A Drivers-Pressure-State-Impact-Responses Framework to Support the Sustainability of Fish and Fisheries in Lake Tana, Ethiopia

: Lake Tana, Ethiopia’s largest lake, has a remarkable and conservation-worthy assemblage of ﬁsh species, requiring ﬁsheries management for sustainable exploitation. However, due to anthropogenic impacts, many of these ﬁsh species are threatened. Hence, an improved management of these resources is recommended. To allow a more sustainable exploitation of natural resources, a better understanding of the cause-effect relationships between anthropogenic impacts and environmental components is fundamental. The Drivers-Pressure-State-Impact-Responses (DPSIR) framework is a useful tool to describe these links in a meaningful way to managers and policy makers. Despite its potential, application of DPSIR is virtually lacking in developing countries. This paper assessed the potential of the DPSIR framework and used it to comprehensively describe the available knowledge and management needs in the lake catchment. Rapid population growth and the economic transformation are the main driving forces leading to various pressures such as water quality and wetlands degradation as well as declining ﬁsh community, which is detrimental to the socio-economic state and health of the local inhabitants. As feedback to the driving forces, pressures, state changes and impacts, optimal multi-level responses are developed. This study aims at providing policy makers a better understanding of the lake catchment in order to bridge the gap between science and decision-making.


Introduction
Ethiopia has more than 24 natural lakes and many artificial reservoirs, with a total surface area of about 7500 km 2 . The major rivers stretch over 7000 km. Based on the composition of their fish fauna, Ethiopian freshwater systems are classified into seven drainage basins: Abay (Blue Nile), Awash, Baro-Akobo, Omo-Gibe, Rift Valley Lakes, Tekeze and Wabi-Shebele Genale [1]. Lake Tana, the country's largest lake, contains four main fish families: Namacheilidae, Cichlidae, Clariidae and Cyprinidae [1,2]. The first three families are represented by a single species; Afronemacheilus abyssinicus, Oreochromis niloticus, and Clarias gariepinus, respectively. Cyprinidae is the most abundant family, which consists of three genera and 24 species. The three genera are: Enteromius (three species) [3], Garra (four species) [4] and Labeobarbus (17 species) [5,6]. Of the 27 fish species in the lake, 21 are endemic [1] and 17 (76%) of the endemic species belong to the genus Labeobarbus, The wide range of different habitats in the geologically young lake [7], in combination with the 40 m high Tissisat We reviewed various sources of DPSIR-related literature [18,21,22,[25][26][27][28] to define the following five elements of the framework: (1) driving forces are identified as general human needs responsible for pressures on the ecosystem, (2) pressures are the actual human activities that affect the lake ecosystem, (3) state changes describe the condition of the lake ecosystem components affected by pressures, (4) impacts are human health risks or socio-economic losses, and (5) responses are the feedback to driving forces, pressures, state changes and impacts. A first step consists of identifying the major driving forces, pressures, state changes and impacts in the lake catchment. To identify the first four elements of the framework, we reviewed the currently present scientific literature on the biology, ecology, and major challenges of the fish communities and fisheries in Lake Tana. The responses were described based on this review in combination with personal experience, informal communication with local inhabitants particularly fishers and by consultation with experienced fisheries experts and scientists. Finally, we adopted the DPSIR framework to understand, summarize and visualize the actual situation of the Lake Tana catchment and to propose optimal management options. To this end we provided a description of the current biological aspects of the local fish and the socio-economic interactions in Lake Tana. In a final section we integrated this knowledge in the DSPIR framework for the Lake Tana area.

