Environmental changes kill fish in Okavango Delta

The mystery surrounding the death of fish in the Okavango Delta has been cleared by the Department of Wildlife and National Parks, who attribute it to changes in water quality and temperature in the river system, following heavy rains experienced in the last two weeks ago.

The North West Regional Wildlife Officer, Dimakatso Ntshebe said upon learning about fish mortality, the department dispatched a team of veterinary experts to investigate. Last week, he told The Patriot on Sunday that they have concluded investigations on the fish mortality and found nothing which might cause the death in fish in large numbers. He said they have concluded that the deaths were caused by a natural phenomenom of changes in the ecosystem of the delta.

He further indicated that it is not something new as it always happens when there are changes in the environment. He added that fish mortality was caused by the heavy rains which were experienced at Shakawe two weeks ago, which caused changes in water quality and changes in temperature of the river system.

He added that a team of veterinarians arrived last Friday and spent the whole weekend sampling up to 400 fish but found nothing, hence they concluded that it is natural death.

A Fish Biologist, Professor Keta Mosepele from Botswana University of Agriculture and Natural Resources, BUAN, sheds some lights on the matter as he explains that the Okavango Delta experiences fish deaths at the onset of new flood waters from Angola. He said these deaths are caused by changes in water quality where anoxic water (i.e. water with low dissolved oxygen content) are flushed out from beneath the papyrus and onto the main water body (i.e. either the river channels or lagoons of the Okavango Delta). ‘This is a common and natural occurrence which, however, does not cause any irreparable harm on the Delta’s fish stocks,’ he noted adding that the same phenomenon is experienced worldwide where fish experience mortalities due to changes in water quality, generally induced by natural dynamics.

He further noted that other potential causes of fish deaths in natural systems like the Okavango Delta are disease outbreaks, sharp temperature changes and fire outbreaks. The EUS (Epizootic ulcerative syndrome) is also a major fish disease whose outbreak has been recorded in the Okavango Delta in the past.

Professor Mosepele however noted that fish dying from this disease have significant lesions on their skins which has not been recorded in the recent fish deaths in the Delta.  ‘Sharp temperature changes can also cause fish mortalities, though the impact of this is usually localised and may not therefore explain the recent reports of fish deaths in different parts of the panhandle,’ he stated

He also stressed that wildfire outbreaks followed by heavy rains where ash is washed out onto the river channel can also result in fish mortalities. This occurs when ash depletes oxygen levels in the water body which may result in fish mortalities, he stated. He further noted that unfortunately, some causes fish deaths remain unknown, especially if these are brief, and in the absence of long-term monitoring of fish populations.

An academic further highlight that currently, there is no monitoring of fish populations in the Delta’s panhandle, which used to be done in the past so that occurrences of this nature can be rapidly observed and analysed.

He noted that this, used to be done by the then Fisheries Division in the Department of Wildlife and National Parks (DWNP) of the Ministry of Environment, Natural Resources and Tourism. However, this Division appears to have been disbanded and its research activities absorbed into the Research Division of the same Department (i.e. DWNP). Possibly due to stretched financial and human resources, there’s currently minimal fisheries research undertaken of wild fish populations, Mosepele explained

He also noted that globally, fish are a key source of cheap and yet highly nutritious food which is a source of livelihoods for socio-economically marginalised communities, including in the Okavango Delta.

Mosepele stressed that optimum utilisation of fish can also contribute to reduction of the national food import bill. He added that therefore, lack of investment in fisheries research will gradually erode the utility of this resource for these marginalised communities, and to the national accounts. These recent fish deaths in the Okavango Delta’s panhandle have exposed our dearth of knowledge on the state of our natural fish populations, he stated

He added that there is a clear and urgent need for investment on research to re-establish long term fish monitoring so that science can provide immediate answers to observed phenomena.  ‘The ultimate benefit of investment in research is that policy development will be science driven,’ he noted.