Impact of solid shrimp pond waste materials on mangrove growth and mortality: a case study from Pak Phanang, Thailand

Study Number:

17

Author:

C. Vaiphasa, W. F. de Boer, A. K. Skidmore, S. Panitchart, T. Vaiphasa, N. Bamrongrugsa & P. Santitamnont

Abstract:

One of the most serious threats to tropical mangrove ecosystems caused by shrimp farming (aquaculture) activities is the poor management of pond waste materials. We hypothesise that mangroves can tolerate chemical residues discharged from shrimp farms and can be used as biofilters, but the capability of mangroves to cope with solid sediments dredged from shrimp ponds is limited. Our study in Pak Phanang, Thailand, confirmed that the excess sediments discharged from nearby shrimp ponds reduced mangrove growth rates and increased mortality rate (extinction). A series of transformed multitemporal satellite images was used in combination with the field data to support this claim. In addition, a comparison between four dominant mangrove species revealed that Avicennia marina could tolerate sedimentation rates of >6 cm year–1, while Bruguiera cylindrica tolerated sedimentation rates of 5 cm year–1 (total sediment depth = 25 cm) before dying, while Excoecaria agallocha and Lumnitzera racemosa performed intermediate. This outcome implied that in our situation A. marina and to lesser extent E. agallocha and L. racemosa could be more effective as biofilters than B. cylindrica, as they may survive the sedimentation longer in the disposal areas. Further studies on the impact of sedimentation and chemical pollution of shrimp farm wastes on mangrove mortality and growth are required.

Main Results and Conclusions:

• Illegal dumping of shrimp pond (aquaculture) wastes in Pak Phanang, Thailand caused severe problems for surrounding mangrove habitats: “Following the aerial survey of the Thai Forestry Department (TFD) during May-June 1996, it was found that the dominant mangroves on the eastern coast of the cape, including Avicennia marina, Bruguiera cylindrica, Excoecaria agallocha, and Lumnitzera racemosa, were severely disturbed by the illegal deposits of shrimp pond wastes dredged from the surrounding shrimp farms (Fig. 2)”(49). 
• Remote sensing analysis was carried out to document the extent of mangrove habitat degradation after the dumping of shrimp pond waste: “An image analysis, using a series of multi-temporal satellite images and aerial photos, was used to study the changes in the spatial extent of the impact of shrimp pond wastes on mangrove forest development. Using aerial photographs of 20 February 1995 and 2 July 2001, the total destructive area could be delineated”(49).
• LANDSAT satellite imaging was also used to capture the transformation of mangrove habitat before and after the dumping occurred: “a series of multispectral LANDSAT satellite images of the delineated area taken respectively on 19 April 1995, 8 April 1997, and 27 April 1998 was transformed into three corresponding Normalized Difference Vegetation Index (NDVI) images… In our case, the area covered with mangrove forests possessed a positive value where the value between 0.0 and 0.3 represented sparse forests, and the value between 0.3 and 0.7 was assigned to dense forests. In contrast, the non-vegetated area (e.g., bare soils, roads, shrimp farms) was assigned with a negative value”(50).
• From studying the satellite imagery as well as sedimentation quantities in soil samples from both control and contaminated mangrove plots, the following conclusions were made:
  • “The field observations demonstrated that the trees in the affected area had a lower growth rate (Fig. 3) and a higher mortality rate (Table 2) than the trees in the controlled plots. These findings confirmed that solid shrimp farm waste depositions have negative effects on the Pak Phanang mangrove development”(52). (Extinction).
  • “The outcome of the multi-temporal NDVI analysis of images taken between 1995 and 1998 (Fig. 5) strengthened this claim. The analysis indicated that the effects of the shrimp farm (aquaculture) waste depositions were not localised within the sampled areas but affected a large extent of the Pak Phanang mangroves”(52).
  • “…it was also found that A. marina in the affected area is the strongest mangrove species, possessing the highest growth and lowest mortality rates.”(52). (Extinction).
  • “Our 5-year sedimentation record in the affected area showed that the amount of sediments deposited from the nearby shrimp farms were well beyond the mangrove tolerance level (i.e., approximately 5 mm of sedimentation per year; Ellison, 1998)”(53-54).
  • “It is therefore likely that the excess in sediment supply is the major cause of the mangrove decline. Furthermore, our results suggest that B. cylindrica was the weakest species to survive, as it tolerated an average sedimentation depth up to only 25 cm while the E. agallocha and L. racemosa species performed better at 31 cm and 33 cm, respectively. This outcome indicates that E. agallocha and L. racemosa, together with A. marina, are able to withstand the effect of sedimentation better, and are therefore more suitable species to be used in biofilter experiments than B. cylindrica. Lastly, it was a surprise that there was almost no natural colonization of the mangroves in the affected area during the 5-year monitoring period (Fig. 4) even though the dumping activity had been banned since 1996. Instead, the areas covered with dense mangroves gradually receded over the years (Fig. 5)”(54). 
  • “Despite the capability of natural mangroves to withstand the chemical constituents of the shrimp farm (aquaculture) wastes, the excessive amount of the waste materials deposited in the Pak Phanang mangroves is beyond the mangrove tolerance levels”(55).

Works Cited:

Ellison, J. C., 1998. Impacts of sediment burial on mangroves. Marine Pollution Bulletin 37: 420–426.