamong geographic populations. To address theseresearch needs, six microsatellite loci wereisolated from A. o. oxyrinchus. Pedigreeanalysis suggested that all are inherited in acodominant Mendelian pattern. The six lociwere tested in ten additional sturgeon speciesfrom three genera and three apparent ploidylevels (4n, 8n, 16n). Approximately 70% ofsuccessful locus-species amplifications werepolymorphic. Polysomy was observed most oftenin 8n and 16n species. Genetic diversity andpopulation structure of A. o. oxyrinchuswere assayed using three polymorphic Aoxmarkers and four markers developed from lakesturgeon (A. fulvescens). A. o.oxyrinchus were sampled from the AltamahaRiver, Georgia, USA north to the St. LawrenceRiver, Quebec, Canada. Gulf sturgeon, A.o. desotoi, were sampled from the SuwanneeRiver, Florida, USA, to assess differentiationbetween the subspecies. Seventy-seven alleleswere observed to segregate into uniquemultilocus genotypes for each of the 392individuals assayed. Mean diversity wasgreatest in the Chesapeake Bay (9.7 alleles perlocus) and Delaware River (7.4 alleles perlocus) collections, and lowest in the St.Lawrence River (4.6 alleles per locus). Meanheterozygosity across seven loci ranged from44.3% (St. Lawrence River) to 62.6% (Altamaha River). Significant allelic heterogeneity wasobserved in 82% of pairwise comparisons aswell as a global test (p < 0.0001) for A.o. oxyrinchus collections. Genetic distancesuggests the presence of at least sixsubpopulations in A. o. oxyrinchus: St.Lawrence River, St. John River, Hudson River,Delaware River, Albemarle Sound, and AltamahaRiver. Genetic and geographic distances werepositively correlated (r = 0.57, p < 0.03) amongA. o. oxyrinchus, suggesting isolation bydistance and philopatry. Hierarchical genediversity analysis indicated significantgenetic population structure at every level. Maximum likelihood assignment tests correctlyassigned individual fish to collection with ahigh rate of success (mean = 87.5%); this andother lines of evidence indicated that theChesapeake Bay collection represents a mixedpopulation of sub-adult sturgeon from northernand southern Atlantic coast populations. Population structure was correlated with thatsuggested by earlier mitochondrial (mt) DNAanalyses. Significant diversity was observedbetween two Canadian populations from whichonly a single mtDNA haplotype has beenreported.     

Genetic population structure and neighbourhood population size estimates of the caddisfly Plectrocnemia conspersa

1. We used both genetic and ecological methods to evaluate the role of history and the scale of colonisation in structuring populations of the caddisfly Plectrocnemia conspersa. There was no genetic differentiation between sites up to 20 km apart, despite population sizes suggesting that genetic drift could create substantial variation at this scale. 2. Genetic differentiation between populations separated by more than 20 km was greater than expected given the contrasting short‐range trend, and implied a neighbourhood population size that is implausibly small. Therefore, the evolutionary processes that affect the short‐range trend do not explain differentiation over greater distances. 3. At small scales (<20 km), relatively short flights by winged adults spread over a number of generations could account for the spread of genes. For instance, dispersing individuals could found small (often temporary) populations, which may then grow and exchange genes with larger and more permanent local populations, amplifying the effects of the initial gene flow. 4. Over larger scales (20–500 km), substantial gaps between regions containing suitable habitat patches could reduce the number of colonisation events. Genetic patterns at this scale may date from the time they were last colonised. Previous ecological studies have rarely examined the dynamics of aquatic insect populations over these larger geographical scales, yet these processes may be central to their persistence and spread.