ENABLING MAHIMAHI AQUAFARMING THROUGH THE DEVELOPMENT OF MALE MONO-SEX CULTURE

SummaryThe U.S. seafood trade deficit in 2012 was over $11.2 billion and is increasing every year. Demand for seafood is increasing, but capture fisheries face closures, declining stocks and increasing regulations. Aquaculture is the only viable solution, but coastal fish farming faces environmental and public policy limitations. Open ocean aquaculture offers tremendous expansion opportunities, but there are few suitable marine species.Mahimahi is a very popular fish with a world catch in 2012 of about 100,000 tonnes, up from 10,000 tonnes in1980. The US imported 25,000 tonnes of mahimahi worth $233 million in that year (NOAA 2013). Mahimahi readily spawn in captivity and large scale hatchery technology is well known. Growth rates are phenomenal (9kg in the first year) and there are strong, established markets. However, earlier commercial trials have not been successful. After six months, the fish become sexually mature. Female growth then slows dramatically and males become very aggressive towards other males. Fish harvested at this small size have low market prices. Mono-sex culture of mahimahi could alleviate these problems and allow more efficient and profitable culture.There are three main pathways to obtain mono-sex populations:Physical Sorting - This is not practical as they do not express secondary sexual characteristics (i.e., externally visible sexual dimorphism) until they are several months old, which is too large to separate them without causing high mortality.Temperature Shock - Exposing developing embryos to lower than normal water temperatures can result in skewed sex ratios, potentially 100% males. This has been noted in several freshwater and marine fishes and there is anecdotal evidence that this can occur in mahimahi.Hormone mediated sex reversal. Hormones applied at the right time in development can result in sex changed fish. These fish crossed with other sexually manipulated fish can result in all-male progeny.At least one of these techniques will hopefully be successful and establish the basis for a new aquaculture industry that can be applied throughout the tropical seas of the world. This new industry would have the potential of producing a massive amount of high quality marine fish in eco-friendly offshore cage systems.