Shrinking guppies could help make cod bigger

There is already a great deal of evidence that commercially exploited fish species such as cod are shrinking and breeding at an earlier age as a result of human fishing activities removing the larger, older females which had previously been evolutionary advantageous due to their extra egg producing capacity.

Now a team from the University of Norway led by evolutionary biologist Beatriz Diaz Pauli have started an experiment to see what other potential changes may occur in ocean species as a result of current fishing practices.

The researchers are maintaining populations of between 500 and 900 of guppies, (Poecilia reticulata) in nine tanks. Diaz plans to remove all fish over 16mm in three of the tanks. The remaining tanks will act as a control for the experiments effects, with fish under 16mm being removed in some, while in others fish of any size will be taken to allow for any effect changing population densities may have.

Detailed records of any changes in growth rate, age and size at maturation, reproductive effort, feeding and breeding behaviours will be kept throughout the process.

The team hope to be able to see whether any changes noted are a 'plastic response' whereby the fish mould themselves to a new environment, or are caused by genetic changes.

Plastic responses differ from genetic ones in not being inherent; factors such as poor diet may cause a stunted adult, but the progeny of this stunted fish would not inherit its small size, but a genetically small fish would remain small regardless of diet.

Understanding the reasons behind size changes in fish will help us better understand how over-exploited species might bounce back if breeding grounds were protected or over-fishing was stopped.