A study published in Environmental Biology of Fishes challenges the prevailing belief that fish cease to grow significantly after reaching maturity due to the energy demands of reproduction. The research, led by Dr. Daniel Pauly, the principal investigator of the Sea Around Us initiative at the University of British Columbia, reveals that temperate fish, such as cod and Atlantic herring, experience a surge in growth after their first spawning event.
Contrary to the widely held notion that reproductive efforts curb fish growth, the study emphasizes the critical role of oxygen intake in the growth process. Dr. Pauly explains that as fish develop, the growth of their 2D gills, responsible for oxygen supply, cannot match the growth of their 3D bodies, which demand more oxygen. This disparity triggers a hormonal cascade leading to gonad maturation and spawning.
Approximately 25 percent of a fish’s total body weight consists of fully mature ovaries or testes until the spawning event. Once this weight is shed, the gills, unaffected by the loss, have a smaller body to supply with oxygen. The increased oxygen supply, coupled with post-spawning feeding, enables the fish to process food more efficiently and compensate for the pre-spawning weight loss.
“It’s like fish revert to their younger selves, akin to teenagers eating a lot and growing quickly,” explains Dr. Pauly. “However, in this case, the accelerated growth is not in length but in weight.”
To arrive at these findings, the researchers conducted a comprehensive literature review, focusing on the ‘gonadosomatic index,’ which measures the weight of ovaries or testes relative to the fish’s body weight before and after spawning events. The study’s conclusions were further supported by original data on gonad weights from Germany’s Western Baltic Kiel Bight.
Co-author Peter Sorensen, a researcher at the University of Minnesota, notes, “The Gill Oxygen Limitation Theory provides a simple explanation for post-spawning growth acceleration, linking the loss of live weight at spawning to increased food conversion efficiency.”
This research challenges conventional wisdom about fish growth dynamics and sheds light on the intricate interplay between reproduction, oxygen supply, and post-spawning growth in temperate fish species.