Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have investigated the interplay between migration and maturation in the field during the downstream migration of female silver eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over 3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder parasites on their swimming performance

A longitudinal study of racing Thoroughbreds: performance during the first years of racing

To develop a profile of the racing careers of Thoroughbred horses in south-eastern Queensland, and to examine factors that affect racing during the first years of racing.A longitudinal study using the racing records of a defined cohort of Thoroughbred horses that were born on or within 12 months following 1 August 1991. Data collection commenced in 1996 and will continue until all horses cease racing. In this paper two measures of performance were examined: race earnings during the first year of racing and cumulative proportion of horses still racing up to 2 years after their first start. Univariable and multivariable methods were used to describe and identify factors associated with the performance of these horses.By 31 July 1997, 1804 horses were enrolled in the study including 916, 701, 152 and 35 horses that first raced at 2, 3, 4 and 5 years of age, respectively. During their first year of racing, half the horses earned no more than A$450 from race earnings, and 710 (39.4%) horses earned no money at all. In comparison to poorly performing horses, well-performing horses were more likely to be male, to have started as 2-year-olds and to have had more starts during this year. Of the horses that first started as 2 and 3-year-olds, only 71 and 46% continued racing for at least 1 and 2 years after their first start, respectively. Length of racing life was associated with performance during the first year of racing (number of starts and average earnings per race), and with sex, date of birth and age at first start.The study confirmed a high wastage among racing Thoroughbreds. As expected, premature retirement from racing was linked to poor performance. During the first year of racing, the race earnings of an estimated 1567 (86.9%) horses were insufficient to cover training costs. The 2-year-old racing cohort outperformed the older racing cohorts in each of the performance measures under investigation. Interpretation of this result, and the long-term effects of 2-year-old racing, was limited by the problem of confounding