3.1

Despite being a large seahorse, and reasonably widespread around both Australia and New Zealand, there is very little known about this seahorse in its natural habitat. There has also been recent taxonomic contention as to whether H. abdominalis constitutes two species: H. abdominalis and H. bleekeri. For these reasons, H. abdominalis is presently listed as Data Deficient, with the recommendation that representative populations throughout the species range be monitored (particularly in New Zealand), given significant and recent declines in the Derwent Estuary, Tasmania (Martin-Smith and Vincent 2005).

No historical data exist for in situ population structure or longevity and little information on growth rates. Only one project has assessed trends in abundance of H. abdominalis where declines of 79–98% were observed in three areas of the Derwent Estuary, Tasmania, Australia between 2001 and 2004 (Martin-Smith and Vincent 2005). Unpublished data from a study of a population of H. abdominalis in Sydney Harbour, NSW, Australia showed no clear trends in abundance between Jan’ 2003 and Jan’ 2005 (K. Martin-Smith pers. comm.). Information from research trawl collections in New Zealand, suggests generally low biomass (e.g., Stevenson and Beentjes 2001), although such trawls are usually conducted away from habitats such as macroalgal stands, where seahorse biomass is likely to be highest. Most available information relates to ex situ breeding, growth, survival and tagging in relation to aquaculture (Woods 2000, 2003a-d & 2005, Woods and Valentino 2003, Woods and Martin-Smith 2004). There is some information available on aspects of reproduction and diet in wild (in-situ and ex-situ) H. abdominalis from Wellington Harbour (North Island, New Zealand) (Woods 2002 [in press], Poortenaar et al. 2004).

The species is thought to face five main threats: 1. bycatch in commercial fisheries (pelagic finfish, benthic shellfish and invertebrate fisheries); 2. unregulated take; 3. risk through intrinsic life history traits; 4. natural predation; and 5. insufficient protection under the CITES 10 cm minimum size limit. Bycatch exploitation appears to be limited in geographical area relative to estimated extent of occurrence and is not suggestive of large-scale exploitation. Unregulated take could become a growing problem if demand in extant traditional medicine markets or in the home aquarium trade increase. In Australia, H. abdominalis would be protected under present legislation. However, the absence of catch limits in New Zealand in terms of numbers or sizes, may present the potential for exploitation to threaten wild populations. This is particularly true in light of the poor protection afforded via CITES. The species has a few life history traits that suggest vulnerability to exploitation (low density), but also other traits thought to confer resilience (rapid growth, short life span, rapid maturation and small size). The synergistic effects of natural predation in combination with exploitation need study.

A relatively conservative measure of the extent of occurrence for H. abdominalis in both Australia and New Zealand can be estimated by using GIS data to calculate the possible area in km2 down to 40 m, which is regarded as the usual depth limit for this species (Paulin and Roberts 1990). For New Zealand, this possible extent of occurrence does not take into account offshore islands (e.g., Chatham Islands, Three Kings Islands etc.) or estuarine areas. This 40 m depth extent of occurrence is calculated at 42,621 km² for New Zealand coastal waters. If the bathymetric constraint is pushed down to 100 m depth (maximum recorded depth for H. abdominalis is 104 m), then the total estimated extent of occurrence becomes 70,117 km² (see Figure 1). Similarly, for Australia, the extent of occurrence is calculated at 176,947 km² for a 0–40 m depth range and 336,696 km² for a 0–100 m distribution. Australian calculations were based on GIS ETOPO2 data with 8 km² grid cell resolution, so numbers should be treated as order of magnitude estimates.

The absence of information on population size, structure and many aspects of the species’ ecology presently precludes the ability to estimate or infer global population trends for H.abdominalis.

Follow the link below for Figure 1: known bathymetric boundaries for the species around coastal mainland New Zealand.