Economic valuation of ecosystem services trade-offs of tilapia cage culture farm and native capture fisheries in Lake Maninjau, Indonesia

Research on the effects of cage aquaculture on native fish populations has revealed its negative and positive impacts. However, a separate analysis of its ecological and economic effects creates incomplete information for the decision-makers. Moreover, the inadequacy of time series data has caused challenges to knowledge-based decision-making in establishing new cage aquaculture sites, especially in low-middle income countries. We use the ecosystem services (ES) framework to analyse the synergy and trade-off of cage aquaculture to a native fish species, Gobiopterus sp. in Lake Maninjau, Indonesia, as an effort to provide comprehensive information to support local decision-makers and to fill the information gap. We engaged some modelling techniques such as the Maximum Entropy Model (MaxEnt) validated with field survey data, Bayesian Networks (BN), and Monte Carlo Simulation (MCS) in this research. The results show that cage aquaculture provides habitat services for the species and temporary forgone fish production due to the accelerated sulfur upwelling. The economic value of habitat provision reached 74,500 IDR per year per cage or 1,128 million IDR per year in 2017. Further, the average value of the forgone benefit of fish production in the current cage aquaculture business extends from more than 550 million IDR per year in the dry season to almost 600 million IDR per year in the rainy season. The results indicate that the negative impacts of cage aquaculture on the native fish population outweigh its benefit. The recommendation for management actions includes applying alternative aquaculture techniques and other technological interventions.


Introduction
The effect of inland water cage culture farm (IWCCF) on native fish populations has been subject to much debate in fisheries science.Previous research has mainly described the impacts of IWCCF based on an ecological perspective.Results are mixed and vary from no signs to low adverse effects (i.e., [1,2]. Most of the research recognized adverse effects of IWCCF, such as predation and alteration of native fish species composition [3,4].Other effects on aquatic environmental include nutrient enrichment and eutrophication, biochemical contamination, alteration of benthic organism population, and adverse effects on fish biodiversity [4][5][6][7][8].Overall, there appears to be a consensus that there should be precautionary actions to prevent adverse impacts of introducing exotic species for cage aquaculture in 1260 (2023) 012040 IOP Publishing doi:10.1088/1755-1315/1260/1/012040 2 various water bodies [9,10].However, some positive impacts are also identified, such as wild fish refugee area provisioning [11].
In most low-middle income countries, where financial motives and economic growth still outweigh ecological considerations in decision making, the precautionary actions before constructing new IWCCF sites are commonly neglected [12].More importantly, the lack of adequate time series data also promotes the absence of knowledge-based decision-making in aquatic ecosystem management in such countries (see [13,14]).As a consequence, the countries face the environmental impacts of the proliferation of IWCCF (see [5,[15][16][17].Therefore, there is necessity to provide scientific consideration on both the economic and ecological aspects to evaluate the continuation of IWCCF programs. We applied an ecological-economic approach to fill this gap by analyzing the ES synergy and tradeoffs of IWCCF management on Gobiopterus sp., a native fish species in Lake Maninjau, Indonesia.IWCCF in Lake Maninjau (Figure 1) started in 1992 and has significantly expanded exceeding the lake's carrying capacity [18].It causes water quality degradation indicated by algal blooms and mass fish kills (MFK) as a result of sulphur upwelling and hypoxia [19,20].In the end, it results in a declining trend of IWCCF profits and even causes financial loss [21].
MFK has been continuing to be reported since 1995 and throughout consecutive recent years [22][23][24].This situation draws national attention since it threatens IWCC operation and it was roughly estimated to kill thousand tons of fishes in 2002, 2008, 2009, 2010, 2011, 2015, 2016, 2017, and 2018 [25,26].Financial lost in February 2018 alone was estimated to be 3.75 billion IDR (262,978 USD) [24].This value was only estimated by the death of culture fish.It will significantly increase if the value of dead native fish species was also counted.Furthermore, the lake environmental degradation also impacted tourism sector which was the prime economic sector for surrounding area [27].
The environmental deterioration also affects the native biotas.Two of the most influenced native biotas are: 1. Gobiopterus sp (ikan rinuak), 2. Rasbora maninjau (ikan bada), 3. Corbicula moltkiana (pensi).There were four main points of the effect of IWCCF that they mentioned; which are: 1. providing habitat (shelter and food) for ikan bada and ikan rinuak, 2. Forgone production of ikan rinuak and pensi for couple months after the sufur upwelling, 3. Appearance and establishment of wild tilapia as a result of escaping tilapia from the IWCCF, 4. Predation of native fish fries and fingerlings by the wild tilapia.Considering limited time and fund, this research is targeted to assess the first and second impacts.
This study is a part of a more extensive study to value the synergies and trade-offs between IWCCF and other lake ecosystem services.The identified ES synergy was habitat provisioning for the fish.The recognized trade-off is the exacerbated sulphur upwelling (locally named tubo belerang) which created temporary forgone production of the species.By assessing the economic value of the synergy and tradeoff, we aim to balance both the assessment of both positive and negative impacts of IWCCF to the fish species.We expect it to capture the decision-maker's attention and provide an alignment between the produced knowledge and decision makers' interests.
This paper is organized as follows: it starts with an overview of IWCCF in the study area which is followed by the methods section.Then, we elaborate on the result of the study, which is continued by a brief discussion.It ends with a body of conclusion to summarise the study.

