Seasonal changes in abundance and age structure of burbot Lota lota (L.) and stone loach Barbatula barbatula (L.) in the littoral zone of a large pre-alpine lake

Abstract – Seasonal changes in the abundance and age structure of the stone loach and burbot populations in the littoral of Lake Constance, Germany were examined. The abundance of both species was lowest during the summer months and highest during autumn. The stone loach population in the littoral comprised age classes 1 to 3 plus, in autumn, age class 0. The burbot population showed distinct seasonal changes in both length and age composition. During spring and summer, only age class 1 was present in the littoral whereas, in autumn, only age class 0 was caught. This remarkably strict habitat segregation of two age classes by season is discussed with respect to a possible mechanism for avoiding conspecific predator risk in a cannibalistic species.     

Prey selection and growth in 0+ Eurasian perch Perca fluviatilis L. in littoral zones of seven temperate lakes

We studied the relationships between the planktonic food base and feeding patterns of juvenile mid-summer/early autumn Eurasian perch Perca fluviatilis L., a common predatory freshwater fish in large parts of Europe and Asia. The feeding of 0+ perch was studied during summer and autumn in littoral habitats of seven lakes with different environmental conditions – four Latvian (Auciema, Riebinu, Vārzgūnes, Laukezers) and three Estonian (Kaiavere, Prossa and Akste) lakes. Simultaneously, the abundance, biomass and structure of zooplankton communities were examined. We focused on the littoral areas because many studies in lakes suggest that littoral habitats are particularly important for 0+ fish growth and survival. We were interested in the question: can the diet and growth of 0+ perch be explained by zooplankton community structure? We also presumed that if the amount of zooplankton is low, more benthic invertebrates will be consumed by 0+ perch. Opposite to expectations, we found that zooplankton always counted for over 90% of diet biomass in perch. There were also clear correlations between the zooplankton biomass in a given lake, the zooplankton biomass in 0+ perch stomachs, and the fish growth rate. The study also suggested that nutrient enrichment can positively impact the 0+ perch feeding conditions in lakes.     

Movement,home range and habitat selection of stocked juvenile muskellunge,Esox masquinongy,in Forbes Lake,Illinois: exploring the effects of latitudinal origin

Abstract Intraspecific, seasonal and diel variation in movement behaviours of three stocks of juvenile (age‐2; 399–610 mm total length) muskellunge, Esox masquinongy Mitchill, were assessed using radio telemetry in Forbes Lake (225 ha), IL, USA. Experimental populations included muskellunge from the Upper Mississippi (Leech Lake, MN, USA) and Ohio (Cave Run Lake, KY, USA) river drainages, as well as progeny from North Spring Lake, IL, a mixed‐origin stock. No differences in hourly movement rates or home ranges were detected among stocks. Movement rates were greatest during spring (mean ± SE = 42 ± 4 m h^?1), lowest during summer (16 ± 3 m h^?1) and intermediate in autumn (28 ± 5 m h^?1). Additionally, movement rates during the summer were greater at night than crepuscular periods. Home range sizes were similar during spring and autumn (mean ± SE = 17–18 ± 3–4 ha) and decreased during summer (5 ± 3 ha). Although habitat selection characteristics were generally similar among stocks, fish from the Upper Mississippi River drainage occupied deeper water more frequently and selected the pelagic zone more strongly during the spring than those from the Ohio River and mixed‐origin stocks. Within the littoral zone, muskellunge selected coarse woody habitat and aquatic macrophytes. Collectively, these findings suggest little behavioural differentiation among genetically divergent stocks when evaluated in a common reservoir environment.     