Fish Species in Lake Tana
Distribution and abundance of fish depends on the quality of their habitat and spawning grounds. However, in Lake Tana, the physical, chemical and biological characteristics of these areas are severely affected by different anthropogenic activities. The lakeshore areas and the tributary rivers and streams are highly vulnerable to these impacts. For instance, damming and sand mining are detrimental to spawning grounds of the migratory endemic Labeobarbus species in Lake Tana [11,15]. Moreover, flood recession agriculture is negatively affecting the habitat and nursery sites of many fish species in the lake [10]. Therefore, degradation of these areas could have a large impact on the survival of the fish species in the lake (Table 1) [29,30]. In this section we briefly describe the reproductive strategy, habitat preference and feeding habit of the fish in the lake. A detailed explanation of the links between anthropogenic activities and ecosystem functions are presented using the DPSIR framework in the next sections. Lake Tana fish species have different reproductive strategies, whereby some species breed in the shore areas and lake-associated wetlands, while others migrate to the floodplains or upstream river [9,13,[31][32][33] (Table 1). Nine Labeobarbus species migrate in search of ideal breeding grounds with high gravel beds, and clean and oxygen-rich waters. Clarias gariepinus migrates to the floodplains for spawning. Chemo-physical gradients may guide the migratory fishes to aggregate at the river mouths [9]. Flooding events, which increase the lake water level and turbidity, are also likely to trigger their migration [13,32]. The non-migratory Labeobarbus species are thought to have adopted a new reproductive strategy [13] and may breed in the lake itself or lake associated wetlands.
With exception of L. osseensis, which was only reported in the Bahir Dar Gulf, the remaining species are found all over the lake. However, their habitat preferences vary; some predominantly prefer rocky substrates, while others prefer muddy or sandy substrates. Some fish species live in areas with water depths of less than 6 m, while others mainly live in areas with depths greater than 6 m (Table 1) [5].
Oreochromis niloticus primarily feeds on phytoplankton, whereas Enteromius and Garra species feed on zooplankton and C. gariepinus is omnivorous (Table 1). Despite the lack of teeth in their jaws, eight Labeobarbus species in the lake are piscivores, while the rest feed on plankton, macrophytes or detritus [31]. Table 1. Aspects of habitat and feed preference, reproductive behavior, spawning grounds and IUCN status of Lake Tana fishes (shore areas also include lake associated wetlands. Streams with high gravel beds, clean and high oxygen water. "-" refers to no data available.)

Socio-Economic Interactions in Lake Tana
Conservation challenges of the fish and fisheries in Lake Tana are a complex product of social, economic and ecological processes. To comprehend the complexity of the lake catchment, to manage human pressures and to optimize alternative management options, the lake fisheries and human-ecosystem interactions must be understood in their entirety. As a first step in this direction, we describe the historical fisheries development in Lake Tana and its economic importance and create a feedback diagram that shows the socio-economic interactions.
Currently, fishing is undertaken in two ways: either using single-person reed boats (a majority of the fishers) or motorized boats. The Negedie Woyto ethnic group has exploited Lake Tana fisheries since the 18th century. Until the mid-1980s, fishing activity was almost completely limited to subsistence [34]. During this period, the fishing activity was restricted to shore areas and only simple single person papyrus reed boats were used. The fishing gear were locally made traps, hook and line, and small gill nets. In 1986, two Dutch Non-Governmental Organizations together with Ethiopian Ministry of Agriculture and Orthodox Church initiated the modernization of the lake fisheries. During this time, modern multifilament gill nets and motorized boats were introduced, which were gradually being adopted by the poor members of the farming communities [34]. Nowadays, approximately 20% of the full-timer fishers own a motorized boat [35]. The mobility of the fishers who own reed boats is limited to the shore area, while those who own motorized boats have access to the open water [34]. Introduction of the motorized boat enabled fishers to access the open water fish resources and distant fishing grounds.
The lake fishery is creating job opportunities and socio-economic benefits for about 6000 fishers. Besides its direct employment and economic contribution to fishers, the Lake Tana fishery has many social values and creates job opportunities for fishery related activities. Many people are employed in fish processing, gill net making, and fish product trading (personal observations). Consequently, one can genuinely assume that a single job on the lake generates at least two fisheries related jobs on land. Local inhabitants and fisheries experts also corroborated this during informal stakeholder discussions. Thus, considering the number of fishers in the lake, the lake fisheries are creating additional job opportunities for about 12,000 people in the lake surroundings. As the average family size of the Amhara region is five, the lake fisheries are, therefore, able to support the livelihoods of about 90,000 people through the job opportunities on both the lake and the land.
Despite the increasing fish consumption demands in the neighboring country of Sudan, the majority of the catches of the commercially important species in Lake Tana mainly serve the local market. The growth, investment and mortality interactions of the lake fisheries are depicted in Figure 2. Both the local market and the market of Sudan, convert catches into investment power, which allows the local economy to grow. The local economy growth supports the society, particularly fishers, through increasing infrastructure development, market linkage and increased availability of all necessary equipment for fisheries. To increase their total catch, fishers can invest their capital in fishing efforts. Furthermore, fishers can invest in other productive assets, which in turn can further reduce vulnerability to poverty. This stimulates fishers' population growth. The fishers' population growth feeds back into the local markets and the local markets invest in fishing gears such as boats and gill nets to ensure their supply of fish. However, the increasing fishing pressure negatively affects the fish population in the lake. Due to their characteristics, the reed boat fishery has mainly impact on the inshore fish community, while motorized fishery has impact on both inshore and offshore fish communities [34]. Socio-economic interactions of the fisheries in Lake Tana. The diagram is adopted from Downing et al. [36]. Dotted arrows (associated with "+" sign) represent growth. Red arrows (associated with "−" sign) represent mortality and black arrows represent investment. FCDSU = fish consumption demand by neighboring country Sudan, F1 = Labeobarbus species from de Graaf et al. [37], F2 = Oreochromis niloticus, F3 = Clarias gariepinus, owners = motorized or red boat owners, M1 = motorized boat fishing effort, R1 = reed boat fishing effort.

