David Lidholm. bioscience explained Vol 7 No 2. Mangrove forests threatened by human exploitation

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1 David Lidholm Department of Biological and Environmental Sciences, University of Gothenburg, Box 463, SE Gothenburg, Sweden Mangrove forests threatened by human exploitation Shrimp aquaculture and forest rehabilitation Tiger shrimp, Penaeus monodon Introduction You might have been told to avoid buying or eating giant prawns, which naturally live in mangrove habitats in tropical and subtropical coastal areas in all three of the major oceans. Giant prawns are also called King prawns and comprise several species from different areas with the common trait that they are big. Some of them have transverse dark stripes on their abdomen and have therefore been called Tiger shrimps. The most common of these species is Penaeus monodon (see figure below) which is thus commonly found in fish stores. It is a distinctly marine species, but also in fresh water tiger shrimps can be found, but they are all much smaller. The reason to stay away from giant prawns is that large areas of mangrove forests are being degraded or even completely destroyed because of prawn cultures in them. These forests form highly productive ecosystems that contribute to significant inputs of organic carbon to the sediment along with several other ecosystem services. Timber and fuelwood, medicines, boats, finfish and edible crustaceans are all examples of benefits from mangrove forests. They also provide protection from coastal erosion, storms and tsunamis, while being important nursery grounds and breeding sites for birds, fish and mammals. There is an estimated equal amount of biomass above and below the ground. The distribution of mangroves is limited by the 20ºC seawater isotherm where winter temperatures never drop below 20ºC (Fig. 1), hence the tropical domination. However, there are local exceptions and mangroves appear in other places as well in small numbers. Correspondence: David_Lidholm@hotmail.com

2 Figure 1 The distribution of mangrove forests along the latitude gradient, where the heavy lines indicate the mangrove forests and the small dashed lines are an estimation of where the 20ºC isotherm would be (Alongi 2002: modified from Spalding et al. (1997)). Aerial roots, viviparous embryos which means they germinate before they part from the parent plant and the fact that they are halophytes, adapted to high salinity, makes mangrove unique, and there are about 70 known species of mangroves across the world. They occupy an area of roughly 180,000 km² and grow between the mean sea level and the highest spring tide. That area is significantly smaller than in the beginning of the 1980 s as a world average loss of 35 % is reported over those 20 years up to the year Reports from Ellison and Farnsworth (1996) suggest that mangrove forests keep degrading by a yearly rate of 1.5%. Therefore, efforts have been made to preserve the forests with conservation and restoration projects. Degraded areas are targets of restoration, while the undisturbed mangrove forests are being managed in conservational ways. The diminishing of mangroves is caused by storms, floods and tsunamis but the biggest contributor is aquaculture, standing for more than half of the degradation. Aquaculture Sites where mangrove forests grow are very convenient places to start aquacultural activities because of the intertidal zone, where necessary flushing keeps the water quality high, while upland areas are more attractive to agriculture. Shrimp, prawn, crab, and different fish ponds are examples of aquaculture engaged by local populations and companies, with shrimp farms likely being the most common. The degradation of mangrove forests between 1980 and 2000 was substantial, and more than half of it (52%) was due to aquaculture. The degrading and often complete removal of mangroves to benefit aquaculture, leaves that area with reduced water quality and excess nutrients. The hole, left in the ecosystem, leads to the release of toxic wastes and excessive nutrients with ultimately increased turbidity as a result. Shrimp farms Shrimp farms are mainly present in Asia (75%), with Thailand and China representing the majority of them. Starting a shrimp farm requires access to juvenile shrimps, which most often are taken from natural stocks outside the coast. To maintain the culture the flow of juveniles must continue and the natural stock is threatened, with the risk of wiping it out. Even fish stocks offshore will suffer degradation, since it is the 2

