Introduction
The flathead grey mullet (Mugil cephalus) has been recognised as an economically important euryhaline and eurythermal species with potential for aquaculture diversification in the Mediterranean and other regions, such as the Republic of Korea, Taiwan, and South Africa. The species positive attributes for aquaculture include good adaptation to captivity, rapid growth (Whitfield et al., 2012), omnivorous and detritivore feeding habits (Vinatea et al., 2018), and a valued food market including "bottarga" (the roe of fully mature females) with a market value exceeding 200 euros per kilogram. However, a bottleneck for mullet aquaculture development is reproductive control in captivity. Depending on the region and the broodstock holding system, reproductive dysfunctions have been reported to arrest reproductive development. In intensive systems mullet exhibited a severe reproductive dysfunction where the females fail to progress through the vitellogenesis phase and males fail to generate fluid sperm. Studies in IRTA have overcome these severe reproductive dysfunctions with a protocol of weekly injections of recombinant gonadotropins (rGth) (recombinant follicle stimulating hormone – rFsh and luteinizing hormone - rLh) to induce gametogenesis and spawning (Ramos-Júdez et al., 2022). The aim of this study was to use the rGth protocol to induce spawning outside the natural spawning period and further characterise the quality of gametes for larval culture of mullet.
Material and Methods
The experiment was conducted between March 20th and July 31st, 2023, to induce gametogenesis in mullet using recombinant hormones. The experiment involved 15 mullet randomly selected from a stock of 38 wild adult mullet introduced into IRTA´s facilities (IRTA Aquaculture, Centre de La Rápita, Spain) between 2015 and 2018. Eight females and seven males were divided into an rGths (supplied by www.raraavis-bio.com) group that was treated species specific recombinant follicle stimulating hormone and recombinant luteinizing hormone and a control group treated with saline. The eight females selected for the study had 105.8 ± 5.81 µm oocyte diameter. The treatments of the females initiated on staggered dates, every two weeks a pair of one rGths female and one control female received the first recombinant hormone and saline respectively. The treatment of each pair commenced on March 20, April 3, April 17, and May 2, 2023. This programmed the spawning of the females to be every two weeks to facilitate live food production for the larval rearing. The rGths (rFsh and rLh) were administered to mullet following the protocol described by Ramos-Júdez et al. (2022). The eggs were incubated under constant conditions, and hatched larvae were transferred to different 100 and 500 L tanks. The larvae were reared with green water (microalgae Tetraselmis Chuii and Isochrysis galbana), rotifers, Artemia and Gemma wean diets (Skretting) based on protocols from Vallainc et al., (2021). Larval growth, development and survival were registered.
Results & discussion
In the present study, both female and male mullet were induced with rGths to mature and spawn in June – July, a period outside the natural spawning season of wild populations, which typically occurs in September — October in the Western Mediterranean. All four females were during a 13-week period induced from the early stages of pre-vitellogenesis to complete maturation and ovulation. Over a 5-week period, all three rGth treated males developed from having no sperm present to producing and releasing sperm with spermiation sustained for 10 weeks. Three females spawned with males to give fertilised eggs that were incubated and larvae cultured. Juveniles were obtained with a maximum survival of 14.1 ± 5.2% to 42 days post hatch (final experiment day). In contrast, control females and males remained in the early stages of gametogenesis and did not produce viable gametes, highlighting the success of out-of-season induction with rGths. The present study has for the first time demonstrated that rGth hormones can be employed to induce gametogenesis, maturation and spawning of a teleost species out-of-season. Egg and larval quality were sufficient to show that, with further refinement, these protocols can be used by the aquaculture industry as a basis for juvenile production. The high efficacy of induced maturation, over 90% observed in the present and previous studies that developed the protocol, indicate that it offers an attractive alternative for breeder management. Therefore, the findings of this study hold crucial implications for aquaculture, as the culture of this species relies heavily on wild juvenile resources, with hatchery-produced juveniles being scarce.
Bibliography
Ramos-Júdez, S., Giménez, I., Gumbau-Pous, J., Arnold-Cruañes, L.S., Estévez, A., Duncan, N., 2022. Recombinant Fsh and Lh therapy for spawning induction of previtellogenic and early spermatogenic arrested teleost, the flathead grey mullet (Mugil cephalus). Scientific Reports 12, 1–15. https://doi.org/10.1038/s41598-022-10371-0
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Vinatea, L., Malpartida, J., Carbó, R., Andree, K.B., Gisbert, E., Estévez, A., 2018. A comparison of recirculation aquaculture systems versus biofloc technology culture system for on-growing of fry of Tinca tinca (Cyprinidae) and fry of grey Mugil cephalus (Mugilidae). Aquaculture 482, 155–161. https://doi.org/10.1016/j.aquaculture.2017.09.041
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Acknowledgements
Funding is from the Spanish Government, MINECO project RTA2021-126070OR-100 awarded to ND. GC was support by a PhD grant from the Government of Panama.