DPSIR Framework to Support Fish and Fisheries Management in Lake Tana
Decision making on natural resources management requires the relevant knowledge to be presented in an accessible and meaningful form. In this regard, we adopted the DPSIR framework to understand, summarize and visualize, in a simplified way, the cause-effect interactions of the lake ecosystem. Furthermore, viable response options for managing and protecting the lake resources were incorporated in the framework. In the Lake Tana catchment, driving forces exert pressures leading to state changes in the ecosystems, which then lead to impacts on humans that will in turn require a societal response ( Figure 3). Different management options appropriate for the Lake Tana catchment were identified and broken-down to different levels, introduced in the DPSIR framework and connected as responses to drivers, pressures, state changes and impacts. The adopted DPSIR framework therefore provides a conceptual understanding of the interactions between anthropogenic pressures, state changes and potential management options in the lake catchment and stimulates efficient communication among policy makers, scientists and the public, improving the cooperation among them. As a result, it can potentially bridge the gap between different scientific disciplines and aid in linking science with policy and management.

Population Growth
Population growth leading to increased needs of food, water and energy determines the development of natural resources in a given area. This is because the majority of the social services and development agendas designed by a government are based on this information. Ethiopia is the second most populous country in Africa. The population growth rate in Amhara region, where Lake Tana is located, is also very high and it is the second most populous region in the country. In the Lake Tana catchment, population growth leading to pressures such as agriculture, urbanization, waste discharge, dam construction, sand mining and fisheries, is the main driving force for natural resource degradation such as fish. About three million people (12.7% of the Amhara region population) live within the Lake Tana watershed [38]. The age group ranging from 0 to 14 years old is the most strongly represented population category, which makes the dependency ratio of the population in the lake catchment very high [39].

National Economic Transformation
The intended economic growth supported by the current national policy is also a potential driving force to various pressures in the lake catchment. Ethiopia, has committed itself to become a low middle-income country by 2025 [40]. In order to realize its national vision, Ethiopia had set up its first Growth and Transformation Plan (GTP I) in 2010 and formulated a second Growth and Transformation Plan (GTP II) in 2016. To achieve this ambitious goal, targets have been established for several sectors including agriculture, energy and manufacture. Among others, emphasis has been given to ensuring rapid and sustainable growth through enhancing productivity of agriculture. Hence, the country intends to increase cultivable land by 13% and this policy necessitates the conversion of forestland and wetlands into cropland [41]. Besides, in order to achieve high productivity increases in agricultural output, fertilizer and pesticides use is expected to increase by approximately 100% [42]. Therefore, this could increase the amount of nutrients such as nitrogen and phosphorus in the lake, which would subsequently lead to algal blooming and invasion of weeds. Furthermore, Ethiopia has planned to increase irrigated lands by more than 400% [41] and this would stimulate dam construction in the lake catchment. Dam constructions increase pressure on habitat and spawning grounds and block the migration routes of the migratory Labeobarbus species [11].

