Photo-thermal manipulation for the reproductive management of pikeperch Sander lucioperca

Abstract

The aim of the present study was to investigate the effect of photoperiod regime combined with thermal treatment (PTT) on maturation of three-year-old pikeperch (Sander lucioperca). To induce the onset of maturation, all treatment groups underwent a wintering phase of 3 months at 12 °C and 12-h light-to-12-h dark. Controls were maintained at 23 °C and 12-h light-to-12-h dark throughout the trial. After the wintering phase, the PTT groups experienced regimes with 8-, 10-, 12-, and 14-h light for 4 months at 14 °C referred to as photo-thermal treatment (PTT). The four PTT groups as well as the control were studied in triplicates, with 12 fish per tank. We assessed gonad development and maturation by histological analysis, sex steroid plasma concentrations (17β-estradiol, testosterone, and 11-ketotestosterone) and, at the end of the experiment, mRNA expression of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in the pituitary. As expected, both male and female pikeperch initiated reproductive maturation during the wintering phase, which was confirmed through histological analysis and sex steroid measurements. During wintering, the onset of maturation in females was confirmed by the increasing diameter of ovarian follicles (217 ± 65 to 800 ± 97 µm), the developmental stages (50 % mid and 21 % late vitellogenic females), concomitant with elevated 17β-estradiol (E2) plasma concentrations reaching 2600 ± 1500 pg/mL, compared to 380 ± 230 pg/mL at the onset of the experiment. Similarly in males, maturation was indicated by peak concentrations of the androgens observed within the first 2 months of wintering (testosterone 21 ± 12 ng/mL first month, 11-ketotestosterone 5.7 ± 3 ng/mL second month). Subsequently, the photoperiod treatments after the wintering phase influenced the progression of reproductive maturation. During the PTT, follicle diameter increased irrespective of the light regime from 800 to more than 1100 µm, but the progression of the vitellogenesis was differentially modulated by the photoperiod. Already after 1 month of the PTT, 92 and 86 % of females reared at 12- and 14-h light per day were in a late vitellogenic stage. After 2 months, females with mainly atretic follicles were observed under long light conditions (14 h of light), indicating overripeness and spawning. In contrast, 82 and 72 % of the females reared at 8- and 10-h light per day were still in the final stage of vitellogenesis at the end of the experiment and thus ready to undergo final maturation. Concomitant to the histological outcomes, highest E2 concentrations were observed under long-day conditions (12, 14 h) in females with peaks of 4200 ± 3300 (12 h of light) and 6800 ± 4200 pg/mL (14 h of light). In male pikeperch, ongoing spermatogenesis was indicated by rising androgen levels especially under long-day conditions (14 h of light) reaching peak levels of 27 ± 21 ng/mL testosterone and 39 ± 91 ng/mL of 11-ketotestosterone at the end of the experiment. In all PTT, the mRNA expression of FSH and LH was significantly elevated compared to the control, confirming the activation of the hypothalamus–pituitary–gonad axis. Here, no effect between the different light regimes was detected, neither in males nor females. Thus, photoperiod revealed a slight influence on male and female pikeperch maturation when combined with an effective thermal treatment and therefore can be explored as a cheap and easy-to-handle fine-tuning tool for artificial pikeperch reproduction.