3 main ingredient in the shrimp s food. Because of the shrimp s poor uptake, a lot of it fuels the eutrophication of the pond and downstream estuaries. The eutrophication together with toxins and other wastes from the aquafeeds and chemical additives, accumulating in the sediments, unfortunately leaves the pond unusable after 5-10 years. The rate of recovery of these scavenged areas is very low. The shrimp farms have then to be moved somewhere else. Copper (Cu), among other trace metals, are present in the aquafeeds that nurture the shrimps to prevent bacteria, parasites or fungal infections. Lacerda et al. presents interesting results in their paper (2006), where they look at Cu emissions in Northeastern Brazil. They measured concentrations of Cu in the aquafeed, in shrimp biomass and inflow and outflow of water as well as sediment samples in a shrimp farm at Jaguaribe River estuary (NE Brazil). The aquafeeds supporting the shrimp farms stand for a majority of the Cu added (Table 1) with agriculture coming up second with 1/8 of the emission. Shrimp biomass accounts for less than 10 % of the Cu load added, so a large part of the remaining load is either exported to nearby waters or retained in the sediments. Cu sedimentation and accumulation can cause toxic problems to shrimps and other organisms. Due to the relatively small area occupied by shrimp farms, the annual emissions/ha is less significant compared to agriculture and urban areas as shown in Table 1. However, the local effect is still substantial and sediment measurements, both upstream and downstream from the studied farm, show ten times greater levels of Cu downstream. Similar results were also found under salmon cages and in lobsters in New Brunswick that can support the findings. Activity Cu emission Cu annual emission Agriculture Wastewater diposal Urban runoff Solid wastes disposal Husbandry Shrimp farming Table 1. Comparison of average Cu emission factors (g ha 1 yr 1 ) and total annual emissions (t yr 1 ) from different anthropogenic activities and processes and shrimp farming at the Jaguaribe River estuary, NE Brazil (Lacerda et al. 2006). Rehabilitation Not surprisingly, mangrove losses are positively correlated with human population and density growth in coastal areas. With fewer people living at or near a for- 3

4 est, the destruction and exploitation is much smaller. However, mangrove forests are also naturally disturbed by cyclones and other storms, lightning, tsunamis and floods, and often take decades to recover. Severe degradation of the mangrove forest, manmade or not, often need a helping hand to be able to recover. Field (1998) explains two types of regeneration used to rehabilitate degraded mangrove areas: Natural regeneration Artificial regeneration Natural regeneration Natural regeneration uses the seeds that naturally occur on the site and the mix of species should be equal to the local conditions. Advantages with this method are firstly; the final result is likely to be close to its original state. Secondly, it is relatively cheap and requires less labour which ultimately results in less soil disturbance. This method should be considered the first option unless there is evidence indicating it will not be successful. Artificial regeneration This method is being used when the natural regeneration is insufficient. It requires moving seedlings from another location or collecting seeds to plant on the site after they have grown under nursery conditions. By using artificial regeneration the species composition and the distribution of seedlings can be controlled and genetically improved species can be introduced, which may be more tolerant to diseases or predators. The natural regeneration is the preferred method, to keep the area as undisturbed as possible when it comes to species composition and soil conditions. The artificial method applies to severely degraded areas with little or no active regeneration, and is in general less successful because more factors and parameters have been altered. The topography of one of these heavily degraded sites may be changed due to erosion/accretion. Tidal waves, sedimentation, soil physiochemistry and hydrology in the area might also have changed. Mangroves are dynamic and can change in abundance as a result of small changes in the natural environment, and when changes appear in those parameters mentioned above the mangroves become more susceptible to diseases and pests. Restoration of eroding shorelines Hashim et al. (2010) executed a restoration project in Sungai Haji Dorani, Malaysia. The area was severely degraded and faced erosion impacts, as do roughly 1/3 of the Malaysian shorelines. The method used in this project was artificial regeneration since the area did not 4