Agriculture
In Ethiopia, agriculture is the most important sector of the country's economy. It is the main source of livelihood for the overwhelming majority of the population. The Lake Tana catchment is highly agricultural, focusing on crop production and, to a lesser extent, livestock. The area of land used for agriculture and the amount and types of agricultural production are critical factors in determining the severity of agriculture as a pressure on the lake ecosystem. Agricultural development proponents in the lake catchment often fail to consider the biological, ecological and socio-economic values of natural resources such as wetlands. Decision makers, who are involved in natural resource conservation, are unaware of the services of wetlands ecosystems [43]. Wetland destruction and conversion to agricultural areas are still widely accepted in the national context. Driven by a lack of certified farming land, due to rapid population growth, people are forced to carry out their farming activities on hilly areas and in wetlands during the dry season. Above all, annually a campaign is organized by development agents, who are responsible for agricultural development, to dry up wetlands for the purpose of agricultural activities. Moreover, there is no policy, which limits use of fertilizers and pesticides. This allows farmers to use them without limitation. Therefore, these activities are potentially affecting water quality of the lake and its tributaries.
Similar to crop cultivation, livestock has a wide variety of functions and is a critical commodity of many households in the Lake Tana catchment. However, in Ethiopia as well as in the Lake Tana catchment, animals are not fenced, but are left free to graze on the communal lands such as wetlands. This could potentially cause environmental effects such as erosion, soil degradation, water pollution, and deforestation. For example, households in the Kurt Bahir Wetland allowed their livestock to graze on average for eight hours per day [43], which affects both the physical and biological components of the wetland through the consumption of plant biomass, trampling of plants, soil compaction and increasing nutrient input and bacterial contamination from their dung and urine.

Dam Construction
Dam construction for irrigation and hydropower purposes, has tremendous economic value and hence the practice is growing worldwide [44]. During the period of its growth transformation, Ethiopia planned to increase irrigated lands and energy supply [45]. This stimulated dam and hydropower plant construction from 2010 onwards. The Lake Tana watershed is identified as a region for significant irrigation development [45]. Currently, there are irrigation schemes including diversion weirs and large dams on more than 18 rivers. However, many dam construction projects are still planned on perennial tributaries of the lake, which could aggravate the effect on the state of the lake ecosystem. The Beles hydroelectric power plant (at the second outlet of the lake) [46] and the Megech-Seraba irrigation pump station are under construction, and intend to use water directly from the lake. This will have severe effects on the water volume of the lake, since 400 ha of land will be irrigated. This will subsequently affect the aquatic life, such as fish communities.
In sharp contrast to the policy principles and guidelines developed by the World Commission on Dams [44] and the International Hydropower Association [47], dams in the lake catchment are constructed in response to the emerging development needs only. Water resource developers are often unaware of the importance of fisheries and the impact of dam construction on fish. The co-ordination and collaboration between biologists and engineers is often lacking. Therefore, dam construction both for irrigation and hydropower purposes are potentially affecting the habitat and spawning ground of the fish species in the lake, leading to their stock reduction [11,48].

Urbanization and Waste Discharge
Currently, Ethiopia is rapidly urbanizing by a growth rate of 4.8% per year [49]. The development of urban areas is also the same in Amhara region. Of the 422 towns in the region, the majority are encroaching on both sides of Lake Tana and are rapidly expanding. For example, to accommodate the growing social and economic needs, Bahir Dar, the capital of Amhara Region, has been rapidly expanding. This has resulted in lake-associated wetlands being transformed for different purposes such as streets, parking lots, hotels, loges, industries and residences.
Besides wetland degradation, waste discharge into Lake Tana has become a serious and highly visible pressure. Waste management, both solid and liquid, in urban areas is unregulated. Consequently, all surface waters in the Lake Tana catchment are receiving untreated municipal and, industrial wastes. Two-third of the households in Bahir Dar discharge waste into streets and floodwater drainages, which ultimately end up into Lake Tana or the Blue Nile River [50]. Besides household waste, the waste also originates from major institutions and industries including universities, factories, hotels, floriculture and hospitals which discharge their untreated waste into Lake Tana [51].

Sand Mining and River Water Pumping
Sand mining in the watershed, a relatively recent phenomenon, is rapidly growing and expanding to all tributaries of the lake and as such, it is becoming an important pressure on the migratory Labeobarbus species [15]. Sand mining has direct effects on habitat and breeding ground degradation and it physically injures the fish. It also accelerates soil erosion, which results in siltation within the lake. The two ways of sand extraction in the inflowing rivers are filtering and digging [15]. The former is mainly practiced in the upstream areas during the rainy season, which is the period during which the peak spawning of the migratory Labeobarbus species takes place. Whereas, the latter is primarily executed at the river mouths during the dry season.
During fieldwork, we observed many small-scale farmer−level irrigation activities. Farmers undertake these activities either by directly pumping water from the perennial tributaries and streams or by building a barrier on the riverbed and divert the river water partly into their farming land. According to the local inhabitants, such kind of activities are very common in the entire lake catchment and they are considered as model practices being encouraged all around the country. Hence, these activities could have severe effect on the natural flow of the rivers and their biodiversity.

