'The Dinosaur Hunters' Deborah Cadbury ISBN; 1-85702-963-1
'Dodo from extinction to icon' Errol Fuller ISBN: 0-00-714572-1
'A fish caught in time-The search for the coelcanth' Samantha Weinberg ISBN: 1-85702-907-0
The mutterings of an Ex-London Fishmonger, a qualified fishery scientist, a lover of cooking and eating seafood, ex fishery consultant, fish farmer, and angler - Guess its going to be fish all the way.
As many of you know I have a background in fish that stretches further than mongery. One of the most enjoyable parts of an old job was to get wet and dirty with the fish. I wrote the account below for my old fisheries management website that details how we removed the fish out of those canals you all drive over – enjoy.
These three images show why water is removed from canal sections. The picture at the top was taken at
There are various ways to de-water a section of canal. Two solid barriers are required; therefore, short pounds with adjoining locks are ideal. Unfortunately this scenario is rare so other water retaining methods need to be employed. Stop planks can be placed in the purpose cut grooves located underneath bridges.
When the use of stop planks and lock gates as dams is not possible temporary structures are required to hold vast quantities of water back. The common choice is the use of an impervious fabric membrane attached to a free standing steel support system. These Portadams are fixed across the canal, shown in the above pictures (
These two images show the completed dams in place. At the top is the dam at Horton on the
The concept is simple. A small boat is pulled along the canal section behind 3 - 4 electrofishermen. The boat contains a generator linked to a control box. From this box run two hand held anodes and one cathode; this is connected to the rear of the boat. 240 volts and 6 amperes are generated and a direct current is implemented. The current is applied via the use of two "dead man's switches", one on each anode. Also within the boat there can be up to four large bins of water which hold the captured fish.; each bin has an aerator pipe that is connected to a battery powered air pump. Up to three stop nets of various sizes can also be carried. Each bin can hold up to 100lbs of large fish or 50lbs of small before they need to be emptied.
With a three man team the outside men will control the anodes, sweeping left to right, whilst the centre man controls the boat. Each team member will also use a small net to collect stunned fish. Large nets are held in the boat if substantial amounts of fish are encountered. If a forth team member is utilized the man can be placed in one of two positions. If the presented canal still has a wide area of water the forth man would be suited at the front alongside his team members. However, if conditions are either 1) good (narrow); he can follow behind netting the smaller fish that rise late (usually perch & ruffe), or 2) Very silty; he will probably have to push the boat if the substrate offers difficult walking conditions.
The length of the canal section is the usual determining factor that dictates where the job will commence. It is beneficial to work into clear water, therefore, if groundwater is entering on the length then the electrofishing should always take place towards the source. However, if the section is being pumped or drained whilst the rescue is in progress then working away from the running pumps or open paddles is again preferable. Incidentally, it is not always possible to fish into clear water as accesses are not always available to launch the boat from.
Once a suitable area has been found for launching, and the water flows and colour has been assessed, the work can commence. The process is fairly simple, however, there is one hard and fast rule. No matter what conditions are presented the job is not complete until all of the fish have been removed. This could mean up to four runs of the whole length; usually two complete runs are sufficient to remove the fish.
These three pictures show a successful fish rescue in progress at Yelvertoft on the Leicester Line of the
The picture on the top gives a clear view of the contents inside the electrofishing boat. The electric box can be seen on the front of the boat (grey) with the yellow cables of each anode attached via specialised waterproof four pin plugs; the UK standard colour for these plugs is blue/grey. The box is connected to the generator (red) which is situated in the rear of the boat. The generator can usually be found in the rear as it counteracts the water bins (yellow) in the front. To the left of the generator the air pump can been seen; the pipes and diffusers from this pump run to the water bins. There can be up to five bins in the boat at any given time. The picture on the bottom shows an electrofisherman (that's me) working an anode whilst netting small roach. He can be seen in a full dry suit and gloves, the only realistic clothing to carry out fish rescues in.
These two pictures show the working conditions in and around solid structures. The image above shows a simple lock system that requires navigating before the next section can be fished. Note the large amount of ice in the lock; never assume all jobs are warm and fun!! The second image gives an excellent view when approaching a portadam.
