Developing and applying size based multispecies models to explore ecological and fisheries consequences of climate change

Size spectrum models are physiologically structured models that model size diversity in species. This is important in marine ecosystems, where individual’s size can change many-fold over the lifespan and where small individuals of different species are often more similar to each other than to big individuals of their own species. Started by Richard Law, Ken H Andersen, Julia Blanchard and others, size spectrum modelling has now been implemented in a user friendly R package mizer and is becoming increasingly popular in research. My work focuses on extending and adapting these modelling tools to study climate change effects. 

Climate change effects come in different shapes – changes in species distributions, changes in ocean productivity, and changes in temperature to name just a few. Together with my colleagues we are exploring how to account for these changes in a methodologically rigorous way and how to predict, minimise and adapt to the changing ocean. 

Climate change is also likely to change energy flows in marine ecosystems, such as the role of benthic and pelagic production in coastal ecosystems. However, to understand this change we first need to understand what this role is in the first place. Are coastal systems mostly sustained by local benthic production or by “imports” from pelagic production? Amy Coghlan’s PhD research explores this topic, looking at the benthic-pelagic coupling along the entire eastern Australian coastline. 

Size spectrum models are not the only physiologically structured models. Previously I worked with an age-structured ecosystem model Atlantis (developed by E. A. Fulton and her team), studying ecological and fisheries consequences of changing fish body sizes. Decreasing body sizes and size-at-age has been reported for many fish stocks globally, so I wanted to understand what that might mean for the stock’s productivity and resilience.  Using Atlantis model, we simulated gradual and slow decreases in size-at-age in five main fisheries species in South Eastern Australia. We found that just 5% decrease in length-at-age over 50 years could increase natural mortality by up to 30%, and consequently hinder recovery after a fishing moratorium. In fact, over 50 years these gradual changes in fish body size had a comparable effect on a stock biomass as introducing a new moderate fishing regime.