Introduction
The production of lumpfish (Cyclopterus lumpus) juveniles which largely relies on wild broodstock collection has continued to increase and there is very little knowledge on its reproductive biology (Powell et al., 2018). To close the life cycle of lumpfish in captivity, successful control of sexual maturation is necessary. Photoperiod and temperature are manipulated to control sexual maturation and ensure a year-round availability of other temperate fish juveniles (Wang et al., 2010). Although photoperiod can alter spawning in lumpfish females (Imsland et al., 2019, 2018), the speculated confounding effect of temperature (Imsland et al., 2018) is still unknown, further, there is no information on the long term gonadal development in relation to environmental manipulations. The monitoring of sexual maturation in many cultured fish species involves invasive methods, crowding and extensive handling, which negatively affect the fish health and reproductive performance (Næve et al., 2019). The use of ultrasound is noninvasive, and it has been applied successfully in monitoring sexual maturation in for example Atlantic salmon (Salmo salar) (Næve et al., 2019). Therefore, this study was aimed at determining the effects of different photothermal regimes on gonadal development and plasma sex steroid levels and evaluating the efficiency of ultrasound in monitoring sexual maturation in lumpfish.
Materials and methods
Two experiments were conducted: Experiment 1: two groups of lumpfish (n = 300, body weight = 697 ± 364.1 g) in four tanks, were exposed to either a continuous or a short autumn-spring signal photoperiod (two tanks each). Experiment 2: two groups of lumpfish (n = 4000, body weight = 157 ± 32.2 g) also in four tanks, were exposed to either a natural annual photoperiod or a nine-month compressed annual photoperiod. Using external appearances, gonadosomatic index and histomorphology, in both experiments, temperature in one tank from each photoperiod was elevated by 3 ℃ during final gonadal maturation in females. There were four and sixteen sampling points in the first and second experiments, respectively, during which body weight and length were registered. Ultrasound was tested in sexing and assigning the fish to different maturation categories. Blood for radioimmunoassay of sex steroids was collected. Gonads were excised, their weights registered, and tissues were collected for histological analyses of gametogenesis.
Results
Experiment 1: gonadal development in females was more synchronized and advanced in the short-autumn signal photoperiod. Temperature elevation resulted in accelerated final maturation and ovulation in both photoperiod regimes. In males, gonadal development appeared to be less affected than it was in females. Experiment 2: fully matured males were observed seven months before matured females were recruited. Gonadal development in females exposed to the compressed annual photoperiod was advanced. Spawning in the compressed annual photoperiod was advanced, and more so with elevated temperature. In males however, gonadal development was not different between the photoperiod and temperature regimes. The ultrasound categorization of sexual maturation in both females and males successfully followed the progression of gonadal development, and blood plasma levels of sex steroids.
Discussion and conclusion
Our findings showed that compressing the natural photoperiods caused temporal shifts in levels of sex steroids that are responsible for advanced gonadal development and leading to earlier spawning. Similar findings were reported for wolffish populations; temporal shifts in sex steroid profiles were observed and, sexual maturation and spawning were advanced under exposure to an eight-month compressed annual photoperiod (Dupont Cyr et al., 2018). In lumpfish, a more predictable spawning and peaks in spawning were observed under a compressed annual photoperiod and a short autumn-spring signal, respectively (Imsland et al., 2019, 2018). The results obtained from accelerated sexual maturation and spawning under elevated temperatures in the present study, is similar to a study on rainbow trout where elevation of winter-spring temperatures caused an increased maturation rate (Wilkinson et al., 2010). Although ultrasound was successful in maturation monitoring, an ultrasound-based quantification of maturity in place of the conventional gonadosomatic index is still unavailable (Næve et al., 2019). Our findings demonstrated that the photoperiod and temperature combination was better than photoperiod alone in controlling sexual maturation in lumpfish. We also showed that ultrasound is a simple and non-invasive technique that could be used to assess sexual maturation in lumpfish.
References
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