Habitat partitioning among three predatory fish in a temperate reservoir

The diversity of predatory species plays a key role in ecosystem functioning but our understanding of the mechanisms underlying their coexistence is limited, particularly in freshwater ecosystems. Northern pike Esox lucius, European perch Perca fluviatilis and pikeperch Sander lucioperca are three widespread predatory species in European lakes, where they often coexist. As potential competitors, we hypothesised that partitioning habitat is a determinant of species coexistence. This was tested by quantifying the variability of their habitat use in tracking adult individuals in the Bariousses reservoir (France, 86.6 ha, mean depth 7.1 m). Specifically, we investigated their distribution along the littoral–pelagic and depth axes along the daily cycle and across seasons. From littoral to pelagic waters were first found pike, then perch and finally pikeperch. Pike was the closest to the surface while pikeperch was the deepest. This general pattern was, however, variable across seasons with the three species located in the upper layer in summer during reservoir stratification. Individuals were more evenly distributed along the littoral–pelagic axis and closer to the bottom when water was mixing (autumn, winter). In summer, perch used more intensively in the pelagic zone during the daytime. Other species did not show any diel change of habitats. Our results highlighted that species coexistence is associated with habitat partitioning among these three predators, with perch showing a more variable behaviour regarding habitat characteristics. Now more than ever, in the context of global change which modifies habitats, it is of crucial importance to understand the coexistence mechanisms of species that shape ecosystems.     

Hydroacoustic assessment of young-of-year perch, Perca fluviatilis, population dynamics in an oligotrophic lake (Lake Annecy, France)

Abstract  A cohort of perch, Perca fluviatilis (L.) was monitored from hatching to first winter in the oligotrophic Lake Annecy, France, by combining hydroacoustic and direct capture methods (fish plankton net and pelagic trawl). The total stock of pelagic fish increased by a factor of 10 during the season, reaching a maximum in August and decreasing in autumn. Juvenile perch were mainly pelagic, being aggregated into schools in the daytime and dispersed at night. Migration of young-of-year (Y-O-Y) perch to the pelagic zone of Lake Annecy in late spring did not significantly affect zooplankton population abundance. It did, however, result in changes in zooplankton vertical distribution, with Daphnia remaining deeper in the water column. The increase in Y-O-Y biomass during summer was possibly associated with collapse of the Daphnia population by late July, and copepods one month later.     

Survival of stocked hatchery-reared brown trout, Salmo trutta L., fry in relation to the carrying capacity of a trout nursery stream

Abstract. During the period June 1982-84 hatchery-reared brown trout, Salmo trutta L., fry were Stocked into stretches of the Owendoher. a trout nursery stream on the east coast of Ireland. These experiments were designed to examine the survival of stocked fry and to estimate the carrying capacity of the system. During the first year fry were stocked into sectors already supptorting wild fish at densities normal for the system. In the following year fry numbers were artificially reduced prior to stocking with the hatchery-reared fish. Mortality of the stocked fry was high after release with less than 33% of the fish surviving beyond the first 3 weeks. No stocked fish survived after October 1982. In the second year, however, 2-9% of the fish survived. The best survival rates were achieved where wild fry numbers were lowest. Regardless of the initial stocking density the various experiments yielded autumn fry densities (0.07-0.7 fish/m²) similar to those at unstocked sites (0.1-0.62 fish/m²).
Stocking did not increase recruitment to the 1+ group and again 1+ densities (0.15-0.35 fish/m²) similar to unstocked sites (0.07-0.39 fish/m²) were obtained at the end of each year. These results suggest that spawning and recruitment in the Owendoher yield population densities approaching the maximum carrying capacity of the stream. The system appears to support a maximum summer fry density in the region of 1 fish/m² and a maximum autumn density of 0.7 fish/m².     

Characterization of the quality of the gametes and sex steroid levels during spring and summer spawning events in pejerrey Odontesthes bonariensis reared under natural conditions of temperature and photoperiod

Pejerrey Odontesthes bonariensis spawning period takes place during the end of winter and spring and, depending on temperature conditions, during late summer and autumn. Nevertheless, the occurrence and quality of the summer–autumn spawning period have not been well documented and are not considered for aquaculture production. In this context, this study aimed to characterize the reproductive activity of pejerrey reared in captivity under natural conditions of temperature and photoperiod throughout a year in order to summarize the spawning quality of the two spawning periods. The present results showed that pejerrey might spawn from late winter to the end of the spring with a peak in October (mid‐spring) and, also during summer and early autumn, interrupted by a period of 56 days without spawning activity in between. The extension of both spawning periods was almost equal (67 and 69 days). Nevertheless, higher relative fecundity and larger egg size were observed during the first spawning period, associated with higher levels of oestradiol measured in females demonstrating that it is better than the second spawning. The fertilization rate was not affected throughout the reproductive periods. These differences in the reproductive parameters are discussed in order to design specific strategies to increase seed production.     

