Influence of social factors on the nocturnal activity pattern of the noble crayfish,Astacus astacus (Crustacea,Decapoda) in recirculating aquaculture systems

The social environment plays a major role in the control of growth in many crustaceans, but little is known on the mechanisms involved. Two‐ and three‐year‐old noble crayfish, Astacus astacus are strictly nocturnally active. In the present article, we studied social effects on the pattern of nocturnal activity, represented by the time which the animals spent outside shelters. For this purpose, the activity of groups of uniformly large (‘dominant’) and uniformly small (‘subordinate’) individuals were recorded at different densities both separately and in mixed set‐ups. In crowded laboratory cultures, the presence of larger (dominant) conspecifics was found to reduce the time which smaller (subordinate) individuals spent outside shelters in search of food and feeding by about one‐third. It is suggested that this asymmetric inhibitory effect is an element of the functional chain which ultimately results in an extremely wide variation in individual growth rates of communally reared A. astacus. The relevance of the findings to crayfish aquaculture is discussed.     

Growth performance and RNA/DNA ratio of noble crayfish (Astacus astacus) and narrow‐clawed crayfish (Pontastacus leptodactylus) fed fish waste diets

New and viable species for aquaponics and integrated multi‐trophic aquaculture (IMTA) in freshwater systems can improve yields and sustainability of aquaculture. Freshwater crayfish species such as Astacus astacus and Pontastacus leptodactylus are omnivorous feeders and considered candidates for feeding on faecal matters in existing aquaculture systems. Feeding trials were conducted to determine growth response and RNA/DNA ratio in freshwater crayfish fed fish waste. Carapace length and wet weight were measured to determine the growth response. Juvenile A. astacus was fed faeces of hybrid striped bass (Morone chrysops × Morone saxatilis) and rainbow trout (Oncorhynchus mykiss), while adult P. leptodactylus was fed with two commercial pellet diets and pikeperch (Sander lucioperca) faeces. The nutritional composition of hybrid striped bass faeces was close to optimal diet composition of A. astacus, and crayfish showed significantly higher carapace growth, weight gain and weight gain per moult as the group fed rainbow trout faeces. The growth of P. leptodactylus was significantly lower in terms of weight gain and weight gain percentage per moult for crayfish fed on pikeperch faeces. Thus, this study can recommend a co‐cultivation of hybrid striped bass and A. astacus within one system, but cannot recommend co‐cultivation of P. leptodactylus with pikeperch. Additionally, this study showed controversial results of RNA/DNA ratio and weight gain of both crayfish species. Thus, RNA/DNA ratio cannot be approved for investigations on crayfish physiological status in controlled feeding experiments if animals are fed with an inadequate diet.     

Nutrient removal efficiency of Hydropuntia cornea in an integrated closed recirculation system with pink shrimp Farfantepenaeus brasiliensis

In this study, we have tested the effect of seaweed stocking density in an experimental seaweed biofilter using the economically important red seaweed Hydropuntia cornea integrated with the cultivation of the pink shrimp Farfantepenaeus brasiliensis. Nutrient removal efficiency was evaluated in relation to seaweed stocking density (2.5, 4, 6 and 8 g fw L^?1). Total ammonia nitrogen (TAN) was the main nitrogen source excreted by F. brasiliensis, with concentrations ranging from 41.6 to 65 μM of NH₄⁺‐N. H. cornea specific growth rates ranged from 0.8 ± 0.2 to 1.4 ± 0.5% day^?1 with lowest growth rates at higher seaweed stocking density (8 g fw L^?1). Nutrient removal was positively correlated with the cultivation densities in the system. TAN removal efficiency increased from 61 to 88.5% with increasing seaweed stocking density. Changes in the chemical composition of the seaweed were analysed and correlated with nutrient enrichment from shrimp effluent. The red seaweed H. cornea can be cultured and used to remove nutrients from shrimp effluents in an integrated multi‐trophic aquaculture system applied to a closed recirculation system. Recirculation through seaweed biofilters in land‐based intensive aquaculture farms can also be a tool to increase recirculation practices and establish full recirculation aquaculture systems (RAS) with all their known associated benefits.     

