Assessing the biodiversity of eels in Tanzania

Background and Objective

Eels are traditionally fished in many coastal regions and are therefore an important source of food and income. However, the populations of several eel species are declining worldwide and it is assumed that various anthropogenic factors play an important role in this decline. As migratory fish that use a great variety of habitats, eels are dependent on the passability of the rivers and are subject to a large variety of anthropogenic impacts. Hence, they are particularly vulnerable.

About 40 anguilliform fish species of the moray, conger and river eel families have been described for Tanzania. However, little is known about their distribution, ecology and use. So far, they have not been the focus of biological and fishery research, and catches are not recorded in detail.

Four species from of the genus Anguilla are known to exist in East Africa: Anguilla bengalensis, A. bicolor, A.marmorata and A. mossambica. However, knowledge about their distribution and other basic biological parameters is patchy, especially for Tanzania, although two of these species, A. bengalensis and A. bicolor, are classified as "near threatened” by the International Union for Conservation of Nature (IUCN).

In addition to habitat loss, these tropical  eels are facing a growing interest for the international live fish trade as seed for aquaculture operations, due to the decline of temperate eel species like European and Japanese eel. Furthermore, the population growth expected for the coming decades could lead to an increased risk of overfishing as well as a progressing degradation of aquatic habitats, especially near the coast (e.g. barriers, pollution).

Sustainable management does not only require a political and administrative framework, but also sufficient knowledge about the status of the resources and their use. In addition to documenting the current status, the project should therefore also develop the basis for a long-term, but cost-effective monitoring concept.

Approach

The project partners have defined four main project goals:

• Biodiversity assessment for specific eel habitats and documentation of the biology and ecology of anguilliform eels in selected riverine and coastal aquatic systems
• Reconstruct the dynamics of abundance and catch trends of eels to assess the impacts of fishing on the eel stocks
• Evaluation of social-cultural and economic factors affecting the sustainability of the eel fishery
• Knowledge transfer and capacity building for the protection of biodiversity and sustainable use

The study, which is funded by the Gesellschaft für Internationale Zusammenarbeit (GIZ) in the MeerWissen program, focuses on sections of the most important coastal rivers in Tanzania, namely Rufiji in the Rufiji district and Pangani in the Tanga region, where eels are frequently fished.

The study will include river sections as well as estuaries and coastal areas. In both rivers, ecological degradation with economic and social effects can already be observed. The species diversity in the rivers and coastal waters will be recorded, documented photographically and analyzed using modern molecular genetic methods. For eels, some biological aspects, e.g. the diet composition and contaminant load will be examined more closely. In addition, first investigations of the oceanic spawning migration will be conducted using pop-up satellite transmitters. The socio-economic, fisheries` and socio-cultural aspects are mainly dealt with by the Tanzanian project partners. Care will be taken to ensure that the local fishing communities are closely involved, for example when describing the habitat, collecting samples and assessing the catch trend. This will mainly be done by using questionnaires and interviews. The results of the study on biological characteristics, the use of species and socio-economic aspects will be communicated to the state institutions and authorities along with proposals for an appropriate and practicable monitoring scheme. This also includes insights into potential threats to the species in the study area.

Results

For the assessment of the diversity of the fish community, 70 species from 59 genera and 40 families were detected in 334 genetic samples. In some cases, morphological and genetic species determination were not in agreement. In these cases, further investigations are necessary.

During the project, the presence of A. bicolor, A. bengalensis and A. marmorata in the studied rivers was documented, which show a distinct distribution pattern along the rivers. Whereas A. bicolor was found in all sampled river sections with a clear tendency towards the lower stretches, A. marmorata and A. bengalensis were only caught upstream.

With a value of 109.3 µg/kg wet weight, the average concen­tration of mercury in the muscle of 42 analysed eels (A. bicolor and A. bengalensis) from different locations in the two rivers 

was in an expected range and, e.g., clearly below the EU thresh­old for human consumption (500 µg/kg for fish, 1000 µg/kg for Anguilla species). No clear differences between species and locations were found. Similarly, the measurements of PAH metabolites in the bile of 21 eels did also not reveal any unex­pected values (mean value for 1-Hydroxypyren 272.9 ng/ml bile) and were slightly lower than in eel samples from German rivers, but higher than in eels from Morocco.

The project also provided the very first, though only anecdotal, information about the early phase of the oceanic spawning migration of A. bengalensis from East Africa. Data received from one PSAT-tagged individual revealed a northward migration of roughly 300 km along the coast within one week after tagging and diel vertical migrations (DVM), which have been described also from other Anguilla-species. During the DVM, the eel achieved depths of more than 550 m during day and only about 65 m at night. Another eel was possibly preyed upon.

The project also revealed first information on the socio-economic importance and on the value chain of the eel fishery at the river Rufiji. Consumers of eels differ depending on eel species. Anguillid eels are locally utilized for human consump­tion while other anguilliform estuarine species like the giant slender moray are mostly utilized as baits for crab fisheries. The eel fishery is characterized by a short marketing chain that supports the livelihood of the communities. While the consumption preference of eels near the coast is low compared to other species due to the snake-like shape of eels, the larger growing species further inland are higher priced.

Links and Downloads

meerwissen.org/partnership-projects/bioeels-tz