Mekong River Commission

Addressing fisheries in the Climate Change
and Adaptation Initiative

By Ashley S. Halls*

The Lower Mekong Basin is getting warmer and more rain is falling in the wet season. In the delta, the sea level has already risen by as much as 3 cm and further increases are expected, possibly by as much as 1 m by the end of this century. More uncertain are future patterns of precipitation and water availability in the dry season. Reducing this uncertainty will be an important first step of a new regional initiative which coincides with a study showing that fisheries in Viet Nam and Cambodia are among the most vulnerable to climate change.

How will the wet and dry season views of the Mekong change as climate change
impacts the Mekong region?

The Mekong River Commission held a regional forum in Bangkok on February 2-3 to formulate a Climate Change and Adaptation Initiative (CCAI) for the basin. The CCAI is being led by the MRC Environment Programme with technical support from the International Water Management Institute (IWMI), Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Southeast Asia Global Change System for Analysis Research and Training (SEA START).

Attended by nearly 200 regional stakeholders and international experts, the forum provided an opportunity to gain feedback from key stakeholders on the planned approach and framework, review existing knowledge, and identify stakeholder needs from the initiative. The forum also sought to promote dialogue on climate change and adaption in the Lower Mekong basin (LMB) among regional development partners and organisations.

Fisheries and climate change
The CCAI forum coincided with the publication of a climate change study by a team of scientists from the WorldFish Center, the MRC, and the Universities of East Anglia, UK, and Simon Fraser, Canada. The study, funded by Britain's Department for International Development and published in the journal Fish and Fisheries, examined the vulnerability of national economies to the impacts of climate change on their fisheries. Using indices of exposure, sensitivity and adaptive capacity, the team mapped and ranked the vulnerability of 132 national economies to climate change (see Allison et al. 2009).

Significantly, the study ranked Viet Nam and Cambodia as two of the most vulnerable countries in tropical Asia (ranking 27 and 30 respectively), along with Bangladesh, Pakistan and the Yemen. Their vulnerabilities arise from the combined effect of predicted warming, economic and dietary importance of fisheries and comparatively limited capacity. Lao PDR was also found to be vulnerable but its ranking at 37 may underestimate its true relative vulnerability because its fisheries are likely to be grossly underestimated in the statistics employed for the study. Thailand ranked 82 in the study because despite the significance of its fisheries, it is better able to cope with climate change impacts having a higher gross domestic product, a more diversified economy, and lower rates of poverty (Allison pers comm.).

Climate change in the Mekong region?
Riparian experts at the CCIA forum presented what appeared to amount to unequivocal evidence that the climate of the LMB has already changed in recent decades. In Viet Nam, for example, annual average temperature was reported to have increased by approximately 0.71°C and sea level by 2.5 to 3 cm during the last fifty years.

Predictions from down-scaled Global Circulation Models (GCM) of future climate change in the region under different scenarios of human development and emissions are broadly consistent, indicating warming throughout the region in the range of 0.6 - 0.7°C with increasing and more variable precipitation in the wet season. Less consistent are the predictions for the dry season. Depending upon the choice of model, precipitation during the dry season may increase or decrease, but greater variability can be expected in both cases.

Uncertainty surrounding the direction of climate change during the dry season will be reflected in predictions of the likely impacts on fisheries (and other sectors) and raise obstacles to the formulation of appropriate and effective adaptive coping strategies. Efforts to reduce this uncertainty as part of the CCAI would therefore appear to be a priority.

What might be the impacts of climate change on fish resources and dependent livelihoods in the LMB and what strategies might exist to cope with these impacts? Of course, these and other sector-relevant questions will form the focus of the CCAI during the coming years. But what can we say now?

The recently published study by Allison et al. (2009) emphasises the difficulty in attempting to predict what are likely to be profound impacts on fish populations, stemming largely from a lack of knowledge and understanding of the response of ecosystems to the anticipated physical changes in the environment.

Potential effects of climate change
These impacts are likely to arise through complex behavioural and physiological responses of fish which may be exacerbated by additional changes to the environment arising from adaptive coping strategies pursued by other sectors, particularly those that compete for water.