Habitat and food choice of Arctic charr in Linnévatn on Spitsbergen, Svalbard: the first year-round investigation in a High Arctic lake

Abstract –  Habitat and diet of Arctic charr Salvelinus alpinus (L.) were studied by monthly sampling from late autumn to early summer in Linnévatn, Svalbard (78°3'N, 13°50'E). This is the first year-round study of a population of charr in the High Arctic, with samples being taken every 5–7 weeks. The ice cover lasted for more than 9 months, from mid-October to late July, with the greatest thickness in mid-May. Although most charr occupied the littoral zone during winter, the highest densities in April and October were found in the deeper areas (20 m) of the lake. The fish fed at all times of the year, but the number of stomachs with food and the stomach-filling indices were lowest during the darkest part of the season. The diet of smaller charr (<15 cm) varied strongly with season, showing a dominance of zooplankton in late autumn and chironomids in winter (larvae) and summer (pupae). The food choice was in accordance with the density of food items available. Larger fish (≥15 cm) were mostly cannibalistic during the entire year.     

Temporal variation of seasonality of egg production and the spawning biomass of Pacific anchovy, Engraulis japonicus, in the southern waters of Korea in 1983–1994

Embryonic mortality, egg production and the spawning stock biomass of Pacific anchovy, Engraulis japonicus , off Southern Korea during 1983–1994, and their biological response to oceanographic features in spring and summer, were analysed. The instantaneous mortality rate (IMR) of embryonic stages decreased in spring and increased in summer, with a range of 0.33–1.23 day^–1 in spring and 0.78–1.69 day^–1 in summer. Egg production in summer was three times that during spring and production was low in the late 1980s. Mean lengths of yolk-sac larvae and adult females were greater in spring than in summer, whereas spawning fraction and spawning stock ratio (spawning biomass:adult biomass) were lower in spring than summer. Estimated mean spawning stock biomass ranged from 141 × 10³ to 380 × 10³ MT in spring and from 221 × 10³ to 557 × 10³ MT in summer. Statistically, the seasonal and long-term trends of embryonic mortality, egg production and spawning stock biomass of Pacific anchovy can be explained largely by spring warming, summer cooling and by less abundant zooplankton in the late 1980s.     

First-year overwinter mortality in Eurasian perch (Perca fluviatilis L.) Results from a field study and a simulation experiment

Abstract– The importance of overwinter mortality of 0+ perch ( Perca fluviatilis L.) in central European lakes was estimated using a stochastic simulation model. The probability of overwinter mortality of a perch cohort was calculated by using a model developed for yellow perch ( P. flavescens Mitchill). Winter duration from a long-term data set and the length of perch at the end of the first year from five lakes were used as input data. After 1000 simulation runs, the total extinction of a cohort in the lakes studied was never predicted. Mortality rates of more than 0.5 were only predicted in two of the five lakes, and rates of more than 0.3 in these two lakes were predicted in approximately 10% of all cases. For two consecutive winters differing in duration, the length-frequency distributions of 0+ perch in the autumn and following spring were compared by a graphical method. No significant size-dependent mortality of smaller individuals could be detected in any of the populations studied. Simulated spring length-frequencies were derived from observed autumn length distributions by the same model that was used for the stochastic simulation. These simulated and the empirical spring length-frequency distributions were not identical. The differences between the two distributions were attributed to growth, which occurred between the sampling dates. The results from the simulation and the analysis of the empirical data suggest that high overwinter mortality caused by starvation is rare in central European lakes.     

