Globally, striped mullet, Mugil cephalus, are a commercially important fish in both wild fisheries and aquaculture. Striped mullet aquaculture began in Taiwan in the 1970’s and has since been employed worldwide. Egypt leads mullet culture with 182,540 t of annual production from 2003-2012. Wild fry are often used in mullet aquaculture due to technological limitations that hinder captive breeding at commercial levels. Although popular abroad, striped mullet has received limited attention in the United States despite the potential for additional markets. These markets include feed in the aquarium industry, sale in the baitfish industry, stock enhancement in support of sustainable wild fisheries, and as a supplement for high-quality fish meal. The focus of this study was to initiate captive maturation research by starting two broodstock populations held in our photothermally controlled, fully recirculating, 28 m³ maturation systems.
To obtain the wild adult mullet, a commercial fisherman was hired to capture fish with cast nets. Early efforts to transport the fish back to the lab resulted in high losses. Fish were transported in 450 L totes supplied with pure oxygen for ~40 mi over land (~60 min). In the first attempt fish transported with no water treatment resulted in 20% survival. A following attempt utilized Aqui-S 20e at 10 ppm resulted in 9.1% survival. Using a product called stress coat at ~130 ppm resulted in 25% survival. We finally achieved high levels of survival (85%) by using MS-222 at 14 ppm buffered with sodium bicarbonate. This transport method allowed us to establish two populations of mullet, population one at 2.3 females to 1 male and two at 3 females to 1 male.
To attempt to mature the fish, both broodstock populations were held at their approximate photothermal maturation conditions of 22℃ and 12 hr of light for three months before they were sampled. No males were implanted/injected at either sampling/spawn. At the first sampling, population one was sampled and 5 of the 12 mature females, those with oocytes presenting at SGfg, were implanted with mGnRHa at a dose of 50 ug/kg. No eggs were harvested following this procedure. The second population of mullet was sampled the following week. Again, 5 of the 16 mature females were implanted with mGnRHa, this time at a dose of 100 ug/kg. Following the implantation of this second population, eggs were harvested 2- and 3- days post-implantation. The first release resulted in ~1.6 million eggs harvested and the second had ~1.3 million eggs; unfortunately, none of the eggs were fertilized. Once the fish were induced, they were phase shift out of maturation (30℃ and 15 hr of light) to rest and given a hyposalinity treatment of 5 ppt for 45 days to combat the Argulus sp. found on the fish at the sampling. Future maturation work will be directed at incorporating mGnRHa dosing for the males, as well as the females, in hopes of achieving not only egg releases, but viable eggs from these inductions.