Cohort‐specific estimates of first‐year survival are positively associated with size at stocking for lake sturgeon Acipenser fulvescens (Rafinesque 1817) stocked in Black Lake,Michigan, USA

The authors conducted a gillnet survey in 2013 in Black Lake, Michigan, USA to evaluate the lake sturgeon (Acipenser fulvescens) stocking programme that began in 2001. Objectives were to (i) estimate year‐class specific abundance of juvenile lake sturgeon in Black Lake; and (ii) determine year‐class specific survival of stocked year classes and determine whether year‐class‐specific first‐year survival was related to average size at the time of stocking. Deployed were 15 and 20 cm stretch mesh gillnets at 72 randomly selected sites in Black Lake over a 3‐week survey using a Schnabel multiple‐mark, multiple‐recapture estimator to determine overall abundance of stocked fish. Ages for captured fish were determined from fin ray cross sections and the presence of coded wire tags, and apportioned the overall abundance estimate of juveniles to year class using an age‐length key. Overall survival estimates were calculated by dividing the year‐class specific abundance estimates by the number of fish stocked that year. Also evaluated was the relationship between first‐year survival and average total length (TL) at time of stocking using logistic regression. Overall survival from stocking to 2013 ranged from 0.03 to 0.53. First‐year survival was positively associated with average TL at stocking, and ranged from 0.05 for fish stocked at 9 cm TL to 0.84 for fish stocked at 22 cm TL. Estimation of future cohort‐specific abundance based on size‐based expected survival allows managers to establish annual stocking targets that should lead to the achievement of long‐term population goals for adult abundance.     

A bioenergetic assessment of the influence of stocking practices on rainbow trout (Oncorhynchus mykiss) growth and consumption in a New Zealand lake

Summary

• 1 To investigate the carrying capacity and factors affecting growth of rainbow trout in Lake Rotoiti, we employed a bioenergetics model to assess the influence of stocking rates, timing of releases and prey abundance on growth and prey consumption. We hypothesised that stocking rates and prey abundance would affect growth and prey consumption by influencing per‐capita prey availability, and that the environmental conditions encountered by fish at the time of stocking would affect growth and consumption.
• 2 Prey consumption of stocked rainbow trout was calculated with the Wisconsin bioenergetics model. We calculated growth trajectories of released trout based on data from stocked trout that were released in spring and autumn from 1993 to 2009 and then re‐captured by anglers. Diet, prey energy density, body mass lost during spawning and lake temperature were measured locally.
• 3 Stocking timing had no effect on return rates to anglers or length or weight of caught fish. Although trout released in autumn were smaller than those released in spring, autumn‐released trout grew at a faster rate and had similar lengths and weights to spring cohorts after 2 years of growth in the lake. Modelled consumption parameters were negatively correlated with trout population size, suggesting that stocking rates (347–809 fish ha^?1 year^?1) caused density‐dependent effects on growth. Although common smelt (Retropinna retropinna) accounted for 85% of total prey consumption, no significant relationship was found between prey consumption by individual trout and adult smelt abundance, possibly because trout are targeting smaller smelt that our abundance estimate did not account for.
• 4 Releasing trout in autumn appears to be advantageous for growth, possibly because (i) temperature is more suitable for growth in autumn–winter than in spring–summer and (ii) prey for small trout is abundant in autumn. Mild winter conditions appear to enhance overwinter survival and growth of rainbow trout in warm‐temperate lakes compared to higher latitudes. This implies that moderately productive warm‐temperate lake ecosystems are highly suitable for trout growth in winter, but less so in summer, when lake stratification and high nutrient levels may create conditions suitable for algal blooms and hypolimnetic deoxygenation. High growth rates of trout in warm‐temperate lakes can therefore be supported by timing releases to coincide with favourable winter conditions.

