Freshwater fishes (Actinopterygii) of Kenyir Reservoir, Peninsular Malaysia: Updated checklist, taxonomic concerns and alien species

Abstract Background A total of 87 freshwater fish species from 30 families were recorded from the Kenyir Reservoir, Peninsular Malaysia, where 75 are native and 12 are introduced species. Few species still have unstable taxonomy identities which urge further studies. Most of the species were categorised as Least Concern (LC) and two were threatened species; Endangered and Critically Endangered (EN and CR). One introduced species, Gambusiaaffinis is widespread in the human-associated area, while other introduced fish species can be considered low in numbers. New information Twenty five fish species are recorded for the first time in the Kenyir Reservoir.


Introduction
Reservoirs have generated high economic impact via inland fisheries, especially in the Asian region (Tessier et al. 2016).The number of reservoirs is also expected to increase due to economic development, climate change and human population (Zarfl et al. 2014).This has led to the fragmentation of the major rivers worldwide because of dam construction (Santos et al. 2013).The concern arises whether this fragmentation will be affecting the aquatic ecosystems at temporal and permanent spatial isolation.
Peninsular Malaysia has more than 70 reservoirs used for hydropower and agriculture purposes (Syuhada et al. 2018).One of them is Kenyir Reservoir, formed by damming the Terengganu River from 1978 until its maximum water capacity in 1985.Cramphorn (1983) had studied fish community of the Terengganu River, before the dam was wholly inundated in 1985.Since then, several researchers have conducted several studies to document the fish species recorded in this reservoir (post-inundation) (Ambak and Jalal 1998, Amirrudin et al. 2002, Kamaruddin et al. 2011).As the taxonomic revision and studies on fish advanced, many scientific names have been synonymised which need the latest update and additional information.Apart from that, some species are rarely been recorded and sometimes recorded only once or twice, for the last 40 years.
In this article, we gathered the previously-published data and our current data to review and improve the information on the fish species richness in the Kenyir Reservoir, coupled with some findings on the taxonomic ambiguity, alien invasive species (AIS) and some new records of the fish fauna.This study is essential and aligns with Malaysian Sustainable Development Goal 15 (SDG) which aims to conserve and restore the terrestrial and freshwater ecosystem, as well as to prevent invasive alien species in the water ecosystems.

Study area
Kenyir is the largest freshwater reservoir in Peninsular Malaysia with approximately 260,000 hectares of catchment area (Ambak and Jalal 1998), located on the east coast of Peninsular Malaysia (5°05'20.1"N102°43'26.5"E) in the Terengganu State (Fig. 1).There were numerous headwater streams with rocky and sandy habitats and more than ten large rivers flowing into Kenyir Reservoir (Ambak and Jalal 2006).The Reservoir comprises of 52.83% forest cover and the other 34.23% is a multitype land-use area (Kamarudin et al. 2018).It experiences northeast monsoon from November until February annually.

Sampling
All sampling events were carried out with the permission by the Department of Fisheries, Terengganu.An electrofishing technique using backpack electro-shocker model LR-24 (Smith Roots Inc.) was used following Aqmal- Naser et al. (in press).The ethical procedure for fish collection follows guideline by Bennett et al. (2016).The data including unpublished data are owned by the corresponding author, collected in 2008, 2013, 2017, 2018, 2019 and 2020.Three units of gill nets (mesh sizes of 1.0, 2.0 and 4.0 inches; 2.5, 5.1 and 10.2 centimeters) were used to collect pelagic fish species and scoop nets were also used to collect small fish-like loaches.Interviews with the boat operators and local fishermen have also been conducted regarding the species that can be found in Kenyir Reservoir (personal communication).

Species identification
The fish were identified in situ when possible based on Zakaria-Ismail et al. (2019).The voucher specimens were fixed in 10% formalin and left for two weeks before rinsing and transferred into 70% alcohol for long-term storage.The preserved specimens were kept in Universiti Malaysia Terengganu Zoological Collection (UMTZC).Species validity and the spelling of the scientific names follow the California Academy of Science's Catalogue of Fishes (Fricke et al. 2021).

Notes:
The distribution of Chitala lopis is restricted to Java, Indonesia ( Kottelat and Widjanarti 2005), which raises questions about the current taxonomic identity of other identified Chitala lopis outside its native distribution ranges.The species is declared extinct in Java (Ng 2020) as it was not recorded for the past 100 years.The individuals resembling Chitala borneensis were also recorded from Peninsular Malaysia in the Endau and Terengganu River (Mohd Ilham Norhakim Lokman, pers. comm.).The identity of Chitala spp.from Peninsular Malaysia needs further taxonomic clarification based on morphology and molecular works (Fig. 2).

Conservation status: NE
Notes: Clupeichthys aesarnensis is restricted to the river systems in Cambodia, Laos, Thailand and Vietnam (Di Dario 2018).The distribution of freshwater clupeid in Peninsular Malaysia, Clupeichthys perakensis is restricted to the Perak River system ( Ng et al. 2019).Hence, the species recorded in this study (identified as Clupeichthys sp.) (Fig. 3) are neither of these species and need further study on their taxonomic identities (Aqmal-Naser et al. 2021).Freshwater fishes (Actinopterygii) of Kenyir Reservoir, Peninsular Malaysia: ... Native status: Native species.