Fisheries
Illegal fishing, which leads to increased fish harvesting, affects the aquatic ecosystem functioning as well as the sustainability of the fish stocks [52]. It likely affects the abundance, composition and growth of fish. The most destructive monofilaments, which are imported from Egypt through Sudan, were introduced into Lake Tana fisheries towards the end of 2000s. Currently, because of their perceived effectiveness in catching fish, more than 98% of fishers are using these gill nets [14]. Thus, illegal, uncontrolled and unreported (IUU) fishing in the lake is becoming a norm. Furthermore, legally banned fishing techniques such as fencing, castnets, and poisonous chemicals (crushed seeds of Birbra tree, Millettia ferryginea and DDT) are used in most of the lake tributaries.

State Changes
Lake Tana catchment is among the top 250 globally most important lake catchments for biodiversity. Wetlands in the Lake Tana catchment form the largest and ecologically main national wetland complex (24,000 ha, 1.6% of its watershed) [43]. These wetlands are unique bird hotspots. Because of this, the lake catchment is a globally recognized wintering site for migratory birds. Moreover, wetlands in the lake catchment are ideal breeding grounds and nursery sites for many fish species including genera Afronemacheilus, Enteromius, Clarias, Garra, Labeobarbus (i.e., L. beso) and Oreochromis [9,13,32]. They are also nursery sites for juveniles of the migratory Labeobarbus species.
However, land degradation, particularly wetlands degradation, has become an environmental disaster in Ethiopia [53]. Despite their socio-economic and ecological importance, many people in Ethiopia still consider wetlands solely as the breeding places for disease vectors like mosquitoes [54]. This perception makes the wetlands susceptible to degradation. The main reasons for Lake Tana wetlands degradation are flood recession agriculture, urbanization, waste disposal, free grazing and removal of macrophytes [43,55]. Cyprus papyrus, the characteristic plant species of the wetlands, has dramatically declined, with even local extinctions in some areas [38]. Consequently, many wetlands in the lake catchment are shrinking at a drastic rate [43,[56][57][58], (Table 2). This is an indication that the wetlands in the lake catchment are not wisely utilized in line with the Ramsar Convention. The negative effect of wetland degradation on diversity and abundance of macroinvertebrates has been described in previous studies [59,60].
The drastic land degradation and deforestation aggravates sedimentation in Lake Tana [1,41]. Despite the 0.48 million tons of sediments (32% of the gross sediment transported) being trapped annually by the floodplains (wetlands), 1.56 million tons of sediment is still being deposited into the lake each year [61] and this negatively affects the habitat and nursery sites of many fish species. Moreover, due to siltation and water pumping, many perennial rivers have now become seasonal to the detriment of the riverine and migratory fish species.
Nowadays, degradation of the physical-chemical characteristics of the Lake Tana ecosystem is intensifying. Due to both point and nonpoint pollutions, the lake water quality is rapidly changing [62]. This was confirmed after bacteriological and chemical pollutions of the lake were revealed near Bahir Dar City [8]. According to Goshu [63], fecal coliforms and Escherichia coli concentrations significantly increased and analysis of Biological Oxygen Demand (BOD5) indicated organic pollution [64] in the Gulf of Bahir Dar. Additionally, a high concentration of the cyanobacterial genus Microcystis [65] leading to eutrophication [66] was observed in the Gulf of Bahir Dar. According to a discussion with local fishers, the taste of the flesh of O. niloticus, which primary feeds on algae, seemed to have changed noticeably in 2012. A plausible explanation could have been the occurrence of high concentrations of cyanobacteria. Of some fish species in the lake, mercury levels exceed internationally accepted safe levels for consumption [67]. Ahrens et al. [68] also reported high concentration of perfluoroalkyl carboxylates such as PFCAs in water, sediment and fish of Lake Tana.
The world's most invasive aquatic weed, water hyacinth (Eichhornia crassipes), infested Lake Tana in 2011 [62]. The plausible reason could have been the changes in the physico-chemical characteristics of the lake. Currently, its coverage is estimated to be more than 20,000 hectares. Due to the rapid reproduction and the high capacity of water hyacinth to utilize nutrients, this species outcompetes the native species [69,70]. Consequently, it is massively affecting the biological and physical-chemical characteristics of Lake Tana [71][72][73][74]. Besides, water hyacinth causing evapo-transpiration from Lake Tana could most likely affect the flow of the Nile water. The negative effects of water hyacinth on ecosystem functions of many African lakes has been repeatedly reported [69,75,76]. Therefore, based on experiences in other African lakes and the rapid increasing distribution of water hyacinth in Lake Tana, there is little doubt that the effects of water hyacinth on Lake Tana fish and fisheries will become severe.
Development activities such as sand mining and dam construction for both irrigation and hydropower purposes undermine the biodiversity of the fish. They are creating immense pressures on survival of the lake fish, particularly on the migratory endemic Labeobarbus species. Dam constructions, for example, do not consider mitigation of fish [11]. At the Gelda and Shini Rivers, because of dam constructions, spawning grounds across 27 km and 22 km of the rivers are now inaccessible to the migratory Labeobarbus species [11]. The dam causes a large number of fish to be trapped and to be more susceptible to predators and illegal fishery. Also hydropower structures [77] and sand mining activities [15] are known to have immense impacts on fish. Many fishes from Lake Tana enter the hydropower structure of Tana Beles where they are exposed to severe risks of injury. Sand mining for dam and house construction purposes has various negative effects on spawning grounds of the migratory Labeobarbus species. Therefore, if the development projects in the lake's watershed are going to be fully implemented, without proper management, the lake water depth and area will be reduced by 0.44 m and 30 km 2 , respectively [78] and this will continue to cause declines of the fish stock. Subsequently, local extirpations of the endemic Labeobarbus species is anticipated. Illegal fishing in Lake Tana is massively contributing to the depletion of the lake's fish stocks [80]. Due to their spawning aggregation [13,32] and ecologically highly specialized endemics [81], Lake Tana Labeobarbus species are highly vulnerable to this effect. This can be best illustrated by the reduction in Catch Per Unit Effort (CPUE). The CPUE for the endemic Labeobarbus species has declined from 63 kg/trip in 1993 [80] to 6 kg/trip in 2010 [82]. Likewise, the CPUE for all the commercially important fish including Labeobarbus species has also declined from 177 kg/trip in 1993 to 56 kg/trip in 2010 [10].
Generally, when habitat and spawning ground degradations are severe, endemic species such as fish have a high chance of extinction. In light of the fact that Lake Tana is home to many endemic migratory fish including six threatened species, anthropogenic activities are detrimental to their stock. These activities can jeopardize the lake ecosystem, leading to severe reductions in productivity and ecosystem services. The overall impacts on the Lake Tana catchment have put the lake at risk. Thus, if degradation of the lake catchment continues at the present rate and measures are not urgently taken, extinction of some fish species is inevitable. The loss of fish is disastrous and those who are relying on its service (fishers) are the first to feel the impact. To avoid another repeat in history, lessons learnt from Lake Haromaya (a dried lake) and Lake Abijata (a rapidly declining lake), need to be incorporated into the national and regional fisheries management plan.

