Three rivers, Juaseh, Labis, and Segamat rivers, flowing off from the western part of the ERL, were assessed for their freshwater fish assemblages (Fig. 1). The average width and depth of Juaseh River was 8.51 meters and 0.4 meters respectively. It is estimated that the Juaseh River runs along approximately 29 km before joining the Segamat River. Similarly, the Segamat River with an average of 11.06 meters of width and 0.4 meters depth, originated from the ERJNP, flows approximately 45 km before joining with the Muar River at the west of the national park. For the Labis River, the dimensions are an average of 5.33 meters in width and depth ranged between 0.15–0.20 meters. The river starts from the hills of the Labis Forest Reserve, then flows to the south of the landscape complex before joining the Muar River. It is estimated that the Labis River runs more than 60-km before flowing into the Muar River.

Figure 1. 

The localities of the 12 substations to study the composition and distribution between freshwater fish assemblages in three rivers in ERL.

Four substations were established in each river with a minimum of 3 km intervals between one substation and another. Each substation sampled its freshwater fish along a 200-meter long transect. All substations #1 were located at the headwater of each river, where they were protected in the forest reserve, and the river’s condition was pristine from any anthropogenic activities. The vegetation conditions in each substation #1 were dense with emerging trees, and the rivers were covered with close canopies. The soil substrates were mostly boulders with sandy banks. Some sites were designated as public recreation sites, such as waterfalls.

The subsequent substations #2, #3, and #4 of each river flowed through developed areas, including open land, shrubs, housing areas, and oil palm plantations. Largely, this section of the river was lightly polluted thanks to the presence of oil palm plantations or human settlements. The vegetation along the substation was mainly shrubs with algae submerged into riverbeds and was not covered by tree canopies. The boulders and cobbles were absent in these substations but were largely covered by sand, silt, and clay (Table 1).

Fish samples were collected from 12 substations (Fig. 1) based on habitat representativeness and accessibility from September to November in the year 2021. Fish sampling in each substation was based on two approaches; electro-fishing and net scooping along the 200-m transect. The first group with a minimum of four persons were tasked with electro-fishing, while the second group conducted scooping using fine-sized scoop nets along the transect. A backpack Root-Smith LB24 electro-fisher unit was used to stun fish in each designated substation by applying a standardized voltage and current for an hour of effort. A single person operated the unit with assistance from three to four persons to collect the stunned fish using scoop nets. The second group was tasked with scooping out fish using fine-sized scoop nets from underneath wood debris, below riverbanks with hollows, submerged vegetation along the river, and any potential area within the river that was unreachable by the electro-fisher. Both approaches were conducted with an hour’s effort of fishing. All the substations sampled its freshwater fish once throughout the sampling session.

The collected fish were temporarily kept in a pail filled with water before being brought back for examination. When possible, the fish were identified to species level and released back into the river. Those that were unpreserved in 10% formalin solution were taken back to the laboratory for examination. Apart from a voucher for identification, additional samples were collected, and tissues from the specimens were excised for molecular analysis. Identification was conducted according to Zakaria-Ismail et al. (2020). All fish specimens were deposited in the Zoological Collection of Forest Research Institute Malaysia (FRIM).

All the data recorded in the field were transferred into a master list in the Microsoft Excel software with additional information on the taxonomic order. Then, the data were sorted into respective substations, and specimens of each species were counted. Also, data on water quality at each substation were analyzed and presented in Suppl. material 1.

We defined the relative abundance as (RA), RA = (Number of individuals for species A/Total number of individuals) ×100%. Species diversity can be defined as the species richness in a certain area during a certain period. Hence, we constructed a comparison boxplot to show the differences in species richness among the investigated rivers.

In order to investigate the differences in fish composition between substations, we combined fish data from each substation into four different groups; namely group 1 which consists of all substations #1, group 2 with substations #2; group 3 comprised of substations #3 and group 4 with all substations #4. Analysis of non-metric multidimensional scaling (NMDS) was conducted to infer the fish composition using the metaMDS function in the R package vegan 2.5–3 (R Core Team, Vienna, Austria). We conducted a permutational analysis of variance (PERMANOVA) to compare the fish assemblages at each substation. Before PERMANOVA, the multivariate homogeneity of group dispersions was tested using the function betadisper, and there was no difference in dispersion between groups/substations (F = 0.63, ρ = 0.61).

In total, sixty-six freshwater fish species, including 841 specimens, were collected in the 12 substations of these three rivers (Juaseh, Labis, and Segamat) flowing off from the western part of the ERL, distributed in 43 genera, 18 families and six orders (Table 2). Among them, the most species-rich order was Cypriniformes (26 genera, seven families and 39 species), followed by the order Siluriformes (six genera, four families and 10 species). Anabantiformes and Synbranchiformes with three families comprise five genera of eight and five species, respectively. Order Gobiiformes only recorded a single family and a single genus and species.

Cyprinidae was the most dominant family, with 17 species, comprising 52% of the total abundance collected from all the substations (Table 1), followed by Danionidae and Cobitidae, with six species each. The families with the lowest number of species were Pristolepididae, Belonidae, Vaillantellidae, Siluridae, Eleotridae, and Synbranchidae, with a single species each.

Crossocheilus obscurus Tan & Kottelat, 2009 was the most abundant species collected in all the substations with a total of 85 collected specimens, followed by Barbodes sellifer Kottelat & Lim, 2021 and Mystacoleucus obtusirostris (Valenciennes, 1842), with 56 and 52 specimens, respectively. Pangio malayana (Tweedie, 1956), Epalzeorhynchos kalopterum (Bleeker, 1850), Osteochilus microcephalus (Valenciennes, 1842), Nemacheilus selangoricus Duncker, 1904, Vaillantella maassi Weber & de Beaufort, 1912, Parachela oxygastroides (Bleeker, 1852), and Leiocassis micropogon (Bleeker, 1852) were the species with singleton sample collected throughout the sampling session.

