LATE HIGH TEMPERATURE PROMOTES FAST GROWTH AND FEMALE SEX DIFFERENTIATION IN EUROPEAN SEA BASS Dicentrarchus labrax WHILE EARLY PHOTOPERIOD HAS NO EFFECT ON EITHER TRAIT

Introduction

The s ea bass, Dicentrarchus labrax is an important species for European mariculture, mainly in Mediterranean countries. In this species, females grow faster and reach higher weight than males. However, most farmed fish batches show a highly male-skewed sex-ratio. The sea bass has a complex system of sex determination, combining both genetic (polygenic) and environmental (temperature) influences. Low initial temperatures (<17°C) favor female sex determination (Piferrer et al., 2005). In contrast, a cold treatment prolonged until the late post-larval phase has been recently shown to promote male differentiation (Vandeputte et al., 2020).  In other species with environment sex determination (ESD) like Leurestes tenuis , both temperature and photoperiod can affect the sex-ratio (Brown et al., 2014). In this experiment, we tested the effect on the sex-ratio of  D. labrax, of two photoperiods (12L:12D and 10L:14D), applied during the larval phase, from 9 to 90 days post hatching (dph ), at a temperature of 16°C, crossed with the effect of four temperatures (19; 21; 23 and 25°C) applied from 90 dph until the fish were about 80 mm length, a time where sex can be considered definitively fixed.

Material and methods

 All experiments were conducted in the Experimental Marine Aquaculture platform of Ifremer at Palavas-les-Flots , France. Fish were from a complete factorial mating of 10 males and 8 females from a West Mediterranean strain. Fertilized eggs (23,000 / tank) were directly dispatched into six 500 L tanks. Photoperiod treatments (12L: 12D and 10L:14D ) were done in triplicate, from 9 to 90 dph, with water temperature kept at 16°C.
Temperature was gradually increased starting at 85 dph. F ish were transferred to another room at 87, 88, 89 and 90 dph , when temperature reached 19, 21, 23 and 25°C, respectively. The o ther room had four independent recirculating systems with four tanks of 110L each, stabilized at 19, 21, 23 or 25°C. Two tanks in each circuit received 240 fish from the 12D:12D photoperiodic treatment and  the other two  240 fish from the 10L:14D treatment. The photoperiod applied until the end of the experiment was 12L:12D for all groups .  Thermal treatment was stopped when fish reached a length about 8 cm (1500 day.degrees, calculated on a 10°C basis – DD10°C ), a size at which sex is completely determined.  At this time fish  were individually weighed, measured and tagged with RFID glass tags, and transferred to 1.5 m3 tanks at 22°C.  All fish from the same temperature treatment were pooled in the same tank, and at 242 dph , were individually measured and transferred to a 5 m3 tank. Fish from the 19°C and 21°C treatments were pooled in a same 5 m3 tank, and a second tank received fish from  the  23 and 25°C treatments. They were grown at 23°C until  sexing at  409 dph , where 1926 fish were measured and sexed. Data were analyzed using R software. Temperature, photoperiod, interactions and tank effect on sex-ratio were tested with a logit generalized linear mixed model fitted with lme4 package in R.  Sex, temperature, photoperiod, interactions and tank effect on final weight were tested with a linear mixed model.  Post-hoc differences between treatments were tested by multiple comparisons of means with Tukey adjustment.

 Results and discussion

Photoperiod had no significant effect on sex-ratio (p>0.45), while temperature effect on sex-ratio was highly significant (p< 0.0001). At 19°C, the proportion of females was 30.4%; it was 36.5% at 21°C, 44.0% at 23°C and 49 .0%  at 25°C (see fig. 1). Compared to 19°C, odds-ratios were 1.31 for 21°C, 1.78 for 23°C and 2.17 for 25°C, meaning t hat at high temperature (23 and 25°C), the probability to be (or become) a female was twice that observed at 19°C.  No interaction between temperature and photoperiod was found (p>0.088).  Up to now, high temperatures during  the sex determination period were known to promote male development, in sea bass but also in most fish species with TSD (Ospina -Alvarez and Piferrer , 2008). Here we showed that high temperatures applied to larvae after a cold (16°C) first period, can significantly increase the percentage  of females in European sea bass. Early photoperiod had no effect on the final weight of fish (p>0.33), while early temperature treatments showed a long-term effect on weight later on, at 409 dph . At this age, the mean weight was 101±1.92g for the 19°C treatment; 107±1.98g for 21°C; 124±1.92g for 23°C and 119±2.13g for 25°C. Sex also had an effect on weight (p<0.001 males: 102±1.18g, females: 123±1.39g). No effect of interaction between temperature, sex or photoperiod, on the weight at 409 dph was found (p>005). Since the differences between 23 and 25°C treatment are not significant neither for sex-ratio nor for body weight, we propose a treatment at 23°C during the post larval stage (90-160 dph), following an initial 90 days at 16°C, to significantly increase both female ratio and final weight, with a moderate energy cost, in a West Mediterranean strain of sea bass.

Acknowledgements

This study was supported by the French Ministry of Environment under grant CRECHE2020

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