Ultraviolet light and semi‐recirculating systems in artificial incubation of noble crayfish (Astacus astacus) eggs: opportunities and limitations

Development of artificial crayfish egg incubation is a milestone in intensive culture of crayfish as commercially important freshwater animals. This study evaluated experimental treatments combining continuous UV lighting, a non‐chemical antifungal treatment, with an initial formaldehyde bath for noble crayfish (Astacus astacus) eggs incubated in semi‐recirculating systems, which requires less than 1 per cent the amount of water necessary for conventional flow‐through systems. The one‐time administration of a pre‐incubation bath to reduce formaldehyde exposure was ineffective. Ultraviolet irradiation of recirculating water provided poor results (13.5% and 35.2% final survival rates to stage 2 juveniles) and led to deteriorating water quality. An inability of hatchlings to successfully moult and the occurrence of limb deformities was observed in UV‐treated groups, and juvenile mortality was found across all experimental treatments.     

Characterization of spaC‐type Erysipelothrix sp. isolates causing systemic disease in ornamental fish

Since 2012, low‐to‐moderate mortality associated with an Erysipelothrix sp. bacterium has been reported in ornamental fish. Histological findings have included facial cellulitis, necrotizing dermatitis and myositis, and disseminated coelomitis with abundant intralesional Gram‐positive bacterial colonies. Sixteen Erysipelothrix sp. isolates identified phenotypically as E. rhusiopathiae were recovered from diseased cyprinid and characid fish. Similar clinical and histological changes were also observed in zebrafish, Danio rerio, challenged by intracoelomic injection. The Erysipelothrix sp. isolates from ornamental fish were compared phenotypically and genetically to E. rhusiopathiae and E. tonsillarum isolates recovered from aquatic and terrestrial animals from multiple facilities. Results demonstrated that isolates from diseased fish were largely clonal and divergent from E. rhusiopathiae and E. tonsillarum isolates from normal fish skin, marine mammals and terrestrial animals. All ornamental fish isolates were PCR positive for spaC, with marked genetic divergence (<92% similarity at gyrB, <60% similarity by rep‐PCR) between the ornamental fish isolates and other Erysipelothrix spp. isolates. This study supports previous work citing the genetic variability of Erysipelothrix spp. spa types and suggests isolates from diseased ornamental fish may represent a genetically distinct species.     

Nanoparticles of Arthrospira platensis improves growth,antioxidative and immunological responses of Nile tilapia (Oreochromis niloticus) and its resistance to Aeromonas hydrophila

Several studies investigated the usage of spirulina (Arthrospira platensis) in aquaculture as a functional dietary additive or plant proteins replacers; however, more efforts still needed to test its nano form inclusion in aquafeed. In this study, Nile tilapia (Oreochromis niloticus) fed dietary spirulina (A. platensis) nanoparticles (SNP) at 0 (control), 0.1, 1 and 10 g/kg diet while the fifth diet was mixed with 10 g/kg of the normal form of A. platensis as a positive control. The final weight (FW), weight gain (WG) and feed conversion ratio of fish fed SNP at 10 g/kg displayed improved values comparing with the other groups (p < 0.05). Further, fish fed 1 g SNP had higher FW and WG than the control and 0.1 g SNP and 10 g normal A. platensis. The incorporation of SNP at 1 and 10 g/kg in Nile tilapia diets significantly (p < 0.05) enhanced total protein, albumin, globulin, lowered the values of serum glucose and activities of ALT and AST compared with control and spirulina groups. The SOD, CAT, GPx, lysozyme and respiratory burst activities were meaningfully promoted (p < 0.05) in fish received 1 and 10 g/kg diet concerning the control and spirulina groups. Nile tilapia challenged with Aeromonas hydrophila showed congestion of the kidney, liver and spleen tissues while SNP alleviated the inflammation induced by A. hydrophila. Different levels of SNP-incorporated group showed lower cumulative mortality than the other groups. In conclusion, the obtained results illustrated that spirulina in nano form is recommended to enhance Nile tilapia well-being.     

