First Rearing of the Hawaiian Seahorse, Hippocampus fisheri at the Waikiki Aquarium
Since December of 1996 the Waikiki Aquarium has had success in raising seahorses thought to be Hippocampus erectus. We have been able to rear H. erectus in small glass aquariums with minimal effort and are able to get them to eat non-living foods by the time they are one month old. Some of our juveniles even consume flake food at this young age. Encouraged by this success, we decided to attempt the captive culture of the “Hawaiian” seahorse collected off the leeward coast of Oahu. Surprisingly, we found this to be much more of a challenge than we had anticipated!
The Hawaiian seahorse has been tentatively identified as Hippocampus fisheri. The life history and conservation status of this unique seahorse are unknown (Lourie, 1999). University students at the Waikiki Aquarium are conducting research projects to investigate the genetic identification of the species and its pair bonding behaviors.
Most seahorses spend their lives in shallow seagrass beds or coral reef habitats. Not so with the Hawaiian seahorse. The Hawaiian seahorses on exhibit at the Waikiki Aquarium were originally collected by fishermen who spend their nights at sea off the Waianae (western) coast of Oahu. The seahorses are attracted to the fishermen’s lights and are collected using scoop nets. These relatively small animals are found quite far offshore in very deep water. Fishermen report finding the bodies of seahorses in the stomachs of tuna and other pelagic fishes. Sometimes these seahorses can be obtained from local pet stores, but they are not at all common in the aquarium trade. Newly collected animals have a beautiful red to orange coloration, and after a few months in captivity they take on more of a yellow hue. Adults reach a length of only 8 cm and are most often found in small groups by the fishermen.
The Hawaiian seahorse is a prolific spawner, producing up to 250 hatchlings every 15 days. The 6 mm hatchlings are smaller than those released by H. erectus which average 9.8 mm. We started our larval rearing trials using the same techniques as for H. erectus and ran into problems almost immediately. While the H. erectus had exceptional survival rates, few of the Hawaiian seahorses survived more than four days. Most mortalities were attributable to small gas bubbles within the esophagus of the hatchlings. Many hatchlings also become “stuck” at the surface of the water, seemingly trapped within the surface tension. We tried various techniques to correct these problems, but to no avail. Then we had a breakthrough!
One of the male Hawaiian seahorses involved in a mate selection study looked ready to release young, so we transferred it to one of our large outdoor larval rearing tanks. We hoped that this 4,000 liter tank with its larger volume and currents would more closely resemble the open ocean habitat of these small seahorses. A gentle current is created by releasing large bubbles of air at the bottom center of the tank. As water is pulled up with the rising bubbles, it flows outward to the walls of the tank and then down to the bottom to once again rise with the bubbles to the surface. The larval tank is constructed of fiberglass and is 2.4 meters in diameter and 1 meter deep. The tank sides and bottom are painted a medium blue. We chose this color because it is light enough to allow us to see the animals and it is dark enough to allow the young fish to see the light colored food items they are hunting. This tank is located outdoors under a fiberglass roof and receives indirect sunlight. The tank is also partially covered with 50% shade cloth.
Once the young seahorses were released from the male’s pouch, we added rotifers, Brachionus plicatilis, as well as copepods, Euterpina acutifrons, to the tank as food for the new seahorses. A microalga, Tetraselmis chuii, was added as a food for the rotifers and copepods. Because the young had hatched out directly into the rearing tank we could not get an accurate total count of the hatchlings. It appeared that this was not a very large batch of hatchlings, but because they looked healthy, we decided to continue. Rearing runs like this one involving a large rearing tank demand large amounts of algae and zooplankton to keep the food density high. This represents a significant investment of effort and it is important to make sure that the hatch is good in terms of number and quality of hatchlings. Otherwise the amount of effort put into the plankton production is not worth the expected outcome.
For the first three days, food densities were maintained at 1-2 rotifers / ml and 0.5 copepods / ml. During this time water was neither added to nor removed from the tank. On the third day, the surface of the water became scummy, so we started a slow water exchange of 0.3 liters / min. We are very fortunate to have an excellent source of water from a saltwater well at the Waikiki Aquarium. A 100 micron mesh screen was placed over the center stand pipe so that water could drain, but the seahorses and most of the food items would not be flushed out. Once we initiated the flowthrough system, we had to add 30-50 liters of algae every day. Rotifers and copepods reproduced within the tank so there was no need to add additional zooplankton. Temperature in the rearing tank ranged from 77.5F to 79.0F during this phase. Although many species of young fish will usually be evenly distributed in rearing tanks, these young Hawaiian seahorses swam together in loose “herds” of ten to twenty individuals.
At the end of the second week, copepods were the dominant food organism in the tank. Rotifer numbers has declined because they are small enough to slowly escape through the 100 micron screen, and because the seahorses had been consuming them. Copepods grow well when they are fed the diatom Chaetocerus gracilis, so we began to add this alga to the rearing tank and discontinued the use of the alga T. chuii. The copepods were attracted to the sunny areas of the tank and we could see the seahorses (which at this age are a lovely golden color with black bands) snapping up the copepods with gusto.
On day 24, we took measurements of the young seahorses, and they averaged 1.6 cm total length. We decided to provide larger foods for them, by adding three day old Artemia enriched on Algamac 2000‚ for 24 hours. The seahorses soon had pink bellies full of the brine shrimp. We increased the incoming water flow and changed the standpipe screen to a 500 micron mesh to allow uneaten Artemia to flush out of the tank. This may sound a bit wasteful, especially considering the price of Artemia, but we wanted to make sure that the seahorses were eating only freshly enriched Artemia.
When the seahorses reached 8 weeks old we saw a change in their behavior. They began using their tails to hang onto the air lines and each other as well as to filamentous algae growing on the sides and bottom of the tank. They would also move slowly across the bottom dragging their tails behind them. This was quite disconcerting at first, it looked like there were dead seahorse bodies sprawled across the tank floor. On closer inspection we could see that they were moving ever so slowly, seemingly studying the bottom of the tank. We immediately started adding mashed up frozen mysids in the hopes that the seahorses would come across them during their surveys of the bottom. We are not
sure if they actually consumed the frozen mysis at this point.
On December 10, 1999, we celebrated completion of the life cycle of the Hawaiian seahorse in captivity. At the tender age of less than 4 months, our tank bred seahorses produced hatchlings of their own. Adults and hatchlings were measured from the tip of the snout to the tip of the tail with the head tilted up and the tail stretched out straight. The adult females averaged 3.8 cm, and adult males 4.2 cm. Although these adults were half the size of those collected from the wild, the average size of the hatchlings was the same as that of the hatchlings from the wild caught adults. The number of hatchlings produced in the first batches was quite small, but we expect that the number will increase as the new parents attain their full size.
We are pleased that our investment was rewarded. Three generations of the Hawaiian seahorses are now on display in the Miniature Marine Life exhibit at the Waikiki Aquarium.
Lourie, S.A., A.C.J.Vincent and H.J. Hall (1999) Seahorses: An Identification guide to the world’s species and their conservation. Project Seahorse. London, UK
Article reprinted with permission. Original publication January 2001, Drum and Croaker vol. 32.