New records of reef fish for Hong Kong have been published in many previous studies about Hong Kong reef fish faunal diversity (Sadovy and Cornish 2000; To et al. 2013), despite the more than 160 years of ichthyological research in Hong Kong (hk-fish.net 2016a). Notably, some sightings of reef fish, which would otherwise be new records to Hong Kong, were very likely artificially introduced, as noted by To et al. (2013). The occurrence of these reef fish might have originated from release by aquarium hobbyists who no longer wish to keep the fish for various reasons (To and Situ 2005). Fish records falling into this category usually have a few observable characteristics.

Firstly, if Hong Kong does not fall into the species’ documented natural range, then this may be an indication that the species was artificially introduced. An example is the sighting of Zebrasoma xanthurus (Blyth, 1852) in the same survey, which is a species from the Indian Ocean (To and Situ 2005).

Secondly, if juveniles of a species have not been recorded locally, but a fully-grown adult is sighted in a publicly accessible location or in a fish sanctuary such as marine parks, then there is a possibility that the individual was artificially introduced. Examples include a previous sighting of an adult Pomacanthus imperator (Bloch, 1787) in an easily-accessible marine park in Hong Kong (To and Situ 2005). The individual’s previous human owners might have chosen the marine park for releasing the fish after having grown tired of keeping it, believing that the marine park would offer it protection (To and Situ 2005). Past reports of a large Naso lituratus (Forster, 1801) and a fully-grown Acanthurus japonicus (Schmidt, 1931) for instance, were both locally unprecedented sightings that were rejected as new species to Hong Kong, as they were found in one of Hong Kong’s most easily accessible, popular dive sites with open-access to anyone, which pointed to the likelihood of introduction by aquarium fish release for those individuals (To et al. 2013).

Thirdly, if the species is popular and available in the aquarium trade in Hong Kong, then there is an increased probability for individual occurrences in the wild to be results of artificial introduction. Although there are no recent studies on the aquarium fish trade in Hong Kong, an earlier study revealed a large diversity of reef fish involved in the trade (Chan and Sadovy 2000). A species’ availability in the local trade can serve as an additional circumstantial evidence to the possible origin of individuals sighted in the wild, including the abovementioned sighting records of surgeonfish and angelfish. An earlier sighting of a large Aulostomus chinensis (Linnaeus, 1766) individual of brown colour-type was regarded as a valid new fish record for Hong Kong, as it was observed in a relatively remote dive site, and was not found in the local aquarium trade (To et al. 2013). These key characteristics, together with other observations such as the presence of injuries which can link to recent captivity, lack of wariness of approaching divers and the suitability of the habitat to the fish observed, can further shed light on the likely origin of the observed reef fish species and their validity as new records for Hong Kong.

Notably, in addition to release by aquarium hobbyists, reef fish are sometimes released in religious activities (To and Situ 2005; To et al. 2013). Based on the authors’ observations, reef fishes involved in this type of fish release mainly include species of Lutjanidae and Serranidae, but any other species available in local fish markets may also be purchased for release. The release of the fish can take place in near-shore areas, such as from public piers or jetties, or further from the shore areas through release from onboard fish-carrying vessels. Individuals introduced into the sea by such fish release activities may also be differentiated from naturally occurring fish by the same characteristics as described in the earlier section. For instance, the sighting of an individual of Epinephelus fuscoguttatus (Forsskål, 1775), which was observed to have injuries around the jaw, and encountered in shallow coral areas, was almost certainly a result of such fish release. The plate-size of that individual and the popularity of this species in the live reef food fish trade, which is centered in Hong Kong (Lau and Li 2000; Craig et al. 2011), further substantiates the evidence pointing towards fish release as the origin of that observed individual (To et al. 2013).

With reference to the above accounts of released fish characteristics, it arises that the present study represents the first valid documentation of the four reef fish species featured in this paper, in Hong Kong. Although these species may be available in the local aquarium trade, the individuals described were encountered in outlying islands (e.g. Tsim Chau and Ninepin Islands), or in sites that are relatively inaccessible to the public (e.g. Clearwater Bay), reducing the probability of release by aquarium fish hobbyists. In addition, fish released by aquarium fish hobbyists are typically bigger in size, abandoned by their keepers when they grow out of their tanks. The specimens observed by the authors, however, were relatively small. Furthermore, all four species presented here have previously been recorded in mainland China, meaning these species naturally occur in adjacent areas of the northern South China Sea regions. Hong Kong therefore falls within the species’ natural ranges. Last but not least, the behaviour of the fishes and their interaction with the respective habitats appeared natural, further supporting the validity of these species as new records for Hong Kong.