     

Seasonal inter‐calibration between acoustic and multi‐mesh gillnets sampling for fish biomass assessment in reservoirs

This paper presents the results of an inter‐calibration between acoustic and gillnets sampling in two North African reservoirs according to seasons. Gillnets with multi‐mesh were designed for sampling fish in lakes while acoustic surveys were performed with a split beam Simrad EK60 echosounder. Sampling events were carried out during summer (September 2015), autumn (December 2015), winter (March 2016) and spring (June 2016) in two Tunisian reservoirs (Kasseb and Siliana) with different depths and shapes. Gillnet catches showed a high proportion of barbell whatever the seasons in Kasseb Reservoir, while at Siliana Reservoir, significant seasonal changes in relative abundances have been evidenced. The highest fish biomass of the entire water column was observed in winter daytime (103 kg/ha) in Kasseb Reservoir and in summer daytime (283 kg/ha) in Siliana Reservoir. Average biomass observed in autumn (December) for the two reservoirs were lower than the other seasons. During spring, density daytime values in Kasseb were higher than during nighttime while it was the opposite in summer and autumn. Fish densities detected in Kasseb Reservoir in vertical beaming was higher than those detected in horizontal beaming (p < .05). The longitudinal distribution of fish in the reservoirs showed that there is no clear trend in fish densities according to strata. High numbers of fish were detected in deep strata and big fish were located in the surface water near the dam of Kasseb Reservoir. A significant linear correlation was showed between acoustic density/NPUE and acoustic biomass/BPUE but the perfect correlation with the 1:1 fit was showed only between acoustic biomass/BPUE.     

Movement and early survival of age‐0 brown trout

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Optimal stocking density for beluga,Huso huso,and ship sturgeon,Acipenser nudiventris during the grow‐out phase

The influence of three different initial stocking densities (SD) in flowthrough systems was evaluated on growth performance and feed utilizaition in beluga, Huso huso, and ship sturgeon, Acipenser nudiventris, juveniles in three different grow‐out phases for 228 days. In each grow‐out phase, fish were randomly distributed in 18 concrete square tanks (2.0 × 1.0 × 1.2 m³; 2 m³ of volume) according to the experimental SD. At the first phase, both fish species were subjected to three SD including 1.5, 3 and 6 kg/m² for 60 days. In the second phase, fish were submitted to three SD including 3, 6 and 9 kg/m², whereas in the third phase, beluga were stocked at 6, 9 and 12 kg/m² and ship sturgeon were stocked at 6, 8 and 10 kg/m² and each phase lasted for 12 weeks. In both species, the growth performance and feed intake (FI) significantly decreased with increasing SD at the end of the first phase (p < 0.05). In the second phase, growth performance and FI in beluga decreased with increasing SD, whereas in ship sturgeon neither growth nor FI were affected by SD. Interestingly, the growth performance and feed utilization in both species were not decreased with increasing SD in the third phase indicating adaptability of these species to high stocking density during grow‐out period. According to the result of this study, the appropriate initial SD for beluga within the range of 100–500, 500–2,000 and 1,500–3,000 g were at 1.5, 6 and 12 kg/m², respectively in an open flow‐throw system. Regarding to ship sturgeon, initial SD at 1.5 and 10 kg/m²recommended for fish within the range of 100–300 and 300–1,200 g, respectively in an open flow‐throw system.     

Post‐stocking pallid sturgeon Scaphirhynchus albus growth,dispersal, and survival in the lower Missouri River

Hatchery augmentation has been used to mitigate declines in fish populations worldwide, especially for sturgeon species. Information regarding stocking success including survival, dispersal, and growth of sturgeon post‐stocking may refine sturgeon augmentation programs and facilitate recovery. Pallid sturgeon Scaphirhynchus albus populations have been supplemented by hatchery‐reared stocks for 25 years in the Missouri River, USA. We assessed survival, dispersal patterns, and growth characteristics of post‐stocked pallid sturgeon in the lower Missouri River. Hatchery‐reared pallid sturgeon stocked at age‐1 (4.1%) and > age‐1 (2.9%) were recaptured at a higher frequency than fish stocked at age‐0 (0.3%). Post‐stocking dispersal patterns suggested dispersal range increase as age increased, but individuals tended to remain in the same river segment as their stocking location. Growth rates varied by year class with younger year classes having truncated growth trajectories compared to older year classes. Post‐stock survival of pallid sturgeon varied by age‐at‐stocking and suggest age‐1 survival has declined through time. Augmentation of pallid sturgeon may benefit from considering dispersal from stocking location and by stocking older individuals which appear to have increased survival post‐stocking. A better understanding regarding environmental drivers of growth and specific habitat features used is needed to better predict optimal timing and location of future stockings.     

