Improvement of natural spawning of black flounder, Paralichthys orbignyanus (Valenciennes, 1839) by photothermal and salinity conditioning in recirculating aquaculture system

Black flounder (Paralichthys orbignyanus) is an important commercial and recreational species inhabiting marine and estuarine waters from Rio de Janeiro (Brazil) to San Matías Gulf (Argentina) from 1 to 30 m depth, with a great potential for aquaculture in coastal or internal brackish waters. It is an estuarine-dependent species whose adults reproduce in marine water during the spring and summer. Since 2003, the reproduction of flounder broodstock in a recirculating aquaculture system (RAS) subject to a photothermal period and constant salinity of 34 psu has been carried out at the National Institute for Fisheries Research and Development (INIDEP). Final maturation of broodfish was regularly achieved through the use of a photoperiod and water temperature regimen simulating natural seasonal changes over a 10-month period, reaching 19–20 °C and 16 h L:8 h D by February. These spawning events were unstable and unpredictable, with wide variations in quality and quantity of eggs. Control of the reproductive cycle and development of cultured broodstocks are basic requirements for a successful aquaculture industry. In order to acquire a more precise control of egg production, the effect of salinity variation upon the quantity and quality of spawning eggs of the flounder broodstock in a RAS during three reproductive seasons was evaluated. For that purpose, salinity was changed according to photoperiod and temperature variations along the year as: a) fall-winter, 16–12 °C, salinity 25–15 psu, natural photoperiod at low light intensity (10 lx), and b) spring-summer, 16–19 °C, salinity 15–34 psu, artificial photoperiod and mean light intensity (200 lx). The result of the present study indicated that the combination of photothermal and salinity regimes effectively extended reproductive activity in the broodstock tank and increased both the number of spawning events and the amount of eggs in relation to previous years. 2015–2016 and 2016–2017 reproductive seasons lasted for >150 days, reporting a total of 52 and 53 spawning events and reaching 14,289,661 and 17,593,888 eggs released, of which 37.8% and 32.5% were viable eggs, respectively. The 2017–2018 reproductive season lasted 106 days with 55 spawning events and 27,548,896 eggs released (61.0% viable eggs), reaching the greatest number of total eggs obtained in any season at INIDEP since 2003.