Generalist versus specialist: the performances of perch and ruffe in a lake of low productivity

Abstract –  To elucidate the performances of perch and ruffe in oligotrophic lakes, we carried out a field study in reoligotrophic Upper Lake Constance. Both these percids used the same habitat, albeit with different activity patterns. Interspecific competition for food was relevant only in summer when both species fed on zoobenthos. Even then, niche overlap was low, while intraspecific diet overlap was moderate to high throughout the season. Perch did not perform fixed, ontogenetic diet shifts, but used a wide range of prey. During spring and early summer, all size classes were planktivorous, then switched to benthivory and cannibalism in summer, and part of the population reverted to planktivory in autumn. Ruffe, by contrast, fed mainly on chironomid larvae and pupae throughout the year. It is suggested that in lakes of low productivity the euryphagous characteristics of perch, including cannibalism, provide a clear advantage over the benthivorous specialist ruffe in two ways: (i) it allows perch to switch to alternative prey types if one prey type becomes scarce; and (ii) reduces both intra- and interspecific competition for food.     

Delayed smolt migration of stocked Atlantic salmon parr

Abstract The timing of the smolt run in the Dale River in western Norway was monitored from 2002 to 2007 after annual stocking in late autumn 2000 to 2005 with 5000–10800, 11–16 g, Atlantic salmon, Salmo salar L., parr. The releases yielded an annual smolt production of 1000–2000 individuals, mainly 1+ smolts. Almost 5700 stocked smolts were trapped during smolt migration and 60% of these were genotyped for family identification. The date for 50% descent varied by 14 days from year to year. For the most part, however, the 2+ stocked smolts and the majority of the wild smolts left the river in May, while the 1+ stocked smolts migrated 23–26 days later in June. It was concluded that the strategy of stocking large parr in late autumn may conflict with the natural timing of smolt migration the following spring.     

北部湾东北部游泳生物资源现状

2009年至2010年对北部湾东北部海域游泳生物调查研究表明,该海域共有游泳生物300种,其中鱼类有209种（硬骨鱼类205种,软骨鱼类4种）,隶属15目、70科、119属;甲壳类76种;头足类15种。底层鱼类100种,近底层鱼类72种,中上层鱼类29种,岩礁性鱼类8种,分别占鱼类总数的47．85％,34．45％,13．87％和3．83％。暖水性鱼类153种,暖温性鱼类55种,冷温性鱼类1种,分别占鱼类总数的73．20％,26．32％和0．48％。游泳生物种类数季节变化为夏季〉秋季〉春季〉冬季;资源密度指数季节变化为夏季〉秋季〉春季〉冬季;渔业资源密度随季节变化为夏季〉秋季〉春季〉冬季,渔业资源年平均资源密度为522．925kg·km^-2,约相当于该海域原始渔业资源密度的1／10;生物多样性指数以夏、秋季最高。估算了北部湾东北部海域潜在渔业资源量为1．092×10^5t·a^-1,最大可捕量为5．460×10^4t·a^-1。比较了北部湾渔业资源的历史变动情况,对渔业资源的进一步养护和管理提出了建议。     

Changes in levels of GnRH in the brain and pituitary and GTH in the pituitary in male masu salmon,Oncorhynchus masou,from hatching to maturation

Levels of two types of gonadotropin-releasing hormone (salmon GnRH and chicken GnRH-II) in the brain and pituitary, and content of gonadotropin (GTHIβ and IIβ) in the pituitary were measured in male masu salmon from hatching to gonadal maturation for three years in order to clarify the involvement of GnRHs in precocious maturation. Underyearling precocious males were distinguishable in summer of year 1 and were marked by an increased GSI. Spermiation was observed among these individuals thereafter every autumn. Pituitary GTHIβ content in both precocious and immature males, and GTHIIβ content in precocious males showed seasonal fluctuations — high in autumn and low in winter. Pituitary GTHIIβ content was low in immature males. Pituitary sGnRH content in precocious males increased from spring to autumn during the three-year period. sGnRH concentrations in discrete brain areas showed seasonal changes — high during autumn to winter and low in summer. Concentrations in the olfactory bulbs and hypothalamus increased significantly in association with testicular maturation during year 3. sGnRH concentrations in the hypothalamus were significantly higher in precocious males than in immature males; this was possibly due to positive feedback of steroid hormones. cGnRH-II was undetectable in the pituitary and no distinct changes were observed in its concentration in the brain in relation to maturation. The phenomenon of underyearling precocious maturation is considered to be triggered before the onset of early summer. It is suggested that males which mature precociously are larger in size and contain much sGnRH in the pituitary before the outward signs of precocity appear; sGnRH may stimulate GTH II synthesis and induce precocious maturation.     

