Blatnjača: what is known about Plav Lake’s own trout

Blatnjača is the local name for the trout of Plav Lake. Almost everything that has been measured about it comes from one paper, published in 1957. This article sets out what that paper says, what the fish has been called since, and what nobody has tested.

The 1957 study

In 1957 the fisheries engineer Đ. Drecun published “Pastrmka »Blatnjača« iz Plavskog jezera” in the journal Ribarstvo Jugoslavije, volume 12, issue 6, pages 135 to 138. The journal is now the Croatian Journal of Fisheries.

Drecun worked from ten fish. His table gives them as 501 to 836 mm long and 1.23 to 6.275 kg in weight, with ages written as 3+ to 8+. He measured the proportions of the body. He counted the scales and found 117 to 120. He counted the pyloric caeca, small pouches on the gut, and found more in the lake fish than in three river trout from the Ljuča and the lake.

What he described

Drecun described a stout, rounded body, shaped like a torpedo, with a broad back and a small rounded head. The back was ash-grey, the flanks lighter and the belly silver-white. There were many irregular black spots, often shaped like an X. Most lay along the lateral line and none were on the belly. The gill cover carried ten to twelve round black spots.

It was not simply a heavier trout. The one river trout he could set beside a Blatnjača of the same length was slightly the heavier of the two: 495 mm and 1,310 g, against 501 mm and 1,230 g.

Beyond his ten fish, Drecun reported that Blatnjača reached 12 kg, and wrote that this figure was verified. Most fish caught were up to 5 kg, and heavier ones, he wrote, were not rare. These are figures from the 1950s. No measured figure from recent years was found.

He found the fish across the whole lake, in the shallowest part and over the deepest water. In the body of the paper he wrote that it lived mainly in the lake and rarely strayed into a nearby water. In his conclusion he wrote “only”. Both are on the record. He recorded one migration: out of the lake and up the Ljuča to spawn.

“Probably”

Drecun did not assign the fish to a species. His own English summary calls it “probably a new type of lake trout, respectively a new endemic trout”. The text in his own language says “novu vrstu”, which is literally “a new species”. In both, the word that carries the weight is “probably”. He based the idea on ten fish and on the shape and counts of the body, and he gave the fish no scientific name.

The names it has been given

The local name comes from blato, mud. Drecun gives two explanations and leans to the first. Local people called the lake itself “Blato”, so this was the trout of Blato; he records the older form “Blatske pastrve”. The second is that the fish was believed to keep over the muddy bed of the lake by day. He adds that local people also called it simply lake trout, jezerska pastrmka.

Three Latin names are on record:

• In 1953 J. Vujačić surveyed the lake’s fishery for the same journal. He wrote the name as “Pastrva blatnjača (Salmo trutta lacustris L.)”.
• A 2017 paper (Nikčević and others) calls it “a lake trout (Salmo trutta) inhabiting lake Plav”.
• A 2024 state monitoring report (Mrdak and others) gives “Blatnjača” as the local name of Salmo labrax in the Tara, which is a river fish.

Drecun, between the first two, gave it none. None is settled, for three reasons. First, none of them comes from a work that set out to describe the fish: one is a catch survey, one is a study of a muscle enzyme, and the monitoring report does not discuss Plav Lake. Second, the local name is not used for this fish alone: the 2024 report uses it for a river fish. Third, Salmo labrax and Salmo trutta may themselves be one species. FishBase still lists Salmo labrax, but notes that a 2018 study (Kalayci and others) treats it as a synonym of Salmo trutta.

What later work says, and does not say

The 2017 paper appeared in Environmental Biology of Fishes. Only its abstract was read for this article. It describes the lake fish as smolt-like, with an “elongated silver colored body”, and compares it with river trout “of the same, confirmed by us, Danubian lineage”. So the one statement about lineage on record puts the lake fish with the river trout, not apart from them. How that lineage was confirmed was not read.

Drecun’s fish was stout. The 2017 fish is elongated. Nothing found explains the difference.

The 2017 authors cite a 1963 paper by D. Janković, “Pastrmka iz Plavskog jezera”, in Arhiv bioloških nauka. It was not read for this article.

What is not known

• No formal description of Blatnjača as a species was found, and it has no scientific name of its own.
• No study was found that tests whether the lake fish differs from the trout of the Ljuča and the Lim.
• What it eats is not recorded in anything read.
• Whether the lake has ever been stocked with trout is not known.
• No count or assessment of the lake’s own fish was found.

That is why Montenegro Fishing calls it Plav Lake’s own trout, and no more. Which fish live in which water is on the Fish and seasons page. To ask about a day on the lake, send a booking request.