Spawning is a stressful and exhausting process for fish, but it also has a surprising benefit: it makes them younger.
A new study published in Environmental Biology of Fishes reveals how fish lose weight and gain oxygen after they release their eggs or sperm, leading to a rapid growth spurt.
The oxygen paradox of fish growth
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Fish need oxygen to grow, but their oxygen supply is limited by the size and shape of their gills. As fish grow bigger, their three-dimensional body volume increases faster than their two-dimensional gill surface area, creating a mismatch between oxygen demand and supply.
This mismatch triggers a hormonal response that causes fish to mature and spawn, according to the study's lead author, a marine biologist at the University of British Columbia.
Before spawning, fish accumulate a large amount of reproductive tissue, such as ovaries or testes, which can make up to 25 percent of their body weight. This tissue also consumes oxygen, adding to the fish's metabolic burden.
Once the fish release their eggs or sperm, they lose a significant amount of weight and free up oxygen for other purposes.
This allows them to breathe more easily and eat more voraciously, resulting in a rapid increase in body weight.
The marine biologist explained that after spawning and the loss of body weight, fish grow faster than what would be expected from an increase in feeding alone.
He compared the fish to teenagers who eat a lot and grow quickly, except that in this case, the growth is not in length but in weight. He said that it was like fish became their younger selves again.
The implications for fish management and conservation
The study, which was based on a literature review and original data on gonad weights of six commercial fish species in Germany's Western Baltic Kiel Bight, challenges the common assumption that fish stop growing after they reach maturity.
The authors argued that this assumption leads to overestimating the size and age of fish stocks, and thus to setting inappropriate fishing quotas and regulations.
The authors suggested that fish managers and conservationists should take into account the effects of spawning on fish growth and oxygen consumption, and adjust their models and methods accordingly.
They also recommended that fishers should avoid catching fish during or immediately after spawning, as this would reduce the potential for growth and recovery of the population.
The study also highlights the importance of protecting the habitats and environmental conditions that support fish spawning, such as water temperature, salinity, oxygen levels, and food availability.
By ensuring that fish can spawn successfully and repeatedly, we can help them maintain their health and productivity, and ultimately, their resilience to human impacts.
How exactly do fish rejuvenate after spawning? The answer lies in the interaction between oxygen and growth hormones.
The researchers found that fish have a feedback loop that regulates their growth and reproduction based on their oxygen availability.
When fish grow larger, their oxygen demand increases, but their gill surface area cannot keep up. This creates a respiratory stress that signals the fish to produce more growth hormone, which stimulates gonadal development and maturation.
Once a fish spawns, it loses gonadal tissue that previously had to be supplied with oxygen and, thus, its relative gill area increases, which facilitates renewed growth until the next spawning season, when the respiratory stress reappears.
The researchers also discovered that fish can manipulate their oxygen consumption by changing their activity level.
For example, fish can reduce their movement rate to save oxygen for growth or reproduction, or increase their movement rate to induce spawning.
This explains why fish farmers can increase the growth rate of fish by selecting for calmer individuals, or induce spawning by increasing the water flow.
The researchers concluded that fish have a sophisticated system that allows them to optimize their growth and reproduction in response to their environmental conditions, and that this system is mediated by oxygen and growth hormone.
They suggested that this system could be used as a biomarker to assess the health and welfare of fish in aquaculture and in the wild.
