Taxonomic Notes
Two phenotypically-distinct forms of this taxon inhabit the upper and middle-lower stretches of the Trebižat River, respectively. They are largely separated by the subterranean stretch of the river, and it is hypothesised that they may represent distinct species (Bogutskaya et al. 2019). Further research is required, since their splitting is not fully-supported by existing molecular analyses (Perea et al. 2010, Geiger et al. 2014, Schönhuth et al. 2018).
Justification
Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)
The Imotski Chub has a restricted range (extent of occurrence (EOO) c. 809 km2, EU 27 member states 681 km2), which meets the threshold for the Endangered category under Criterion B1 (EOO < 5,000 km2). It is present at four locations where the quality of habitat and number of mature individuals are estimated to be declining.
Therefore, this species is assessed as Endangered under Criterion B (B1ab(iii)), both globally and for the EU 27 member states.
Geographic Range Information
This species is endemic to the Trebižat River catchment in the Dinaric Karst region of Croatia and Bosnia and Herzegovina. The Trebižat is a sinking river that goes by several different names along its course, and is a right-bank tributary of the lower Neretva River.
The river rises from a spring in the western part of the transboundary Imotsko polje, Croatia, where it is named Vrljika until its confluence with a tributary at the village of Kamenmost. After this point, it is known as the Matica until it submerges underground at the southeastern extremity of the polje in Bosnia and Herzegovina, emerging c. 1.5 km further east as the Tihaljina River, which is subsequently referred to as the Mlade and eventually the Trebižat once it enters Ljubuško polje near the village of Otok.
As the Trebižat flows through Ljubuško polje, some of its water drains southward towards Vrgorsko polje, to which it is connected via subsurface karstic conduits. This water represents the main source of the short Vrgorska Matica River, which flows through Vrgorsko polje for c. 15 km before terminating in an extensive wetland.
The Imotski Chub has in the past been reported to occur throughout this complex system, with the prominent Kravica Waterfall on the Trebižat main stem assumed to represent the natural downstream limit of its range. However, the majority of recent (post-2000) records pertain only to Imotsko polje and a series of associated wetlands comprising Prološko Blato (Prološko swamp) plus lakes Galipovac, Knezovìća and Mamića in Croatia, and Lake Krenica in Bosnia and Herzegovina. Further downstream, it may currently be restricted to a small part of the Tihaljina River near the village of Klobuk in Bosnia and Herzegovina (see 'Population').
Records from the Ričina River in the upper Trebižat catchment, including the Tribistovo and Zeleno artificial accumulation lakes, may result from translocations and are therefore considered to be of uncertain origin for the purposes of this assessment. Elsewhere, historical reports from the Baćina Lakes wetland complex near the Neretva River delta plausibly result from misidentification, translocation or individuals entering from Vrgorsko polje via a man-made tunnel (see 'Threats'). Contrary to some earlier suggestions, this species has never been reliably observed in the Neretva River itself.
Population Information
This species' current population size and trend have not been quantified. However, the results of field surveys carried out since 2004 are indicative of an ongoing decline. Its population status in Croatia was reported as Unfavourable-Inadequate under the 2013-2018 European Union Habitats Directive-Article 17 reporting process.
In Imotsko polje, it might today only be abundant in lakes Galipovac, Knezovìća and Mamića, with a small subpopulation still present in the Vrljika/Matica River. It was formerly plentiful in the Prološko Blato wetland and Lake Krenica, but the last confirmed reports are from 2008 and 2011, respectively, and it may now have been extirpated from both locations.
All known records from the Tihaljina/Mlade/Trebižat River were obtained between the villages of Tihaljina and Grabovnik, but since 2011 individuals have only been collected from a short stretch of the river downstream of its confluence with Klokun Spring near Klobuk.
It is considered extinct in the Vrgorska Matica River, since the last confirmed records correspond to the late 19th century and no individuals were observed nor detected via eDNA during field surveys carried out between 2017-2019.
Outside of its purported native range, this species has not been reported from the Ričina River system since 2008, or from the Baćina Lakes since the mid-20th century.