Overview of IWCCF in Lake Maninjau
IWCCF in Lake Maninjau (Figure 1) started in 1992 and has significantly expanded exceeding the lake's carrying capacity [28].It causes water quality degradation indicated by algal blooms and mass fish kills (MFK) as a result of sulphur upwelling and hypoxia [19,20].In the end, it results in a declining trend of IWCCF profits and even causes financial loss [28].
MFK has been continuing to be reported since 1995 and throughout consecutive recent years [22][23][24].This situation draws national attention since it threatens IWCC operation and it was roughly estimated to kill thousand tons of fishes in 2002, 2008, 2009, 2010, 2011, 2015, 2016, 2017, and 2018 [25,26].Financial lost in February 2018 alone was estimated to be 3.75 billion IDR (262,978 USD) [24].This value was only estimated by the death of culture fish.It will significantly increase if the value of dead native fish species was also counted.Furthermore, the lake environmental degradation also impacted tourism sector which was the prime economic sector for surrounding area [27].

Data collection
Collected data is divided into three categories.The first one is secondary and interview data to obtain the general pictures of impacts of IWCCF to native fish, and to find the focus of the research.The second category is ecological data including environmental and abundance data to validate the pictures obtained from the literature and interview data.The third category is economic data including but not limited to production, price, and cost of the fish.

Literature review and interview.
The research was started by conducting literature reviews about impacts of IWCCF to native fish and water quality degradation in various inland water ecosystems and in Lake Maninjau.This includes reviewing 26 studies conducted in Asia, South America, and Africa and 10 papers for Lake Maninjau.Thereafter, informal interviews with 15 local fishers and three local fisheries agency's staffs were conducted in March 2019 to capture their knowledge about what effects of IWCCF to native fish.As a conclusion, the most influenced biotas: 1. Gobiopterus sp (ikan rinuak), 2. Rasbora maninjau (ikan bada), 3. Corbicula moltkiana (pensi).There were four main points of the effect of IWCCF that they mentioned; which are: 1. providing habitat (shelter and food) for ikan bada and ikan rinuak, 2. Forgone production of ikan rinuak and pensi for couple months after the sufur upwelling, 3. Appearance and establishment of wild tilapia as a result of escaping tilapia from the IWCCF, 4. Predation of native fish fries and fingerlings by the wild tilapia.Considering limited time and fund, this research is targeted to assess the first and second impacts.

Ecological data.
The ecological data includes habitat, the extent of habitat, and abundance of ikan rinuak, and impacts of metrological conditions and sulphur upwelling to the probabilities of ikan rinuak forgone harvest.Details of the ecological data collection related to the habitat of ikan rinuak is elucidated in [29] While, the methodology to collect data to build a model to predict the fish forgone production is described in [30].Due to limitation of time, the paper does not include the impacts through predation and food competition between tilapia (the farm fish) and the native fish.

Economic data.
Monthly production data for ikan rinuak were attained by recording daily production from 62 traders among 83 identified traders in the area.Thus, fish caught for domestic consumption is not recorded in this research.There are several types of ikan rinuak fishers.Some of the fishers catch the fish in daily and routine basis and use their catch to be sold to the traders, some others use their catch for both consumption and production, and some sporadic fishers just catch when they are willing to consume the fish.The fisheries are common pool resources and there are no official records of the number of the fishers; thus, trader production data is used as the approach for the production.

