Aquaculture of the Big-Bellied Seahorse Hippocampus Abdominalis Lesson 1827 (Teleostei: Syngnathidae)
Authors:   , Woods Chris M C 
Published: November 2021

<p>Seahorses (Teleostei: Syngnathidae) are subjects of worldwide demand for medicinal use, as curios, and as live ornamental aquarium fish. Aquaculture has the potential to replace or at least supplement potentially unsustainable wild exploitation as the supply source of seahorses. The primary aim of the research within this thesis was to determine techniques for improving the technical and economic feasibility for commercially culturing the big-bellied seahorse Hippocampus abdominalis in New Zealand. In a preliminary investigation, the breeding of wild H. abdominalis in captivity and rearing of juveniles was examined, as difficulties have been encountered with these in historical attempts at culturing H. abdominalis. Breeding was found to be facilitated by providing tanks with a water height of 1 m. This depth of water allowed females to transfer their eggs to male seahorses during the vertical rising stage of mating. Growth rates of progeny to sexual maturity were reasonable with seahorses reaching an average 11 cm in standard length (SL) at one year of age, but high juvenile mortality was observed in the first few months of age, with an average 10.6% of juveniles surviving to one year. Further on-growing of these first generation progeny to seven years of age (average of 27 cm SL for both sexes) demonstrated the robustness of the species in captivity and potential to supply large seahorses to the medicinal trade where large size is desirable. To improve juvenile survival and growth, the effects of initial tank colour, lighting arrangement and stocking density on early juveniles were tested. Juveniles at one week of age were found to have higher attack rate and capture success on Artemia nauplii in clear jars than those contained in white- or black-wrapped jars, but this effect of tank colour had less affect on one month-old juveniles. Juveniles were also found to suffer fewer incidences of air bubble ingestion in side-illuminated tanks due to positively phototactic prey (Artemia) being drawn away from the water surface. The rearing of juveniles from birth to two months of age in glass aquaria with side-illumination and tank surfaces blacked-out above the waterline resulted in survival rates of >80% due to increased feeding efficiency and reduced risk of air bubble ingestion. Juvenile growth and survival at stocking densities of 1, 2 and 5 juveniles l-1 demonstrated that increasing stocking density resulted in reduced growth and survival, due to the greater occurrence of juveniles grasping and wrestling each other with their prehensile tails. Producing live foods for fish is a significant cost in finfish culture. This has led to concerted efforts to develop appropriate artificial or inert diets to reduce culture costs. To determine whether juvenile seahorses could be weaned from live food to inert diets, two inert diets (Golden Pearls and frozen copepods) were tested. It was demonstrated that one and two month-old juvenile H. abdominalis could ingest and survive on these inert foods. Co-feeding the inert diets with live Artemia improved feeding on the inert foods. However, growth and survival rates of juveniles on the inert diets were inferior to those fed only on live enriched Artemia. Cultured live foods such as Artemia are often enriched with various enrichment media to boost their nutritional value. However, enrichment media can vary in their nutritional value relative to the final target organisms they are being fed to, as well as their relative cost-effectiveness. Therefore, the effect of different Artemia enrichments on the growth and survival of H. abdominalis and their relative cost-effectiveness was tested using three commercial enrichment products (Super Selco®, DHA Protein Selco® and Algamac-3050®) and a low-cost Artemia on-growing diet (EPABSF/Spirulina platensis). On a cost/benefit basis, EPABSF/S. platensis worked out to be the most cost- effective for H. abdominalis, with comparable growth rates to seahorses fed Artemia enriched with DHA Protein Selco® and Algamac-3050®. Juvenile growth rates were poorest on Artemia enriched with Super Selco®. Feeding seahorses frozen mysid shrimp may help reduce culture costs and also increase cultured seahorse marketability to the aquarium trade, but their efficacy in seahorse culture is largely untested. Frozen mysids (Amblyops kempi) were shown to be an acceptable alternative to live enriched Artemia for H. abdominalis, providing comparable rates of seahorse growth and survival. When daily rations of frozen mysids at 5%, 10%, 15% and 20% wet body weight (wbw) were tested there was no growth advantage to feeding seahorses more than 5% wbw per day in terms of increase in seahorse length. There was a wet weight gain and Condition Factor (CF) advantage associated with increasing feed ration >10%. Feed conversion ratios (FCR) became less efficient as feed ration increased based on the total amount of mysids offered to seahorses, with increasing food wastage. However, when actual mysid consumption was taken into account there were no significant differences in FCR between rations. The natural diet and male reproductive output of H. abdominalis in Wellington Harbour was examined for use as aquaculture benchmarks. Natural diet consisted mainly of epibenthic and epifaunal crustaceans (e.g. amphipods, mysid shrimp and caridean shrimp). There were no sex-related differences in diet although there were some size- related differences with smaller seahorses consuming more amphipods. Some of the prey species eaten by wild H. abdominalis may show potential as cultured foods. Wild males produced an average of 271 juveniles per brood, with brood size increasing with parent male size. Comparison of wild reproductive output data with those of cultured male H. abdominalis revealed that cultured male output was approximately 27% lower than that of wild males. However, there were no differences in the quality (size and weight) of the juveniles produced by wild and cultured male H. abdominalis. It is suggested that cultured female reproductive output is the primary determinant in lower cultured male seahorse reproductive output. The research within this thesis has contributed to improving the technical and economic feasibility for commercially culturing the big-bellied seahorse Hippocampus abdominalis.</p>