Forty-year-old concepts around fish respiration regain prominence in light of climate change

Forty-year-old concepts around fish respiration regain prominence in light of climate change

Forty-year-old concepts around fish respiration regain prominence in light of climate change

Common carp. Photo by Bernard Spragg. NZ, Wikimedia Commons.

Before Dr. Daniel Pauly, now the principal investigator of the Sea Around Us initiative at the University of British Columbia, became a doctoral student, he spent two years doing fisheries work in Indonesia.

Having done his academic studies in Germany, he was surprised to discover a near absence of information on the growth of tropical fish. Thus, upon his return to Kiel University’s Institute of Marine Sciences, he decided to find out how fish grew; the idea was that if general patterns emerged, they could be applied to the many species in Indonesia and elsewhere in the tropics.

His doctoral dissertation was, consequently, built around identifying the factors that govern fish growth.

Using published data on the growth parameters of over 500 fish species in more than 1500 populations, Dr. Pauly was able to show that local conditions can influence the growth patterns of fish, but that their intrinsic growth performance is not at the mercy of such local conditions. These patterns, he showed, are determined by a major anatomical feature – the surface area of their gills.

Thus, he set the foundations of what is now known as the Gill-Oxygen Limitation Theory (GOLT), which proposes that as two-dimensional surfaces, gills can’t keep up with the growth of three-dimensional bodies of fish. This constraint means that fish must work hard to extract – via their gills – the oxygen they need to grow and maintain their body functions as even the best-aerated water contains less dissolved oxygen than the air at the top of Mount Everest.

The Role of Proteins

This gill limitation means that in water-breathing animals it is primarily oxygen, rather than food supply, that limits the process known as ‘anabolism,’ that is, the synthesis of the body’s proteins. This is a process requiring energy, derived mainly from ‘burning’ the proteins and other substances in their food. Anabolism requires oxygen. However, there is another process going on in the bodies of all animals: after a few days or weeks, all the proteins that make up their cells, tissues and organs spontaneously lose the complex shapes that enable them to function and turn into useless ‘random coils’ of amino acids.  This process, known as ‘denaturation,’ doesn’t require oxygen; however, denatured proteins must be resynthesized, lest the animals wither away. Denatured ‘random coils’ can be used as building blocks and/or fuel for re-synthesis but this process does consume oxygen.

The description and mathematization of biological organisms’ metabolic growth process was first described in the early 20th century by August Pütter and Ludwig von Bertalanffy. However, these authors did not specify that anabolism – which requires oxygen and is thus surface-limited – is countered by a process that is mass-proportional (because denaturation occurs throughout an animal’s body), but which is not dependent on oxygen. Thus, they wrote that anabolism was countered by some form of cell or tissue “breakdown,” which was later interpreted as also requiring oxygen. These views produced all sorts of contradictions.

Dr. Pauly seems to be the first to have postulated, in 1979, that Pütter’s and von Bertalanffy’s equations make sense only if their “breakdown” consists of the spontaneous denaturation of proteins and nothing else. Also, given that the surface area of gills cannot – as a surface – keep up with the weight of the fish and other water-breathers, Pauly’s reconceptualization explains why the growth of fish and other water-breathers gradually slows down as they get larger because their gills deliver less oxygen per weight to their bodies. This explains why fish generally remain smaller when in warmer temperatures, as spontaneous denaturation is extremely sensitive to temperature.

Climate Change

At the time, however, these ideas – which are not really intuitive – were not well understood and researchers continued to argue about the mechanisms at play in the growth of fish and other aquatic organisms. For 30 years, Dr. Pauly continued working on his theory but mostly as a side project. By the late 2000s, however, global warming gave the GOLT a new breath of life.

For example, fishers, aquaculturists and fisheries scientists started noticing that fish remain smaller when the waters they live in become warmer and less oxygenated. In terms of the GOLT, this is due to their 2D gills supplying a smaller amount of oxygen to 3D bodies that require more of it.  Thus, fish stop growing at smaller sizes because they need more oxygen for their maintenance and invest less in growth.

Numerous scientists working on issues of this sort started validating the principles of the GOLT using their own data. At the same time, the theory also elicited critiques, most of which, unfortunately, were not based on actual premises of the GOLT, but subjective perceptions derived from it, which do not represent what the theory is about.

Given these two trends, Dr. Pauly decided it was time to revive his doctoral thesis, as those drawn to applying the GOLT to their research would be able to better understand how it was developed. A ‘new’ version of such ‘founding document,’ thus, has been created to make it readily available online.

Edited as a Fisheries Centre Research Report, the content of the dissertation remains the same as that in the 1979 typewritten version but with minor typographical errors corrected and tables and figures reorganized for clarity.

Two forewords, one by Dr. William Cheung, renowned climate change researcher and director of UBC’s Institute for the Oceans and Fisheries, and the other by Dr. Johannes Müller, an environmental historian at Leiden University, reflect on the relevance and applicability of the thesis content and, therefore, of the GOLT.

“It is not often that one sees the second edition of a doctoral thesis, let alone one that is 45 years old,” Dr. Pauly wrote in his preface. “However, there are good reasons why this work, originally conceived as providing a methodology for inferring the growth parameters of fish exploited by tropical fisheries, is being re-issued when our main environmental problem is global warming.”

To access the document “Gill size and temperature as governing factors in fish growth: A generalization of von Bertalanffy’s growth formula (2nd edition),” follow this link.