Zebrafish population changes in a future, multi-stressed environment

Monday, 2 March 2015

There are significant challenges ahead for the ecology of our planet. Economic development and human population growth are driving climate change. But it is particularly difficult to predict the potential effects of temperature change in combination with other environmental stressors such as habitat destruction, the effects of chemical stressors, predation, or changes in the physiology of the animals themselves.

Environmental scientists have long recognised these future challenges and there is a growing body of scientific literature speculating on the threats and impacts that these combined stressors pose to a healthy planet. However, there is surprisingly little reliable data available to begin to address these questions in practice. In the absence of high quality data, scientists cannot begin to build models to help understand which of the environmental stressors are the most important in order to develop appropriate management options and science-based policy.

To better understand the impact of multiple stressors, together with colleagues at the University of Exeter, we have examined the potential relationship between the impacts of climate change on vulnerable fish populations in concert with chemical exposure.

Pharmaceuticals excreted by patients are found in the environment, and the challenge has been to understand the potential risk that these compounds and other stressors pose to wildlife1. In research terms, fish are an important model for the pharmacological and toxicological characterisation of human pharmaceuticals in drug discovery, drug safety assessment and environmental toxicology. In a recent study we helped to demonstrate that only when the internal concentration of a medicinal product within fish reaches levels similar to that which is medicinal in human patients does the pharmacology equate to that in man2. In reality outside the lab, the environmental concentrations of pharmaceuticals in rivers are usually many orders of magnitude below those needed to cause pharmacological responses.

Whilst it is known that compounds that affect the hormonal axis (endocrine disrupters) can have significant effects on reproduction in fish, the concentrations in the environment for most of these compounds are much lower than the concentrations that cause adverse effects in the laboratory; hence, the risks are also low.

However, these are not the only factors that can affect populations3. Population size itself can be a significant factor in the future survival and vigour of fish populations. Small populations are at risk of inbreeding (mating between close relatives) and over several generations this can un-mask deleterious recessive alleles and reduced genetic diversity can leave the population at increased risk from environmental stressors.

One model species used increasingly as a research tool in laboratories around the world is the zebrafish (Danio rerio). This little fish is native to India and Bangladesh, and has proven a very valuable research tool; it even has a role in assessing the safety of new candidate pharmaceuticals before they are tested in mice, and is one of the key species used to help understand the environmental risk of pharmaceuticals required by regulators. But zebrafish (like many fish species) have some unusual biology; one aspect is that their sex is not only genetically determined, but can be driven by environmental factors, including the temperature of the water in which they swim. In warmer water, the population consists of more males. Exposure to some chemicals can also result in a male dominated population. However little is known about the combined effect of increasing temperature and exposure to such chemicals, particularly in small populations where in-breeding can potentially increase sensitivity to these stressors. This is the fundamental question addressed in a collaborative study between environmental scientists at AstraZeneca and University of Exeter, and published today in PNAS.

This new work demonstrates empirically that an increase in temperature (5°C) does indeed lead to more males being produced. Furthermore, exposure to a representative endocrine disrupting compound also led to a male bias, and the physiological mechanisms appear the same. Combined, increased exposure temperature and chemical exposure led to very few females in the resulting populations. The collaborative research team spent significant time generating 40 separate families of fish with known in-bred and out-bred characteristics from the same background of fish. For the first time this enabled the researchers to establish the effects of these combined physical and chemical stressors, with the biological complexity of in-bred populations. Taking the data generated in the laboratory, it was used to develop a model that enabled the researchers to predict the impact of the combined effects.4

This research illustrates how multiple stressors can have additive effects and highlights the need to consider a holistic approach to understanding man’s interactions with the environment.  Climate change and chemicals in the environment are normally considered separately but our research shows that they may be linked and this has potential implications for our future approaches to environmental risk assessment as we anticipate a future with rising temperatures, increasing human population and potentially associated increasing environmental exposure to chemicals.

In order to market new pharmaceuticals, part of the registration process requires the formal (regulatory) assessment of environmental effects and exposure. Here at AstraZeneca, the Global Environment team has been proactive in advancing the science through its Safety, Health and Environment Research Programme. In addition, as a global company employing more than 50,000 people in over 100 countries, our commitment to helping to mitigate climate change is recognition in the A list of CDP Climate Performance Leadership Index 2014.

The new work published in PNAS seeks to understand the combined effects of temperature and a model hormonally active compound, and in particular the potential to impact vulnerable populations. Importantly, these data can be used to make predictions about future impact. Focusing on the predictions made by the model, one remarkable aspect is the apparent robust nature of these fish; even with very few individuals contributing to the next generation, the populations can survive, but it is the inbreeding that presents the increasing risk. We believe that our work demonstrates a forward thinking approach to the science. Partnering with world class academics has facilitated the first substantial evidence that climate change, and pollution can interact, and further that biological factors such as inbreeding can be tested in the laboratory.

The study was funded by the Natural Environment Research Council, the Biotechnology and Biological Sciences Research Council, the University of Exeter and AstraZeneca’s Global Safety Health and Environment Research Programme.


  1. Boxall ABA, Rudd MA, Brooks BW, Caldwell DJ, Choi K, et al. (2012) Pharmaceuticals and personal care products in the environment: what are the big questions? Environ Health Perspect. 2012;120(9):1221–9.
  2. Brown AR, Hosken DJ, Balloux F, Bickley LK, LePage G, Owen SF et al. (2009) Genetic variation, inbreeding and chemical exposure—combined effects in wildlife and critical considerations for ecotoxicology. Phil Trans Roy Soc B 364(1534):3377-3390.
  3. Margiotta-Casaluci L, Owen SF, Cumming RI, de Polo A, Winter MJ, et al. (2014) Quantitative Cross-Species Extrapolation between Humans and Fish: The Case of the Anti-Depressant Fluoxetine. PLoS ONE 9(10): e110467. doi:10.1371/journal.pone.0110467
  4. Brown AR, Owen SF, Peters J, Zhang Y, Soffkar M, Paull GC, et al (2015) Climate change and pollution speed declines in zebrafish populations. PNAS