Skip to main content
Creating a better place

Preparing your fishery for the summer

Posted by: , Posted on: - Categories: Fisheries and biodiversity

Spring and summer can trigger a range of problems for fisheries, and it’s important to understand how to prepare and protect them. It’s vital that anglers and fishery managers recognise the risks posed by warmer weather; remain vigilant for signs of fish in distress and take time to prepare. A little extra care can go a long way.

Causes of fish kills

The causes of fish kills are many and varied, but most often occur due to poor water quality or disease or a combination of the two, with warmer weather increasing the likelihood of problems occurring.

Stressed fish are also less able to cope with changes in their environment - they can become more susceptible to disease and less able to recover from infection or damage. Stress can be caused by many things: poor nutrition, poor water quality, fishing pressure, lack of habitat, overcrowding - even the weather!

Water quality

Poor water quality can affect fish with varying levels of severity. Severe problems can result in large scale fish mortalities, whilst bad water quality concerns can lead to persistent problems with fish health and fishery performance.

  • Low dissolved oxygen is a major cause of fish mortalities. The amount of dissolved oxygen that water can naturally hold is closely linked to water temperature, with levels decreasing as temperatures rise. Very young fish and more sensitive species (such as salmon or pike) can be amongst those worst affected, as well as larger fish that need more oxygen due to their size.
  • Algal blooms can significantly alter water quality, varying oxygen levels throughout the day due to natural photosynthesis. This natural fluctuation can cause significant stress to fish populations, and extremes can cause mortalities. Some species of algae are also capable of releasing toxins or are thought to cause direct damage to the delicate gills of fish.
  • Varying temperatures can impact some fish more than others. Species such as salmon and trout prefer cool chalk rivers, streams, and headwaters. Pike and other predators use lots of oxygen and can be the first to show signs of stress in warmer temperatures, whilst other species like barbel can also be very sensitive. Many anglers take note of this and impose their own voluntary closed season, choosing to target more sensitive species in the colder months - which is recommended.

Fish diseases and parasites

Fish diseases and parasites are a natural part of any freshwater environment. They play an important role in keeping fish populations healthy by removing weak, old or vulnerable fish and allowing fitter individuals to take their place. In healthy fisheries, fish and fish diseases exist in a balance; but if that balance changes, disease can be a real concern.

Many diseases and parasites need certain temperatures to become ‘active’. Koi Herpesvirus is a well-known example of a warm weather disease that can have huge impacts on carp fisheries; but there are others. Outbreaks rarely occur simply due to the presence of a pathogen, with many problems starting when fish are stressed, damaged, or exposed to adverse environmental conditions. Knowing what can trigger disease outbreaks is key to understanding how they can be avoided.

We have some great advice for managing disease here.

How to prevent problems and prepare your fishery for warmer weather

Good fisheries management practices help reduce the risk of problems occurring at any time – but are especially important in the summer months. Maintaining sustainable stocking levels, following a biosecurity plan, and providing an environment in which fish can thrive will go a long way.

Ten top tips

  1. Prevention is better than cure – do what you can to prevent problems before they happen
  2. Ensure stock levels are sustainable – high stock densities increase the opportunity for diseases and parasites to spread, contribute to poor water quality and increase competition for resources such as food and habitat
  3. Practicing good biosecurity will lower the risk of transferring diseases and parasites between fisheries. Remember:

    - Check, clean, dry
    - Net dips will only work if used correctly. Ensure guidance for their use is followed.
    - Fishery owned nets are a great way to stop the spread of diseases and parasites

  4. Reduce fishing pressure - repeated capture can cause stress and excessive handling can cause physical damage.
  5. Contact us for advice or help
    - For an incident call 0800 80 70 60
    - For general advice call 03708 506 506
    - Every fishery/club should have their own dissolved oxygen meter – monitoring levels can help you foresee and address issues before they escalate
  6. Understand your fishery and learn how it reacts to changing environmental conditions

  7. Avoid disturbing silts in summer – avoid work parties and in-water work
  8. When buying fish, think…

    - Are they really needed? Can the fishery support them?
    - Is the species appropriate for the fishery?
    - Are the fish healthy?
    - Is it legal – have I got the right permits and does my supplier have them too?
    - Is it the right time of year – to reduce stress and risk of disease the colder months are the ONLY time to introduce fish

  9. Be prepared for when it might go wrong

    - What aeration kit do you have?
    - Have enough people got access to it?
    - Do you know how to deploy it, when and where?
    - Is it serviced?

  10. Know who to call and when

We provide a 24/7 incident response service backed up by our National Fisheries Laboratory. We aim to be able to provide advice, assist on the ground, assess any health or disease concerns, and give guidance to help keep fisheries healthy. This service is funded by fishing licence income, and for many anglers, angling cubs and fishery owners it is the most valuable service we offer.

If you see dead fish or fish in distress, please contact our Incident Hotline on 0800 80 70 60 and alert the fishery owner or angling club.

We have recorded a presentation - Preparing Your Stillwater Fishery for the Summer Months here -


Sharing and comments

Share this page


  1. Comment by Andrew Thurgood posted on

    Stop allowing the release of sewage into our rivers - that will help the fish considerably.

  2. Comment by Mark frost posted on

    Stop polluting the rivers! The rivers are a mess!!


Leave a comment

We only ask for your email address so we know you're a real person

By submitting a comment you understand it may be published on this public website. Please read our privacy notice to see how the GOV.UK blogging platform handles your information.