Categories: Sanger Science9 January 20134.3 min read

A recipe for better fry and better fish

9 January 2013

by Nicola Goodwin

Zebrafish are easy to breed and relatively easy to genetically manipulate, and have therefore become a key model organism in genetics and development. Recently, our work on Zebrafish husbandry was awarded a poster prize by The National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs) at the Laboratory Animal  Science Association winter meeting. This is an important recognition of the work we do: as a researcher working with Zebrafish, I work on several projects to uncover better ways to care for our animals.

In the wild, Zebrafish are an adaptable species able to populate and thrive in a variety of water conditions, including ditches and brackish water. Feeding on the available nutrition and producing high numbers of embryos, the Zebrafish Danio rerio has a reproduction strategy that accepts the loss of many embryos and young fish during development.

Two fish same age compared with ruler: smaller less than 1 cm on standard feed, larger 1.7 cm on Paramacium feed

Growth difference between Zebrafish at three months of age raised on our Paramecium-enhanced diet (top) compared to our normal food

In research, custom-designed aquatic systems carefully regulate the environment in which we keep our Danio rerio. Many factors such as water quality and breeding schemes are highly controlled to try to ensure the quality and development of the fish through to adulthood. However, research facilities can still suffer from unexplained mortality rates, gender bias and deformities within Zebrafish lines.

Work with this and other laboratory species suggests that many factors influence the animals’ productivity and wellbeing. Dietary content and availability are key components of development and survival. At the Sanger Institute we conducted trials to establish whether the diets we provided could affect the development, quality and survival rates.

We discussed strategies with other international facilities such as the Zebrafish International Resource Center ( and reviewed a variety of different feeding regimes that could potentially enhance the survival rate and quality of life for Zebrafish fry. In the work reported here, we compared three different dry diets (Kyowa, Zeigler and ZM) and Paramecia, live ciliate protozoa. We fed fry on these diets from 5 days after fertilization: we provided Paramecia only from 5 to 15 days after fertilization before being replaced with a dry diet. Artemia (brine shrimp) was provided as a standard dietary component to all fry during these trials.

The results were striking.

Our survival rates on Paramecia were 91 per cent, compared to 30–82 per cent on dry diets. We also found that characteristic tail deformities regularly observed in fry reared on dry food were absent from fish reared on Paramecia. This suggests that Paramecia are a more accessible and nutritious food source for the fry. In addition, we found that fish reared on dry diets showed variable growth rates compared to fry reared on Paramecia: these developed at a more controlled and more comparable rate.

An important benefit is that we do not need to breed or maintain as many adult Zebrafish, reducing the numbers we use: by feeding Paramecia from 5 to 15 days after fertilization, juvenile mortality rates have decreased significantly and we no longer need to add large numbers of fry to our nursery.

Moreover, in addition to improving survival and stock quality we have significantly reduced gender bias present in our lines. We can therefore plan and execute better breeding and experimental programmes as well as increase the welfare of the Zebrafish, factors that sit well with our commitment to reduction and refinement, key components of the 3Rs.

We also found during these trials that, by better meeting Zebrafish nutritional requirements, our fry now grow at a faster rate: they are able to breed in half the time – at 3 months of age instead of 6. An additional benefit is that Paramecia are a living organism for the fry to chase and therefore act as good environmental enrichment – a more natural way for the fish to hunt and eat.

This approach has now been introduced to other facilities and could, therefore, in the future lead to standardising feeding regimes for raising Zebrafish fry.

Zebrafish, like other laboratory species such as the mouse, are naturally very robust species able to inhabit and survive in many different environmental conditions. As with the mouse we are continuously working to understand the optimal conditions for their production, development and wellbeing. In doing so, we not only improve the welfare of our animals, but also enhance the  quality of our science by reducing confounding factors such as developmental variability between animals.

Next we are looking at nutritional requirements and feeding regimes for adult Zebrafish, in order to ensure optimal egg production and to continue optimising welfare conditions. This would, in turn, help decrease the amount of breeding required, and therefore reduce stress, while still allowing us to continue to improve the quality of our research.

Nicola Goodwin is a Principal Animal Technician and Named Animal Care and Welfare Officer of the Aquatics area in the Research Support Facility. Nicola works more...

Related Links

Sanger Institute Zebrafish Mutation Project

The National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs)

Laboratory Animal Science Association