Impacts
Fisheries in Lake Tana contribute to the livelihoods of fishers as food and an income source via employment. In Lake Tana, fishers are often poor and the income received from selling fish is the only option to provide them access to basic needs such as food, health, education and clothing. Moreover, the lake fisheries have various social and cultural benefits to the fishers. Since the nature of the fishing activity is often social, it strengthens the bond among the fishers. Fishers have a long history of social gathering such as "idir" and through this, they have been sharing knowledge, skills, culture and other assets.
However, the lake fisheries are highly vulnerable to external pressures and these are detrimental to their attributes. Consequently, these have various impacts on socio-economic values of the local inhabitants. In particular, the drastic reduction of fish populations [80], for example, negatively affects livelihoods of the fishers in Lake Tana.
Nowadays, thefts of fish products and fishing gear are on the rise in Lake Tana. This is most likely linked to the fish population reduction. Fishers and fisheries experts also corroborate this and point out that this was not the case until the early of 2000s prior to which fish production was deemed abundant. This situation has become the root cause for conflicts among the fishing community. As a result, it is negatively affecting the community cohesion. Due to this, many fishers have doubts regarding the continuity of their social and economic values. Therefore, policy makers should recognize the importance of fisheries for socio-economic values of the local inhabitants and should ensure its sustainability.
Additionally, health risks, due to contamination by pathogens coming from non-point and point sources of pollution, are wide reaching impacts in the lake catchment. The direct discharge of untreated waste into the lake negatively affected the water quality leading to algal blooms including harmful algae. The lake water pollution is therefore detrimental to many people who are directly fetching water from the lake for drinking and washing. The effect is worse for the fishers whose day-to-day activity is in the lake, and to many people particularly children who are swimming in the lake. Furthermore, mercury levels higher than the internationally accepted safe levels for consumption [67] and high concentration of the perfluoroalkyl carboxylates [68] in some fish species are a great health concern for fish consumers.
Water hyacinth is a suitable media for mosquito reproduction [74,83] and for snails, which serve as vector for the parasite of Schistosomiasis [84]. It also provides cover to reptiles such as poisonous snakes [70]. The occurrence of water hyacinth in Lake Victoria, for example, accounted for a larger proportion of the cholera cases in Kenya [85]. Therefore, based on experiences in other African countries, there is no doubt that the newly introduced and rapidly expanding water hyacinth in Lake Tana could be detrimental to the health of the local people.