Timing of predation by rainbow trout controls Daphnia demography and the trophic status of a Minnesota lake

1. Stocking of lakes with rainbow trout is a common practice that presents a potential conflict for lake managers who must balance the interests of anglers with those concerned that zooplanktivory by trout may trigger a trophic cascade and result in decreased water clarity. 2. This study examined how the timing of trout stocking (autumn versus spring) in a Minnesota (U.S.A.) lake affected (i) the population dynamics of their zooplankton food supply (Daphnia pulicaria), (ii) phytoplankton biomass and water clarity and (iii) trout survival. Sizes of both Daphnia and trout populations were estimated acoustically with high‐frequency (192 kHz) sonar. 3. Daphnia were nearly eliminated from the lake during winters after trout were stocked in autumn. In both of these years (1996 and 1997), the Daphnia population was small in the spring, and grew during the summer and into the autumn as the trout population diminished. 4. The lake was then stocked in spring for 2 years (1998 and 1999). This fisheries manipulation alleviated predation over the winter, but increased predation on D. pulicaria during the spring, summer and autumn. However, the high mortality caused by the spring‐stocked trout was offset by even higher rates of reproduction by the relatively large populations of fecund Daphnia that survived the winter in 1998 and 1999. 5. Grazing by these dense populations of Daphnia produced clear‐water phases during May and June that were inhibited in autumn stocking years. In addition, the large Daphnia populations present during the spring and early summer of 1998 and 1999 provided abundant forage for trout. 6. This fisheries manipulation achieved seemingly mutually exclusive management objectives: a robust planktivorous sport fishery, and clear water for other forms of recreation.     

Growth and contribution of stocked channel catfish,Ictalurus punctatus (Rafinesque, 1818): the importance of measuring post‐stocking performance

In this study it was sought to quantify post‐stocking growth, survival, and contribution of advanced size (178 mm total length [TL]) channel catfish Ictalurus punctatus fingerlings, something rarely done. Channel catfish populations were evaluated before (May 2010) and after (May to August 2011 and 2012) stocking. Relative abundance, stocking contribution, and growth were different (P < 0.05) in the two study impoundments (lakes Lone Chimney and Greenleaf, Oklahoma). For fish stocked in Lake Lone Chimney, stocking contribution was lower (3–35%), and average length and weight of stocked fish by age‐2 reached 230 mm TL and 85 g, whereas the stocking contribution (84–98%) and growth in length (340 mm TL) and weight (280 g) were higher by age‐2 in Lake Greenleaf. Given these unambiguous differences of post‐stocking performance, benchmark metrics that represent population‐level information such as relative abundance and average length and weight of the sample masked these significant differences, highlighting the importance of marking hatchery‐fish and then following them through time to determine the effectiveness of stocking. These results suggest that stock enhancement programmes would benefit from studies that quantify post‐stocking performance of hatchery fish.     

The importance of aquaculture to nase Chondrostoma nasus conservation in Poland

Nase Chondrostoma nasus L. is one of the most threatened reophilic fish species in Poland. At present it has almost disappeared from the northern and central parts of its range while in the south its abundance has decreased. Protection of the species through closed seasons, catch and size limits has not been sufficient, and active enhancement through stocking is now applied. Developing the technology of hormonally induced spawning and adapting carp pond culture techniques for the rearing of nase enabled substantial enhancement efforts. According to Polish Anglers Association, stocking has increased from 50 000 of hatchlings in 1994 to 840 000 of hatchlings, 568 000 summer and 900 000 autumn juveniles, and 1720 two year‐old fish in 2002. At present the problems to be solved urgently are preserving the genetic diversity of cultured and wild populations, and evaluating stocking effectiveness.     