Higher temperatures reduce oxygen solubility in water but can raise the oxygen and food intake demand of fish as their metabolic rates are raised. Associated rises in gill ventilation rates can lead to increased uptake of aquatic pollutants, potentially rendering the flesh unfit for human consumption. Higher water temperatures can also favour the survival of parasites and bacteria. All these responses combine to potentially reduce fish survival, growth (in food limited environments) and reproductive success both in wild populations and aquaculture systems. Studies have also shown that the reproductive success of tropical species can be directly affected by elevated temperatures. Species with relatively narrow thermal tolerances may therefore be displaced to regions where water temperatures more closely match their thermal optima, to be replaced by more temperature tolerant species (Ficke et al. 2007).

The combination of reductions in river flow and sea level rise is anticipated to change salinity profiles in the Mekong Delta in Viet Nam and lead to greater upstream salinity intrusion. These changes may displace stenohaline (narrow salinity tolerance) species further upstream and increase the upstream range and biomass of euryhaline (wide salinity tolerance) species inhabiting the basin, including those that depend upon brackish water environments to complete their life-cycles such as the giant river prawn Macrobrachium rosenbergii. The net effect on wild fish production and fishing opportunities remain uncertain but anticipated to be small (Barlow & Burnhill undated). The expansion of existing aquaculture systems based upon species such as M. rosenbergii may become an important adaptive strategy option for farmers inhabiting the delta.

Changes to river flow in response to changing spatial and temporal patterns of precipitation in the basin are likely to have the most profound impact on the basin's fisheries resources. Increasing flows during the flood season will translate to more extensive and prolonged floodplain inundation potentially increasing overall system productivity including the fish component (Junk et al. 1989; Welcomme 1985). Recent research (Halls et al. 2008) has shown that the growth of fish in the LMB is strongly linked to flood extent and duration (see chart below). Longer more extensive floods are likely to provide greater and more prolonged feeding opportunities for fish. Improved growth can favour survival and reproductive potential (fecundity).

Figure 1. Fish and floods

Mean body weight of fish caught by the Cambodian bagnet fishery
plotted as a function of a flood index. (FI)

Source: Halls et al. (2008)

However, not all species may benefit. Increasing river flows may hamper upstream spawning migrations, erode spawning beds or sweep eggs and juveniles past downstream nursery and feeding habitat. Overly-rapid changes in water level can also lead to diminished reproductive success of channel margin spawning phytophil and nest-building species. Changes to the timing of flows also have the potential to disrupt spawning behaviour (Welcomme & Halls 2001).

The dry season is a period of great stress to many river fish species arising from diminished feeding opportunities and water quality, and elevated risk of predation or capture. Fish survival during this period is therefore likely to be density-dependent (Welcomme & Hagborg 1977). Increased precipitation and water availability during this period might favour fish survival and ultimately exploitable biomass, whilst drier conditions would have the converse effect (Halls & Welcomme 2004). Increasing dry season water levels also have the potential to diminish primary production and habitat diversity within the system by permanently inundating fringing forests and vegetation, leading to permanent die-back and by effectively reducing the size of the flood margin or aquatic terrestrial transition zone (ATTZ) for nutrient recycling (Junk et al. 1989).

Can we predict impacts on Mekong fisheries? Considering all of these potential impact pathways and their interactions in the context of existing stressors (e.g. exploitation, habitat modification, pollution and infrastructure development for power generation, flood control and irrigation), predicting the overall net effects of climate change on exploitable fish biomass and its variability through space and time is a major challenge.

Views of the Mekong in Vientiane, Lao PDR. High flow photo was taken
on 9 August 2005, and the low flow photo on 4 November 2006.

Empirical studies may prove effective in addressing this challenge. For example, the historical response of fish populations to variations in the basin's hydrology should provide some indication of how they are likely to respond to future changes in flow arising from climate change and basin development activities. The Fisheries Ecology, Valuation and Mitigation Component of the MRC Fisheries Programme is pursuing this empirical modelling approach, building on the results illustrated in Figure 1 above to include the response of fish survival, recruitment and capture vulnerability to flow (flood) variability. This model will greatly assist in predicting future trends in fish biomass and yield under different climate change and basin development flow-change scenarios.

