Written by Charlie Gregory
Sustainable and efficient aquaculture practices will be essential to meet the demands of a growing human population. However, to achieve this the natural biological clocks of the organisms being cultivated must start to be properly taken into account.
All organisms share a common theme, whether it’s a plant closing its leaves at the dawning of dusk, a sparrow singing at first light or even a human feeling drowsy in the afternoon sun, almost all species follow a daily pattern of rhythmicity. Although this pattern – known as the circadian rhythm – has only just developed a foundation of knowledge within a few key species, it is remarkably observed across all phyla and any interference to these rhythms can cause stress to the disrupted organism. In humans for example, disruption to our circadian rhythm is linked to insomnia and depression. As a result it has become increasingly important to study the cycles and patterns of different organisms, known as chronobiology.
Chronobiology & the science of internal biological rhythms
Chronobiology is actually one of the oldest disciplines of biology, beginning almost 300 years ago, well before Darwin’s theory of natural selection. The first observation of a circadian rhythm was made by the French astronomer de Mairan in 1729. He realised that the daily leaf movements of the touch-me-not plant (Mimosa pudica), which closes its leaves during the night and opens them again in the morning, continued even after being placed within a cupboard absent of light. This was the first proof that something other than environmental factors were influencing its behaviour. Centuries later we now know that that this mystery force was the plant’s genetic code and today research in chronobiology has shown that virtually all multicellular organisms share a similar genetic basis for biological clocks. Now research is shifting from laboratory-based studies into ones of a more practical utility.
The importance & limits of aquaculture
The global seafood commerce is a vital source of nutrition, acts as a fundamental sector within economies and is considered a key aspect of many cultures. Seafood demand is progressively rising due to development and growth in the global human populace, affluence, and per capita consumption. Seafood supply is also growing, despite declining wild capture fisheries stocks, with phenomenal advances in aquaculture. In 2012, aquaculture systems provided an incredible 42% of the world’s fish and it is estimated to eclipse capture fisheries supply by 2030. As a result, sustainable aquaculture expansion is a priority for meeting protein demands of a growing human population and continues to remain the fastest growing food sector.
However, aquaculture is still far from the perfect solution and there are still problems that need to be overcome. Two of the greatest barriers to intensification of aquaculture are infectious disease outbreaks and inefficient fish nutrition. Currently, disease is a substantial economic burden, costing the aquaculture industry US $6 billion a year and is a growing concern for farmed fish welfare. Furthermore, efficient use of feeds is increasingly important as traditional reliance upon fish-derived proteins and oils, supplied from capture fisheries, is ever more unsustainable. It is critical that innovative management strategies are developed to promote efficient fish feed utilisation and sustainable ingredients, whilst also improving fish stock innate immunity and ultimately fish health and welfare.
Whilst it has long been evident that the management of living organisms requires detailed biological understanding, incorporation of the time-base of circadian rhythms has yet to be fully explored within aquaculture. Past research has shown that biological clocks can be hugely important for aquatic species and that it is possible to manipulate them within aquaculture. However, the use of chronobiology within aquaculture has so far failed to address the key issues facing the industry.
Combining chronobiology & aquaculture
Currently, the main way chronobiology has been utilized within aquaculture is to manipulate the natural circadian rhythms of target species to maximise growth rates and reproductive output. This has mainly been achieved by photoperiod manipulation, which means controlling the light cycles animals experience with artificial lighting. That is because despite biological clocks having a genetic component, a lot of behaviours are still triggered by daily and seasonal light patterns.
Early examples of photoperiod manipulation focused on farmable crustaceans, such as prawns and shrimp. Studies found that, when exposed to irregular or out-of-synch light/dark cycles, grass prawns (Palaemon elegans) actually have impaired growth and mortality rates. Whereas brown shrimp (Crangon crangon) can actually have skewed sex ratios as a result. With a regular 12-hour light/dark cycles the shrimp are born at a 1:1 ratio male to female, but in 8-hour cycles the offspring become predominantly male.
Aquaculture practices have since concentrated extensively on the influence of light periods within commercial fish species, such as the Atlantic salmon (Salmo salar). Experimental modification of seasonal light cycles and photoperiod manipulation are both tools commonly used to augment year-round supply of eggs, whilst also improving the ability to manipulate growth rates. Despite success in exploiting such rhythms, the fundamental physiological mechanisms underlying commercial practices are still poorly misunderstood and are yet to be fully explored. This gap in our knowledge has prompted studies investigating the circadian rhythms of feeding within rainbow trout (Oncorhynchus mykiss), finding that fish fed at midnight demonstrated lower growth performance and nutrient retention than fish fed at dawn.
Whilst controlling light cycles does show that circadian rhythms can be utilized within aquaculture, now is the time to start thinking of more positive ways chronobiology can influence the industry. One way natural circadian rhythms can be beneficial to aquaculture is in addressing the issues surrounding inefficient feeding. Circadian rhythms offers the possibility to enhance growth patterns and nutrient retention by feeding at optimal times relative to food conversion efficiency. Aquaculture feeding should therefore take note of wild species feeding cycles and attempt to mimic them, as well as factoring in natural changes in appetite due to seasonal variations of food availability and predator abundance. By doing so we can use the natural circadian rhythms of target species to our advantage, rather than try to go against them.
The other main issue chronobiology could help tackle is the issue of infectious disease outbreaks, particularly prevalent in fish farms – by combining the use of immunostimulants and prebiotics with optimal circadian feeding, known as ‘chronotherapy’. If done in the right way this can enhance beneficial gut microflora, which is key to the healthy functioning of teleost digestive systems. Whilst this will not eliminate the issue of infectious diseases, which will always be a problem when cramming lots of animals into a small space, it can improve the overall health of the population and reduce the risk. Moving away from photoperiod manipulation will also reduce disease outbreaks as it can negatively impact the immune systems of target species.
However, there is still much more work that needs to be done in order to enable the use of chronobiology across all aquaculture systems. One major consideration which needs to be addressed is the individuality of species. Circadian rhythmicity within teleost fish is not fixed and there is substantial differences in between the biological clocks of different species. As a result it can be contended that an optimal feeding regime has yet to be recognised for any one species. Ultimately, the major problem facing the augmentation of circadian feeding is the plasticity of fish circadian physiology and lack of knowledge understanding the mechanisms behind it. Therefore, it is necessary that research is conducted to consider the effects of circadian feeding not only for growth, but also with the aim of improving the health of the fish.
To sum up
There has been a considerable amount of work accomplished in the past 10 years on circadian time-based rhythms within aquaculture. From several researchers working on some key species, we now have research efforts underway in at least six countries. There is little doubt about the potential of circadian feeding regimes. Although a great deal more needs to be understood regarding the impact on health, disease resistance and microbiomes in order to evaluate its future role within commercial aquaculture and its utilisation alongside a sustainable form of capture-based fisheries in fulfilling the needs of an ever-growing human population.
Charlie Gregory is an MscRes student at Bangor University studying chronobiology and the influence of feed timing on rainbow trout growth and gut microbiota. Alongside this he also works in an aquarium research facility as a research technician studying methylation and phenotypic plasticity in the pharyngeal jaws of African Cichlids. To find out more about his work you can email him at firstname.lastname@example.org.