**4. Discussion**

*4.1. Identified Ecological State Elements*

Practically all fish species have a definite range of habitat preference delineated by abiotic attributes such as substrate, depth, velocity, floods, temperature, dissolved oxygen, etc. [134–141]. The spawning periods of fish also differ with respect to such distinct ecological elements as stagnant or running water, as well as altitude, temperature, quality of water, and etc. A fundamental property of fecundity is its raise during the development of the fish; a big fish produces more eggs than a small one [142,143]. If no relative large habitats, including refuge-type habitats, exist, no large fish can exist, and consequently reduced fecundity of fish populations can appear in the rivers.

As stream discharge lessens, decreasing water quantitative and qualitative characteristics may exceed resilience limits, pushing fish to search for and use refuge habitats. These can be characterized as sites where the adverse effects of disruption are absent or reduced than in more damaged sectors. For example, as discharge diminishes, the fish species which prefer riffles leave and search for refuge in pools. In these general circumstances, it is self-evident that the refuge habitats play a critical role in reducing the disruptive effects of drought in terms of individual endurance of fish and population survival. However, the quality of the pools varies in relation with many elements such as abiotic suitability, trophic opportunities, predation risks, etc.

Drought drying is a critical disruption circumstance in many small streams creating irregular or isolated lotic habitats [144]. We assessed and found that, based on the ratio between the fish numbers in refuge habitats and in the near lotic sectors, the remaining pools act as needed refuge habitats, especially during drought episodes.

In the researched area, there are two fish species of conservative interest, namely *Barbus meridionalis* Riso, 1827 and *Sabanejewia aurata* (De Filipi, 1863). The barbels, Ord. Cypriniformes; Fam. Cyprinidae are influenced by stream habitat quality such as, for example, drought periods [145]. *Barbus meridionalis* species terra typica is the Mure¸s River basin to which the studied Târnava River tributaries belong. This fish is a bottomdweller, relatively fresh, cool, and well saturated with oxygen water species. It prefers hard substrate [134]. The loaches, Ord. Cypriniformes; Fam. Cobitidae are also influenced by the streams' habitat quality, inclusive of drought effects [146]. *Sabanejewia aurata* is a mainly nocturnal, demersal, freshwater, bottom-feeder species. The sand presence in the riverbed is an important habitat precondition, with individuals spending extended periods in the sand. Its presence in muddy or silted areas is rare. It needs a quite warm water temperature especially in the summer season, but not over 20 ◦C [134].

When habitat characteristics match the favored range, anticipated fish species will appear in good abundance [147]. The first fact, which was observed in the research in the field, was that in hydrological drought periods, the number of fish was higher in the identified refuge habitats, a critical aspect for fish conservation, in contrast with transitional lotic sectors and especially in riffles, which are usually the first habitats to dewater at low flows. It is clear that the pools offer refuge from the most negative effects of drought (i.e., isolations and stranding of fish). Specifying a niche is equivalent to defining habitat conditions [148,149] that allow a species to remain in space and time. That is why proper habitats in general and especially the highly needed refuge habitats should be identified, characterized, and inventoried for considering present-future climate changes.

Together with flow fluctuations and low flow prolonged periods, a second stressor which can be important for the fish communities' deteriorated ecological status is the temperature, which can differ among river sectors with refuge habitats bordered by dense riparian vegetation and sectors with no such refuge habitats and no or rare vegetation. These sectors are exposed to the sun heat and have a higher temperature and evaporation rate; in consequence, both the water's quality and quantity decrease.

The rise in frequency and magnitude of hydrological droughts has quantitative and qualitative negative effects, lowering the refuge habitats' surface and volume, the lateral connectedness between rivers and floodplains, the longitudinal connectedness, and the

quality of water. Drought also impacts the relative habitat availability (i.e., ratio of pools to riffles), reproduction sectors and activities, and diminishes the trophic base support surfaces. The shallow habitats may dry pools, decreasing them in dimension, but still hold water for longer after the surface flow ends. Drought also intensifies density-dependent biotic interactions such as competition and predation, as fish are congested into a smaller volume of water.

