| Resumo: |
Intensive culture of teleost fish introduces changes over the environmental conditions found in their spawning grounds, leading to reproductive dysfunctions. The manipulation of these parameters (photoperiod, temperature) represents a common technique to alter the reproductive rhythms and enhance the spawnings in terms of quality and quantity. Nevertheless, for each species some modifications or adjustments have to be done according to their reproductive strategies. The advantages of manipulating those parameters are primarily to schedule the spawning for a desired period of time and to obtain good quality and quantity eggs. Many factors such as tank dimension, light intensity, nutritional status and husbandry of broodfish and minimum stress level have been proved to greatly influence the reproductive behavior and the production of eggs of Paralichtys orbignyanus broodstock. The reproductive peak for this species under controlled conditions occured between October and February, when temperature ranged from 19~' to 22~'C and the photoperiod was 14 hs light. Under these conditions, fertilized eggs (23.9%-80.9%) from natural tank release were collected at a frequency of 1-3 days. Natural spawnings allows a better egg and larval production compared to stripping. The eggs and larvae obtained from natural spawnings were significantly (0.01<p<0.05) bigger in diameter and total length, respectively, than those obtained through stripping. The total number of eggs collected by natural spawning was three times bigger than by stripping, while the average fecundity rate doubled. The extend of the maturation period and the photoperiod increase, together with the supply of fresh squid as the main food item, must be crucial for the reproductive success in terms of quality and quantity eggs produced by flounder P. orbignyanus. Moreover, broodstock females under captive conditions have a fecundity of 300,000 to 580,000 total eggs/female/ reproductive season, and those weighing < 2.9 kg produced eggs with a fecundity rate >50%. Egg diameter, fecundity and hatching rate and total larvae length are considered good criteria for determining egg and larval quality of P. orbignyanus. In juveniles of this species, early gonad differentiation occurs between 41 and 75 mm TL, corresponding to 105 days post hatching (3.5 months of age) and 169 days post hatching (5.6 months of age), respectively. Ultrasound is a non-invasive diagnostic tool used in sex determination and maturity assessment of broodstock. The ovaries are readily distinguishable and characterized by two ovoid and granular greyish-white lobes, while testes are much smaller and darker. Lab-reared males of this species produced average sperm volumes of 0.1 to 1.3 ml / stripping. Juvenile females reared at 23~'C were heavier and bigger in size than those reared at 20~'C, while the sex ratio was almost 1:1. It was shown that the reproductive potential of flounder P. orbignyanus in captivity is significantly decreased, compared with wild specimens. This would be related to increased atresia processes due to the stress generated by captive conditions. Moreover, there was no relationship between broodstock size, reproductive potential and spawning quality, which also may have been masked by the physiological condition of spawners who stay longer in captivity.
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