Are we killing our carp?

By Pat Haughton

Deadly Addictive Diet Experiment

‘Supersize me’ was an award winning documentary by Morgan Spurlock who ate nothing but supersize McDonald meals for 30 days to see what impact it would have on his body.

Figure 1 Morgan Spurlock supersize diet

At the end of 30 days he was sick; 25lbs heavier, listless, no libido, massive cholesterol levels and an acutely toxic liver. Interestingly, he was also ‘craving’ his next meal (fix). In other words, not only was it shortening his lifespan but it was dangerously addictive.

Carp Diet experiment

In an experiment at Sparsholt college, we put 2 groups of 20 mirror carp on a 5 week feed trial. One group were fed a standard carp diet of 33% protein and 6% oil.

Figure 2 A standard carp pellet

The nutrient requirements for carp have been investigated by many researchers and are given as 30-35% protein and 5-15% oil.

The other group were fed a high specification trout diet of 46% protein and 28% oil.

Figure 3 A high specification trout diet

The purpose of the trial was to investigate the impact that this ‘unnatural’ diet would have on the growth and wellbeing of the fish.

Rocket Fuel?

These high ‘energy’ diets are specified for fish that are carnivorous that have a digestive system (engine) designed especially to process them. They grow fish fast usually for the table market.

Carp are NOT carnivorous, they are omnivorous i.e. they eat a wide variety of living organisms ranging from invertebrates to plants. This mixed diet requires a different ‘engine’ (i.e. they have no stomach but a very long extended hind gut) and it follows that they need to be run on a different type of ‘fuel’.

Figure 4 Contrasting colour of the two diets

Oil Addiction

The reason for running the experiment is because there is a lot of controversy about using high energy baits to catch carp. Just like in the Morgan Spurlock’s case, these baits are very attractive, even addictive to the fish. Most animals are genetically programmed to store energy for hard times. So if you put a meal in front of a carp with 3 times the ‘normal’ level of oil it will be attracted to eat it.

The Results

Growth

The fish all started at 225 grams weight. Looking at Figure 5 you can see that the high specification diet grew the fish consistently faster. At the end of 5 weeks the 20 fish on the high specification diet were averaging 353 grams and the fish on the carp diet averaged 322 grams a 10% difference.

Figure 5 The growth of the average fish on each diet

However, this extra weight gain is not necessarily a good thing. The two key issues are; what impact does the diet have on the environment and what structural tissue has been formed. i.e. is the structural tissue muscle, bone and organs or a deposition and storage of fat in the gut cavity?

Pollution

Interestingly when comparing how the protein of each diet was being used, the carp diet was much more efficient.

Figure 6 Comparison of how efficiently the protein in each diet was utilized

Protein efficiency is very important to the welfare of carp in a fishery. Inefficient digestion loads the environment with toxic waste products (ammonia). Just like inefficient cars on the ‘wrong’ fuel, it adds to the atmospheric pollution. Breathing in polluted air causes humans chronic and sometimes acute ill health with many symptoms such as asthma attacks.

For carp, extra ammonia in the water will damage their sensitive gills and make them vulnerable to secondary infections, lesions etc. This is especially important in a fishery where carp mouths can be damaged and need to heal.

Body Mass Index (BMI)

We know that there is a ‘correct’ relationship between our height and our weight shown in figure 7.

Figure 7 Body Mass Index chart for assessing your healthy 'shape'

This chart is what our doctors refer to to see if we are in good shape. We do not want to be underweight or overweight for our height as either will result in long term sicknesses such as diabetes and critically it causes a shortening of lifespan. It is no coincidence that life assurance companies use the BMI as a key indicator for their premiums.

Figure 8 shows that after just 5 weeks the shape of the carp on the two diets had changed significantly. (In fish, the shape of the fish is calculated as the condition factor.)

Figure 8 Comparison between the shape of the carp

Now we all know that carp come in a huge variety of shapes and sizes. One of the great joys of carp angling is the unique shape of each fish. So it follows that the ‘healthy’ range for a carp’s condition factor is broad. What is important here is that relatively the fish, all from the same stock, had rapidly and significantly changed in shape. The fish fed on the high specification diet were bigger but they were also deeper and fatter. In order to see what was causing this change the fish were dissected and the guts examined.

Figure 9 Comparison of the fat deposits in the gut of the carp

The top carp in figure 9 has been grown for just 5 weeks on a high energy diet and already the extra fat deposits can be seen clearly laid down around the digestive tissues.

So What?

The message is simple. If you feed an ‘unbalanced’ diet to a carp you will affect its condition factor, it’s health and it’s environment. In the short run this is not worrying because it can be quickly counterbalanced by changing the diet. However, fish fed predominately on high specification diets for an extended period will pollute their environment and become obese. They will therefore become susceptible to stress infections and they will have a shortened life span.

Figure 10 Frensham's famous ancient double linear

If we want to angle for carp that are old, long and healthy like the famous Fensham’s double linear we must review the use of high energy baits. Used sparingly they are fine, like most tasty things it is all about moderation! So maybe we should use the ‘halibut pellet’ on the hook but not as ground bait. Fishery owners who are responsible for the welfare and health of their stock pay heed.

Pat Haughton wears two hats. He is a long term fish lecturer at Sparsholt College and a business partner of Chris Seagrave and Hampshire Carp Hatcheries.

Disclaimer

This article is based on an experiment carried out by Sparsholt College’s National Diploma fishery studies students as part of their summer course work. The design of the experiment was not very scientifically rigorous and further research should be carried out to ‘support’ the data.