Scientific fitness trials show exercise helps fish reach sexual maturity
Exercise has been proved to make farmed fish happier and healthier, in turn providing consumers with better-quality food. Now, a team of European researchers have applied the fitness model to reproduction studies in fish in a bid to speed up their sexual maturity and increase stocks. The research has already shown increases in rates of growth in zebrafish.
Researchers in fisheries science believe that improving the fitness of fish helps to protect species from environmental changes and provides them with greater strength and endurance to deal with increased aquaculture production. As a result, fitter fish are deemed to be happier and healthier – benefitting not only the fish themselves but also the consumer.
Research has shown that sustained exercise at optimal speeds enhances muscle growth and has consequences for flesh quality. Exercise represents a natural, non-invasive and economical approach to improve the growth, flesh quality and welfare of aquacultured fish.
In addition to providing fitter, better-quality fish, exercise is also an important factor in reproductive health and maturity in fish – another area of great interest to commercial fish farmers. “Exercise, such as performed by migratory species, can be used as an experimental factor in studying the interaction between the metabolic processes and the onset of sexual maturation,” says Dr Arjan Palstra, one of the lead researchers on the EU-funded REPRO-SWIM project. “This knowledge is important in optimising aquaculture of commercially interesting species.”
How the metabolic status of a fish determines the onset of its sexual reproduction, which often coincides with the start of reproductive migration, is a long-standing question for those studying the reproductive physiology of fish. The REPRO-SWIM team hypothesise that swimming, in that case, triggers metabolic and hormonal changes critical for the induction of maturation. Studies on migrant fish like eel and salmon support the hypothesis that swimming triggers sexual reproduction, mobilises energy reserves for fuel and positively influences the quality of gametes (cells that fuse with one another during fertilisation).
“This project has provided important evidence for swimming-enhanced growth, specifically in zebrafish, and extension of the growth stage by suppression of reproduction, in rainbow trout for example,” says Dr Palstra. “These conclusions contribute to our integral understanding of the exercise effects in fish and their application for farming a fitter fish in sustainable aquaculture.”
Testing times in the water
The REPRO-SWIM team carried out a number of tests on trout and zebrafish to determine the effects of swimming-induced exercise on all the important tissues involved – gonad, muscle, liver, fat, brain and pituitary. In addition, the tests set out to identify the main genes involved in both reproductive and metabolic functions.
In the first tests, pubertal autumn-spawning seawater-raised female rainbow trout were rested or swum at a near optimal speed of 0.75 body-lengths per second in a 6000 litre swim-flume under natural reproductive conditions. Fish were sampled at the start and subsequently after 10 days (resting or swimming 307 km), 20 days (resting or swimming 636 km) and 40 days (resting or swimming 1176 km).
Ovarian development was then histologically examined and genetic analysis tests were carried out on the red and white skeletal muscle of exercised and non-exercised rainbow trout with the specific aim of identifying expressed genes and quantify the effects of swimming-induced exercise.
Individual swimming experiments were also performed with adult zebrafish in swim-tunnels to quantify their swimming economy and determine the optimal swimming speed at which energetic efficiency is highest. “To demonstrate the exercise effects on growth in adult zebrafish, we have performed a group-wise experiment with zebrafish swimming at optimal swimming speeds for six hours per day for five days per week for four weeks,” explains Dr Palstra. “For the first time, highly significant exercise-induced growth was demonstrated in adult zebrafish.”
It is hoped that the research into producing fitter, healthier and more fertile fish with optimised reproductive cycles will provide many benefits to the aquaculture sector. Implementation of exercise in aquaculture may improve profits with the generation of greater quantity and higher-quality products with less input and at relatively low investment rates. Fitter fish will also increase sustainability and social acceptance of farming methods.
“This knowledge based on exercise effects in fish can also be applied to vertebrates in general and can be increased and disseminated,” Dr Palstra concludes. “Zebrafish can be used as an exercise model for enhanced growth, with implications in both applied sciences, such as aquaculture, but also in basic, biomedical science.”