Climate Change

Shrinking Fish in a Warming Ocean

Johansen, J.L., Mitchell, M.D., Vaughan, G.O., Ripley, D.M., Shiels, H.A. & J.A. Burt. Impacts of ocean warming on fish size reductions on the world’s hottest coral reefs. Nature Communications, 15, 5457 (01 July 2024). https://doi.org/10.1038/s41467-024-49459-8.

People and fish: Forever intertwined

The world relies heavily upon fish stocks as both food sources and economic resources. But as the global oceans warm, scientists predict that fishes around the world will get smaller, which is in line with the Temperature Size rule. This will impact economies and food availability, but how this happens in fishes is not understood.

The Temperature Size rule states that cold-blooded animals––like reptiles and fishes, which rely on external heat sources to maintain their internal body temperature––reach a smaller adult size under higher temperatures. This phenomenon is well documented from the poles to the equator.  In accordance with this rule, scientists have recorded that reef fishes have reduced in body mass worldwide by up to 5-30%, depending on the increase in temperature– but the mechanism that controls the reduction of body size is debated.

How increasing temperatures impact size

Johansen and his team examined two possible hypotheses for the 5-30% size reduction that has been documented. As temperatures rise, less oxygen is available for fishes to utilize because warmer water readily releases oxygen in its gaseous form. So, the two competing hypotheses for body size reduction of fishes take into account the reduction of oxygen availability to fishes in the ocean, and are as follows:

  1. The Gill Oxygen Limitation hypothesis, GOL, states that fish body size is limited by the physical area of the gills; that is, larger fishes need larger gills, which must uptake additional oxygen for energy purposes.
  2. Maintain Aerobic Scope and Regulate Oxygen Supply, MASROS, assumes that at some point in the early life stages, gill growth and shape will be regulated to the proper size based on the oxygen availability; that is, there may be larger fishes if there is enough oxygen in the water and the gills are the maximizing efficiency to support their energy needs.

Each of these hypotheses propose different pathways by which fish are able to compensate for less oxygen availability. In GOL, when there is less oxygen available, gills will be smaller to maximize the energy acquisition efficiency, which results in smaller adult fishes. On the contrary, MASROS dictates that gill shape, and therefore efficiency, is adaptable and will always maximize energy acquisition to maximize final adult size.

The world’s hottest reef

To measure the two hypotheses, the team examined two species of reef fishes that live in both the Gulf of Oman and the Persian/Arabian Gulf, Lutjanus ehrenbergii and Scolopsis ghanam.

 Figure 1. A school of Ehrenberg’s Snapper, Lutjanus ehrenbergii, on the Dangerous Reef, St. John’s Reef, in the Red Sea, Egypt. Photograph: Derek Keats, Wikimedia Commons.

These two sites were prime candidates, as there is a temperature difference of ~9 ℃ (~16 ℉) between the Gulf of Oman (27 ℃, ~81 ℉) and the Persian/Arabian Gulf (36 ℃, ~97 ℉). Additionally, the Persian/Arabian Gulf is the hottest reef on the planet, which makes it an excellent site to understand how higher temperatures impact fish size. The scientists measured the size of about 21 adults of both species in each Gulf (85 total specimens) and then measured the efficiency of swimming and energy acquisition of all specimens at temperatures higher than the Gulf from which they came.

Figure 2. A map of the sampling locations and the temperatures in degrees Celsius. Johansen et al., 2024.

To understand the swimming efficiency, the scientists examined swimming speed and fin movement, in a variety of ways. To understand the energy acquisition, the scientists measured how much oxygen was used and how much carbon dioxide was expelled in a five minute period.

Heat, swimming, and size

The team determined that L. ehrenbergii had no problems with swimming efficiency or energy acquisition at higher temperatures, regardless of which Gulf the individuals came from. Not only this, but L. ehrenbergii was about the same size in each Gulf. So, neither GOL or MASROS explained the size of this species. The scientists also discovered that only smaller specimens of S. ghanam were found in the hotter Persian/Arabian Gulf. However, because they were not experiencing any impact on energy acquisition or swimming efficiency based on their size, the team determined that MASROS better explains this discovery.

The scientists demonstrated that neither GOL nor MASROS are the perfect explanation for why fishes are getting smaller under higher temperatures because both failed to explain the observations of L. ehrenbergii. This is an essential step in understanding how fish stocks will change as temperatures increase and how climate change will impact future food availability and economic stocks. Some fishes will become smaller, like S. ghanam, but others will not reflect an immediate change under higher temperatures, like L. ehrenbergii, which demonstrates that the Temperature Size rule may be subject to evolutionary lineage, and that additional future work should focus on this factor as an explanation. .

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