Swimming for survival

The United Arab Emirates lies within an arid zone with very hot summers followed by cooler winter months and very little rainfall. In a country without rivers and lakes, wadis are the main freshwater habitats. In the northern UAE mountains there is only one species of native freshwater fish.

Wadi habitats in the UAE are rarely continuous, connecting only during flooding events. Consequently, habitats are isolated from each other and can vary considerably in terms of size, depth, substrate and flow. These physical variations also influence the chemistry and biology of water bodies which directly affects the fish populations inhabiting them.

In order to maintain a healthy population, fish are thought to adapt to the habitat in which they exist. Within a wadi system several habitat types exist ranging from bedrock, gravel and reedbed flowing streams to isolated, deep boulder and gravel pools and series of small, connected pools.

Studies were carried out on fish living in two different habitats; still, deep pools and flowing, shallow streams. Initial results have implied that the fish are able to adapt to where they live. The main areas in which fish differ between pools and streams are their behaviour and their body shape. Pool fish move slowly in shoals and will swim close to the surface. A small, still, clear pool allows predators such as birds and snakes to see the fish easily. By shoaling they could be attempting to confuse predators by adopting a 'safety in numbers' survival approach. As pool habitats have smaller surface areas than streams, they may swim near the surface to take advantage of any insects or food which fall on the water -- in a small habitat they need to exploit opportunities of any possible food source.

In comparison, stream fish rarely shoal and spend the majority of their time swimming quickly between rocks and hiding. This could be to avoid predators which could hunt them easily in shallow water. They may also be saving energy by remaining under rocks, if they were to shoal near the surface they would need to swim continuously against the flow. When food is scarce in these restricted habitats, fish may need to conserve energy as much as possible.

Fish in pools displayed more breeding behaviour than stream fish and fish fry were observed several times throughout the winter months, particularly after flooding. This may be because pools present preferable conditions for reproduction, possibly due to more space and a greater food supply.

Length and weight was measured for 150 fish from pools and 150 fish living in streams. There was no difference in average length between pool and stream fish. However, there was a large difference in their weights -- pool fish were around a quarter heavier than stream fish. This lower weight in stream fish produces a narrow, more streamlined body shape which would be more suitable for living in a shallow habitat and hiding under rocks and algae.

Stream fish need to swim in flowing water so may use more energy, producing 'thinner' fish. As their habitat is shallower and flowing there may be less food available, leading to lower weight.

The 'fatter' pool fish need to exert less energy in their habitat as there is no flow against them when they are swimming. They have a deeper habitat and may have more food available to them- more insects live in pool habitats, food falling on the surface does not get washed away and there also more algae found in pools.

This early research suggests that the type of environment in which the fish are living influences their body shape and behaviour. This is a very important survival strategy as fish may be isolated in pools for several years but they, or their eggs and young, may be washed into streams during flooding so they have to be able to adapt. Wadifish in the UAE have perfected their survival techniques for living in one of the worlds toughest freshwater environments!