Seasonal and size-related changes in the diet of perch,Perca fluviatilis L., in the shallows of an Australian river,and their implications for the conservation of indigenous teleosts

1. Perca fluviatilis was sampled monthly over 2 years from the shallow waters (<2 m deep) of the Collie River in south-western Australia to determine the diet of this introduced species and the extent to which it comprised indigenous fish species. 2. As P. fluviatilis increased in size, its diet shifted from predominantly planktonic crustaceans to benthic invertebrates, with decapod crustaceans constituting between 55% and 88% of the volume of the food ingested by larger fish (> 120 mm long) in spring, summer and autumn. 3. All size groups of P. fluviatilis fed on the small indigenous teleosts Edelia vittata and Bostockia porosa. 4. The prevalence with which the different size groups of P. fluviatilis ingested indigenous fish species in the main channel of the river was as high as 14% in fish < 50 mm long in summer, 25% in fish 50–120 mm long in spring and 17% of fish > 120 mm long in autumn. 5. Despite appreciable predation and a relatively high prevalence of P. fluviatilis, indigenous fish species have coexisted with perch in the Collie River since the early 1900s. Such coexistence has apparently been facilitated by a high density of invertebrate prey (as a result of eutrophication) and limited interspecific dietary overlap. 6. The absence of E. vittata in those parts of a nearby river system now occupied by P. fluviatilis and the extremely high prevalence of fish that is sometimes found in P. fluviatilis stomachs, suggest that under certain extreme conditions, such as when a marked depletion in alternative food sources occurs, the presence of perch could pose a threat to the conservation of indigenous fish species in certain river systems in south-western Australia.     

Influence of environmental factors,season and size at deployment on growth and retrieval of postlarval lion's paw scallop Nodipecten nodosus (Linnaeus, 1758) from a subtropical environment

The effect of seasonal variation of environmental factors on daily shell growth rates (DGR) of postlarval Nodipecten nodosus was studied at the southern distribution limit of the species in Santa Catarina State, Brazil. Five deployments of hatchery produced postlarvae (initial shell height 0.5 mm) in the sea-based nursery were carried out from August 2000 to September 2001, and DGR and percent retrievals were recorded. Chlorophyll-a, seston, salinity, dissolved oxygen and turbidity were measured weekly, and temperature was recorded hourly. Additionally, DGR and retrievals were compared for postlarvae maintained simultaneously in the sea- and land-based nurseries (initial shell height 0.5 mm), and also for post-larvae deployed in the sea-based nursery at different initial sizes (0.29–1.1 mm). Mean DGR was significantly lower in late winter–early spring 2000 (0.045 mm day⁻¹), intermediate in late winter–early spring 2001 (0.078 mm day⁻¹) and significantly higher in the other seasons (late spring–early summer, 0.152 mm day⁻¹; late summer–early autumn, 0.149 mm day⁻¹ and late–autumn early winter, 0.130 mm day⁻¹). Temperature was the best predictor of growth, which was least at temperatures below 20 °C. Growth rate was also minimal during a period of low salinity and high turbidity. Mean DGR was significantly higher in postlarvae deployed in the sea-based nursery than in those maintained in the land-based nursery. Loss of postlarvae in the sea-based nursery was initially higher in collectors transferred earlier to the sea (ca. 2–3 weeks post-set; shell height 0.5–0.8 mm), but percent retrievals were similar after postlarvae deployed to the sea ca. 4–5 weeks post-set (shell height 1.1 mm) were retrieved simultaneously with those deployed earlier. At retrieval, postlarvae deployed approximately 2 weeks post-set were larger than those deployed subsequently, but spat deployed 1 week post-set attained a similar size to those deployed 2 weeks post-set. A strategy to deploy postlarvae in the sea-based nursery at a size circa 0.5 mm is proposed as more advantageous than keeping them longer in land-based facilities. In southern Brazil, there is a wide window of opportunity to deploy post-larval scallops in the sea-based nursery in which growth is maximized, except when water temperatures drop below 20 °C.     

