Study area selection This research was carried out in the Shyamnagar Upazila (sub-district) of Satkhira district in Bangladesh. Satkhira district is located in the southwest part of Bangladesh. Among the seven Upazilas in the Satkhira district, Shyamnagar Upazila was picked because it is highly vulnerable to natural disasters (cyclones, tidal surges, and saline water intrusion). The Upazila is situated between latitude 21°36′ N and 22°24′ N and longitude 89°00′ E and 89°19′ E, covering an area of 1968.24 km2. Most farmers in this area are engaged in farming different species of fish such as shrimp (Penaeus monodon), crab (Scylla serrata), white pomfret (Pampus argenteus), white mullet (Mugil curema), barramundi (Lates calcarifer) etc. Combined fish production or fish polyculture is becoming popular among farmers as the main adaptation strategy because it is profitable. The research used a convenient sampling technique to select the Munshiganj, Atulia, and Buri Goalini union within the Upazila as the final study area for data collection.
Fish polyculture production and sample selection The coastal region of Bangladesh contains intensive large-scale commercial shrimp production, which contributes significantly to the national economy by earning foreign exchange. In recent years, it has been associated with serious environmental and social impacts such as intrusion of saline water, destruction of mangroves, change in agricultural patterns, livelihood displacement, etc. Nevertheless, shrimp mixed culture has emerged as a method for sustainable shrimp farming in the coastal region of the country. Recently, shrimp production has shifted from intensive to extensive production, and other species of fish (crab, white pomfret, white mullet, etc.) are produced alongside the shrimp. Hence, this study has focused on farmers engaged in shrimp production along with other fish species. After identifying these farmers, random sampling was applied to select 85 of them. A pilot survey was conducted to understand the overall context and to validate the questions and variables in the questionnaire. Data were collected by face-to-face interviews, and a pre-tested semi-structured questionnaire was administered. Key information and observation method was also applied to collect data. The final data collection was carried out from February to April 2019.
Data analysis methods To analyse the collected data, the research applied several statistical techniques such as mean, standard deviation, minimum, and maximum in order to ascertain the socio-economic background of the respondents. This research used profit function, benefit–cost ratio (BC), a Cobb–Douglas production function model, and priority index (PI) to analyze the data.
Profit function Using profit Equation 1, the profit level from fish production has been calculated as follows.
π = TR -TC (1)
where π = Profit level in BDT TR = Total revenue (calculated from total fish yield (kg) x unit price) TC = Total cost (calculated from the summation of total variable cost and total fixed cost)
Benefit–cost ratio (BCR) To evaluate the profitability of fish production, benefit–cost ratio has been used according to Equation 2 as follows.
BCR = Total Revenue (TR) / Total Cost (TC) (2)
Here, BCR = Benefit–cost ratio TR = Total revenue from fish polyculture TC = Total cost of fish polyculture