Regarding the origin of these new fish records, a previous study on Hong Kong reef fish assemblage compared the reef fish species in Hong Kong with those of adjacent areas, and suggested that some of the rarer or sporadically occurring tropical species occurring in Hong Kong may have been transported as larvae from south of Hong Kong by the Hainan current (Cornish 1999). The presence of rare species at their juvenile stages in Hong Kong was suggested to further support the legitimacy of this possible pathway (Cornish 1999). Such transport of juveniles of tropical species into subtropical regions has also been noted in other places, such as the occurrence of tropical and subtropical fishes in temperate southeastern Australia (Booth et al. 2007), and in New Zealand (Francis et al. 1999). Likewise, the Taiwan current from the East China Sea invades Hong Kong in winter (Morton and Morton 1983), which potentially brings subtropical or temperate fish larvae species into Hong Kong (Cornish 1999).

Many reef fishes are known to produce pelagic eggs, including species in the families Pempheridae and Labridae, to which Parapriacanthus species and Halichoeres hartzfeldii belong (Sadovy and Cornish 2000; Allen et al. 2015). Species in the families Gobiidae and Tetraodontidae, although are known to produce demersal eggs (Sadovy and Cornish 2000), once hatched the pelagic fish larvae may also become dispersed (Sadovy and Cornish 2000; Lecchini 2005). Depending on the species, the larval stage of reef fishes can last for weeks or months (Leis and McCormick 2002). Notably, all four species reported in this study have been recorded in places nearby, such as mainland China and Taiwan. The close proximity of Hong Kong to places where these species naturally occur could have facilitated the transport and settling of fish larvae of these species in Hong Kong. Therefore it is reasonable to suggest that the occurrence of these four species may have been due to the transport of pelagic fish larvae. However, whether these rare or sporadically occurring reef fish species can establish and become self-sustaining populations in local waters is yet to be investigated.

Existing studies indicate that new fish records can be associated with the addition of new artificial structures. For example, a study conducted in the Mediterranean coast of Israel laid support to the possibility that impoverished areas offered “unsaturated” environments for new fish species, and in that particular case it was filled by Red Sea immigrants, such as sweepers (Diamant et al. 1986). New structures may have been placed in a way that intercepts incoming larvae originally destined for natural reefs down current (Cenci et al. 2011), therefore new fish species could be found in these artificial structures rather than in natural reefs. Notably, although fish assemblage in certain areas can largely be an extract of a common pool of larval recruits, the fish species diversity at a certain site can be affected by microhabitat selection by different species (Kaufman and Ebersole 1984). New artificial structures may offer to these recruits a suitable habitat to settle that is unique in the area concerned (Carr and Hixon 1997). Artificial structures therefore have been suggested to potentially facilitate the settling of new fish species.

Among the four new species found in Hong Kong as reported in this study, the single individual of Canthigaster papua was found in artificial structure–a man-made sea-wall for sheltering yachts. Although this study did not investigate the uniqueness of the micro-habitat offered by this particular sea-wall, it is worth noting that, from a macro point of view, such artificial structures is not unique within waters of Hong Kong where underwater reef fish surveys are conducted. In addition, this artificial structure has been put in place for at least 15 years. Therefore it is doubtful if this structure should be considered as a new artificial structure. Whether this artificial sea wall offers an “unsaturated” environment for new species to inhabit is difficult to judge. With limited information and research into this area, the current study is unable to quantify the significance of this artificial structure to the new fish species record.

Hong Kong’s reef fish diversity, including other marine species, remains, to date, relatively unexplored. The discovery of the four species presented in this paper demonstrates the need for Hong Kong’s reef fish, as well as other understudied marine fauna in Hong Kong, to be surveyed and documented, to produce an updated inventory of local marine species.

Currently, it is the priority of governments worldwide to manage the spread of non-native species (Bax et al. 2003; Thresher and Kuris 2004), and from these attempts the challenge of defining what a “native” species is, is realized (Gilroy et al. 2016). Without a comprehensive and up to date inventory of local species, Hong Kong will soon come upon the same roadblock in its marine conservation efforts.

Any future work in Hong Kong’s marine biodiversity conservation, whether in the elimination of invasive species or the protection of native species, will greatly benefit from a clear understanding of the marine fauna present in local waters. Research into the presence and distribution of species within Hong Kong territorial waters, and the identification of biodiversity hotspots to be prioritized for protection, is hence recommended.