Climate related size shifts in aquatic species: mechanism, prediction and mitigation

Body size is linked to nearly all aspects of an animal's life, be this metabolism, reproduction or survival. Similarly, the structure of food webs, competition, predator-prey interactions and population productivity can all be influenced by body size. For these reasons, body size is often described as a 'master trait', and variation in the size of species has fascinated biologists for over a century. The size at which a species matures can change depending on the environment, and shifts in the size of animals and size-spectra of biological communities as a result of climate change are likely to have worldwide ecological and economic impacts.

In ectotherms, individuals of the same species regularly grow to a smaller adult body size in the warm than in the cold when reared in the laboratory. This near-universal biological phenomenon, known as the Temperature-Size Rule (TSR), occurs in over 80% of ectothermic species, from bacteria to fish and amphibians. Similar patterns in body size have also been seen in nature; larger species are often found at higher colder latitudes, whilst adult body size has been shown to vary seasonally with temperature over an annual cycle, as subsequent generations experience different environmental conditions during growth and development. With average global temperatures predicted to rise by more than 2 degrees Celsius by the end of this century, reduced body size has been described as the third universal response to climate warming.

Size reduction with warming is much greater for aquatic species than for species living in air. This has been attributed to oxygen availability, which is much more limiting in water than in air. Consequently, aquatic species struggle most to meet their metabolic demands in the warm, and growing to a smaller adult size is thought to be an adaptive response to cope. In addition, reduced oxygen availability independent of temperature has also been shown to decrease size at maturity. Deoxygenation is increasing in geographic extent and severity in regions of the world's oceans and in freshwater systems, and is predicted to significantly worsen over the coming decades. Clearly, climate warming combined with reductions in oxygen concentrations present a double jeopardy to aquatic species. There is an urgent need to quantify, understand, predict and develop mitigation strategies to deal with warming and oxygen-induced changes in body size in aquatic ecosystems. Our proposed research aims to tackle these issues by addressing the following key questions:

Q1. How do changes in temperature and oxygen concentration influence body size in ecologically and economically important but under-represented aquatic species, and do aquatic species adapted to environments with low and high oxygen availability adjust their size differently?

Q2. How have body sizes changed in aquatic species in relation to temperature and oxygen availability over recent decades? Are these responses similar to patterns observed in the laboratory and across seasons and latitudes? We cannot rely on laboratory and seasonal estimates to predict future shifts in size. Describing body size changes over decades in natural populations is a critical next step, and importantly, will increase the accuracy and reliability of our predictions.

Q3. What are the most important traits (e.g. feeding mode, reproductive strategy, mortality risk) associated with variation in the strength of temperature- and oxygen-induced body size change, and can we use this information to accurately predict body size change in the future?

Q4. Does body size reduction with warming fully compensate for increased metabolic demand at higher temperatures, and how might this affect the total productivity and efficiency of transfer from food to flesh that can be supported in warmer conditions? Can we use this information to contribute to informed decision making in the aquaculture and fisheries industries?

Beyond a wide range of academic beneficiaries, our proposed research will benefit a variety of other groups, both nationally and internationally. These include but are not limited to:

1) Organizations interested in predicting and mitigating the effects of climate change, such as the Intergovernmental Panel on Climate Change (IPCC).

Our research will be of great interest to these organisations; we will provide new insights into the effects of temperature and oxygen availability on the body sizes of aquatic species, producing novel quantitative data that can be directly used to predict long-term climate-induced changes in body size.

2) General public.

From potential changes in the size of the seafood on our plates, to the possible effects of body size change on important ecosystem functions and services, we will disseminate our findings to the general public through engagement activities and various media outlets. Public interest in and awareness of climate change continues to grow, and we have an excellent track record in public engagement, which includes national and international press coverage, including a BBC One Show nature feature, and have played an active role in public outreach activities, such as the Science Uncovered event at the Natural History Museum in London. Specifically, we will run a series of open days and demonstration sessions for school children at the SAMs Ocean Explorer Centre. These hands-on sessions will allow students to observe aquaculture work and interact with marine organisms, and are designed to educate and inspire, with an emphasis on the impacts of a changing climate on aquatic species. We will create a lay-friendly website related to all aspects of the project, including a blog and summary of our latest research and findings. All of the partners in the project have active communication teams with strong links to national and international media; this includes SAMS' highly qualified filmmaker, who will provide a series of short documentaries popularizing our scientific work. These films will be available online and in the Ocean Explorer Centre at SAMS.

3) The aquaculture and fisheries industry.

By investigating body size responses in important aquatic taxa, including whether body size reductions with warming fully compensate for increased metabolic demand at higher temperatures, we aim to provide valuable insights into how total productivity and efficiency of transfer from food to flesh might be affected in warmer conditions. This information should be extremely useful for aquaculture management, contributing to informed decision making regarding which types of species may be least affected by changes in temperature and oxygen availability. During the first half of the project, we will foster links with aquaculture industry experts by attending conferences and trade meetings. This will culminate in a multi-day workshop during the final year of the project, in which we aim to bring UK scientists and industry experts together under one roof. This will provide an excellent opportunity to disseminate our findings, and to discuss the physiological challenges faced by commercially important aquatic species with relevant stakeholders. We believe this workshop will also add value beyond the project term, leading to collaborations with industry and encouraging the development of follow-on research. We will share our findings and the ideas discussed in the workshop through a news article in Aquaculture Europe, the magazine of the European Aquaculture Society.