三都澳四个水产养殖点水质周年变化分析

2011年8月-2012年7月对三都澳四个水产养殖点的营养盐(N、P)、溶解氧(D0)、化学需氧量(COD)、pH值进行了分析,结果表明,无机溶解氮(DIN)组成结构在春、秋、冬季节中主要以NO3-N为主,其次为NH4-N和NO2-N.夏季营养盐结构,NO3-N所占比例明显减低,NO2-N所占比例超过NH4-N.营养盐NO3-N浓度的周年变化为:冬季＞秋季＞春季＞夏季;NO2-N的浓度为:夏季＞秋季＞春季＞冬季;NH4-N浓度:夏、秋偏低,春、冬较高;P04-P的浓度:冬季＞秋季＞夏季＞春季.水体中DO浓度的周年变化为:冬季=春季＞秋季=夏季.pH变化的周年变化为:春季＞冬季＞夏季＞秋季.三都澳四个养殖点周年属于中度营养状态.     

Effect of density on survival and growth of cyprinid fish fry

One of the bottlenecks in Israel's cold water ornamental fish industry is the large loss of fish during the post-larval stage. As a first step towards increasing survival rates, the optimal stocking density in earthen ponds for fry of each species should be determined. The results of five consecutive experiments with fry of goldfish (Carassius auratus), common carp and koi (respectively, edible and ornamental morphotypes of Cyprinus carpio) are presented here. The experiments were run in 18 cages of 1 m³ and 0.8 mm mesh size placed in a 0.02 ha earthen pond, stocked at 50,100 and 200 fry per cage. The variables analyzed were growth rate, harvesting weight, biomass and survival. Equations to estimate growth rates under different conditions are given. The three fish types present different growth rates and survival patterns in relation to stocking density and season, which lead to different management implications. Koi can be stocked up to 2 million fry per ha without showing negative density effects. Goldfish should be stocked at low density (500,000–1 million fry per ha) in spring and at higher rate (2 million fry per ha) in summer. Common carp stocking density should be adjusted to obtain the required fish size at harvest.     

东海区太平洋褶柔鱼生殖群体结构特征的季节差异

依据1997~1999年东海区春、夏、秋3个季节的底拖网资料,分析了东海区太平洋褶柔鱼生殖群体的性别组成、性腺成熟度和初次性成熟胴长等群体结构特征。结果表明:在东海区春、夏、秋三季均存在太平洋褶柔鱼的生殖群体。各季节生殖群体的雌雄比例均不符合1∶1的关系(P<0.05),雌性比例小于雄性。生殖群体中性成熟个体胴长的季节性变化较为明显,雌、雄平均胴长均以秋季最大,夏季最小。雌性个体的平均胴长均显著大于雄性。各季节太平洋褶柔鱼生殖群体中胴长与体重关系的性别差异极显著(P<0.01)。此外,不同季节雌、雄太平洋褶柔鱼的初次性成熟胴长差异较大,雌性为:春季223.4 mm、夏季170.4 mm、秋季202.3 mm;雄性为:春季174.0 mm、夏季164.3 mm、秋季165.3 mm。雄性初次性成熟胴长小于雌性,夏季雌、雄初次成熟胴长均小于春、秋季。依据春季生殖群体的胴长范围、初次性成熟胴长等群体结构特征推测,在东海区可能存在着小规模的春生群。     

桑沟湾浮游植物群落结构时空变化特征及影响因素

为了解北方典型养殖海湾——桑沟湾水域浮游植物群落结构的时空变化特征及其影响因素,于2017年4月(春季)、7月(夏季)、11月(秋季)和2018年1月(冬季)对桑沟湾水域21个站点进行4个航次的大面调查.结果显示,调查期间,该湾共采集到浮游植物31属51种,其中,硅藻(Diatom)24属43种,甲藻(Dinoflag...