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Increasing demand for seahorses for use in traditional medicines has prompted interest in finding ways of "farming" these fragile creatures.
Recently there have been numerous media articles surfacing concerning the exploitation of seahorses by humans and the conservation issues related to this exploitation.
Such articles are a reflection of the worldwide concern over the large number of seahorses which are taken from the wild, and the associated decline in some wild stocks of seahorses which have been observed.
Involving at least 39 countries worldwide, the international trade in seahorses is relatively large, but not well documented. The majority of exploited seahorses are destined for use in the traditional medicine trade in Asia, where they are used as an ingredient for balancing the body's internal energies and curing a range of ailments such as goiter, high cholesterol and various skin disorders.
As with many other natural ingredients which form part of traditional medicine, not a lot is known about the efficacy of seahorses as a medicinal component by western medicine.
In 1994, Amanda Vincent of the international organization Project Seahorse estimated that at least 20 million seahorses were being collected each year for this medicinal trade. Hundreds of thousands of seahorses are also collected each year to be sold in the aquarium trade, and as dried curios and trinkets.
In addition to such exploitation, wild stocks of seahorses are also being seriously impacted upon by a range of other human activities such as pollution and seine-netting by commercial fishers through seahorse habitats. With their relatively low fecundity, lengthy brooding period and low rates of migration, seahorse populations in many areas simply cannot cope with these devastating anthropogenic impacts.
The reduction in some seahorse wild stocks, and the clear need to protect these wild stocks, has focused attention on how little we know about seahorses. Despite the fact that most people know what a seahorse looks like, seahorses seem to have escaped widespread serious biological investigation.
In view of this, Project Seahorse has initiated a number of international programs to increase our understanding of seahorses and how best to conserve wildstocks while still protecting the livelihoods of fishing communities which depend on seahorses for part of their incomes.
One of these programs is the examination of the feasibility of culturing seahorses.
The aquaculture of seahorses may provide a useful tool in helping to conserve wildstocks, while at the same time providing a new source of economic revenue; aquaculture and the conservation of seahorses are certainly not mutually exclusive courses of action.
Seahorse aquaculture is of particular interest to those communities who already exploit seahorses, and whose livelihoods and welfare are being affected as a result of their exploitation. It is also of interest to marine farmers in countries such as Australia and New Zealand who can tap into established trade connections and utilize existing aquaculture infrastructures.
However, because of certain difficulties with variable rates of juvenile mortality and the ability to supply enough high quality live foods to feed seahorses, which are commonly encountered with seahorse aquaculture, the aquaculture of seahorses is certainly not a panacea for halting the decline in seahorse populations in the immediate future.
Between 1997 and 1999, NIWA undertook a preliminary investigation into the aquaculture of the seahorse Hippocampus abdominalis. This species is common around coastal New Zealand and is one of the largest of all seahorse species (up to 30-35cm in length).
The aim of NIWA's investigation was to gain experience with the basic husbandry and maintenance of adult seahorses and the subsequent rearing of the juveniles which they produced so that this knowledge could be passed on to potential aquaculturists, aquarium owners and conservationists alike.
Initially, 12 adult seahorses were collected from Wellington Harbour and maintained in NIWA's Mahanga Bay hatchery. Here they were held in conditions which promoted courtship and breeding.
Like all other seahorse species, reproduction in H. abdominalis is unusual in that it is the male who becomes "pregnant" and broods the developing young in a specialized brood pouch until they hatch and are then released. Interestingly, male seahorses are keener to get pregnant than females are to give away their eggs.
Twelve broods of juvenile seahorses were produced over a six-month period by these 12 adults, with an average number of 270 juveniles produced per brood. Measuring at an average of 1.5 centimetre in length and 0.008 grams in weight, these juveniles were then raised in the hatchery for a year on a diet consisting of cultured brine shrimp and wild-caught glass shrimp and amphipods.
After a year of age, these young seahorses had grown to an average 11 cm in length and 3.4 g in weight, and were reproductively mature. This size is ideal for the aquarium trade but not for the traditional medicine trade where the larger a seahorse is, the higher the price it commands.
Although the results obtained in this small-scale investigation were encouraging, and have provided useful information for advising people interested in farming seahorses, they also highlighted certain areas which require further investigation if the farming of H. abdominalis is to become an economically viable option in New Zealand.
Of particular concern is the need to increase survival of the juveniles in the first few months of life when they are most vulnerable. Like many other seahorse aquaculture trials, we encountered the highest mortality within the first few months (33% survival at 1 month, 19% survival at 2 months), after which time the survival rate stabilized. After one year of culture, 10.5% of the original juveniles had survived to reach reproductive maturity.
Also of concern is determining ways to promote the most rapid growth rates through optimal culture conditions and diet. Because they usually only eat live prey and must consume a relatively large amount of prey, to culture seahorses in any significant numbers requires the large scale cultivation of live foods, and/or access to a large supply of wild food. The possibility of producing an acceptable artificial food which can be made to resemble common prey items also needs to be investigated.
Over half of the seahorses which were raised in NIWA's first generation were subsequently released into the waters their parents were originally collected from, once they passed a year of age. The remaining seahorses were retained for further study and are now producing their own broods of juveniles.
We are currently conducting trials aimed at increasing early juvenile survival rates among this second generation, which, while still in the early stages, are showing promising results with much higher survival rates being recorded than for the first generation.
Chris Woods is a researcher with NIWA, specialising in aquaculture.
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