January 12, 2023 0 Comments

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In this article we dive into the external and internal defences of a Carp and Coarse fish, raising some interesting questions around fish care and the impact of angling on the natural defences. Let's start by looking at the layers of the fish and the role that they play.


The first line of defence for the fish is their layer of protective mucus. Mucus is far more than a yucky, smelly external lubricant that allows fish to effortlessly glide through the water, it’s full of anti pathogen chemicals! 

The mucus serves the purpose similar to how snot serves us in capturing virus/bacteria to protect us from infection.

The mucus forms part of the non-specific immune system, meaning that safe chemicals are combined to create a lethal mix which kill all good and bad pathogens without any specific target.

Riverine species like bream and roach are sensitive to external stressors, so they have a thicker layer of mucus to help them cope with their variable environment where they’re naturally exposed to more external threats than fish in a still-waters. 

Research suggests that fish upstream, closer to the water source produce less mucus than populations at the lower reaches of the river system. This is because the water quality closer to source is of far better quality and purity than the water at the lower reaches. Water further down the River system obviously picks up more and more bacteria and viruses in the form of pollution, run off and water discharge. Therefore, the further down a watercourse you get, the more external challenges there are for the fish populations, so the fish will respond by producing more mucus to strengthen their anti fungal and anti bacterial defences.

These non-specific defences of fish are unaffected by water temperature.


The layer below the mucus is called the epidermis. This layer contains goblet cells, these are the pores which produce the mucus. Most fish will continuously produce mucus, shedding any excess into the water in an effort to clean themselves and remove unwanted external parasites. 


Next we have the Dermis layer. This is the layer where scales are formed which function as physical plates of armour that protect the lower layers of tissue. The dermal layer of skin, including the scales, contain chromatophores which are the pigment containing cells which give the fish their colouration.

Scales can also act as calcium reservoirs which the fish can mobilise in the event of a nutritional calcium deficiency.

We’ve all seen lifted scales particularly on mirror carp and you’ve probably spent time drying the area to apply antiseptic treatment to the wound. A healthy fish will regenerate that scale in a matter of weeks and I’ve already explained the natural antiseptic properties of a healthy fish, so drying the wound and applying antiseptic can be counter-productive…. but I’ll come back to this point later.


Below the dermis is a layer called the hypodermis which connects the dermis to muscle tissues. The Hypodermis contains sensory receptors, blood vessels and energy storing fat cells. If the hypodermic layer gets damaged then the first line of the fishes defence has been breached which creates a potential site of infection and it’s now down to the 2nd line of defence to come into play.

The 2nd line of defence.

I’m sure we’ve all become familiar with how the specific immune system works with the covid pandemic, but i’ll give a brief explanation of this defence system which is also utilised by fish as their 2nd line of defence.

A response from the specific immune system involves the recognition of the pathogen which sends white blood cells to the site. The white blood cells produce small proteins called antibodies which will form a shape to fit the antigen to eliminate it. The antibodies then remember the shape of the antigen and become far more efficient next time it encounters this antigen.

The specific immune system is less effective in colder water temperatures, although bacteria and virus are also less active so the threat of infection is far lower in cooler months. However, bacteria are fast to reproduce as water temperatures warm up, whereas the fishes immune system is a little slower to respond which is why the Spring is the most vulnerable time for fish regarding bacterial infections.

The other time that the immune system is less effective is after capture. When fish are stressed they produce the stress hormone ‘cortisol’. Cortisol helps fish get through periods of stress by shutting down essential systems including the immune system. 

With this in mind we can relate what we’ve learnt here to the impact of anglers on fish condition. If we start with the scenario of carp angling. It questions the benefit of spending time to apply antiseptic to a wound, particularly if mucus has natural antiseptic properties. Surely keeping the fish out of water, dabbing the wound dry to remove the mucus is extending this period of stress, which then makes the fish more reliant on the artificial antiseptics.

My personal opinion is if you can minimise stress on the fish and preserve the natural defences of the fish (ie the mucus) then that’s far more beneficial than spending time preparing wounds for treatment. This further strengthens our argument as to why we encourage anglers to deal with their catch in the water to protect that mucus, only removing the fish if absolutely necessary. Removing that fish will cause physical damage to the external mucus, which is the first line of defence and a key part of the non specific immune system, and the impact of this stress is further weakening the specific immune system therefore making the fish more vulnerable to infection.

Now if we look at how these systems might be affected in the scenario of match angling. Although match venues are higher stock and generally more stressful environments for fish, each catch is usually dealt with and returned far quicker than they are by specimen carp anglers. However, in a match scenario, the fish are then stored in a keep net after capture. This is where the stressful event is extended and where the first line of defence is compromised with the removal of mucus at weigh in, before being released back into the stressful environment of a high stock commercial. So the specific immune system is constantly weakened with the production of this stress hormone that’s only briefly produced at the time of capture in the scenario of specimen carp fishing.

I think the efficiency of the immune system is crucial in the protection of fish condition and key to the repair of hook damage. If fish stress is kept to a minimum then the specific immune system will functioning well and strengthen. If the fish are exposed to a stressful living environment then cortisol will be produced more frequently, therefore weakening the immune system and allowing infection to compromise the successful repair of mouth tissues. This infection will lead to scar tissue upon scar tissue with poor repair and ultimately lead to deformed mouths. So reduce that stress and make anglers responsible for the safety of their catch and you will have healthy immune responses of both the specific and non-specific immune systems.