Distribution of common carp in a Spanish reservoir in relation to thermal loading from a nuclear power plant

Arrocampo Reservoir is used as a cooling basin for a nuclear power plant located in southern Europe. Its annual mean temperature is about 30 °C with maxims that achieved 41.5 °C near the hot water effluent. Common carp (Cyprinus carpio, Linnaeus 1758) is the dominant pelagic fish species in this reservoir. Hydroacoustic surveys were conducted bimonthly over 9 years to characterize common carp abundance and distribution in the reservoir on an annual and seasonal temporal scale. Mean fish density during the period was 0.029 fish m⁻³, varying from a maximum of 0.038 in 2003 to a minimum of 0.012 in 1998. There were no significant differences in the mean fish density among years. Each year, fish distribution showed significant seasonal variation. Carp were homogeneously distributed during autumn and spring; in winter carp occupied warmer outfall areas, whereas in summer, fish were distributed in the coldest area of the reservoir (dam area) but avoided the warm outfall area. These seasonal tendencies of carp distribution was repeated every years. Behavioural reaction appears to be particularly important in explain carp distribution in this reservoir.     

Patterns of lake occupancy by fish indicate different adaptations to life in a harsh Arctic environment

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Larger temperature response of autumn leaf senescence than spring leaf‐out phenology

Climate warming is substantially shifting the leaf phenological events of plants, and thereby impacting on their individual fitness and also on the structure and functioning of ecosystems. Previous studies have largely focused on the climate impact on spring phenology, and to date the processes underlying leaf senescence and their associated environmental drivers remain poorly understood. In this study, experiments with temperature gradients imposed during the summer and autumn were conducted on saplings of European beech to explore the temperature responses of leaf senescence. An additional warming experiment during winter enabled us to assess the differences in temperature responses of spring leaf‐out and autumn leaf senescence. We found that warming significantly delayed the dates of leaf senescence both during summer and autumn warming, with similar temperature sensitivities (6–8 days delay per °C warming), suggesting that, in the absence of water and nutrient limitation, temperature may be a dominant factor controlling the leaf senescence in European beech. Interestingly, we found a significantly larger temperature response of autumn leaf senescence than of spring leaf‐out. This suggests a possible larger contribution of delays in autumn senescence, than of the advancement in spring leaf‐out, to extending the growing season under future warmer conditions.     

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

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Direct and indirect effects of predation on the genetic structure of a Daphnia population

The effects of fish predation on zooplankton communities arewell documented, but relatively little is known about how predationstructures the genetic composition of individual populations.This study illustrates how a perturbation in the timing andstrength of predation by rainbow trout directly and indirectlyaltered the genetic composition of a Daphnia pulicaria populationin a Minnesota (USA) lake. Trout were stocked in autumn in thefirst 2 years of the study and in spring in the second 2 years.In autumn stocking years, predation was highest over winterand in spring but relatively low during summer stratification.In contrast, in spring stocking years, predation was low overwinter and high during summer stratification. In all years,the Daphnia population became genetically differentiated withrespect to depth, as summer stratification became more pronounced.In addition, allozyme analyses of trout stomach contents revealedselective predation on Daphnia in the metalimnion. In springstocking years, when trout were abundant during the summer,this directly caused a shift in the dominant clone type froma metalimnetic to a hypolimnetic specialist. The fisheries manipulationindirectly affected the genetic composition of the Daphnia populationby altering the importance of recruitment from diapausing embryosin the sediment (the egg bank). In autumn stocking years, whenthe over-wintering population was small, genotype frequenciesin early summer indicated the recent emergence of sexually derivedindividuals from the egg bank. Conversely, in spring stockingyears when over-wintering populations of Daphnia were large,no emergence events were detected.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Consumption of crucian carp (Carassius carassius L., 1758) by restocked pike (Esox lucius L., 1758) in a lake with frequent winter hypoxia

To assess the effects of stocked pike (Esox lucius L.) on crucian carp (Carassius carassius L.) biomasses, the annual consumption of pike was estimated and compared with removal fishery catches. The studied lake, Lake Savijärvi, is a small (40 ha), shallow and eutrophicated lake in southern Finland with frequent algal blooms during summers and fish kills during winters. Until the 1980s, the fish fauna consisted of pike, perch (Perca fluviatilis L.), roach [Rutilus rutilus (L.)], crucian carp, and tench (Tinca tinca L.). Since 2003, when crucian carp were abundant in the extreme and with only a few roach and tench individuals, the lake has been biomanipulated by removal seining. To enhance the effects of seining, piscivorous pike were restocked in the spring of 2008. During the 3‐year study, the catch of crucian carp decreased from 243 to 136 kg while the catch of pike increased from 0.1 to 5.7 kg per seining hectare due to their spawning in the lake as of 2009. The total crucian carp consumption by captured pike was 588 kg during the 2008–2010 study period. When applying the number of pike estimated with the mark‐recapture method, consumption estimate increased to 917 kg, or 22.9 kg per lake hectare in 2010. Thus, pike consumption of crucian carp was about 17% of the removal catch in 2010.     