Given the small-scale, largely unmechanised nature of the fisheries of the LMB, the scope for mitigating climate change by the fisheries sector by reducing CO2 emissions to slow or reverse global warming appears very limited. However, formulating and implementing national adaptation programmes of action in the context of integrated water resources management (IWRM) are likely to be key functions of the CCAI. These may include options such as (re) building the resilience of overexploited or vulnerable populations to shocks and perturbations by reducing the rates at which they are exploited, and by adopting ecosystem and precautionary approaches to co-management partnerships (FAO 2008; Brander 2007).

Careful consideration will have to be given to both planned and autonomous adaptive coping strategies pursued by the agricultural sector. Less predictable flooding patterns and reductions in dry season flows may force small-scale farmers to build makeshift levees to protect their crops from flood damage and to increasingly rely upon surface water bodies to meet their irrigation needs. Planned adaptation may favour the construction of large-scale storage reservoirs, flood control embankments and irrigation schemes. As already experienced by millions of fishers in Bangladesh, these types of adaptations typically impact negatively on the fisheries sector by obstructing fish migrations and diminishing dry season habitat availability and quality (Halls et al. 1998; 1999). As alternatives, crop diversification, sustainable groundwater management systems and other softengineering solutions may yield win-win outcomes for both sectors (Shanker et al. 2005).

The IWRM approach to basin planning adopted by the MRC should prove effective in addressing these types of interactions in the context of existing stressors and thereby help to formulate and implement effective national adaptation plans and guide local autonomous coping strategies.

* Dr Halls is Coordinator of the Fisheries Ecology, Valuation and Mitigation Component of the MRC Fisheries Programme.

References
Allison, E.H., Perry, A.L., Badjeck, M.C., Adger, W.N., Brown, K., Conway, C., Halls, A.S., Pilling, G.M., Reynolds, J.D., Andrew, N.L. & Dulvy, K.N. (2009). Vulnerability of national economies to the impacts of climate change on fisheries. Fish and Fisheries 10: 173- 196 .

Barlow, C. & Burnhill, T. (undated). The consequences of climate change on the fisheries of the Mekong River system. Contribution to Intergovernmental Panel on Climate Change.

Brander, K.M. (2007). Global fish production and climate change. PNAS 104:19709-19714.

Ficke, A.D., Myrick, C.A., & Hansen, L.J. (2007). Potential impacts of global climate change on freshwater fisheries. Reviews in Fish Biology and Fisheries 17: 581-613.

Halls, A.S., Hoggarth, D.D. & Debnath, D. (1998) Impact of flood control schemes on river fish migrations and species assemblages in Bangladesh. Journal of Fish Biology 53 (Suppl. A), 358-380.

Halls, A.S., Hoggarth, D.D. & Debnath, K. (1999). Impacts of hydraulic engineering on the dynamics and production potential of floodplain fish populations in Bangladesh. Fisheries Management and Ecology 6: 261-285.

Halls, A.S. & Welcomme, R.L. (2004). Dynamics of river fish populations in response to hydrological conditions: a simulation study. River Research and Applications 20: 985-1000. 

 Halls, A.S. Lieng, S., Ngor, P. & Tun, P. (2008). New research reveals ecological insights into the dai fishery. Catch & Culture 14: 8-12.

FAO (2008). FAO expert workshop on climate change implications for fisheries and aquaculture. FAO Fisheries Report 870: 32pp. 

 Junk, W.J., Bayley, P.B., Sparks, R.E.(1989). The flood pulse concept in river-floodplain systems. In Proceedings of the International Large Rivers Symposium, Vol 106, Edited by Dodge, D.P. pp 110-127.

Shankar, B., Halls, A.S., & Barr, J. (2005). The Effects of Surface Water Abstraction for Rice Irrigation on Floodplain Fish Production in Bangladesh. International Journal of Water 3: 61-68. 

Welcomme, R.L. & Hagborg, D. (1977). Towards a model of a floodplain fish population and its fishery. Environmental Biology of Fishes 2: 7-24.

Welcomme, R.L (1985). River Fisheries. FAO Fisheries Technical Paper 262: 330pp. 

Welcomme, R.L., & Halls, A.S. (2001). Some considerations of the effects of differences in flood patterns on fish populations. Ecohydrology and Hydrobiology 1: 313-321.