A new study by Chinese and Canadian researchers has provided fresh evidence supporting the notion that fish reach sexual maturation in reaction to growth-induced respiratory stress.
The research team found a consistent metabolic ratio across 133 fish species in Chinese marine and freshwater environments. Findings hint that energy diversion from growth to reproduction seems to be the key predictor of sexual maturation.
The study was done by scientists from the Chinese Academy of Sciences in collaboration with counterparts from the University of British Columbia. It appeared in the Journal of Fish Biology.
Falling oxygen supply
Researchers estimated the ratio of oxygen use in fish species at two sizes. They worked out the ratio using the maximum and mean sizes of more than 200 fish populations belonging to 133 species at first maturity.
The team found that fish enter into adulthood when the oxygen consumption ratio is roughly 1.40. This metabolic ratio was consistent across the species of fish that were studied.
“The consistency of this ratio across the species we looked at – and other species studied in the past and now being studied – supports the idea that reproduction is initiated by changes in the balance between oxygen supply and demand,” senior author Dr. Daniel Pauly said.
Researchers said that maturation and first-time spawning seemed to be triggered by a drop in oxygen supply relative to individual fish’s weight. At the identified threshold, fish begin to respond to external stimuli for them to become sexually active and spawn.
Gill Oxygen Limitation Theory
Pauly proposes what is called the Gill Oxygen Limitation Theory (GOLT). This goes that the supply of oxygen in fish drops as weight increases since their gills have surfaces growing in two dimensions while their bodies develop in three dimensions.
Gills are organs in fish that help them to pull oxygen from the water for their bodies’ use.
As said by Pauly, an increase in fish’s weight results in the reduction of relative gill surface area. This is what brings about a decline in oxygen supply. In turn, reduced oxygen sets off a “hormonal cascade” that causes fish to become mature for breeding in response to environmental stimuli.
The GOLT counters the belief that it is only environmental stimuli at the start of a spawning season that make fish become sexually mature and spawn.
“Being able to corroborate with empirical evidence that respiratory stress is what prompts fish to reach first maturity when they do is a nice achievement,” stated study co-author Dr. Cui Liang. “This has important implications for aquaculture practitioners, who have long known the importance of dissolved oxygen in fish production.”
The team also pointed out that the findings can explain why fish affected by heat surges and deoxygenation brought on by climate change are more likely to reproduce at rather smaller sizes.