A fall in fish numbers among refuge-sampled habitats goes along with descending discharge and growth in the sampled refuge habitat areas.

Over the course of the summer low-flow period, numbers of fish in the sampled nonrefuge habitats were lower in comparison with the refuge habitat, indicating degradation in abiotic and biotic conditions linked with drought.

Interest and struggle are increasing on improving environments that are humandominated [150]. In this study, the skewed natural fish communities' structure, or even disappearance/local extinction of some fish species on some studied lotic systems sectors, had complex causes, among which climate change and human impact synergy in these basins can play a central role, inducing as a mitigation measure the completion of the needed refuge habitats proposed herein.

The traditional land and water use best practices in the studied area, i.e., lotic systems with natural courses, using the phreatic rather than the river water for household needs, thick riverine natural vegetation (especially *Alnus incana*, *Telekia speciosa*, and *Chrysosplenium alternifolium* Erika Schneider-Binder *in verbis*), terraces with vineyards with anti-erosional effects on slopes, full coverage with forests on the tops of the hills, soft agricultural practices in small family farms in lowland areas, raising of cows, etc., were replaced by new, modern activities with negative effects on the environment.

The traditional best practices related with the land and water use were replaced by new ones which are aggressive with the aquatic environment: habitat fragmentation with riverbed barriers with no fish ladders or passages; aquatic lotic habitats over sedimentation due to improper riverine forestry and agricultural area use; the riverine vegetation destruction on some sectors reduces their capacity to retain sediments carried down by floods from the basin; overgrazing mainly with sheep and, to a lesser degree, with cows also induces sediment mobilization in the basin; the relatively big riverine industrial farms and the small family farms overflowing their animal manure direct into the river without any cleaning treatment; household water and solid waste overflows without any treatment; the lotic natural courses being affected in some sectors by modifications of the course, banks, and riverine habitats; the escape of alien fish from adjacent fish farms/ponds, etc.

All these quantitative shortages in refuge habitats and the qualitative problems induce significant pressure on fish fauna in the context of climate change and should be addressed with specific in situ adapted management measures.

The shortage of refuge habitats did not allow the presence of proper conditions for fish related first to the needed hard and coarse sand substrata due to fine sediments and mud over sedimentation and clogging, and second with water temperature and oxygenation due to the rising of water temperature and decreasing of oxygenation in lotic sectors not protected by riverine vegetation, exposed to direct sun heat, without refuges, and which are shallow.

Human activities in drought conditions, which synergically induces erosion regularly, deliver massive quantities of fine sediments into streams and rivers, forming large static bodies of sediment known as sand slugs, which smother in-stream habitat, alter community structures, and decrease biodiversity [151–153]. Based on our findings of such droughts plus a high variety of human impacts in the studied area, a strategic basin management should be designed and put into action to accomplish effective and viable aquatic refuge habitats used for the associated fish communities. Correct protection of the fish populations should rely on the protection of their environment through integrated management, which should solve the following identified problems: riparian zones were reduced on the surface or removed by the agriculture expansion and destructive agriculture works, lessening river shading and raising aquatic habitat temperatures and decreasing water oxygenation; severe sedimentation problems due to basin erosion, channel cuttings and strong weather water runoff raised by the insufficiency or absence in some sectors of riparian vegetation; accentuated erosion and sedimentation issues that develop from a lack of riparian vegetation along long sectors of river corridors and which can lead to gravel bed siltation, critical to the insectivorous fish species; disturbed hydrologic regimes in some sectors by water over extraction and use; the man-made stream barriers which have repercussions on the ability of fish to move among refuge habitats; lasting inputs of pollutants; habitat loss-induced native fish species decline; stream sectors channelization and isolation from their alluvial plain, important for cyprinids; habitat deficit easing fish overcrowding circumstances, which lead to occurrences and outbreaks of diseases; etc.

Based on the biological and ecological characteristics of the two sampled fish species of conservative interest and on the identified problems in relation with this species throughout the six years of monitoring activities, some management directions can be highlighted so that basin managers can have the key information and recommendations needed to preserve a good quantitative and qualitative conservation status of the local water resources and fish populations in synergic climate change and anthropogenic impact conditions.