Bacillus subtilis CK3 used as an aquatic additive probiotics enhanced the immune response of crayfish Procambarus clarkii against newly identified Aeromonas veronii pathogen

To identify the aetiology of the disease outbreak in cultured Procambarus clarkii, a dominant bacteria strain from the diseased crayfish hepatopancreas was isolated. It was identified as Aeromonas veronii based on biochemical identification and 16S rDNA sequencing. The symptoms of artificially infected crayfish were similar to those of naturally infected P. clarkii. The A. veronii was sensitive to norfloxacin, ciprofloxacin, ceftazidime, cefuroxime and ceftriaxone antibiotics, while resistant to penicillin, ampicillin, oxacillin and piperacillin. Virulence genes such as the enterotoxin genes (act, alt and ast) and haemolytic toxin genes (hlyA and aerA) were detected. After A. veronii infection, the connections between some liver tubules disappeared, the vacuoles appeared in the brush border and the mitochondria were enlarged. The antagonistic action of previous identified B. subtilis CK3 against A. veronii was also detected. The supernatant of B. subtilis CK3 exhibits a significant bactericidal effect on A. veronii. After B. subtilis CK3 immersion, the antioxidant enzymes and immune-related enzyme activities in hepatopancreas were significantly higher than control. The accumulative mortality caused by infection of A. veronii can be significantly reduced by adding B. subtilis CK3 into the aquatic water. These results demonstrated that B. subtilis CK3 could act as a water additive to improve the immune response of P. clarkii against newly identified A. veronii. The present study will provide a new way for the prevention and control of crayfish bacterial diseases and also provide technical support for the healthy cultivation of P. clarkii.     

An automated recirculation aquaculture system based on fuzzy logic control for aquaculture production of tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis>)

The development of an automated recirculation aquaculture system (ARAS) is presented here. The production system facility was built in a 672-m² polyethylene greenhouse using four 20-m³ tanks, each of which was stocked with 1,000 tilapia (O. niloticus). The growth of the fish was evaluated from a starting weight of 2 g to a final average weight of 418 g, during a production cycle from July 2008 to January 2009. Our objective was to develop new technological approaches, based on fuzzy logic control, to integrate into novel automated equipment for recirculation aquaculture systems used for fish production under intensive conditions. This technique allows farmers to incorporate their knowledge and experience into the control system. The results demonstrated a substantial water savings of 97.42%. There was a total fish biomass production of 1,655.3 kg and an average feed conversion rate of 1.55. The environmental variables were kept within acceptable ranges for tilapia production. We conclude that it is possible to increase the productivity and profitability of aquaculture systems, by applying new approaches based on modern control techniques, such as fuzzy logic control, that promote resource optimization.     

Evaluation of limb autotomy as a promising strategy to improve production performances of mud crab (Scylla olivacea) in the soft‐shell farming system

