Opsarius bernatziki

Opsarius: apparently from an Ancient Greek word meaning 'small fish'.

bernatziki: named for Dr. H. Bernatzik who collected the type series in 1936 and 1937.

The precise extent of this species' distribution is unclear. It appears to occur in river systems on both sides of the Tennaserim Hills in Ranong, Phang-nga and Pattani provinces in peninsular Thailand, plus Tanintharyi (formerly Tennaserim) Division, Myanmar, all located south of the Isthmus of Kra.

Type locality is 'Kapa, northwestern peninsular Thailand', which may refer to modern-day Kapoe district in Ranong province.

Inhabits well-oxygenated, moderate to fast-flowing rivers and streams with substrates of gravel, cobbles, larger boulders and exposed bedrock, and is absent in coastal drainages tending only to be found 10-20 km or more away from the sea.

In the Sai Buri river, Pattani province, southern Thailand it was collected from a stretch less than 2 metres deep with a muddy bed and sandy margins alongside Nemacheilus masyai, Tuberoschistura baenzigeri, Balitoropsis zollingeri, Acantopsis sp., Labiobarbus lineatus, Cyclocheilichthys heteronema, Mystacoleucus chilopterus, Mystus nigriceps, Xenentodon cancila, Glossogobius giuris and Brachirus harmandi.

An aquarium with minimum surface area of 120 ∗ 45 cm48 ∗ 18 in or equivalent should be the smallest considered.

The aquarium should ideally be designed to resemble a flowing stream or river with a substrate of variably-sized rocks, sand, fine gravel, and some larger water-worn boulders. This can be further furnished with driftwood roots and branches if you wish but be sure to leave plenty of open swimming space.

While the majority of aquatic plants will fail to thrive in such surroundings hardy genera such as Microsorum, Bolbitis, or Anubias spp. can be grown attached to the décor.

This species is intolerant to the accumulation of organic wastes and requires spotless water at all times in order to thrive. It also does best if there is a high proportion of dissolved oxygen and moderate degree of water movement meaning external filters, powerheads, airstones, etc., should be employed as necessary.

As stable water conditions are obligatory for its well-being this fish should never be added to biologically-immature aquaria, and weekly water changes of 30-50% aquarium volume should be considered mandatory. A tightly-fitting cover is also essential as Opsarius spp. are prodigious jumpers.

Opsarius spp. are predominantly surface-feeders preying on aquatic and terrestrial insects in nature, with some small fishes and benthic invertebrates probably taken as well.

In the aquarium good quality dried products can be offered but should be supplemented with regular meals of live and frozen fare such as chironomid larvae (bloodworm), Artemia, chopped earthworms, etc. Drosophila fruit flies and small crickets are also suitable provided they are gut-loaded prior to use.

Unsuitable for the general community due to its environmental requirements, and likely to outcompete or intimidate slow-moving and less bold species at meal times since it is an extremely fast swimmer and vigorous feeder. Much smaller fishes should also be omitted as they may be predated upon.

The best tankmates are similarly-sized, robust, pelagic cyprinids such as Dawkinsia, Barilius or larger Devario and Rasbora spp. while bottom-dwellers could consist of Garra, Crossocheilus, Botia and Schistura spp., for example. Many characids and loricariid catfishes should also work well, but be sure to perform thorough research before purchase.

Although gregarious by nature this is a shoaling rather than schooling species which develops a distinct pecking order and therefore should always be maintained in a group of five or more individuals. If only two or three are purchased the subdominant fish may be bullied incessantly whereas solitary specimens may become aggressive towards similar-looking species

Adult females are less-brightly-coloured, tend to grow a little larger and are thicker-bodied than males, especially when gravid.

Nuptial males develop orange pigmentation in the lower portion of the body.

Unreported.

This species is occasionally available in the ornamental trade and is normally sold as 'blue hillstream trout'.

It possesses 6-7 bluish vertical bars on each flank and a similarly-coloured, relatively large blotch at the base of the caudal-fin, plus very short pairs of rostral and maxillary barbels.

O. dogarsinghi and O. signicaudus are the only two Southeast Asian congeners to possess a large blotch at the caudal-fin base but in the former the blotch is vertically-orientated and hardly extends onto the fin itself (vs. laterally-elongate and extending onto the basal fin rays in some specimens of O. bernatziki) and long (vs. short) barbels, while the latter has 36-39 (vs. 33 or less) scales in the lateral row.

The generic placement of species currently referred to Opsarius has been open to question since Howes (1980) concluded that the Barilius grouping, within which they were included at the time, was not monophyletic and identified two separate lineages. The first group contained B. barila, the type species, plus B. bendelisis, B. radiolatus, B. vagra, and B. shacra, while the second included all other species, and initially these were referred to the subdivisions 'group i' and 'group ii' within Barilius itself.

The composition of these groups is confusing since in a later work Howes (1983) included B. evezardi and B. modestus in 'group i' whilst omitting B. bendelisis, B. radiolatus, and B. shacra without explicitly stating why.

Rainboth (1991) assigned the members of Howe's 'group ii' to the revalidated generic name Opsarius based on the fact it was the oldest available with 'group i' species retaining the name Barilius due to the presence of the type species.

This system has been followed by some subsequent authors, e.g., Tang et al. (2010) and Collins et al. (2012) but not gain wider usage until Kottelat (2013), who included all former Barilius from Southeast Asia in Opsarius.