On occasions it is possible to carry out a drag down with a net before a dam is put in place. These pictures, taken at
A canal drag-down is usually undertaken with a small seine net rarely exceeding 25 metres in length. Again the concept is simple. The net needs to be a sufficient depth to cope with the drag-down; therefore, a net exceeding 1 metre at the deepest point of the canal is required. A man works each bank from the water by pulling the net towards the dam frame; it is imperative that the net stays of equal distance between workmen. The middle of the net needs to be in the centre of the canal. The images above show the net being pulled towards the dam frame (top) and of the excess net bulging behind as the net is pulled (centre). Incidentally, this method is not always feasible as dragging nets is both time consuming and energy sapping. In reality very long stretches cannot be netted as the efficiency decreases over long lengths due to snags.
As a net is pulled through a section numerous obstacles will be encountered. Many areas of canal are used as dumping grounds for all types of rubbish; this will affect the efficiency of any drag-down. As these snags are uncovered the leads attached to the base of the net need careful manoeuvring over the obstacle. The worst type of snags include bicycles, tree branches and brambles and the mandatory car engine. However, if the canal is not to deep most problems can be solved. This image (bottom) shows a large branch being removed from the net.
These two images taken at
Occasionally some very difficult conditions are put in front of a rescue procedure. As many rescues are linked with hugely expensive civil engineering projects the time scale available is always short. Therefore, problems other than deep water, which cannot be solved, require tackling and over coming. These two images were taken in January 2002 at Yelvertoft on the Grand Union canal Leicester Line. This stretch was approximately 2200 metres in length and when presented had 2 inches of ice across the surface. For obvious reasons a rescue could not be undertaken, the only solution was to launch the boat, find some heavy poles, and then tow the boat whilst breaking the ice.
‘Before starting to fish for their 500kg of cod winners will have to receive an EU log book and permit letters'How bizarre is all this? Why could anybody think the allocation of fish quotas in this day and age is dated when such games are used to decide people’s livelihoods? Not to forget the sleepless nights poor Charles Clover will have when he finds out “his fish” are been given as a prize in a raffle!!
Fish species are present in many varying forms all over the world. The main difference between shapes of individual species would firstly be concern the need for them to travel through a fluid with varying densities pressures and drags. With a combination of these problems and in tern making best use of there form inside their chosen environment each species has developed, sometimes radically, different body profiles.
Figure 1: Comparisons of flow around objects in viscous fluids. Source: Hosford (1997), p.16
As this is the case the inducing of a turbulent boundary layer can also reduce drag. It has been suggested that some species such as the mackerel (with the use of added dorsal fins), and the rainbow trout (with the use of an adipose fin) tend to induce a change in boundaries. Figure 2 shows that many years of development in aerofoil technology have created a design identical to the shape of a rainbow trout. The solid line is the aerofoil where as the dotted line is the trout. This concludes that the rainbow trout has formed into a highly streamlined structure.
The position of the shoulder will determine the type of flow over the body of the species. The shoulder of the trout is somewhat further back than the likes of the common bream (Abramis brama). This seems to verify that the form compares to the habitat and lifestyle of the species. The final point on the subject of drag would involve the use of the mucus covering the skin of the fish. The use of mucus in filling irregularities on the body could improve the flow characteristics of the boundary layers.
Figure 2: Comparison of a trout body form and a typical aerofoil. Source: Blake (1983), p.61
In normal observations fish swimming can be split into steady speeds, where the fish moves in one direction at a constant velocity or more commonly un-steady swimming which is the continuous changing of speeds and directions (Hoar, 1978). The critical speed of a species can only be measured from steady swimming but it is important to note that three other phrases, prolonged, burst and sustained swimming will take place in either of the firstly mentioned descriptions. It must be mentioned that potential growth response is effected by the cost of locomotion, which in tern contributes to the overall metabolic load (Ware, 1975), therefore efficient energy use is essential. Evidence has been found that juvenile fish do feed by moving at the appropriate speed to maximise their production rate (Ware, 1978). Figure 3 shows the fatigue curve of a rainbow trout (formally Salmo gairdneri). It clearly defines the three states of swimming used by all species of fish. Sustained swimming includes routine activities such as schooling and cruising and can be maintained in excess of 200 minutes. Prolonged swimming can continue between 200 minutes to 15 seconds and entails cruising with short bouts of vigorous movement. Burst swimming is the most inefficient form of activity with respect to energy use and can only be achieved for short periods of up to 15 seconds (Wilson, 1994).
