The following blog comes from Dave Rogers, Site Manager at Lakenheath:

Survey work doesn’t stop when the summer migrants fly south for the winter.  We monitor waterfowl and waders through the winter for the Wetland Bird Scheme or WeBS run by the BTO and Wildfowl and Wetland Trust, we also undertake marsh harrier and crane roost counts.  These are regular co-ordinated surveys carried out across the whole country.  As well as these there are also occasional surveys such as David mentioned in his blog on Sunday.  Last Friday we had Matt, Graham and Sarah over from the Lodge to electro-fish many of the water courses on the reserve.  We like to look at all aspects of the ecology of the reserve and fish, as well as being interesting in themselves, provide a food source for many of our key species like bitterns and otters.  Electro-fishing provides a way of sampling the fish population in a way that does not affect the fish, with all the fish being returned to where they were caught, hopefully unharmed by their electric shock.

Matt and Graham sampled three main areas of the reserve.  They went out in a little boat with the fishing gear and swept an electrode through the water in front of the boat as it moved through the pool or ditch.  The electric current stuns the fish temporarily and these are then netted and put into buckets of water in the boat according to the size of each fish.  This is important for two reasons - big fish won’t go into small buckets and big pike will eat smaller fish!  The team sail a set route or transect in each location so that comparisons can be made of fish numbers over the years.

 

Photo credit: Dave Rogers - here Matt (left) and Graham (right) sort the fish out.  The pike in Graham’s hand was around 2kgs, so big but not huge.

 

Once each transect is completed the fish are all recorded, with species being identified and each animal weighed before they are let go again into the pool.  A range of fish were caught during the day and we are still awaiting the detailed records from Matt.  When I was with the team at New Fen viewpoint they had one decent sized pike plus a number of smaller ones.  They also caught a number of lovely medium sized tench.  These fish feed mostly on the bottom of the pools and ditches and so are out of reach of many of the avian predators, but probably not otters or cormorants.

 

Photo credit: Dave Rogers - tench

 

We also caught a few rudd, which are a key species for us as they are prime bittern food.  Unlike the tench, they live in mid and upper levels of the water so are much more accessible for fish eating birds like bitterns who will eat the medium sized rudd, and kingfishers who will take the fry (young fish) and small rudd.

 

Photo credit: Dave Rogers - rudd

 

Finally a close up photograph of a pike.  This is a small pike, but you can clearly see the enormous mouth this fish has enabling it to swallow smaller fish whole.  This is the top fish predator at Lakenheath and will be competing with the otters and bitterns for the smaller fish.

 

Photo credit: Dave Rogers - pike

 

We want a nice mix of species and sizes.  Lots of small fish and fry for kingfishers to eat and prey for the larger fish.  Lots of good sized rudd in particular for our bitterns.  We don’t want too many medium sized pike which will eat the smaller fish so a few large pike are good as these will prey on the smaller pike.  Ducklings and other small water birds had better watch out though as the big pike will take them as well as fish!

From the results so far, New Fen North and Joist Fen North (East) looked good with a nice mix of sizes and species.  We don’t have many fish in the west of the reserve so we may need to restock in those areas to boost fish numbers as well as providing more fish refuges, which are bundles of sticks sunk into the pools to provide shelter from predators.  It just goes to show the importance of surveying.  If you don’t know what you have or have not got, you can’t do anything about it.