Habitat and Ecology Information
The Trebižat is a karstic river system of which several stretches, including the entirety of the Vrljika/Matica and Vrgorska Matica, flow through polje, i.e., depressed flat plains surrounded by limestone ridges. Under natural conditions, these areas are partially inundated from late winter to early summer due to increased river discharge in addition to rising groundwater entering through estavelles and/or ponors located around their margins.
The Imotski Chub is likely to enter such temporal wetlands during flood periods (but see 'Threats'), while its fluvial habitats comprise flowing stretches with clear water and often abundant submerged vegetation. In Imotsko polje, Prološko Blato and Lake Krenica are perennial wetlands which fluctuate considerably in size depending on groundwater levels, while Galipovac, Knezovìća and Mamića are small, steep-sided sinkhole lakes with subsurface connections to Prološko Blato.
Field observations indicate that many Imotski Chub individuals utilise estavelles and ponors to access subterranean water bodies, and that this species is abundant only in the vicinity of such openings. It has been recorded at depths of > 50 metres in lakes Galipovac, Knezovìća and Mamića (D. Jelić, pers. comm.). However, it is unclear whether underground environments represent an integral component of its autecology, or if this abundance pattern is being driven by increasingly adverse habitat quality elsewhere within its range (see 'Threats').
This species feeds primarily on invertebrates and smaller fishes. Maximum lifespan is at least six years, but its life history is otherwise not well-studied. Congeners tend to be fractional spawners with an annual reproductive period extending from spring to summer. Nuptial individuals form polygamous spawning aggregations at spawning sites characterised by shallow, flowing water and coarse stony substrata, or beds of submerged vegetation.
Threats Information
This species is plausibly threatened by a range of anthropogenic factors. For example, the construction of accumulation dams on the affluent Ričice River has interfered with the complex natural hydrology of Imotsko polje. Water is currently redirected from the Ričice to Prološko Blato via an artificial canal, which has reduced downstream discharge in the Trebižat River.
The water level at Prološko Blato itself is partially controlled by a retention dam and canal system, and a number of stretches of the Trebižat (including its source spring in Imotsko polje) have been canalised and/or receive diffuse and point-source agricultural and domestic pollution. Most of the water draining from Imotsko polje to the Tihaljina River first accumulates in an artificial lake before passing through a tunnel and hydroelectricity plant instead of the natural ponor. Vrgorsko polje is almost entirely turned over to agriculture, and the Vrgorska Matica River is extensively impacted by eutrophication and water abstraction.
All poljes within its range have been at least partly modified to reduce the extent of annual flooding and provide reclaimed land for agriculture, which has significantly reduced the extent of temporal wetlands which comprise spawning and nursery habitat for native fish species.
The increasing frequency of drought periods driven by climate change is likely to be exacerbating several of these threats.
Non-native fish taxa established within the Imotski Chub's range include Pumpkinseed (Lepomis gibbosus), Largemouth Bass (Micropterus salmoides), Northern Pike (Esox lucius), Eurasian Perch (Perca fluviatilis), Eurasian Pikeperch (Sander lucioperca) and Common Carp (Cyprinus domestic strain), all of which are documented to adversely affect native fish abundance through resource competition, habitat degradation and/or predation of various life stages. The presence of these species is understood to have driven its putative extirpation from Prološko Blato and Lake Krenica.
Use and Trade Information
This species is not used or traded.
Conservation Actions Information
This species is included in Appendix III of the Bern Convention (as Leuciscus microlepis) and Annex II of the European Union Habitats Directive.
It is nationally-protected in Croatia and strictly protected at the regional scale in the Federation of Bosnia and Herzegovina.
It is currently assessed as Critically Endangered for the National Red List of Croatia.
In Croatia, it occurs within the boundaries of three protected areas included in the European Union's Natura 2000 network (sites HR2000932, HR2000933, HR2001315). It is listed as a target species at site HR2000932 (Prološko blato, which includes lakes Galjipovac, Knezovìća and Mamića within its boundaries), but no specific conservation actions are in place.
A deeper understanding of its population trend, life history and response to the identified threats (particularly its interaction with non-native fish species and autecological relationship with subterranean water bodies) may also prove useful in terms of identifying priority conservation sites and the design of any structured management plan.