Solving Problems in Seahorse Culture
More than 20 million seahorses are traded worldwide each year and there is every indication that seahorse sales are on the rise. Although the vast majority of these are destined for use in Chinese traditional medicine, substantial numbers are also taken for the aquarium trade and shell shops. Seahorses’ low fecundity, monogamous mating behavior, and limited species ranges, place them in a precarious ecological position. The sustainability of this fishery has not been fully determined, but it is very likely being exceeded for many species.
Although seahorses have been spawned and reared in captivity for many years, a simple, effective, and reliable culture protocol is still sorely needed. Many articles to date document overly complex rearing systems or lack essential details to protect a commercial interest on the part of the author.
Large-scale aquaculture operations with high-tech systems may relieve some of the stresses placed on wild populations, however many of the species most at risk of over-harvesting are collected by subsistence fishermen in island nations such as Indonesia and the Philippines where the latest aquaculture technology may not be an option. Project Seahorse has organized an initiative to teach aquaculture methods to these fishermen, but technology remains a problem. The development of rearing techniques utilizing simple, inexpensive equipment would be in the best interest of these indigenous peoples and the wild seahorse stocks being exploited. It would also be helpful for the home hobbyist who would love to raise the young seahorses inevitably produced as a result of good seahorse husbandry, without spending a small fortune. The purpose of this article is to report on my preliminary rearing trials and the solutions to some of the problems I have encountered.
At the Hofstra University Aquaculture Laboratory in Hempstead, NY an effort is underway to solve some of the problems preventing efficient seahorse culture. Two of the major stumbling blocks encountered in the rearing stage are: the inability to provide a sufficiently nutritious first food, and an apparent gas bladder problem that can afflict more than 50% of a brood within the first few days. The affected juveniles become trapped at the surface and eventually die.
Newborn seahorses will only eat live food, and, to date, Artemia has been the most popular choice in culture attempts because of its availability and convenience. Unfortunately, Artemia are separated from ocean food webs by several million years of evolution. So it should come as no surprise that marine organisms are not well adapted to digesting them and utilizing their nutrients. Copepods, on the other hand, are probably the most abundant multicellular organisms in the oceans and are heavily exploited by innumerable marine species. In aquaculture, they have proven time after time to be nutritionally superior to Artemia as a food source for larval and juvenile marine fishes. It is generally accepted that the reason for this is related to the levels and proportions of highly unsaturated fatty acids (HUFAs) in copepods versus Artemia. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are of particular importance. A number of enriching products are available that can be used to enhance the nutritional value of Artemia. Most of these products attempt to impart a fatty acid profile similar to that of a copepod. Unfortunately the use of these products still has not achieved the success provided by copepods in terms of growth, survival, and overall health of cultured fishes. The problem with copepods is that, in spite of their abundance, we have not yet developed a technique to culture them in high enough densities to make their use economically viable. Some researchers and aquaculturists have resorted to catching wild copepods for use in fish rearing, however this can be labor-intensive and cost-prohibitive. It can also introduce parasites and fouling organisms into the rearing system. In the 1970s, using wild plankton (dominated by copepods), Moe and Young became the first and only people to rear larvae of the French angelfish (Pomacanthus paru), successfully. Amazingly, in spite of raising thousands of them, they were unable to turn a profit, partially because of the cost of obtaining sufficient numbers of copepods.
A detailed experiment, aimed at examining the copepod/Artemia trade-off in seahorse culture has been designed and will be conducted at the Hofstra University Aquaculture lab over the next few months. To date, three preliminary rearing trials have taken place. The purpose of these trials was to investigate some feeding regimes and filtration designs that might ultimately be used in the experiment.
Four pairs of adult Hippocampus erectus were collected off Long Island, New York, and housed in a 55- gallon aquarium. Filtration consisted of a power head-driven undergravel filter with 2 inches of crushed coral, and an Ehiem canister filter with activated carbon and nitrate sponge from Kent Marine. The broodstock was fed four times per day with frozen mysids (Mysis relicta) and chopped shore shrimp (Palaemonetes pugio). A light timer provided a 15-hour light photoperiod. Courtship behavior was observed virtually every morning, but will not be discussed here. Receptive males indicate their readiness to spawn with a swollen pouch. Actual copulation was rarely observed, but if the same male was seen with a swollen pouch for more than 3 days, he was considered to be pregnant and placed in a rearing tank. Occasionally the pouch would be deflated a few hours later, but most of the time this criterion indicated an actual pregnancy.
All rearing trials were conducted in standard 10-gallon tanks, each with an entire brood of Hippocampus erectus. In trials 1 and 2, small air driven foam filters were used. In trial 3, one end of the tank was sectioned off with a Penn Plax tank divider. A piece of 500-micron nylon screen was used in place of the plastic divider insert. The divider was placed at an angle of approximately 30 ° to the end of the tank.