Soft‐shell crab, a newly moulted crab having non‐calcified and hydrated soft exoskeleton, farming has been increasing tremendously in recent years throughout the world due to its high market price and consumer preferences. However, high aggressiveness and cannibalistic behaviour of mud crab limit its culture to an individual chamber system which ultimately makes the soft‐shell mud crab farming space and labour‐intensive. Therefore, the aim of this study was to improve the efficiency of soft‐shell crab production by autotomy of the claw and/or walking legs to reduce the moulting duration and intensify the culture system through double stocking in the existing individual chamber culture system. To achieve our objectives, a series of experiment were conducted at both indoor and on‐farm levels by removing the claw and/or walking legs of Scylla olivacea for two consecutive moulting cycles. Results showed that both claw ablation significantly (p < .01) shortened the moulting duration and increased the carapace width, weight gain, specific growth rate, feeding efficiency and yield parameter compared to the non‐ablated crabs more prominently at 1st moulting than the 2nd moulting in both indoor and on‐farm trials. Ablation of all walking legs while keeping the claw intact did not significantly influence (p > .05) the moulting duration and carapace width, but significantly (p < .01) improved the specific growth rate, weight gain, feeding efficiency and yield parameters at 1st moulting compared to the non‐ablation and unilateral three walking leg ablation groups. However, ablation of all appendages (both claw and walking legs) showed significant (p < .01) improvements of carapace width, weight gain, specific growth rate, feeding efficiency and yield parameters, and shortened the moulting duration up to 23 days from 34 days during 1st moulting and up to 27 days from 40 days at 2nd moulting. The survivals of different autotomized mud crabs were statistically non‐significant (p > .05) to the non‐autotomized crabs in various indoor and on‐farm trials. Besides improving the production performances, ablation of bilateral claw and all appendages allowed to double the stocking density in the existing individual chamber‐based soft‐shell crab farming that enhanced the production about threefold higher than the existing individual chamber‐based culture system. Finally, our results suggest that autotomy could be a promising technology for the commercial farming of soft‐shell mud crab in near future.     

Estimating the human impact on populations of the endangered noble crayfish (Astacus astacus L.) in north‐western Poland

• 1. The Pomorskie Lakeland close to the town of Miastko shows the highest density of stocks of the noble crayfish Astacus astacus (L.) in the lowlands of north‐western Poland. In summer 2002, the present status of this highly threatened species was analysed by means of mark and recapture trapping.
• 2. Nine lakes with known occurrence of A. astacus were investigated. Each lake was surveyed with regard to the crayfish population density, water quality and potential recent and historical impact of fisheries and recreation. The estimated population density was evaluated statistically with respect to a set of potential environmental variables.
• 3. The estimated densities of A. astacus populations varied from very abundant (6.6 adult individuals per trap per night) to absent. Two lakes were found to contain the non‐indigenous crayfish Orconectes limosus (Raf.). The population density was not correlated with conductivity, pH, transparency, alkalinity, the concentration of ammonium, calcium, chloride, nitrate, nitrite, total nitrogen, total phosphorus, or with the trophic status of the lakes. A. astacus occurred in lakes ranging from mesotrophic to eutrophic.
• 4. The results suggest that both occurrence and density of A. astacus populations depended on criteria associated with human activities that have altered the aquatic environment. Abundance of A. astacus was negatively correlated with eel (Anguilla anguilla) stocking intensity (r = ?0.88; p < 0.01) and number of yearly net catches (r = ?0.66; p < 0.05) and positively correlated with distance to nearest town (r = 0.72; p < 0.05).
• 5. It is concluded that the introduction of O. limosus most likely resulted from fishery activities, emphasizing the need for regulations to reduce the negative impact of lake fisheries on native crayfish stocks.

Copyright © 2005 John Wiley & Sons, Ltd.     

Incidental capture of leatherback sea turtles in fixed fishing gear off Atlantic Canada

相似文献   

The in vitro efficacy of oxytetracycline against re‐isolated pathogenic Aeromonas hydrophila carrying the cytolytic enterotoxin gene through hybrid catfish,Clarias macrocephalus (Günther, 1864) × Clarias gariepinus (Burchell, 1822) in Thailand