Potential Responses to Increase the Sustainability of Lake Tana
Many institutions share ownership of Lake Tana and its resources. There is no institution clearly responsible for management of Lake Tana and its resources [86]. Resources such as fish and wetlands are public goods. This makes it difficult to sustainably exploit these natural resources. The problem of shared or lack of ownership is clearly reflected when it comes to the management of the lake resources. For example, when an attempt was made to manage the water hyacinth, the issue of ownership made it difficult to address the responsible partners. Despite the reality that solving problems in Lake Tana appears to be very much dependent on institutional collaboration, the current cooperation among the institutions with a similar scope is weak [11]. These institutions do not communicate very well about conservation of the lake and its biodiversity. For example, although local universities and research institutions have conducted many studies on the fish species and the challenges they are facing, there is no organized system, in which these institutions co-operate and share their findings. Furthermore, sustainable management of the lake resources involves participation of the local communities and fishers in particular as main actors. However, in the Lake Tana watershed, cooperation between the concerned institutions and local communities is generally weak [11,38]. Despite the fact that a bottom-up and top-down approach is crucial during problem identification, decision-making and policy enforcement, merely a top-down approach has been implemented instead. For example, fishers have not been consulted during the development of fisheries regulations and their attitudes and perceptions towards the management and policy enforcement, which are vital for effective fisheries management, have not been considered and included in the environmental management (personal observation by the authors). Moreover, implementation of the national and regional proclamations such as the Federal Fish Resource Development and Utilization Proclamation, the Amhara Regional State Fisheries Proclamation, and the Fisheries Resource Development Protection and Utilization Proclamation Enforcement is lacking.
Thus, the lack of proper policy implementation, participation of the local communities and institutional collaborations are leading to ineffective fisheries management in the lake. A timely enforcement of the policy, a close co-operation and mutual support among governmental institutions that have a similar scope and the local communities is therefore crucial for conservation of the lake resources. To this end, we have identified viable multilevel responses including, family planning, policy revision and proper implementation of wastewater treatment plants, habitat restoration and subsiding stakeholders as feedback to driving forces, pressures, changes of state and impacts. More explanation about the responses is presented in the DPSIR framework ( Figure 3). Besides, ecotourism development helps to reduce pressures on the natural resources through diversification of livelihoods. The Lake Tana watershed has various key destinations for ecotourism development including biodiversity hotspots, natural forests, monasteries and wetlands. The rich natural resources coupled with the unique cultures and histories of the local communities provide a foundation for ecotourism development. As a result, more than 84,000 local and international visitors travel to the watershed each year, substantiating the potential of Lake Tana as emergent destination [38]. This can make the local people aware of the importance of protecting and conserving natural resources. Therefore, it is very crucial to integrate ecotourism development into the sustainable management of the Lake Tana fish resources. Furthermore, we have described the major threats of the lake ecosystem and their potential management options (Table 3). To help facilitate conservation of the resources, we also suggested that the responsible organization(s)/communities should address these issues (Table 3). Hence, each organization and the community should take their responsibility and strengthen their co-operation. Furthermore, we have described several research issues in ecology and fisheries, which require further investigation. The main research gaps are related to: (1) the ecosystem services of wetlands in the lake watershed and the consequence of their degradation for biodiversity, (2) impacts of the rapidly declining Labeobarbus species particularly the piscivorous species on food web of Lake Tana, (3) quantitative estimation of the effects of development activities on both biodiversity loss and environmental degradation particularly the lake water quality, (4) the ecological and socioeconomic impacts of water hyacinth in the lake, (5) potential of the native plants to out-compete water hyacinth, (6) overall as well as species based contribution of Lake Tana fisheries to the local livelihood and regional economy, and (7) scientific and feasible recommendations regarding suitable fish migration designs for the migratory Labeobarbus species. Therefore, the local universities and research institutions in collaboration with other institutions should take the responsibility to resolve the research gaps and to combat the conservation challenges of the Lake Tana fish resources. Table 3. A description of the major anthropogenic threats in the Lake Tana catchment and their optimal solutions and the responsible organizations/community. For proper implementation of these solutions, the available policies should be revised and properly implemented. In addition, budge has to be supported by the local Government and NGOs.