The resulting compartment was filled with small plastic beads, which were slightly negatively buoyant. The beads were kept in motion with aeration, forming a simple fluidized bed filter. In this tank, an 8-inch air diffuser was placed on the floor of the tank, at the end with the filter compartment to prevent seahorses from being drawn into the netting. This method of aeration created a vertical circulation pattern in the tank that kept most of the newborn seahorses from becoming caught on the surface of the water, as was experienced in trials 1 and 2. In each trial, live Isochrysis galbana was maintained at a density sufficient to keep a 2-cm secchi disk out of sight at a distance of 50 cm. All tank bottoms were siphoned daily. In the first trial, nauplii of Artemia salina, enriched with Super HUFA (Salt Creek, Inc.) were offered as the exclusive food. After 2 weeks 100% mortality was observed in this tank. In the second trial, copepod-dominated wild plankton (CDWP) was substituted as a live food for the first 2 weeks, after which time the diet was abruptly switched to enriched Artemia salina. At the end of the 60-day trial, 190 seahorses were counted. In the third trial CDWP was offered for only 3 days, followed by enriched nauplii of Artemia franciscanis. After 60 days, 214 seahorses were counted and moved into a 40-gallon grow-out tank. Food densities were maintained at 0.5-2/ml. Synthetic seawater in all broodstock, rearing, growout and plankton culture tanks was maintained at 25ppt with Instant Ocean synthetic sea salt. Artemia cysts were decapsulated with household bleach (5.25% Sodium Hypochlorite). Isochrysis was cultured according to the methods outlined in the Plankton Culture Manual by Hoff and Snell.
Obviously these preliminary trials do not represent controlled experiments. Percent survival could not be calculated or even estimated for trials 2 and 3 because the initial brood size was not counted. All that can be said is that copepods appear to play a crucial role in the early diet of H. erectus. The controlled experiments planned for the coming months are aimed at determining the effects of variations of the copepod/Artemia feeding regime during the first week, on the growth and survival of H. erectus after one month. The results of these experiments will be discussed in a future Sea Scope article.
Although I have not had a problem with parasites associated with the use of wild plankton, a number of other pests were introduced to the tanks along with the desirable plankton. The worst of these were hydroids, gammarid amphipods, and gastrotrichs. Hydroids were probably introduced as larvae or medusae. Within a few days of introduction, fuzzy colonies could be seen on the walls of the tank. Their stinging tentacles can injure and even kill small seahorses, but they probably do more damage by competing for food. By the time a hydroid colony reaches a few inches in diameter, it can become a serious drain on your plankton supply. Three hydroid genera have been observed in our aquaria: Bouganvillia, Tubularia, and one other, which has not yet been identified. Scraping the colonies off the walls only helps temporarily. Within a few days, numerous new colonies are formed around the tank as a result of fragmentation.
In an attempt to learn more about these hydroids, we began culturing them in separate tanks. After a few weeks of culturing, a tiny nudibranch, identified as Tenellia fuscata, appeared in some of the hydroid tanks. White egg clusters also appeared among the hydroid tentacles. The 6-mm nudibranchs were observed feeding on the tentacles of all three hydroid species. Within about a month of their initial appearance, hundreds of nudibranchs could be seen in these tanks and virtually all of the hydroids had been consumed. Hydroid-eating nudibranchs such as Tenellia have appendages called cerata on their backs. As they feed on their cnidarian (corals, anemones, hydroids and jellyfishes) hosts, they are able to keep the cnidocytes (stinging cells found in cnidarian tentacles) intact. They transfer the cnidocytes into their cerata to sting would-be predators that dare to bite into them. Hopefully, with the help of T. fuscata, hydroids will no longer be a problem in our rearing tanks.
The amphipods came in as small but fast-growing juveniles. Although they are an important component of the seahorses’ diet in the wild, in a culture tank they will quickly grow too large for a young seahorse to ingest and are very competitive planktivores. Siphoning seems to be the best technique for removing amphipods. Once removed they can be used as a nutritious, live treat for adult seahorses.
Gastrotrichs are an obscure phylum of free-living worm-like organisms that normally live interstitially in sediment. I have often seen large populations of an unidentified gastrotrich crawling on the skin of young seahorses that have been exposed to wild plankton. I don’t believe that the gastrotrichs are parasitizing the seahorses, but they do appear to be a source of stress as they stimulate incessant scratching. They can be removed with a fresh-water bath.
If you are interested in keeping and/or breeding seahorses, I encourage you to take some time to research their requirements before making a purchase. In many respects seahorses are hardy fishes, however they have a number of unique needs that must be met if they are to survive and thrive in the home aquarium. One of the most important of these is the need to be in a non-competitive environment. The sad fact is that the vast majority of fishes in the trade will not be suitable tank-mates for seahorses. You should also make every attempt to acquire captive-bred seahorses as these will be well adapted to aquarium life and will not have been subjected to many of the diseases and other stresses that contribute to the high mortality rate of wild-caught animals.