Impacts of freshwater aquaculture on fish communities: A whole‐ecosystem experimental approach

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Landscape controls on the timing of spring,autumn, and growing season length in mid‐Atlantic forests

The timing of spring leaf development, trajectories of summer leaf area, and the timing of autumn senescence have profound impacts to the water, carbon, and energy balance of ecosystems, and are likely influenced by global climate change. Limited field‐based and remote‐sensing observations have suggested complex spatial patterns related to geographic features that influence climate. However, much of this variability occurs at spatial scales that inhibit a detailed understanding of even the dominant drivers. Recognizing these limitations, we used nonlinear inverse modeling of medium‐resolution remote sensing data, organized by day of year, to explore the influence of climate‐related landscape factors on the timing of spring and autumn leaf‐area trajectories in mid‐Atlantic, USA forests. We also examined the extent to which declining summer greenness (greendown) degrades the precision and accuracy of observations of autumn offset of greenness. Of the dominant drivers of landscape phenology, elevation was the strongest, explaining up to 70% of the spatial variation in the onset of greenness. Urban land cover was second in importance, influencing spring onset and autumn offset to a distance of 32 km from large cities. Distance to tidal water also influenced phenological timing, but only within ~5 km of shorelines. Additionally, we observed that (i) growing season length unexpectedly increases with increasing elevation at elevations below 275 m; (ii) along gradients in urban land cover, timing of autumn offset has a stronger effect on growing season length than does timing of spring onset; and (iii) summer greendown introduces bias and uncertainty into observations of the autumn offset of greenness. These results demonstrate the power of medium grain analyses of landscape‐scale phenology for understanding environmental controls on growing season length, and predicting how these might be affected by climate change.     

The tempo-spatial variations of phytoplankton diversities and their correlation with trophic state levels in a large eutrophic Chinese lake

Lake Chaohu is one of the most eutrophic lakes in China. Research on this lake's seasonal and spatial variations in phytoplankton diversity is needed to understand the distribution of eutrophication, as well as to find appropriate comprehensive biodiversity indices to assess the eutrophication status of the lake. The present study indicated that the Margalef index of all samples was as low as 0.799 ± 0.543 in summer (August 2011) and as high as 1.467 ± 0.653 in winter (February 2012). The Margalef index of the river samples had a high mean value and substantial variation compared with the lake samples. The Peilou index of the lake samples was higher than that of the river samples in summer and autumn (November 2011) but lower than that of the river samples in winter. In spring (May 2012), the Peilou index of the western samples was lower than that of the eastern samples. The spatial distribution of the Shannon–Wiener index was more similar to that of the Peilou index in autumn and winter, while in spring and summer, the spatial distribution was affected by both species richness and evenness. High eutrophication levels occurred in the western lake in spring and summer, whereas high levels occurred in the eastern lake, especially in the middle of the lake, in autumn and winter. The total trophic state index (TSI) in all samples exhibited a significant negative correlation with the Margalef (r = −0.726) and Peilou (r = −0.530) indices but a significant positive correlation with the Shannon–Wiener (r = 0.654) index. The partial correlation analysis results implied that these phytoplankton biodiversity indices could serve as synthetic ecological indicators to assess the eutrophication condition of Lake Chaohu.     

Dispersal of mass‐reared sterile,laboratory‐domesticated and wild male Queensland fruit flies

Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) (‘Q‐flies’) were released as sexually immature adults from a point within an orchard. Marked male Q‐flies were recaptured in the trap furthest from the release point (1087 m) by 2 weeks after release, although 98.25 ± 1.04% of recaptured males were trapped <500 m from the release point. Comparison of gamma‐irradiated (sterile), laboratory‐adapted and wild male Q‐flies indicated that dispersal distance was not significantly affected by fly type. There was no significant correlation between temperature and mean dispersal distance, but total recaptures were significantly negatively correlated with increasing daily maximum, minimum and average temperature.