Osteochilus vittatus (Valenciennes, 1842)
Notes: New record to Kenyir Reservoir (Fig. 12).A common fish that thrives well in flood-plain or man-made habitats (Aqmal-Naser and Ahmad 2018a, Aqmal-Naser and Ahmad 2018b).Previously, this species is known as a native species in Peninsular Malaysia (Mohsin and Ambak 1983).However, it has been treated as an introduced species by Khan et al. (1996) and all recent studies without any justification.The identity of this species will soon be determined and the article on its status is being prepared.Hence, at the moment, we regarded this species as introduced species.

Conservation status: NE
Notes: New record to Kenyir Reservoir.The Rasbora sp. 1 in this study has a thicker and anteriorly tapered mid-lateral stripe and the subdorsal blotch is absent, which did not fit into the description of Rasbora notura by Kottelat (2005) (Fig. 13).

Conservation status: NE
Notes: New record to Kenyir Reservoir.There were two forms of Rasbora sp.collected in this study.A more common form has an incomplete mid-lateral stripe (not reaching gill opening) which is different from the real Rasbora paviana (Fig. 15A).It has the midlateral line that begins to diffuse at the mid-section of the body, known as a midhumeral diffuse patch (MDP) (Lumbantobing 2014) (Fig. 15B).One specimen displayed a unique set of characteristics: high and pointed dorsal fin (vs.rounded), long and, when adpressed, reaching beyond the base of the anal fin (vs.only half); pectoral fin long and overlapping pelvic fin (vs.not reaching the pelvic fin); pelvic fin elongated and reaching half of the dorsal fin base (vs.not overlapping) and possessing diamondshape caudle peduncle blotch (vs.longitudinally elongated diamond) (Fig. 15C).Due to a lack of specimens, a detailed comparison was not made at this moment.Native status: Native species.

Conservation status: LC
Notes: New record to Kenyir Reservoir (Fig. 22).In his study, Cramphorn (1983) recorded the presence of Mystacoleucus chilopterus, but the post-inundation surveys failed to record this species.This species could have been present before, but was diminished or reduced as a result of impoundment or we did not hit the right spot to collect this species.This species is a common species found in the larger rivers, especially in the eastern part of Peninsular Malaysia (Zakaria-Ismail et al. 2019).

Mastacembelus favus
Other species, such as Barbichtyhs laevis and Wallagonia leerii, were only recorded during the early post-inundation (Yusoff et al. 1995, Ambak andJalal 1998).It was known that Barbichthys laevis cannot cope with the impoundment (Rainboth 1996), while Wallagonia leerii is a highly migratory species Riede (2004) where the impoundment could have impacted the migratory pathway for these species.Further assessments are needed to confirm the presence of these species through comprehensive field sampling.One species, Scleropages formosus, is now facing the threat for its highly ornamental values.
More studies also are encouraged to be done on other species especially in the Data Deficient category especially Tor tambra ( Walton et al. 2016) and Betta stigmosa ( Low 2019) for the updated status on their distribution and populations.We also did find any species in the ghost nets in the streams that are vulnerable to migratory fish species especially, cyprinids fish, concurrent with the finding of Aqmal-Naser and Ahmad (2020), which found dead Tor tambra entangled in the ghost net.Nevertheless, it is very difficult to monitor or control the fishing activities in Kenyir Reservoir due to its massive waterbody and limited enforcement.
There were three native species which were introduced for fisheries enrichment (i.e.Pangasius nasutus and Probarbus jullieni) and sports fishing (i.e.Lates calcarifer).Both P.
jullieni and Lates calcarifer are thriving well in the lentic ecosystem of Kenyir Reservoir and were collected by the local people for artisanal fisheries but no information regarding Pangasius nasutus till now.The mosquito-fish, Gambusia affinis is the most widespread especially in the human-associated area.It is unknown how this species ended up in Kenyir Reservoir, but probably a result of aquarium dumping, one of the major practices which lead to the alien fish species introduction (Aqmal-Naser and Ahmad 2020).However, other introduced species are rarely seen or collected, which sparks a debate on their ability to survive.For example, the juveniles of Barbonymus gonionotus and Hypopthalmichthys nobilis have been released by the Department of Fisheries (Yusoff et al. (1995), but they have never been reported or collected till the present day.The fish could be present and caught by the local fisherman, but there are no documentation and records.
All six fish species highlighted required taxonomic revision and molecular evidence to elucidate their true taxonomic lineage.The cryptic diversity amongst freshwater fishes, especially from the genus Rasbora, is one of the interesting subjects to begin with.We recommend more studies that integrate both morphology and the molecular aspect to resolve the taxonomic ambiguity of freshwater fishes, generally.The new fish records in Kenyir Reservoir revealed its importance as one of the conservation areas in Peninsular Malaysia.The presence of successful alien fish species like Gambusia affinis which can have negative impacts on the native aquatic fauna should be monitored.

Figure 1 .
Figure 1.The location of Kenyir Reservoir on the eastern part of Peninsular Malaysia within the red square (left).Streams and rivers sampled haphazardly in 2008, 2013, 2017, 2018, 2019 and 2020 are represented by the red circles (primary data).

(Fowler, 1937)
Introduced species.The species was introduced into Kenyir Reservoir through the restocking programme in 1988-1990 by the Department of Fisheries.