Data analysis
3.2.1.Habitat provisioning services.Abundance data of ikan rinuak were tabulated and descriptive statistically described by using Microsoft Excel.Unequal variance T-test is then performed to find occurring significant differences of fish abundance from three types of habitat by using Stata 15.1.Next, habitat modelling was performed by operating Maximum Entropy model (MaxEnt) to validate if cage culture is a significant factor in providing habitat for the fish [31,32].

Sulphur upwelling impacts on the forgone production of ikan rinuak.
Prediction of frequency of forgone production of the fish was conducted by building a Bayesian Belief Network (BNs) model published in [33].

Abundance and CPUE data.
Abundance data is calculated by dividing the weight of capture fish with the extent area of the scoop net.Meanwhile, CPUE is calculated by dividing the abundance data with the catch time.

Average production of ikan rinuak.
Considering that the fish is not a seasonal species and there is no data series of its production and catch efforts, then it is assumed that ikan rinuak production remains stable as long as there is no sulphur upwelling.The average production is calculated by dividing the fish' monthly production and the extent of the habitat area.Then, average production per cage area is assessed the products of the average production for each square metre multiply with the average cage extent area.The maximum value (right boundary) is the expected value of the probability of MFK and ikan rinuak disappearance generated from the BBN model for Gale-storm state 3.2.5.The expected value of habitat provisioning service.The habitat provisioning service is calculated using: To include the uncertainty generated by the variability of the probability of sulphur upwelling in dry and rainy season, the data is simulated with Monte Carlo Simulation (MCS) using Model Risk Software (Vose Software Corp.).
Beta4 distribution is selected as the distribution of ikan rinuak forgone production in both rainy and dry season because of its ability to represent the probabilities of proportions and its flexibility [34,35].This distribution represents the random variables and its probability distribution function generated by mixing both discrete and continuous functions.We use the distribution to exemplify the seasonal probability of MFK and the fish disappearance characterized by the number of days with specific wind state in a season (discrete functions).Using the cut-off point of 26% of MFK to determine when MFK is happening, the coefficient a (number of successes) of the distribution is obtained from the number of days with strong and gale-stormy days in each season [36].The cut-off points of 26% is the optimum cut-off point of MFK in BBN model, see [33].Whilst the minimum value of the distribution is obtained from the seasonal probability of MFK and ikan rinuak disappearance in calm state for each season (continuous function), the maximum value is obtained from the probability MFK and ikan rinuak disappearance in gale-stormy days from the BBN.
The values to fill the minimum values of the distributions are the expected value of the probability of MFK and ikan rinuak forgone production for the calm wind state for each Scenario.The mean values are obtained from the average percentage of MFK and ikan rinuak disappearance.The maximum value (right boundary) is the expected value of the probability of MFK and ikan rinuak disappearance generated from the BBN model for Gale-storm state.

Results of the interviews
The interview results show that the fishers are aware of the impacts of IWCCF on providing habitat and food to ikan bada and ikan rinuak.They mentioned that harvesting ikan rinuak in the IWCCF area is easier.In addition, they noted that when the IWCCF owners harvest their fish, they can harvest significant numbers of ikan bada at the same time from the cages area.In addition, [37] observed that IWCCF density impacted pensi abundance; the denser IWCCF, the less its abundance is observed.
More importantly, all interviewed fishers mentioned that the upwelling incident severely impacts ikan rinuak.They stated that a day after a severe up welling incident, the fish are found dead, and they cannot catch the fish for about 6 to 9 months after severe upwelling incident.Meanwhile, ikan bada can still be captured after the incident.[38] studied that this species can migrate to stream areas to avoid bad water quality conditions.Thus, it explains the survival ability of ikan bada during the upwelling incident.It is also concluded from the interview that the biotas are not seasonal.Ikan rinuak can be captured all year round except after upwelling events.
There were no fish captured in open water areas which highlight the roles of IWCCF in providing habitat for the fish (Figure 2).Further, t-test results presented in Table 1 confirm that IWCCF provides habitat and gives a significant contribution to the fish abundance.

The likelihood of ikan rinuak forgone production
The wet season associated with frequent heavy rain and strong wind compared to the dry season, significantly increase the probability of ikan rinuak forgone production (Table 3).Meanwhile, the number of active cage aquaculture does not substantially change the likelihood of the fish forgone production due to the severity of the present water quality condition.Other factors such as feeding frequency are also not considered significant in reducing the probability of forgone capture of the fish.Further details on the forecast on the likelihood of the fish forgone production have been elaborated in [33].