Figure 3: Fatigue curves Source: Blake(1983), p.38
The rainbow trout can be classified into the group of subcarangiform fishes. This defines the type of body movement the species experiences when swimming. Most fish species swim with lateral body undulations running from head to tail. These waves run more slowly than the waves of muscle activation causing them, reflecting the effect of the interaction between the fish’s body and reactive forces from the water (Wardle, 1995). The undulations of the side to side movement in the body are slight in the anterior but there is a significant increase in the rear of the body. The snout of the fish does not travel in a straight line but tends to oscillate at small amplitudes along the mean path of the fish (Hoar, 1978). Figure 4 illustrates this type of movement and clearly shows that no part of the body travels in a straight line, but tends to follow a curving path through the water.
Figure 4: Body propulsion of subcarangifom mode. Source: Hoar (1978), p.10
Only at speeds of under 1 to 2 body lengths per second does the amplitude of the body undulations change, usually there is no change with swimming speeds. The frequency that the tail beats and the velocity at which waves are passed to the rear of the fish directly effect the speed of the fish. Speeds of up to 25 body lengths per second have been recorded for fish below 1 metre in length. Although this measurement is for fairly small fish, Webb (1975) discovered that maximum acceleration rates for rainbow trout (40-50cm/sec2) were in the same order for other fish from a wide range of sizes. This suggested that maximum acceleration rates may be relatively independent of size. Although acceleration and size may not be comparable the wavelength of the body undulations remain constant when relative to the body length within species. When the size of the fish increases the attainable maximum frequencies decrease. In relation to tail beats and swimming style, Webb (1991) found that over a period of 226 completed tail beats from trout no constant speed was registered. Figure 5 shows the results and clarifies that sustained swimming is never constant and will always involve acceleration, deceleration or turning.
The caudal fin is probably the most important attachment used for acceleration. During Webbs (1977) experiment involving fin amputation of rainbow trout he clarified that the large caudal fin is required for maximum acceleration performance and creates the majority of generated thrust during fast starts. The dorsal and anal fins are also important in generating thrust but they are not as nearly as significant.
Some of the important aspects involving the movement of trout have been discussed, finally but just as significant is the production of energy needed to create these exercises. The thrust force of the fish is produced by contractions of the propulsive musculature. The velocity that the muscle contracts dictates the power that the fish can produce. Muscle structures of fish can be split into white and red types. Red muscle consists of up to 20% of the total belonging to the trout (less for lower active species such as carp (Cyprinus carpio)). This muscle has a low rate of fatigue and is used for all of the low speed sustained cruising. The white muscle fatigues more rapidly but gives maximum power output, which is used for burst speeds. Due to the speed of fatigue the white muscle can be used only for short periods of time.
Blake, R.W. (1983). Fish locomotion.
Hoar, W.S., and Randall, D.J. (1978). Fish physiology, volume VII: Locomotion. Academic press,
Hosford, M.B. (1997). Fluid dynamics of ship resistance and propulsion. Institute of marine studies,
Wardle, C.W., Videler, J.J., and Altringham, J.D. (1995). Tuning in to fish swimming waves: body form, swimming mode and muscle function. Journal of experimental Biology, Vol 198, p. 1629-1636.
Ware, D.M. (1975). Growth, metabolism and optimal swimming speed of pelagic fish. Journal of the fisheries research board of
Ware, D.M. (1978). Bioenergetics of pelagic fish: Theoretical change in swimming speed and ration with body size. Journal of the fisheries research board of Canada, Vol 35, p. 220-228.
Webb, P.W. (1975). Acceleration performance of rainbow trout (Oncorhyncus mykiss). Journal of experimental Biology, Vol 63, p. 451-465.
Webb, P.W. (1977). Effects of median-fin amputation on fast start performance of rainbow trout. Journal of experimental Biology, Vol 68, p. 123-135.
Webb, P.W. (1991). Composition and mechanics of routine swimming of rainbow trout, (Oncorhyncus mykiss). Canadian journal of fisheries and aquatic services, Vol 48, no. 4,
Wilson, R.W., and Egginton, S. (1994). Assessment of maximum sustainable swimming performance in rainbow trout. The journal of experimental biology, Vol 192, no.1, p. 299-305.