The reproductive cycle of female wild marble trout Salmo marmoratus was studied in a prealpine watercourse (Northeast Italy). Gonadosomatic index, oocyte growth and plasma levels of 17b-oestradiol and testosterone were bimonthly measured during one year to obtain a detailed dataset and to gain useful tools for the identification of the reproductive stage without impact for the species. Monitored features showed significant variations during the study period: gonadosomatic index and oocyte size increased slightly but significantly during the first part of the cycle (from February to June), while steroid levels remained quite constant; for all parameters, major increases were highlighted between August and the spawning season (which occurred from the middle of November to the middle of December). Ranges and trends observed for gonadosomatic index (from 0.87 ± 0.41% to 10.91 ± 3.37%) and oocyte diameter (from 0.692 ± 0.031 mm to 4.624 ± 0.208 mm) were generally in agreement with literature regarding salmonids, while plasma levels of 17b-oestradiol (from 0.214 ± 0.015 ng ml-1 to 78.090 ± 23.882 ng ml-1) and testosterone (from 0.327 ± 0.086 ng ml-1 to 71.800 ± 29.406 ng ml-1) showed wider ranges. A strong non-linear relationship was found between oocyte size and 17boestradiol plasma concentration (r2 = 0.890) and especially between oocyte size and testosterone plasma concentration (r2 = 0.947). This last relationship could likely be used to obtain information relative to the gonadal development, especially in the case of an endangered species like Salmo marmoratus, which needs non-invasive tools for management.