Drought is a natural disruption including aquatic ecosystems in terms of physical and chemical water quality, increased fish death rates, and decreased fish birth rates and/or expanding migration rates, and can be an important factor in destruction of aquatic communities; for fish to remain in affected lotic systems, they must have refuge from interfering factors. Refuges' dimension, commonness, and connectedness play an important role in fish populations' ecological status and persistence. Population dynamics of fish using refuges during drought are best modeled by modified source-sink dynamics, but such dynamics are likely to change with spatial scale [154–158].

For the studied lotic systems, habitats, and their fish populations ecological status recovery, as a part of "a best practice" management of the researched area, relatively numerous new refuge habitats should be created and properly managed: nine times more than the existing number on the Dupu¸s River, 10 times more on the Biertan River, 0.4 times more on the Valchid River, 10 times more on the Laslea River, eight times more on the Mălâncrav River, and six times more on the Fel¸ta River. Among all of these rivers, the Valchid River state reveals a potential good status to be reached by all lotic systems in the area. We can understand now how this area's conditions were in the past, and why, in some localities, coat of arms waterfowls were present (Erika Schneider-Binder *in verbis*).

A complex, integrated, and permanent monitoring system should be implemented in the studied area, based on the fact that the physic-chemical and biological variables can show and anticipate the human impact actions and climate change effects on lotic systems' basin ecologic status and perspectives [159]. For the efficiency of the proposed monitoring activities, people with appropriate fish-related taxonomic skills should be involved [160].

The torrential nature of the superficial flow in the studied lotic ecosystem basins set off high maximum flows especially in rainy periods and minor flows, with very low-flow episodes, during drought periods of the year [98], periods which are more numerous and longer under the present climate changes. The researched area's geomorphologic processes are of high amplitude, have a high frequency, and an intensity of manifestation that confers on them a risk character on the local geomorphosystems [151], especially in the relatively new appearance of climate change effects, including on the refuge habitats of fish.

Sediment motion, flow, amassing, and clogging are influential phenomena in lotic habitats and their associated fish communities due to their significance for: riverbed features, river channel morphology and stability, and refuge habitats availability and quality. In the studied area, the sediment entrainment, transport, and deposition induced a significant refuge habitat loss with associated fish communities, decreasing living conditions. This situation is stimulated especially in the climate change context of change in flow regime, respectively, the diminishing of the water flow in the drought periods, through affecting entrainment, transport, and deposition processes. Changes in the frequency, duration, and

magnitude of flows competent to move the pre-existent bed materials have significance for substrate character formation and maintenance. For example, if riffles are not sufficiently flushed on a regular enough basis, fine sediment deposits amass in the interstices among the gravels and rocks, lowering or even removing the interstitial micro-habitats. Over the longer term, the hydrographical net substrate is buried, depth sectors/refuge habitats are clogged, and some disappear, and all of this leads to the loss of a natural series of river sectors along with the refuge habitats. Reductions in the frequency, duration, and magnitude of flows sufficient for transporting tributary sediment inputs further downstream have significance for river channels sedimentology and morphology, as sediment bars tend to expand, including in the confluence areas with tributaries, reducing fish access and mobility to long sectors of habitats and their resources.

The geomorphologic balance adjustment and sustenance in the studied area can be dealt with in an anthropic way by a set of measures for the prevention and diminishing of the effects of geomorphologic risk processes, but the witnessing of their manifestation remains for long periods, and needs a reconversion of the lands and a realistic management of the soil and sediments in the resilience limits of the area [151].

#### *4.2. Fish Fauna Refuge Habitats Management Elements Proposals*

It must be highlighted that no such research approach regarding the south-east Carpathians small rivers fish refuge habitats was previously realized.

Appropriate basin management measures should begin from the following elements: water is part of the environment and for that reason, different users can adapt only to the surplus quantity that is naturally available from rivers. In nature, there is more water in riverine systems than is needed for support of the riverine ecosystems; if the main features of the natural flow regime can be identified and adequately maintained in a changed flow regime, then the valuable biota should be preserved.

