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News Article

Stunted forms of tilapia adaptation to stress


Stressed tilapia can adjust size at first maturity downwards

According to a new study published in the open access Asian Fisheries Science, tilapias living in crowded aquaculture ponds or small freshwater reservoirs adapt so well to such stressful environments that they cease growing and reproduce at a smaller size than stress-free counterparts.

The study conducted by researchers at the University of Kelaniya in Sri Lanka and the University of British Columbia (UBC) in Canada, explains that while most fish species die when stressed, tilapias are able to survive in tough conditions by stunting and carrying on with their lives in dwarf form.

According to lead author Upali Amarasinghe, professor at the University of Kelaniya, tilapia and others in the Cichlidae family do not spawn 'earlier' than other fishes, as commonly perceived. Instead, he says, they are unusually tolerant of stressful environmental conditions but with elevated metabolism. Fish species which are not as hardy generally die when subjected to  stressful environmental conditions rather than surviving in a stunted form; stunted or dwarf forms do not exist in such species and neither do they spawn at small sizes.

"Gill surface area grows in two dimensions, that is, length and width, but they cannot keep up with bodies that grow in three dimensions - length, width and depth," said Daniel Pauly, co-author of the study and principal investigator of the Sea Around Us initiative at UBC's Institute for the Oceans and Fisheries. He explained that as fish get bigger, their gills provide less oxygen per unit of body weight; therefore in order to stay alive in stressful environments, which increase their oxygen demand, fish have to remain smaller. This observation, Pauly says, corroborates the gill-oxygen limitation theory (GOLT), which identifies spawning as an event rather than a determinant of fish growth.

In tilapias, the stress experienced under suboptimal conditions adds to the stress they experience from their gill surfaces not keeping with the increasing oxygen demand of their growing bodies. As a result, hormonal processes that lead to maturation and spawning are triggered at smaller sizes than under optimal conditions. Therefore the spawning does not occur at a 'younger age,' as the fish's growth process has already ended.

The researchers analysed the length at first maturity and maximum lengths reached in 41 populations of nine fish species such as tilapia and other cichlids found in lakes and aquaculture ponds across the world, from Brazil to Uganda, and from Egypt to Hong Kong.

When the ratio between the maximum lengths these fishes can reach and the lengths when they reproduce for the first time was examined, it was found to be the same ratio identified previously in other freshwater and marine fishes.

Amarasinghe explained that this ratio indicates that tilapias in stressful conditions do not spawn 'earlier,' they merely adjust their size downward, continuing their life cycle.

Pauly says that the findings will be of interest to to fish farmers, specifically in Asia, whose ponds are often full of wildly reproducing, small tilapia for which there is no market.

The relationship between size at maturity and maximum size in cichlid populations corroborates the Gill Oxygen Limitation Theory. Amarasinghe US, Pauly D. Asian Fisheries Science 34 (1): 14-22, doi:https://doi.org/10.33997/j.afs.2021.34.1.002

Article details

  • Date
  • 08 April 2021
  • Source
  • Institute for the Oceans and Fisheries, UBC
  • Subject(s)
  • Animal breeding and genetics