5 Figure 2. Percent of the remaining surviving saplings versus date (Hashim et al. 2010). hold enough seeds to support the plantation, and seedlings had to be taken 4 km north of the site. They planted 10,000 pre-grown seedlings, 10 times the normal density of mangrove forests, so that the expected high initial mortality will not result in a sparse forest. 3 out of the original 10 species were planted in the area, where the first one represented more than half of the total plants (Avicennia marina). That was also the most common species originally and had constant recruits from neighboring areas, whereas the other two (Bruguiera and Rhizophora spp.) were planted at the second phase as supporting species to improve diversity. The area was controlled once a month from July 2008 to February 2009 when living plants were counted. They also measured lengths of the stems which were used as an indicator of growth performance. The restored mangroves was expected to trap and build up the sediment and reduce erosion of the area. The possibility to trap waterborne mangrove seeds that might contribute to further expansion of the mangrove forest was also considered. Roughly 30 % of the original saplings survived to the last date of the controls and that was also the amount of saplings that did not make it through the first month (Figure 2). The results were of moderate success after 7 months. The monitoring was suggested to continue over 3-5 years, but this was expected since eroding shoreline exposed to wave action is not the most fitting target for restoration. The estimated growth was 15 % after 3 months and 30 % after 7 months. This enhances the ability to trap sediments and hinder the erosion. After 3 months there were no erosion or accretion observed in the area and significant amounts of sediment was deposited, and this effect increased after 7 months of the experiment had passed. Competition and problems with restoration Competition between non-mangrove and mangrove species is rarely a problem in mangrove restoration, since non-mangrove species have a hard time invading tropical coastal estuaries. That should equate to solid grounds for rehabilitation of lost mangroves. But, projects rarely use the full complement of available species (as Hashims example above) and often choose those optimal for afforestation, not ecosystem restoration. A switch in biodiversity that eventually can affect the composition of the whole ecosystem, which include species involved in mangroves well-being. For instance, animals in mangrove ecosystems can contribute significantly to mangrove pollination. However, little attention has been paid to the reappearance of the fauna in re- 5

6 habilitation areas, and if it is, often only a few target species are investigated. There is a need of publishing results, in refereed journals, both the successful projects and the failures. To be able to learn from our mistakes and make results easily accessible for those interested, more of the regeneration projects need to be documented properly. The necessary requirement is knowledge of the processes essential to developing and supporting the productivity of the system as a whole, rather than its parts (Field, 1998). And as Field states, it is not likely that an ecosystem will go back to the natural state, the conditions it had before the large disturbance, but that does not mean it is not going to be sustainable (1998). It just needs careful planning considering the changed conditions on the site. Conclusion Mangrove forests are very dynamic and can change in distribution, abundance and diversity as a result of small changes in the environment. The increasing human population in and around coastal areas has increased the pressure and exploitation of the mangroves as it constitutes an important base for the locals livelihood. Aquaculture in general, and shrimp farms in particular has caused roughly half of the 35 % degradation of mangroves between 1980 and The effects of aquaculture often lead to wiped out local ecosystems unable to naturally recover, while the business moves to the next site. Also, the high levels of copper, incorporated mostly through aquafeeds, in shrimp farms could reach toxic levels and accumulate in sediment and biomass of shrimp and other organisms. The degradation of mangroves through aquaculture, but also storms, tsunamis and floods has brought the attention to conserve and protect undisturbed mangrove areas but also rehabilitate damaged areas. The rehabilitation is complex, and dependant on so many factors to be successful. The more degraded an area is, the more likely it is for parameters to be permanently altered. Changes in tidal waves, sedimentation, soil physiochemistry and hydrology need to be taken into consideration when constructing a rehabilitation project. The goal of restoration is often to restore or uphold timber production or some other financially important business, which could lead to less consideration regarding species composition, diversity and fauna in the mangrove ecosystem. The lack of publications is according to Field (1998) a problem, and prevents the people involved in restoration to be able to learn from each other s findings and results. The knowledge and preservation is key to uphold a sustainable way of utilizing the mangrove ecosystem, and beneficial both for the nature itself and for the people making a living out of it. 6

7 References Alongi, D. M., Present state and future of the world s mangrove forests. Environmental Conservation. 29, Ellison, A. M. & Farnsworth, E. J., Anthropogenic disturbance of Caribbean mangrove ecosystems: Past impacts, present trends and future predictions. Biotropica. 28, Field, C. D., Rehabilitation of Mangrove Ecosystems: An Overview. Marine Pollution Bulletin. 37, Hashim, R., Kamali, B., Tamin, N. M., Zakaria, R., An integrated approach to coastal rehabilitation: Mangrove restoration in Sungai Haji Dorani, Malaysia. Estuarine, Coastal and Shelf Science. 86, Lacerda, L. D., Santos, J. A., Madrid, R. M., Copper emission factors from intensive shrimp aquaculture. Marine Pollution Bulletin. 52, Spalding, M. D., Blasco, F., Field, C. D., World Mangrove Atlas. Okinawa, Japan: International Society for Mangrove Ecosystems. Picture sources More reading 7