The management measures should mostly include: basin soil stabilization and conservation, slopes and tributaries erosion diminishing measures, treatment of point sources, etc.

The silting of deep stream habitats is a natural physical phenomenon which cannot be avoided, but by understanding the effects of hydro-morphological pressure due to a lack of basin sediment management, mitigation of this situation's effects on fish refuge habitats can be obtained based on basin sediment management measures: buildups of sediments could be managed by physical intervention like mechanical raking or excavation; restoration of more natural levels of sediment inputs reduce the demand imposed on streams to transport and rework elevated sediment loads; creating outside-of-river-beds storage ponds in which, through some mobile diversion dams, the high-flow period water will be channeled for the deposition of sediments before they can clog the refuge habitats; and all the torrent and soil erosion control works in the basin should be carried out.

Numerous forestry management measures can be useful for basin sediment management: the forest stands' vertical and horizontal structure should be as close as possible to its natural model (multi-layers, groups, and patches) in order to reduce soils being destabilized because of slope characteristics, loss of moisture, loss of large canopy cover, loss of root strength, and etc.; permanent watercourses will be protected by commercial intervention with a minimum 50-m-wide stripe on both sides of the watercourse. It is important to keep and respect the natural dynamics of habitats (i.e., reed beds, sedges and forests), together with their necromass (i.e., died wood), in banks (i.e., major/minor riverbed and alluvial terraces) and in water. The cross-over, in accidental or extraordinary cases when it cannot be avoided, will be made over the shortest distance on installations adopted to keep the water clean and soil erosion at a minimum; riparian forest and the natural bushy and grassy, local vegetation and flora will be strictly protected; any timber harvesting or transport activity is recommended to take place on a frozen substrate, covered by snow and/or ice and on well-maintained forestry roads with a layer of mineral aggregates on top of the road; the network of forestry roads must be optimally dimensioned for maximum efficiency with the minimum amount of damage; existing roads should be preserved and

maintained in order to avoid or minimize erosion, sediment transport, and accumulation in the area; building of new forestry roads should be avoided; ecologically friendly, low-impact logging technologies (oxen, horses, cableway-skidders) are always preferred; timber will not be removed from forests during or after precipitation while the soil is moist; skid-roads will always be designed, built, and monitored in order to avoid soil-erosion as much as possible, ensure the protection of permanent and temporary watercourses, and protect remaining trees; sensitive areas like potential land-slide zones, talus, cliffs, steep slopes, etc., will be identified, protected (excepted from logging), and monitored; it will be prohibited to store timber (even for short periods of time) in the riverbed, on its banks, and on the minimum 50-m-wide protection stripes adjacent to both perennial and ephemeral waterways. All the vegetation within the protection belts shall remain intact; intervention cuttings will be applied in openings with a diameter up to a tree height, from which the old trees can be completely removed; timber harvesting will not exceed 10% of the volume of the stand (exceptions can be allowed in special accidental situations, for example windfalls and/or snowfalls, etc.), and the harvest volume will be correlated with the condition of the stand, the dynamics of natural regeneration, and with assigned conservation requirements; logging techniques will be adopted to minimize the level of injuries to the remaining trees and soil; if there are stands in which the natural regeneration is very difficult or stands are affected by calamities, replanting or direct sowings will be carried out using only seminological material of local origin or, if not possible, from identical ecotypes; timber harvesting access corridors should run parallel to the lotic and lentic ecosystems; pioneer species will not be extracted as they are important for soil improvement; illegal logging will be controlled with the aim of eradicating illegitimate logging activities; during logging and timber transport activities, sediment traps will be installed on the main watercourses (with around 500 m distance between them), and will be cleaned as often as necessary, with their evacuation/transport in areas that do not influence the degree of sedimentation in waterways; construction/monitoring/maintenance of drainage ditches for liquid and sediment flows from the transport routes designed to manage excessive and rapid precipitation events that are characteristic of the mountain area; installation of sediment traps (with around 500 m distance between them) on ditches used to evacuate liquids from the transport routes, cleaned as often as necessary, with the anticipation that the proposed sediment traps will stop, or at least diminish, the potential of these channels' networks to be an unwanted sediment delivery system directly to downstream water bodies; leafy branches and debris left over from the logging will be placed on remaining stumps; full reforestation of the watershed with canopy projection over 0.8 (to reduce the kinetic energy of raindrops on the soil surface, surface runoff from precipitation, air currents, solar radiation, and minimize large temperature differences, etc.) and a subsequent forest management regime; excluding human activity in the riverbed of watercourses and preserving permanent or temporary riparian wetlands as natural sediment traps during periods of increased precipitations and high flows; the construction of new bridges, if needed, should not narrow the waterway to avoid increasing water speed and its capacity for erosion and sediment transport to downstream sectors; ecological restoration of sediment deposits formed at high waters in the riparian areas by fixing sediments or planting on sedimentary areas will prevent sediments from moving downstream in subsequent episodes of high flows; efforts should be made to favor existing flora, undergrowth, shrubs, and the herbaceous bed; it is acceptable to intervene with sowing or planting in critical areas; leaving the stumps in situ; and controlling waste management [152,153].