Major Threats Possible Solutions (Responses) Possible Actors
Illegal fishing

General Discussion
Globally, freshwater resources have been affected by human induced activities causing biodiversity loss and hydro-morphological alterations. These changes also impact people, which are depending on these resources. The anthropogenic impacts still remain major challenges in many parts of the world [87]. Due to the rapid increase in human population leading to various pressures, the fish communities in many African lakes such as Lake Malawi, Lake Tanganyika and Lake Victoria have markedly declined [88,89].
Ethiopia has abundant surface inland water resources harboring an impressive diversity of fish species. However, human population is rapidly increasing, while natural resources remain limited.
As a result, the use of the available resources has been intensified and has led to overexploitation. Activities such as overfishing, land degradation and environmental pollution have negative impacts on survival of fish species. In many cases, wastes are discharged indiscriminately into the nearby waterbodies without any treatment, intensifying the Ethiopian water resources problems [90]. This is not different from Lake Tana, Ethiopia's largest lake, where fish communities are rapidly declining due to anthropogenic activities [1,10,11]. Illegal fishing coupled with environmental degradation has already led to drastic stock reduction of the endemic Labeobarbus species. The fish stock reduction has very serious implications on the lives of thousands of people who are strongly dependent on fish communities. For sustainability of the lake fisheries, implementation of the environmental management is vital.
Fisheries management is virtually lacking in Lake Tana. This is because the managers and policy makers, who are responsible for biodiversity conservation, underestimate the importance of the lake fish and fisheries to the local economy and residents. Additionally, the collaboration between scientists and managers or policy makers is poor and the available knowledge on Lake Tana fish and fisheries is not distributed enough to managers and policy makers.
The DPSIR framework is a useful tool to comprehensively organize the available information in a meaningful way for use by fisheries managers and policy makers. As a result, it has been widely used to identify the major fisheries problems, adopt standardized indicators, improve fisheries management and guide management decisions [21,[91][92][93]. Therefore, its application in the Lake Tana catchment will help to bridge the communication gap among stakeholders and provide decision makers with scientifically underbuilt information.
Nevertheless, it also has some limitations. These are related to terminological unclarity, lack of methodological description to analyze disturbances and oversimplification of problems when dealing with complex environmental issues [21,94,95]. These limitations can be overcome by integrating it with other conceptual frameworks [94,95] and more specific modelling tools [96]. Although the DPSIR framework has particular limitations, it has for many cases shown to be an effective tool to summarize and visualize the cause-effect relationships between environmental components and anthropogenic impacts, and is a first step to define and trade-off responses at different levels of the impact chain.

Conclusions
The local scientific community urgently needs to be informed about the severe fish abundance reduction in Lake Tana and its consequences. Preventive measures must be initiated now, before extinction of some species occur. We should implement lessons learned from experiences elsewhere such as Lake Haromaya, Lake Abijata and Lake Victoria. The conservation challenges of the Lake Tana fisheries must be addressed for the general health of the ecosystem and its users. In order to support the conservation practices, we have summarized and visualized the cause-effect interactions between human pressures and the environmental components in a manner familiar to fisheries managers and policy makers using the DPSIR framework. Thus, this helps to bridge the gap between research and decision-making in the lake catchment. The Ministries of Agriculture and Livestock Resources and all other concerned authorities, therefore, should incorporate the developed conceptual frameworks into the Lake Tana fish resources management plan. As overfishing and environmental degradations are common in many African freshwater bodies, this document can also serve as a reference for the application of DPSIR in these freshwater bodies.