Estimating the Economic Loss Due to Vibriosis in Net-Cage Cultured Asian Seabass (Lates calcarifer): Evidence From the East Coast of Peninsular Malaysia

Abstract

This study aims to estimate the economic loss due to vibriosis in the production of Asian seabass in floating net-cages on the east coast of Peninsular Malaysia. Asian seabass has contributed significantly to Malaysia's economic activities and food security. However, its production can be hindered by the occurrence of diseases, such as vibriosis, causing severe economic losses to farmers. A questionnaire-based survey was conducted on 14 small-scale monoculture Asian seabass net-cage farms. Using a stochastic bioeconomic model and inputs from the survey, existing literature, and expert opinion, the economic losses were determined. Moreover, this model considered the prevalence of Vibrio spp. at a farm on the east coast and the risk posed by its infection from hatcheries. The results showed that 71.09% of Asian seabass simulated in the stochastic model survived. The mortality rate due to vibriosis and other causes was at 16.23 and 12.68%, respectively. The risk posed by Vibrio spp. infection from hatcheries contributed to 2.77% of the increase in Asian seabass mortality. The stochastic model estimated that the total cost of producing a tail of Asian seabass was €2.69 per kilogram. The economic loss of vibriosis was estimated at €0.19 per tail per kilogram, which represents 7.06% of the total production cost of Asian seabass per kilogram. An increase in the prevalence of clinical vibriosis and vibriosis case fatality rate at 42 and 100%, respectively, will lead to an increase in the cost of grow-out Asian seabass by €0.29 per tail from the default value. An increase in pellet price per kilogram by €1.38 and feed conversion ratio pellet by 0.96 will consequently increase the cost of grow-out Asian seabass by €2.29 per tail and €0.82 per tail, respectively. We find that the occurrence of Vibrio spp. infection at the hatchery level can contribute to an increased risk in the mortality of Asian seabass during the grow-out phase. Hence, we also need to focus on the control and prevention of vibriosis infection from hatcheries.

Keywords: Asian seabass; economic loss; net-cage culture; stochastic model; vibriosis.

Figure 1

A framework for the estimation of economic losses due to vibriosis in Asian seabass in floating net-cages on the east coast of Peninsular Malaysia. ¹In the real world, fish mortality is one of the major problems experienced by fish farms, which causes economic losses. A stochastic bioeconomic model is formulated to simulate this issue. ²Bioeconomic modeling is used to simulate the problem of low production due to vibriosis: first, we state the relevant assumptions; second, we construct the model in Microsoft Excel using @Risk add-on; third, we select the suitable parameters of the model using biological and economic inputs from existing literature, survey results, and expert opinion; fourth, we interpret and validate the outputs to modify the model where necessary. ³Following our assumptions, the bioeconomic model considers small-scale farm management on the east coast of Peninsular Malaysia; we conduct a survey that may help in model formulation. ⁴We adapt the bioeconomic model from that of a previous study based on Asian seabass grow-out in cage farms on the west coast of Peninsular Malaysia. Details of the modeling of economic losses are shown in Figure 2 and Supplementary Figure 1. ⁵We obtain biological and economic inputs by conducting farm surveys, analyzing the prevalence of vibriosis from fish sampling, existing literature, and expert opinion (see Tables 1–5). ⁶We interpret the model output in terms of economic losses from vibriosis (see section Analysis of the economic loss due to vibriosis using a stochastic model). ⁷Survey results (see section Survey results) and expert opinion are used to validate the model output. ⁸We modify the model by adapting small-scale farm management on the east coast of Peninsular Malaysia. This model includes the risk of Vibrio spp. infection from hatcheries and grow-out cage culture, assuming that the diagnosis or treatment for infected Asian seabass are not viable (assumption in the model refers to the inputs used in Tables 1–5).

Figure 2

This transition matrix was adapted from a previous study (6). The state of fish at stage (n + 1) is dependent on its state at the previous stage (n). From the figure above, we can see that at stage n, a fish in a healthy state (cell A) can be subclinically infected with vibriosis (cell 2) at stage (n + 1), determined by the prevalence of subclinical vibriosis (A2). Within the following 2 weeks of subclinical infection with vibriosis (cell B), we assume that there will be a response from the fish's immune system, such that it will either become healthy (cell 1) or clinically infected with vibriosis (cell 3), determined by the prevalence of clinical vibriosis (B3). Within 2 weeks after acquiring a clinical infection with vibriosis (cell C), a fish can either become healthy (cell 1) or die (cell 4), which is determined by the case fatality rate (C4). If the state of the fish implies that it was dead in the previous stage (n), it will remain the same in the following stages (D4 and D5). ¹Indicates differences in the rate of infection, depending on the month of grow-out (11).

Figure 3

Impact of change in biological inputs on the costs of grow-out in Asian seabass per tail. ¹When the FCR for pellet increases by 0.96 from its default value, the costs of grow-out increase by €0.82 per tail. ²When the FCR for pellet decreases by 0.27 from its default value, the costs of grow-out decrease by €0.37 per tail. ³When the case fatality rate due to vibriosis is 100%, the costs of grow-out increase by €0.29 per tail. ⁴When the case fatality rate due to vibriosis is 0%, the costs of grow-out decrease by €0.12 per tail. ⁵When the prevalence of clinical vibriosis is 42%, the costs of grow-out increase by €0.29 per tail. ⁶When the prevalence of clinical vibriosis is 11%, the costs of grow-out decrease by €0.08 per tail. ⁷When the prevalence of subclinical vibriosis is at 60%, the costs of grow-out increase by €0.12 per tail. ⁸When the prevalence of subclinical vibriosis is at 0%, the costs of grow-out decrease by €0.09 per tail. ⁹When FCR for trash fish increases by 0.16, the costs of grow-out Asian seabass increase by €0.02 per tail. ¹⁰When FCR for trash fish decreases by 0.47, the costs of grow-out decrease by €0.03 per tail.

Figure 4

Impact of changes in economic inputs on the costs of grow-out in Asian seabass per tail. ¹When the price of pellet per kilogram increases by €1.38 from its default price, the costs of grow-out increase by €2.29 per tail. ²When the price of pellet per kilogram decreases by €0.46 from its default price, the costs of grow-out decrease by €0.75 per tail. ³When the price of trash fish per kilogram increases by €0.53, the costs of grow-out increase by €0.75 per tail. ⁴When the price of trash fish per kilogram decreases by €0.07 less, the costs of grow-out decrease by €0.10 per tail. ⁵When the fingerling price per tail increases by €0.06, the costs of grow-out increase by €0.09 per tail. ⁶When the fingerling price per tail decreases by €0.04, the costs of grow-out decrease by €0.06 per tail. ⁷When the wage of labor per month increases by €70, the costs of grow-out increase by €0.01 per tail. ⁸When the wage of labor per month decreases by €70, the costs of grow-out decrease by €0.01 per tail.