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Impacts of Flood Control Schemes on Inland Fisheries in Bangladesh: Guidelines for Mitigation

A. S. Halls
Aquae Sulis Ltd, Midway House, Turleigh, Wiltshire BA15 2LR, UK

A. I. Payne
MRAG Ltd, 18 Queen Street, London W1J 5PN, UK

S. S. Alam
Bangladesh Centre for Advanced Studies, House 10, Road 16A, Gulshan-1, Dhaka, Bangladesh

S. K. Barman
Bangladesh Centre for Advanced Studies, House 10, Road 16A, Gulshan-1, Dhaka, Bangladesh

Abstract/ Executive Summary:

Flood control, drainage and irrigation (FCDI) schemes are widespread in Bangladesh. They are built to control water levels to improve agricultural production based on high-yielding varieties (HYV) of rice that cannot tolerate rapid inundation or that require irrigation and to provide protection from extreme flood events. The benefits to the agricultural sector can be significant. At some sites in Bangladesh, farmers report up to 80% more agricultural production inside the schemes than outside. However, fish production and species richness are typically lowered by these structures. Fish yields inside a typical flood control compartment can be 50% lower than outside, with up to 25 species of fish absent or less abundant. Lower rates of recruitment of migratory whitefish species, whose lateral migrations are obstructed by the embankments, were found to be largely responsible for these differences. With a risk of more extreme flooding during the monsoon season but hotter and more arid dry season conditions predicted as a consequence of climate change, more FCDI schemes may need to be constructed to provide flood protection and to meet increasing irrigation needs. Based on fisheries monitoring and mark-recapture studies undertaken at 3 sluice gates, nine recommendations for operating sluice gates to mitigate the impacts of FCDI schemes on fish production and biodiversity are described. These recommendations aim to improve the access of migratory whitefish to modified floodplains and to improve or sustain the production of resident (non-migratory) blackfish whilst minimizing agricultural sector losses.

Keywords: Migrations, Tagging, Sluice gates, Flood control, Depletion

Location:

Start Date:

End Date:

Program Area: Food Safety and Security

Non-commodity: Impact

Objectives:

In effect, we aim to provide a set of simple and cost-effective recommendations for (i) improving the access (recruitment) of migratory fish to FCDIs and (ii) sustaining or improving the production of resident fish populations, whilst minimizing impacts on the agricultural sector.

Methodology:

The access and passage of migratory whitefish species to FCDIs were studied at three undershot-type sluice gates. Two of the gates, Talimnagar and Baulikhola, control water levels inside the Pabna Irrigation and Rural Development Project (PIRDP) flood control drainage and Irrigation scheme located in Pabna District, NW Bangladesh, at the confluence of the Padma and Jamuna Rivers. The third, the Jugni Gate on the Lohajang River in Tangail, NC Bangladesh, controls inflowing water from the Jamuna River into the Compartmentalisation Pilot Project (CPP). The PIRDP protects a total area of nearly 2000 km2 from the floodwaters of the Padma and Jamuna Rivers by an embankment of over 200 km in length. Water levels inside the PIRDP are controlled by the Bera and Koitola pumps and by 15 sluice gates, Talimnagar being the largest. The gates are infrequently opened during the rising flood period (June-September), but all are often fully opened at the end of the high water season (October) to allow the floodplain waters to drain back to the main river channels. The CPP was completed in 1991 to provide controlled flooding to 13000 ha of land on the east bank of the Jamuna River. The compartment is divided into 16 sub-compartments or hydrological units, each of which can be managed independently according to the specific hydrological needs of their inhabitants. Water flowing into the compartment via the Lohajang River, a tributary of the Dhaleshwari River, is controlled by the Jugini sluice gate. Data were collected from the beginning of the flood season in June 2003 to the end of the dry season in March 2004 and then again from June 2004 to the peak flood period in August 2004. A hydrological survey provided, on both sides of each sluice gate (inside and outside the FCDI), estimates of hydrological variables hypothesized to affect fish migration passage success through sluice gates (Table 1). A catch assessment survey (CAS) provided information on the timing, magnitude and direction of fish migrations at each sluice gate during the hydrological cycle. A daily interview-based survey of all fishers operating within the vicinity (100 m) of each sluice gate provided a complete census of fish catches (by species) taken by interceptors gears during the study period. These gears are deliberately positioned to intercept the seasonal migrations of fish towards or through the sluice gates. Their placement, orientation (facing towards or away from the sluice gate) and catches can therefore be used to determine the magnitude, timing and direction of the fish migrations during the hydrological year. This assumes that the orientation of the gear reflects the origin of the fish caught. At the study site, these interceptors' gears included liftnets, magnets and jump nets. Whilst catches from seine nets and traps were significant, the orientation in which these gears are fished relative to the sluice gate is often difficult to determine. Based on the catches of fish taken with interceptor gears positioned inside and outside the sluice gates, their orientation relative to the sluice gates and the direction of water flow through the sluice gate, four categories of migrations can be identified after Hoggarth et al. (1999) i.e. passive and active immigrations and emigrations. The CAS also aimed to provide estimates of the optimal attraction velocity. Whilst inward migrations into flood control schemes during the ebb flow period (active immigration) will be constrained by the maximum swimming speed of the immigrating fish (see below), many species of fish are also stimulated (attracted) to swim against such flows (rheotaxis) and, in this case, towards the sluice gate. Attempts were therefore made to estimate mean optimal attraction velocities of immigrating fish by comparing estimates of catch per unit of effort (an index of the biomass) for lift nets positioned outside Talimnagar gate, against ebb flow velocity during 2003. A biological survey provided data on the size (age) structure and life history strategies of migrating fish populations to determine the relative importance of the four types of migrations and to provide estimates of the maximum sustainable swimming speeds of the predominant immigrating fish species. Catches of the predominant species from interceptor gears were sub-sampled bimonthly for length (only 2003), somatic and gonad weight, and maturity state. The fork lengths of 200 fish of each species were measured to the nearest millimeter with a steel rule. For a sub-sample (30) of these fish stratified according to size (small, medium and large) and sampling location (inside and outside the sluice gate), somatic and gonad weights were measured to the nearest gram using an electronic balance and their maturity state identified according to Bagenal (1978) as being immature, mature, ripe or spent. Because of small sample sizes, data were pooled across both sampling years and sluice gates. This assumes that spawning and maturation behavior did not vary significantly between the sampling locations and periods. Length frequency distributions were plotted against time (not illustrated here for brevity) to estimate the mean length of fish during each migration phase and to assess the size (age structure) of the populations.

Source of Information: Hydrobiologia (2008) 609:45–58

Article Link (Online Journal): DOI 10.1007/s10750-008-9402-4

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Many of the recommendations listed above were adopted by sluice gate management committees at the PIRDP with the support of local non-government organizations shortly after the completion of this research. Billboards displaying the recommendations in Bangla were erected at the sluice gates. Fisher communities within the vicinity of the gate subsequently reported improvements in fish yields but formal monitoring programs would be required to substantiate these claims. Furthermore, many of the recommendations have been incorporated into the national Bangladesh Open-Water Capture Fisheries Strategy formulated by the Department of Fisheries, Government of Bangladesh, and are to be implemented shortly.

Knowledge Management: Journal