Sediment management in watersheds is necessary for their water quality, but indirectly, sediments are also the sources of other problems. Sediments are not unmixed and can be adhered to or can bear pollutants. To prevent such situations, various measures can be enforced: monitoring tributary torrents, slopes, and banks, etc., specifically those prone to accidental or permanent erosion; in sectors with accidental erosion, its effects can be drop off with blankets, sandbags, gravel bags, rugs, plastic materials barricades, etc., until the ecological state is secured through ecological rehabilitation and reconstruction;

development of dense, vegetated fencerows, due to their function as sediment traps; banning the extraction of mineral resources in the basin; banning damming or regulating riverbeds, with the exception of debris basins, settling ponds, and other similar structures which can catch and retain sediments, which should then be frequently cleaned up; banning burning vegetation and trimmings; banning grazing and watering domestic animals in the forest; in pastoral lands, grazing will be organized to prevent the destruction of flora, soil compaction and the onset of erosion phenomena and will even be banned in sensitive seasons or sectors; a ban on the substitution forests and pastures with intensive agriculture lands; small-scale family farms are preferable to big-scale, industrial farms; and natural water filtration sectors and retention ponds should be protected and created and largely used in localities and in opposition solid sectors, growth runoff, and the transportation of sediments should be reduced, particularly hard sectors linked with the construction of buildings and roads infrastructure [152,153].

The general indiscipline with the dispositions commanding the wastewater management and the deficiency of dilution is one of the main causes for some of the researched sectors' environmental bad situations. In this circumstance some management elements are suggested: rising water use performance through general use of contour meters and trustworthy transport pipelines; the development of a hazardous waste site assessment and monitoring unit; the protected lotic sectors must be big and abundant enough to admit the river natural self-cleaning processes to work; the lotic ecosystems must be used like ecological capital seeking to decrease the costs for water cleaning technologies and low-cost fish protein for local consumers.

The present hydrotechnical works' impact should be reduced following some main management directions: increasing the river assimilative capacity thorough its restoration activities; revitalization of the traditions for land protection and use; avoiding the impact of wetland loss; and restoration of sectors of typical lotic ecosystems.

The riverine land exploitation should follow some main directions in this respect: determining the policies for cultivation of multi-year cultures; rehabilitation of the riverine forest corridor, hoping also in areas where the forest and other ecosystems in contact can be allowed to evolve according to natural dynamics; prohibiting access to the upper parts of the catchment areas so that spontaneous perennial vegetation could regenerate in good conditions and limit erosion damages; and rotating sylviculture and grazing (diminishing forestry, agriculture, and livestock impacts) with regard to seasonal conditions, especially on the river adjacent areas.

To sustain the protection of the conservative interest fish species and their habitats, their shelters should to be protected from all man-related aggression; the fact that proper protection is a useful help to the economic development of the rural communities should be highlighted; and complementarily should exist between human society's development and conservation.

The collector river, The Târnava Mare River, which belongs, together with some of its tributaries, to the Natura 2000 Sighi¸soara-Târnava Mare protected area, cannot be properly managed if its tributaries are not in the ecological state to offer permanent refuge habitats for fish reproduction, development at early growing stages, feeding, and refuge in periods of accidental pollution and high flows.

