Abstract:
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.