Aeromonas hydrophila is a pathogen infecting farmed hybrid catfish, Clarias macrocephalus (Günther, 1864) × Clarias gariepinus (Burchell, 1822) which incurs substantial economic losses in Thailand. The study aimed at a genetic tracking of A. hydrophila infection and the in vitro assessment of the efficacy of antibiotics against its virulent strains. Five clinical strains from catfishes and Nile tilapia were employed. They were 3‐passage re‐isolated through healthy hybrid catfish and the cytolytic enterotoxin gene (AHCYTOEN) of individuals was traced. Each of the re‐isolates at a dose of ~6.67 × 10⁵ CFU/g was intraperitoneally injected into ~15 g‐healthy hybrid catfish and their pathogenicity were observed for 7 days. It was found that AHCYTOEN was carried over whereas typical signs of motile aeromonas septicaemia were found in the specimens. The bacterial strains of Nile tilapia origin did not induce mortality but those of catfish origins (80%–100% rate of mortality). The strains were susceptible to the tetracycline antibiotics, and oxytetracycline produced MIC₅₀ and MBC as low as 0.007–0.031 μg/ml and 1–8 μg/ml respectively. As oxytetracycline specifically inhibited pathogenic A. hydrophila in vitro, it is recommended that an appropriate dosage regimen of the drug should be established.     

Effects of soluble dietary cellulose on specific growth rate,survival and digestive enzyme activities in three freshwater crayfish (Cherax) species

The current study evaluated the effect of soluble dietary cellulose on growth, survival and digestive enzyme activity in three endemic, Australian freshwater crayfish species (redclaw: Cherax quadricarinatus, marron: C. tenuimanus, yabby: C. destructor). Separate individual feeding trials were conducted for late‐stage juveniles from each species in an automated recirculating freshwater, culture system. Animals were fed either a test diet (TD) that contained 20% soluble cellulose or a reference diet (RD) substituted with the same amount of corn starch, over a 12‐week period. Redclaw fed with RD showed significantly higher (P < 0.05) specific growth rates (SGR) compared with animals fed the TD, while SGR of marron and yabby fed the two diets were not significantly different. Expressed cellulase activity levels in redclaw were not significantly different between diets. Marron and yabby showed significantly higher cellulase activity when fed the RD (P < 0.05). Amylase and protease activity in all three species were significantly higher in the animals fed with RD (P < 0.05). These results indicate that test animals of all three species appear to utilize starch more efficiently than soluble dietary cellulose in their diet. The inclusion of 20% soluble cellulose in diets did not appear, however, to have a significant negative effect on growth rates.     

Organic acids blend as dietary antibiotic replacer in marine fish olive flounder,Paralichthys olivaceus

The present study was conducted to evaluate the efficacy of organic acid blends as dietary antibiotic replacer in marine fish olive flounder, Paralichthys olivaceus. Fish averaging 3.5 ± 0.05 g (mean ± SD) were fed one of the four experimental diets: (1) without antibiotic or organic acid (Control/CON); (2) with antibiotic—50 mg oxytetracycline per kg body weight per day (OTC); (3) with organic acid blend A—4 g/kg diet (OA_A); and (4) with organic acid blend B—4 g/kg diet (OA_B), for 10 weeks. At the end of the experiment, total intestinal bacterial counts in fish‐fed OA_A, OA_B and OTC were significantly lower than that of fish‐fed CON diet (p < 0.05). Further, the group of fish‐fed organic acid blends (OA_A, OA_B) or antibiotic (OTC)‐supplemented diets exhibited lower intestinal Vibrio sp. counts compared with fish‐fed CON diet. Disease challenge test with bacteria Edwardsiella tarda showed significantly lower cumulative mortality rates for the group of fish‐fed OA_A, OA_B or OTC than that of fish‐fed CON diet (p < 0.05). There were no negative effects on the growth, serological characteristics and proximate composition among the group of fish‐fed different experimental diets. Therefore, the present experiment demonstrates that blends of organic acid could be a promising alternative to dietary antibiotics for the preventive and/or curative health management in marine fish olive flounder aquaculture.     