The elements of the design used for the management of these river basins management can be followed as a general suggested model for other similar Carpathian Basin river basins of conservation interest facing similar environmental issues.

#### **5. Conclusions**

The global increasing temperature and other human activities impact induced the hydrologic net and habitats alteration, with a decrease of needed refuge habitats for freshwater fish, diminishing the fish distribution and abundance in the studied area.

In the new climate change scenario, an ongoing pattern in the context of the alreadypresent human impact stress, especially the small hydrological nets, their lotic refuge

habitats, and associated fish fauna are under a significant increasing environmental and anthropogenic risk due to an accentuated, new game changer, the drought/low flows, even in what have been, until recently, considered safe geographical areas like the Carpathian Basin.

New proactive special in situ adapted assessment, monitoring, and management integrated systems should be designed and implemented in such risky, potential hot spots to prevent and mitigate these present and future negative effects for the conservation and restoration of fish communities.

Poor water and land use imposed long-term effects on natural lotic systems, namely a change in the physical structure and substratum cover of fish habitat and the fish themselves. This is due to the human-induced, increase in fine sediment clogging in the refuge habitats of fish, which had an adverse effect on them, a process in a continuous acceleration due to climate change-induced diminishing of liquid flow on streams.

A composite model of climatic and anthropogenic induced threats and pressures of the researched river basins has significantly jeopardized the ecological status of its fish. As indirect (climate changes) and direct (various human activities in the basin) human impact affects the lotic habitats, they have become, in some sectors, critically altered or degraded, and the fish populations of conservative interest, and others, have been affected.

For the studied lotic systems' fish populations ecological status recovery, relatively numerous new refuge habitats for fish should be created: nine times more than the existing number on the Dupu¸s River, 10 times more on the Biertan River, 0.4 times more on the Valchid River, 10 times more on the Laslea River, eight times more on the Mălâncrav River, and six times more on the Fel¸ta River.

The habitat managers are required to specifically monitor the extent to which the changes in physical structures and cover for fish refuge habitats will affect these fish populations in the future, and a key management element should be the refuge habitats' proper management and the creation of others.

The proposed fish refuge habitat monitoring and the creation of new ones should increase the fish survival rate and the recovery of species populations experiencing climate change-induced environmental disturbance.

**Author Contributions:** Conceptualization, D.B. and A.C.-B.; methodology, D.B., A.S., K.C., S.B. and A.C.-B.; software, A.S.; validation, D.B. and A.C.-B.; formal analysis, D.B., A.S., K.C., S.B. and A.C.-B.; investigation, D.B., A.S. and A.C.-B.; resources, D.B., A.S. and A.C.-B.; data curation, D.B. and A.S.; writing—original draft preparation, D.B., A.S., K.C., S.B. and A.C.-B.; writing—review and editing, D.B. and A.C.-B.; visualization, D.B., A.S., K.C., S.B. and A.C.-B.; supervision, A.C.-B.; project administration, D.B.; funding acquisition, A.C.-B. All authors have read and agreed to the published version of the manuscript. The first and the last authors have equal contributions.

**Funding:** The APC was funded by Lucian Blaga University of Sibiu, Applied Ecology Research Center, and Hasso Platner Foundation research action LBUS-RC 2020-01-12.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available within the article.

**Acknowledgments:** The project was financed by Lucian Blaga University of Sibiu, Applied Ecology Research Center, and Hasso Platner Foundation research action LBUS-RC 2020-01-12. The first and the last authors thank to their students (Ceau¸su M., Tchoumta R. and Nagapin S.) for their active involvement in the field activities. Last but not least the authors thank for the editorial English language support to Baumgartel D., writer/editor in Lausanne, Switzerland, and to Atmosphere journal editors and reviewers support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.





**Table A2.** Biertan River Habitat

Characteristics.

139


140


*Atmosphere* **2021**, *12*, 1209




#### *Atmosphere* **2021** , *12*, 1209

**Table A2.** *Cont.*

144


**Table A3.** Valchid River habitat

characteristics.





149


