Area Selection
The area was selected on the basis of concentration of freshwater pond fish farming in Bangladesh. The land conversion from crop to fish farming is mostly concentrated in the northern part of Bangladesh. Therefore, the most land-converted area, Mymensingh District, from the north part of Bangladesh was considered for this project. Among 64 districts in Bangladesh, Mymensingh district is an area of 4363.48 square km and consists of 12 sub-districts. The soil formation of the district is flood plain, gray piedmont, hill brown and terrace. There are small valleys between the high forests; annual average temperature maximum 33.3°C, minimum 12°C; annual rainfall 2174 mm. Agriculture is the main occupation (57.67 %), followed by transport, commerce, service and others. The amount of land used for cultivation is 346117 hectares; single crop 18.58 %, double crop 70.20 % and treble crop land 11.22 %. Four major concentrated sub-districts such as Mucktagachha, Trishal, Phulpur, and Bhaluka were considered among 12 sub-districts of Mymensingh district. In total 230 samples were collected from four sub-districts following the purposive sampling procedure.
Farm Size Selection
The sampling units were classified into different groups based on their land holding status. Most of the farmers in Bangladesh are small and medium-scale farmers (98.45 %) who have only less than 7.5 acres or 3 ha land and some few large-scale farmers (1.55 %). According to agricultural census of Bangladesh, a farm household was classified into three categories such as small (up to 2.4 acres), medium (2.5 to 7.4 acres), and large (7.5 acres or more). The landless and the farmers not using their land in pond fish farming were excluded from the sample units. All farmers produced fish for commercial purposes except some small amounts kept for home consumption, but crop farming was not commercial farming for the small and medium-scale farmers. The primary target of crop farming is to fulfill the family food demand and only sell the surplus amount to the market.
Commodities Selection
A crop calendar year represents two major growing seasons namely summer season (locally named Kharif season from April to October) and winter season (locally named Rabi season from November to March). The summer season is mostly rain-fed and the winter season is mostly irrigated in Bangladesh. The major portion of the cereal food supply comes from winter crops. Rice is the staple food in Bangladesh. Therefore, every farmer produces rice in both seasons along with other crops and vegetables. The major growing crops in Mymensingh District are rice, jute, and some winter vegetables [8], but the farmers who convert their land to fish farming confined themselves only to rice and fish farming activities. Although some farmers produced some vegetables, potatoes, sweet potatoes, and mustards those are excluded in the study because of using a negligible amount of land. Farmers cultivate different varieties of rice and fish during summer and winter seasons. The name of the rice varieties are Hori dhan, BR11 (locally called Mukta), BR22 (Locally called Kironmala), BRRI hybrid3 and BRRI hybrid4, BR28, and BR 29.Most of the farmers practice Hori dhan followed by Mukta and Kironmala. A very few farmers cultivate hybrid rice because of high production costs though its output is highest among all winter rice varieties. Among winter rice varieties, BR28 is the popular rice variety for the farmer though its output is the lowest among all winter crops varieties. The BR29 rice variety is also popular with those farmers who have the ability to apply more fertilizer to the field, which means comparatively large farmers have the capability of cultivating this variety. Like the summer rice, the winter rice hybrid variety is also unpopular to the farmer. There is a significant difference in fish farming varieties. One variety is significantly different from other varieties in stocking rate, yield and in price also which is not mentioned here. Particularly, in the study areas, farmers are performing polyculture instead of monoculture to avoid the risk. Six polyculture combinations were selected for the research. The name of the fish varieties are Pangus (Pangasius hypophthalmus), Koi (Anabas testudineus), Shing (Heteropneustes fossilis), Magur (Clarias gariepinus), Rohu (Labeo rohita), Catla (Catla catla), Mrigel (Cirrhinus cirrhosis), Silver carp (Hypophthalmicthys molitrix), Common carp (Cyprinus carpio), Tilapia (Oreochromis mossambicus), Gulsa (Mystus cavasius), Shorpunti (Barbodes gonionotus), Bata (Labeo bata).
Data Collection
Combinations of the participatory, qualitative and quantitative methods were used for primary data collection. The primary information was obtained through interview with key informants and questionnaire survey with farm households. At first, the information was collected from the key informants and then followed the household surveys. A short background of the research, study objectives, and data requirements was briefed with the participants. The secondary information was collected by reviewing the literature (publications and research articles) and obtaining through visiting some organizations such as BBS (Bangladesh Bureau of Statistics), DAM (Department of Agricultural Marketing), DOF (Department of Fisheries), Upazila Fisheries Officer (UFO), WorldFish center etc. Some important secondary data obtained from the experts of this field through email communication.