When Temperatures Rise: Summer Challenges in Aquaculture

Summer is a glorious time of the year; the days are long and warm, and it is holiday season.  However, on an aquaculture farm certain of the challenges that we face are heightened at this time of the year.  Running a successful aquaculture venture requires us to anticipate and recognise these factors, implementing the necessary actions timeously, to ensure they do not compromise the health, wellbeing and growth of our stock.
 
The most obvious summer-related change on the farm is the rising of the water temperature.  An increase in temperature results in the metabolic rate of living organisms speeding up, which means they need more oxygen and more feed.  Unfortunately, the solubility of oxygen reduces as water temperature increases; which means there is less oxygen available when it is needed the most.  Also bear in mind the optimal water temperature for the species you are farming- it is important to keep them at, or close to, that temperature; outside of the optimal temperature their growth will be compromised.  At higher than optimal water temperatures the metabolism of the fish increases such that they feed more heavily, but their ability to utilise the feed is reduced.  The consequence of this is heavier feeding, but the lower dissolved oxygen in the water reduces their ability to convert the feed into growth, resulting in robust feeding but poorer feed conversion efficiency. Compromised growth affects not only production costs, but also reduces potential earnings.
 
In earth ponds warmer water increases the demand for oxygen by all forms of life but because the algae in the pond are photosynthesising, there is lots of oxygen in the water by day.  At night, however, there is no sunlight so the plants can no longer photosynthesise and produce oxygen.  Instead they respire, using oxygen, and adding to the overall biological oxygen demands on the pond.  As a consequence, the level of dissolved oxygen in a pond tends to fall overnight and rise during the day.  If the nighttime drop goes below 5mg/? this is stressful for the fish and must be mitigated by aerating the ponds at night. 
 
An associated day/night change caused by algal activity in ponds is the pH swing.  All aerobic life forms give off carbon dioxide (the plants do this only at night) which mixes with pond water to form carbonic acid, lowering the pH.  During the day the plants require a source of carbon dioxide for their photosynthesis, and so strip it from the carbonic acid, causing the pH to rise.  In a pond with lots of algae but too little alkalinity, this pH swing can be several points on the pH scale, which is stressful to the fish, even potentially fatal.  This is controlled by careful weekly management of both the algae levels and the buffering capacity of the pond water.
 
Another factor at play in earth ponds is that the sunlight warms the surface warmer, causing it to become less dense and form a distinct layer above the cooler, deeper water.  This boundary is known as a thermocline and is often characterised by cool water at the bottom of the pond which has a low concentration of dissolved oxygen, and oxygen from the surface cannot readily replenish the deeper water as the different density layers do not mix naturally.  Fish may then be forced to remain in the area of warmer surface water, reducing their available habitat and increasing their stress.  Under severe conditions, this can compromise fish health, growth and survival.  Again, the formation of this undesirable thermocline is prevented by aerating the water in the pond.
 
Fish feed and digest the protein in their diet, releasing ammonia as a waste product.  Ammonia nitrogen is naturally present in two forms in water, toxic ammonia and benign ammonium, with an equilibrium relationship between the two forms.  As the water temperature increases, the equilibrium shifts towards the toxic ammonia, placing further strain on the fish.
 
Another consideration of summer warmth is that many of the pathogens and parasites that could impact the fish, and plants if you are practising aquaponics, grow faster and have a shorter life cycle.  As such they are potentially more problematic in the summer and close management is required to identify the presence of pests early on and to implement measures to control or eliminate them.
 
A final thought relating to warmer water is that the handling of fish can result in heightened stress.  One therefore needs to consider water temperature whenever the routine tasks of size sorting, moving and sampling fish are performed, to minimise stress on the fish.
 
Summer is a wonderful time of the year, and many species of fish naturally thrive during this season.  However, warmer water also brings associated potential hazards that need to be understood and taken into consideration when operating an aquaculture business.  Being aware of these risks and taking the necessary precautions can help protect the health, wellbeing and growth of our stock, while ensuring that our businesses remain productive and profitable.