Sulawesi stream fish communities depend on connectivity and habitat diversity

Abstract

Streams provide an array of habitat niches that may act as environmental filters for fish communities. The tropical island of Sulawesi in Indonesia is located in the Wallacea, a region isolated by marine barriers from the Asian and Australian faunas. Primary freshwater fishes are naturally absent in the Wallacea, including Sulawesi's numerous coastal streams. Diadromous species are in contrast species-rich in the area. The knowledge available on stream fishes in the Wallacea is largely restricted to taxonomic work and studies targeting single species groups, whereas baseline data on fish ecology remain extremely scarce. Such data and a deeper understanding of stream fish ecology are, however, urgently required for purposes such as informed management. We assumed that the stream fish assemblages are dominated by recruitment from the sea and are structured by macro- and microhabitat diversity. To test this hypothesis, we quantified the occurrence of individual fishes by point abundance electrofishing at 33 streams across Sulawesi. The 4632 fishes obtained represent 58 species out of 24 families. The native fishes recorded are mainly amphidromous (34 species), euryhaline (five species), and catadromous (five species). Gobiiformes make up the vast majority of records, dominated by Oxudercidae (22 species) and Eleotridae (five species). Only two of the species recorded are endemic to Sulawesi, including a single species strictly confined to freshwaters. Ten species, making up 6% of the fishes caught, are not native to Sulawesi. The outlying mean index (OMI) and BIOENV analyses suggest that effects on the scale of macro- and microhabitat shape fish assemblage composition, ranging from pH, conductivity, and temperature to current velocity, substrate, canopy cover, and elevation. Habitat niche use of species along the first two OMI axes is complementary and fine-scaled, covering a wide range of the available habitat space. Juvenile and adult conspecifics share similar habitat niches in most of the cases. Niche breadths overlap, but niche specialization is significant in most of the species. Non-native fishes link into the assemblages at the margins of habitat space, with substantial niche overlaps to native species. The present findings show that the native fish communities in coastal streams of Sulawesi are largely composed of species depending on access to the sea, highlighting the importance of connectivity down to the estuaries and sea. The ichthyofauna shows a rich diversity in habitat use, and the availability of alternative habitats along the altitudinal gradient provides plausible filters for species establishment. Non-native fishes are locally abundant, pose substantial potential for changing communities, but are still stocked intentionally. We stress the need for incorporating the need for connectivity and maintained habitat quality into management decisions, and a critical evaluation of stocking activities.

Keywords: community assembly; ecology; freshwater fishes; habitat use; point abundance electrofishing.

FIGURE 1

Sulawesi (a) is located in the Wallacea (insert map, Sulawesi highlighted in red). The tropical island is characterized by steep mountain formations with numerous coastal drainages (blue). Species accumulation curve (b) based on point abundance sampling at 33 sites located in different drainages (numbered green circles in map (a)) shows clear saturation of species diversity along the 2246 points bearing fishes; insert picture shows a male Sicyopterus longifilis (photograph by T.K.), the most abundant species recorded. Habitat diversity covered ranges from broad and mostly open streams (c) (site 13) to narrow, swiftly flowing streams with small cascades (d) (site 4) and forest creeks dominated by gravel and sand (e) (site 24) to rocky pool‐and‐riffle formations (f) (site 14) (photographs by L.L.W.).

FIGURE 2

Composition of the stream fish fauna recorded, in relative abundances, with the most frequent species per family in parentheses. Most abundant were the gobiiform families Oxudercidae (Sicyopus zosterophorus), Eleotridae (Belobranchus segura), Rhyacichthyidae (Rhyacichthys aspro), and Gobiidae (Glossogobius celebius), in total 81.8% of the individual catches. Moderately abundant were Kuhliidae (Kuhlia marginata), Cyprinidae (Barbodes binotatus), Ambassidae (Ambassis interrupta), Anguillidae (Anguilla celebesensis), Poeciliidae (Poecilia reticulata), and Zenarchopteridae (Nomorhamphus sagittarius). Non‐native families are marked with an asterisk (photographs by T.K.).

FIGURE 3

Cluster analysis with similarity profiles of fish community data per stream revealed two major clades. Left, Taxonomic composition (family level; for taxonomic composition at species level see Supp. Figure S4) based on abundance data; sampling site numbers given in the centre, as encoded in Figure 1. Right, results of a CLUSTER/SIMPROF analysis indicating differences in community composition among sites at a significance level of p = 0.05 with two significant clusters.

FIGURE 4

Multivariate OMI analysis of fishes of Sulawesi coastal streams, with respect to environmental parameters. Canonical weights of habitat variables (clockwise: Ca, canopy cover; Co, conductivity; El, elevation; pH; St, stone; Di, distance to shore; Cu, current velocity; De, depth; Gr, gravel; T, temperature; O2, oxygen; Ve, vegetation; Wo, wood; Sa, sand), along the first two principal component axes (PCs; 46 fish entities analysed in total). Fish species subdivided to maturity stages where appropriate. Species illustrated per family as in Figure 2, complemented by the less abundant Scorpaenidae (Tetraroge barbata: light grey), Syngnathidae (Microphis leiaspis: dark grey), Ophichthidae (Lamnostoma sp.: black). Non‐native families marked with an asterisk. Vectors visualise relative importance and direction of habitat variables in the ordination space. Axis loadings are shown in the inlay box; loadings of the first two PCs sum up to 66% of the total variance. Effects of macrohabitat, mainly vectors associated with altitude, and combined effects of microhabitat variation, are associated with fish communities covering a wide array of the habitat space available (Photos by T.K).

FIGURE 5

Niche position of Sulawesi streams fishes along the first two PCA axes based on outlying mean index (OMI) analyses. The position of each species and maturity stage (dots; colors encode families, as specified in Figure 3) is the weighted mean of species distribution in each point (the bottom vertical bar in the panel). Non‐native fishes are marked with an asterisk. The position of the black vertical bars at the bottom of the panel corresponds to the respective loading of each sampling point in the habitat parameter space, with increasing pH and conductivity (from the left to right) for PC1 (a), and with increasing current velocity along with a decreasing presence of sandy substrates (from the right to the left) for PC2 (b).