Introduction
The pikeperch is valuable for aquaculture production due to its flesh quality and high market value (Blecha et al. 2015; Nebeský et al. 2016). However, more than 95% of the market supply has been provided by capture (FAO, 2018; Policar et al. 2013; Steenfeldt et al. 2015), leading to the decline of wild populations in central, eastern, and northern Europe (Policar et al. 2016).
One of the concerns in pikeperch populations dynamics is the disproportion of the sex ratio, where females dominate in a size group above 550 mm TL (total length) (Raikova-Petrova and Živkov 1998; Gröger et al. 2007; M’Hetli et al. 2011). Gillnet fishing has the most significant impact on the disproportion, as the most prominent individuals are usually among fisheries catches (Gröger et al. 2007; Vainikka and Hyvärinen 2012). Such imbalances occurring within the wild population of pikeperch could favour a decrease in reproduction efficiency. It can drastically affect the amount of energy the population invests in reproduction since larger females contribute disproportionally more to population replenishment (Barneche et al. 2018; Olin et al. 2018); therefore, the most productive specimens pair with an unequally sized partner.
This study aimed to investigate the preference of male size by the female during natural spawning on the nest. We hypothesise that variations in spawning behaviour related to the different sizes of partners could influence the reproduction process and the success of pikeperch.
Material and methods
In late October, adult pikeperch individuals were collected from the production ponds of the Nove Hrady Ltd. and transferred to a small earthen pond (0.25 ha) for overwintering. At the beginning of April, females with oocytes in stage V (BW = 920 ± 69; Bodyweight) and males of two sizes: large (BW = 1388 ± 163 g) and small (BW = 566.7 ± 66.9 g); released milt, were selected for spawning in the nests. A set of fish consisting of one female and two different sized males were transferred to an outdoor flow-through aquaculture spawning chamber: Length: 2.8m; Width: 1.5m; Height: 0.8m; total volume of 5.6 m3) with two artificial spawning nests (0.9 m in diameter) made of stiff long hair brush (Malinovskyi et al. 2018). The water temperature was measured at 1-hour intervals using Minikin Tie digital thermometer (Enviromental Measuring System s.r.o., Czech Republic) placed inside the inflow water pipe. Nests in the spawning chamber were checked twice a day with a 12-hour interval until the female had paired with one of the males. Male size and occurrence of the spawning were recorded. All nests were photographed, and photos were processed using QuickPHOTO MICRO 3.0 software. The empty nests processing included calculation of: (I) cleaned nest area, % of total nest area; (II) cleaned area centring, mm from the centre of the nest. For the nest with egg processing, they further included (III) calculation of egg distribution area, % of total nest area; (IV) clustered eggs area, % of total nest area; (V) clustered eggs area, % of the distribution area. Subsequently, the nests with eggs were transported to the RAS system for further incubation under controlled conditions.
After each successful spawning, blood samples from all fish were collected to determine testosterone for males and estradiol for females in the blood plasma. The testosterone and estradiol levels in the blood plasma were measured by immunoanalytical ECLIA methods (Cobas 6000, Roche, Manheim, Germany).
Results and discussion
Observed behavioural patterns have indicated that the differences in cleaning efforts of pikeperch males were recognised by females and determined the selection of the partner. Generally, pikeperch females have preferred bigger males in 77% of recorded spawning events. However, statistical analysis has indicated that the mechanism of selection was not related to the bodyweight of the male but rather to its efforts in nest preparation. All big males have exhibited cleaning efforts toward the nest, compared to just 50% among small males, confirming the suggestion of better consistency and efficiency of spawning behaviour expression in larger fish. Despite appropriate body weight and cleaning efforts, 33% of the big males were not selected for spawning due to their inability to efficiently recognise and utilise the nest area. The preferred small males were significantly more precise in identifying and using the nest area compared to unpreferred big and small fish. This could be crucial for expressing paternal care and an explanation for females’ selection.
The statistical model indicated the male size, plasma testosterone, and average daily temperature significantly affected the parameters of cleaned nest area, % and cleaned nest area centring, mm (Table 1). Higher testosterone values were associated with the larger cleaned size of the nest and its more precise placement relative to the total nest area, e. g., the lower distance between centres of the areas. Similarly, a lower temperature (range: 11.3 – 12.5 °C) was associated with a better cleaning performance of pikeperch males, regardless of the size (Table 1; Figure 1). Therefore, females preferred males cleaning larger nest areas and more precise placements on the nest.
The statistical analysis has also indicated that the water temperature significantly influenced the testosterone levels of males (Figure 1). Females paired with big males showed an insignificant tendency of higher estradiol levels than those selected small males. In addition, testosterone in male blood plasma had increased within a temperature range of 11.6 - 12.5 °, which explains the better cleaning performance of males in this temperature range.
There were no significant effects of the factors mentioned above on parameters of egg distribution, egg clustering and larvae production (Table 1). These characteristics were stable through the experiment, suggesting that selection availability allowed females to exhibit consistent spawning behaviour.
To the best of our knowledge, this is the first study on pikeperch describing a relation between water temperature and testosterone levels in the blood plasma of males. This study confirms the importance of male spawning behaviour in the natural reproduction of pikeperch. Such data could be valuable in predicting the reproduction output in wild pikeperch populations and may guide the controlled reproduction of wild and pond-cultured pikeperch broodstock.
Acknowledgements
The study was supported by the Ministry of Agriculture of the Czech Republic, project NAZV QK22020144.