Minimum water exchange spares the requirement for dietary methionine for juvenile Litopenaeus vannamei reared under intensive outdoor conditions

We examined if minimum water exchange could spare dietary methionine (Met) required for maximum growth performance of juvenile Litopenaeus vannamei reared in an intensive outdoor system. Shrimp of 1.98 ± 0.13 g were stocked at 70 animals/m² and reared for 72 days in 50 tanks of 1 m³ under flow‐through (14.4% a day) and static (1.4%–2.9% a day) green‐water conditions at 32.0 ± 3.7 g/L salinity. Five diets with a minimum inclusion of fishmeal supplemented with a dipeptide, dl ‐methionyl‐dl ‐methionine, were formulated to contain increasing levels of Met, 4.8, 6.2, 7.2, 8.1 or 9.4 g/kg (on a dry matter basis). Each of the five diets were fed four times daily to five replicate groups. Dietary Met and water exchange significantly influenced shrimp survival, gained yield, apparent feed intake, food conversion ratio and final body weight (p < .05). Raising shrimp under limited water exchange, i.e., static versus flow‐through spared the dependence on higher levels of dietary Met to maximize shrimp body weight, from 9.4 g/kg to 8.0 g/kg (14.0 and 12.6 g/kg Met+Cys respectively). In an intensive rearing system, a reduction in water exchange is desirable as it leads to a lower need for supplemental dietary Met.     

An assessment of the west winter–spring cohort of neon flying squid (Ommastrephes bartramii) in the Northwest Pacific Ocean

In the Northwest Pacific, the squid jigging fisheries targeted the west winter–spring cohort of neon flying squid (Ommastrephes bartramii) from August to November. Total annual catch by the Chinese mainland squid jigging fleet during 2000–2005 ranged from 64,100 to 104,200 t. The unique life history of this squid species makes the use of traditional age- or length-structured models difficult in evaluating the effect of intensive commercial jigging on this stock. We fitted a modified depletion model to the Chinese jigging fisheries data to estimate the squid stock abundance during 2000–2005. Monthly biological data were randomly sampled from the five squid jigging vessels during the fishing seasons. Effects of using different natural mortality rates (M) and three different error assumptions were evaluated in fitting the depletion model. Based on sensitivity analyses, the log-normal error model was found to be preferred for the squid assessment. The assessment results indicated that the initial (pre-fishing season) annual population sizes ranged from 199 to 704 million squid with the M value of 0.03–0.10 during 2000–2005. The proportional escapement (M = 0.03–0.10) for different fishing seasons over the time period of 2000–2005 ranged from 15.3% (in 2000) to 69.9% (in 2001), with an average of 37.18%, which was close to the management target of 40%. Thus, the current fishing mortality of the squid jigging fishery was considered to be sustainable. We inferred its annual maximum allowable catch ranging from 80,000 to 100,000 t. This study suggests that the modified depletion model provides an alternative method for assessing short-lived species such as O. bartramii.     

Effects of production intensity and production strategies in commercial Atlantic salmon smolt (Salmo salar L.) production on subsequent performance in the early sea stage