Among the three rivers investigated, Segamat River recorded the highest species richness with 45 species, followed by Juaseh River with 36 species, and Labis River with 34 species of freshwater fishes. Among the investigated rivers, based on standardized effort, the highest number of specimens were captured in Segamat River (355 specimens), followed by Labis River (262 specimens), and the least was in Juaseh River (224 specimens) (Fig. 2).

Figure 2. 

The boxplot shows the species richness of freshwater fish for all three investigated rivers in the western part of the Endau-Rompin Landscape.

The fish composition among the substations varied significantly. Substation #1 recorded 33 species, as the species richness increased in substation #2 (with 40 species) and substation #3 (41 species). However, the species richness recorded at substation #4 showed a marked reduction to 29 species compared to the other substations (Table 2). The fish compositions in substation #1 of the three rivers were grouped separately compared with other substations, as shown in the NMDS ordination (Fig. 3). The fish structure in substations #2, #3, and #4 overlapped.

Figure 3. 

NMDS analysis for each substation examined along study areas. The composition of fish on substations #1 differs significantly from substations of #2, #3 and #4. Each convex hull represents Group 1 = substations #1, Group 2 = Substations 2, Group 3 = Substations #3 and Group 4 = Substations #4.

The community’s major economy in the study area is oil palm plantations and agricultural estates. The current land-use changes in the study areas, which humans mainly cause, would directly impact negatively on freshwater fish communities. It was suggested that any form of logging activity, including selective logging, widely practiced by foresters, could create a positive ecosystem for freshwater fish (Iwata et al. 2003; Wilkinson et al. 2018). Although there was no significant difference in fish diversity between logged forest and oil palm catchments in the Sabah River reported by Wilkinson et al. (2018), the forests can be constructive and influence the diversity of freshwater fish communities by increasing the heterogeneity in physical structures and food materials in freshwater habitats (Lo et al. 2020). The anthropogenic land cover change could also serve to increase the sedimentation, changes in water pH, and the availability of food materials that directly influence the number of aquatic organisms (Lo et al. 2020). Consequently, the understanding of habitat alteration, especially between forest and aquatic ecosystems, should be enhanced to provide a systematic framework for conservation programs.

Disturbances in the riparian zone along the rivers could have harmful impacts on the terrestrial communities and indirectly affect the freshwater ecosystem (Grimm et al. 2008; Vörösmarty et al. 2010). Riparian zones are the landscapes that form an interface between terrestrial and aquatic ecosystems, with disproportionate influences on the dynamics of the food web and the function of the ecosystem in both habitats. The plant communities along the riverbanks and margins are known as riparian vegetation. They are very significant for fish communities, as they influence light regimes, water quality, thermal dynamics, habitat, and food availability. The riparian buffer along Juaseh, Segamat, and Labis rivers, especially downstream, was not clearly visible as most zones were destroyed or encroached by human activities. The loss of riparian vegetation situated along these rivers could eradicate the food sources and breeding areas of the fish communities. A successful idea of adapting buffer zones has been reviewed in India, where the program, Freshwater Fish Safe Zone, has effectively aided in freshwater fish conservation (Gupta et al. 2014). Therefore, safeguarding the network of the river-riparian zone would be essential in rehabilitating the degraded habitats, which resulted from the urbanization (Coelho et al. 2014).

Freshwater fishes are important sources of protein to the rural communities that live in the inland and riverine rural areas. However, many freshwater fishes in Malaysia have been overharvested, especially the large-sized and game fishes, including Mahseer and Probarbus jullieni Sauvage, 1880 (Chong et al. 2010). Overharvesting mostly occurred in habitats that lack surveillance, which could lead to the decreasing number of freshwater fishes or extinction of the species in the wild. For certain species, such as Betta, it is believed to be overharvested due to trading for the ornamental industry since Betta is very famous and has always been a popular preference among ornamental fish enthusiasts because of its color and shape in Malaysia. Therefore, relevant authorities should take proactive measures in curbing overharvesting by enforcing, monitoring, and guarding the natural habitats besides introducing seasonal harvesting, which could assist in maintaining the wild population of freshwater fish.

The establishment of policies or management plans for the freshwater ecosystem is a matter of requirement, but enforcement could be more crucial. Even though several conservation policies have been put in place in Malaysia for many years, the government continues to poorly manage most of the ecosystems due to the lack of stakeholder participation, inefficient conservation programs, and failure to raise community awareness (Abdullah et al. 2014). Engagement by pertinent stakeholders and authorities should be more holistic and covered by all layers of administration. In this study, the relevant authorities responsible for freshwater fish and ecosystem rely on the Department of Fisheries, State Forestry Department, and the district official since the rivers transcend multi-jurisdictions. These authorities should be advised based on scientific facts to help them design, develop, and execute any rehabilitation effort within their jurisdiction. The involvement of local communities is crucial in ensuring that any conservation and management plan in place is successful. According to Rampheri and Dube (2020), local communities play an essential role in executing management plans since most locals have extensive knowledge and experience in managing and conserving natural resources while safeguarding their livelihoods. The local community should be further informed on nature and its benefits that will likely influence their lives and improve their awareness of conserving the surrounding ecosystems. The scientific society in Malaysia should be more engaging in conserving the freshwater ecosystem by conducting in-depth research, especially on long-term monitoring. The relationship between species and the ecosystem is complex, and the diversity of monitoring requirements must be considered and combined (Deines et al. 2017; Allison and Mills 2018) . The conservation measures should be evidence-based to implement the best strategy to protect and rehabilitate the existing ecosystems within the country.