Thermal effects on the dynamics of infection with metacercariae of Diplostomum baeri and D. spathaceum were monitored between May and September, in a year-class of perch Perca fluviatilis, in an artificial lake receiving warm water discharges from a nuclear power station and in an unheated reference site, for a two year period. In the heated area the prevalence of infection of the retinal form, D. baeri, was always 100%, whereas in the unheated site there was an increase from 93% in May to 100% in June 1986. The relative density of D. baeri was found to increase gradually during the first summer in both areas, although the accumulation rate of metacercariae was significantly increased in the heated area. In August of the first year the relative density of D. baeri peaked in the heated area, whereas it continued to increase in the unheated control. However, in September of the following year, the relative densities were at the same level in both thermal regimes. A concomitant decline in the degree of overdispersion of metacercariae within the host population was observed in the heated area as the population density of metacercariae decreased, whereas the index of dispersion remained at the same level throughout the study in the unheated area. However, it was not possible to sample perch in the unheated area between October 1986 and May 1987 and changes in the parasite population could have occurred during this period. The prevalence and relative density of D. spathaceum, the lens form, was on the contrary low, especially in the unheated site where it was recorded only occasionally. The infection of D. baeri exhibited a convex pattern in both thermal regimes, although the peak infection was noticed earlier in the heated area. Regulation of the parasite infrapopulation may have been achieved by the combined effects of a decreased transmission rate of cercariae with increasing age of the host, the mortality of metacercariae as a natural termination of the life span and to selective predation of heavily infected hosts. However, regardless of an increased accumulation rate of metacercariae in the heated area, the relative densities of D. baeri became equal in both thermal regimes at the end of the study. The mortality rate of metacercariae in the heated area was therefore presumably increased as compared with the unheated reference site.

A wide range of factors can be attributed to the syndrome of fatty liver observed in some cultured fish species. The objective of the study was therefore to quantify different hepatocyte ultrastructural features as potentially influenced by twelve nutritional and husbandry factors, in order to discriminate the most influent factors in Eurasian perch (Perca fluviatilis), a typical carnivorous temperate fish species. Twenty-four groups of juveniles (initial weight 57·6 (sd 14·4) g) were intensively reared for 116 d and fed sixteen different isoproteic diets. The distribution of the experimental treatments was based on a multivariate fractional factorial design (L24 212) with either high (+1) or low ( − 1) level of each of the following factors: diet (lipid and protein sources, lipid content, astaxanthin enrichment), feeding level, daily and weekly distribution frequency, fish density, initial weight heterogeneity, temperature, photoperiod, and light spectrum. Liver lipid droplets, glycogen, mitochondria and rough endoplasmic reticulum (RER) were semi-quantified and analysed by a soft imaging system using transmission electronic microscopy photographs. Important variability of hepatocyte ultrastructural features was observed. The present study confirms that the rearing temperature, through its influences in the general metabolic activity, seems to be the main factor modifying mainly lipid droplet accumulation and RER development. However, factors that could be pooled under the designation of factors leading to food accessibility and lipid and protein quality intensify or compensate the effect of temperature.

In Eurasian perch (Perca fluviatilis), females grow significantly faster than males. Moreover, gonadal development has a significant negative impact on somatic growth and fillet yield. In order to induce sterility, triploidy induction was attempted by subjecting fertilised eggs to heat shocks. Different combinations of temperature (28, 30, 34, 35 and 36 °C), duration (2, 5, 10 and 25 min) and time of shock initiation (TI = 3, 5 and 7 min post-fertilisation) were tested. Flow cytometry analysis was used to assess ploidy level of control and heat-shocked larvae. Low intensity (28–30 °C) and long duration (10 and 25 min) shocks lead to significantly higher survival (44 ± 26%) and triploidisation (71 ± 26%) rates than high intensity (34–36 °C) and short duration (2 and 5 min) shocks (17 ± 19% and 21 ± 26%, respectively). The most effective conditions for efficient triploidy induction were low intensity shock of 30 °C, applied 5 min post-fertilisation for 25 min. This treatment led to the production of all-triploid populations (100%) with up to 43% survival rate.

From April 1995 to April 1996, the annual reproductive cycle of male Eurasian perch Perca fluviatilis was studied at the Lindre Center (Moselle, France). At monthly intervals, five males (mean body weight of 133 ± 43 g and total length of 175 ± 9 mm) were caught. From sampled organs, the gonadosomatic (GSI), hepatosomatic (HSI) and viscerosomatic indexes (VSI) were calculated and plasma testosterone (T) and 11-ketotestosterone (11KT) levels were measured. After the spawning period in mid-April, GSI and HSI dropped and VSI increased to 3.8 ± 0.1 %. In September, GSI reached its maximum (8.5 ± 1.8 %). At this time, spermatocytes, spermatids and spermatozoa were abundant, whereas in June only spermatogonia were observed. During winter, GSI was stable at approximately 5 %, HSI reached its maximum (1.9 ± 0.3 %) and VSI was low (2.6 ± 0.2 %). From April to November 1995, plasma T and 11KT concentrations were low (< 0.5 ng·mL–1). Plasma T levels increased significantly in December and reached peak levels (12.3 ± 2.1 ng·mL–1) in January, then decreased in February and increased again until spawning in April (6.8 ± 2.1 ng·mL–1). This second elevation could be related to the beginning of a new spermatogenic cycle. Plasma levels of 11KT increased significantly from October to February 1996 (4.9 ± 1.1 ng·mL–1). From February to the spawning period (April 1996), plasma 11KT decreased significantly, but values were significantly higher than those measured in fall. Males were spermiating from January to spawning in April.

Les études menées au cours de quatre années sur la composition corporelle de la perche (Perca fluviatilis, L.) et du gardon (Rutilus rutilus, L.) de la retenue de Pareloup ont révélé certaines années, pendant la période estivale et essentiellement chez la perche, une diminution des lipides somatiques accompagnée soit d'un ralentissement soit d'un arrêt de l'accrétion protéique. En fin d'automne, on a pu observer une croissance compensatrice chez les animaux qui auraient subi des conditions nutritionnelles défavorables en été.La composition biochimique du plancton au cours du cycle saisonnier montre que la valeur nutritive du phytoplancton et du zooplancton est maximale en été. Par contre, on a observé une diminution de la quantité disponible au cours de l'automne, ce qui implique un élargissement du spectre alimentaire des deux espèces. La variabilité interannuelle de la structure du peuplement et de la biomasse planctonique disponible en été pourrait expliquer l'utilisation des réserves énergétiques chez les poissons à cette saison.