A data set from commercial Atlantic salmon (Salmo salar L.) producers on production intensity and production strategies in smolt tanks (N = 63–94) was obtained during 1999–2006. The effects of production intensity on subsequent fish mortality and growth during the early sea phase (90 days) were examined by principal component analysis and subsequent generalized linear model analysis. Levels of accumulated metabolites (CO₂, total ammonia nitrogen and NH₃), and information provided by producers (production density (kg fish m³⁻¹), specific water use (l kg fish⁻¹ min⁻¹) and oxygen drop (mg l⁻¹) from tank inlet to tank outlet), were used as predictor variables. In addition, several other welfare relevant variables such as disease history, temperature during freshwater and sea stage; season (S1) or off-season (S0) smolt production; and the use of seawater addition during the freshwater stage were analyzed. No strong intensity effects on mortality or growth were found. CO₂ levels alone (P < 0.001, R ² = 0.16), and in combination with specific water use (R ² = 0.20), had the strongest effect on mortality. In both cases, mortality decreased with increasing density. For growth, the intensity model with the most support (R ² = 0.17) was O₂ drop, density and their interaction effects, resulting in the best growth at low and high intensity, and poorer growth at intermediate levels. Documented viral disease outbreaks (infectious pancreatic necrosis and two cases of pancreas disease) in the sea phase resulted in significantly higher mortalities at 90 days compared with undiagnosed smolt groups, although mortalities were highly variable in both categories. The temperature difference between the freshwater stage and seawater had a small, but significant, effect on growth with the best growth in groups stocked to warmer seawater (P = 0.04, R ² = 0.06). S0 and S1 smolt groups did not differ significantly in growth, but the mortality was significantly (P = 0.02) higher in S1 groups. Seawater addition as a categorical variable had no significant effects, but when analyzed within the seawater addition group, intermediate salinities (15–25 ppt) gave the best results on growth (p = 0.04, R ² = 0.15). Production intensity had small explanatory power on subsequent seawater performance in the analyzed smolt groups. If anything, the analysis shows a beneficial effect of intensive production strategies on subsequent performance. Analysis of the various production strategies indicates better survival of S0 compared with S1 smolt groups, improved growth when stocked in seawater warmer than freshwater, and a negative effect of viral disease outbreaks on survival. The results clearly demonstrate the difficulty of extrapolating results from experimental work on fish welfare and production intensity variables to commercial production. On the other hand, the presented results may simply demonstrate that the traditional fish welfare criteria growth and mortality may not suffice to evaluate welfare consequences of suboptimal water quality or production strategies in the aquaculture industry.     

A tentative new species Aphanomyces fennicus sp. nov. interferes with molecular diagnostic methods for crayfish plague

Several isolates of an unknown oomycete resembling the genus Aphanomyces were obtained into laboratory culture from samples of noble crayfish (Astacus astacus) in 2016–2017. The crayfish were kept in cages in connection with a study on an eventually persistent crayfish plague infection in a small Finnish lake, following an acute episode of the disease in 2010. Despite the close resemblance of the isolates to the causative agent of crayfish plague, Aphanomyces astaci, and the positive results obtained in OIE recommended A. astaci‐specific ITS‐based conventional PCR and qPCR molecular assays, the isolates can be distinguished from A. astaci by morphological features concerning hyphal structure and chlamydospore formation, as well as using the randomly amplified polymorphic DNA‐polymerase chain reaction (RAPD‐PCR) method, microsatellite‐based genotyping, the pathogenicity test and phylogenetic analysis based on ITS sequencing. The name Aphanomyces fennicus sp. novum is proposed for this close relative of A. astaci. The detection of this tentative novel species giving false‐positive results in existing diagnostic assays for the crayfish plague highlights the importance of careful interpretation of the results from molecular methods, especially concerning crayfish with low‐level infections, excluding the possibility to verify the results from clinical or sequencing data.     

Trait‐related variation in the reproductive characteristics of female pikeperch (Sander lucioperca)

Maternal characteristics typically affect the recruitment of an exploited fish population. The size and age at maturity, as well as the effects of maternal traits on relative fecundity and egg dry weight, were studied in six exploited pikeperch populations in Finnish lakes. The among‐lake variation in the maternal characteristics was substantial. The estimated total length at maturity (L₁₀, L₅₀, L₉₀) varied between 318–444, 403–423 and 444–527 mm, respectively, largely depending on the average growth rate and body condition of pikeperch. The estimated L₅₀ was generally close to the recently imposed national minimum size limit (42 cm). The estimated age at maturity (A₅₀) ranged from 4.2 to 6.9 year. Both relative fecundity and egg dry weight significantly increased with female size and age, indicating size‐ and age‐dependent maternal effects on egg characteristics and quantity, and emphasising the importance of large individuals for reproduction. The observed among‐population differences in the size‐dependent maternal influences highlight the need for stock‐specific management of pikeperch fisheries. The conservation of large females should be promoted to increase recruitment and reduce its variability.