Fish live faster in the tropics and slower in the poles because of mortality risks

Biological organisms must acquire resources to enable them to grow, mature and reproduce before they die. The study of how organisms allocate resources to each of these requirements is known as life history theory. Organisms with a ‘fast’ life history grow to smaller sizes, mature early and reproduce at both smaller sizes and younger ages before dying. A ‘slow’ life history suggests the opposite end of the spectrum; organisms have slower growth, mature and reproduce later and live for much longer than their ‘faster’ counterparts.

A new study from The Centre for Geometric Biology at Monash University and international collaborators has found that fish in tropical regions suffer high mortality and so optimise their fitness by diverting energy into reproduction earlier in life — their fast life histories are driven by mortality risk.

The research team led by Dr Mariana Álvarez-Noriega have used life history optimisation models to predict global patterns in the life histories of marine fish. They then tested and confirmed these predictions with a global dataset of marine fish life histories.

They found fish in polar regions have a lower chance of dying and therefore optimise their life history by growing longer and larger, and delaying maturity and reproduction. But when they do start reproducing they have a disproportionately greater investment in reproduction in relation to their size. That is, the relationship between body mass and reproductive output increases much more steeply for fish living in colder waters than those in warmer environments.

The model predicted that fish would mature later at the poles (higher latitudes) (graph a) and the real-life data the team collated from existing records confirmed this relationship (graph b).

So, for example a 10-kilogram fish is predicted to produce approximately 2.2 million eggs in the tropics and 3.5. million eggs in polar regions. But a 20-kilogram fish is predicted to produce approximately 4 million eggs in the tropics and 8.1 million eggs at the poles. The difference is greater as the fish get bigger.

Bigger fish produce more eggs and bigger fish at higher latitudes (polar regions) produce more eggs than fish of the same size at lower latitudes (tropical regions)

Dr Álvarez-Noriega emphasises that these global patterns have important consequences: “the impacts of fishing and Marine Protected Areas will be different in different latitudes”.

What is more, the team modelled the impacts of climate change and found that if CO2 emissions remain high their model predicts that a 25-kilogram fish at 60° latitude will produce 300,000 fewer eggs by the end of the century.

Our model predicts that global warming will profoundly reshape fish life histories, favouring earlier reproduction, smaller body sizes and lower mass-specific reproductive outputs, with worrying consequences for population persistence

Dr Álvarez-Noriega

This research was published in PLOS Biology.