The expected value of habitat provisioning services of IWCCF for ikan rinuak
It is calculated that the IWCCF area contributes 38% of the fish production.Monthly production data collected from the traders showed that in 2018 the fish production approached 16,490.85kg/month.Considering that the fish is not a seasonal species and there is no data series of its production and catch efforts, then it is assumed that production remains stable as long as there is no upwelling and MFK.Thus, each square metre of IWCCF is assumed to contribute 0.013074 kg of ikan rinuak production.The economic value of habitat provision is calculated with increment production approach by using this data.The results are described in Table 2.It is revealed that habitat provision generates quite significant benefits; the calculated economic value is more than 1 billion IDR in 2016.The forgone benefit of Gobiopterus sp.production is slightly higher in the rainy season (Table 4 and Figure 3).The average annual value of the fish forgone production is more than one billion IDR. Figure 3, in particular emphasizes that there is a high probability that the forgone benefit can reach more than 500 million IDR in the dry season and 600 million IDR in the wet season.The values in the

Discussion
The results have shown ES synergy (habitat provisioning) and trade-off (forgone production) of the IWCCF to the ikan rinuak production.It is also revealed that the synergy is far less significant than the trade-off.However, with the increase of IWCCF, the value of habitat provisioning services can compensate for the forgone benefit of fish production.Overall, the figures still indicate that, in general, the ecological and economic negative effect of IWCCF on ikan rinuak fisheries is still more prominent.The IWCCF's adverse impact to the fish production is mainly caused by the detrimental effect of sulphur upwelling, which severely affects the fish due to its low swimming ability to avoid the event [29].The effects may be similar or different to other fish species in the lake depending on the species' swimming ability and other behavioural factors (cf.[39,40]).Further research is necessary to elaborate the effects of IWCCF on other native fish species and to compare these values with the value of the farm fish production.The positive economic impact generated by habitat services is proportional to the increment of the number of cages; nevertheless, the increase is also prone to the rise of financial loss from the mass fish kills (MFK) and is also limited by the ecological carrying capacity.The economic loss from the native fish forgone production is significant.The loss may be multiplied if research related to other native fish species is thoroughly conducted.The occurrence of MFK and ikan rinuak forgone production after sulphur upwelling event signifies the importance of conducting a restoration program in the lake.
Considering this circumstance, the future policy formulation to regulate the optimal number of cages must balance the economic gain and the loss.The current recommendation of 6,000 cages means much lower monetary gain than the forgone benefit.Thus, the management must compensate for the loss by creating alternative income sources.The compensation program can help reduce social impacts resulting from the policy implementation.
Some alternative actions which can be applied are integrated multi-tropic aquaculture (IMTA) [41] and other technological interventions such as sediment capping [42,43].IMTA specifically can reduce environmental costs, increase farm profits, and support a positive attitude toward aquaculture [44,45].

Conclusion
Although positive impacts of IWCCF on the native fish catch are also recognized, the adverse effects still outweigh the benefit.Suppose the current policy recommendation to reduce the number of IWCCF is arbitrated, future policy formulation and management plans should be addressed to overcome the economic loss and mitigate further social impacts.Therefore, alternative technological intervention and lake restoration is imperative to be done to alleviate the consequences.

Figure 3 .
Figure 3. Cumulative probability of the expected value of annual ikan rinuak forgone production.
) = the expected value of Gobiopterus sp.forgone production i = the season (dry or rainy season) y = the production of ikan rinuak when there is no MFK or when there is very low probability of MFK.The value is 16,491 kg/month kg/month or 98,946 kg/season ri = the probability of ikan rinuak disappearance event in dry and rainy season obtained from the BBN model p = the average market price of ikan rinuak which is 18,850 IDR/kg

Table 1 .
T-test results of ikan rinuak abundance data in IWCCF, littoral zone (LZ), and open water (OW) areas.

Table 2 .
The economic value of habitat provision functions of IWCCF for ikan rinuak based on the increment production approach.
Table and Figure show that the IWCCF business of usual creates devastating impacts on the fish capture.

Table 4 .
The annual expected value of forgone production of ikan rinuak in dry and rainy season.