Shrinking Nemo – global warming to make fish smaller

From the University of British Columbia , a fish story inspired by a model:

Fish getting smaller as the oceans warm: UBC research

Changes in ocean and climate systems could lead to smaller fish, according to a new study led by fisheries scientists at the University of British Columbia.

The study, published today in the journal Nature Climate Change, provides the first-ever global projection of the potential reduction in the maximum size of fish in a warmer and less-oxygenated ocean.

The researchers used computer modeling to study more than 600 species of fish from oceans around the world and found that the maximum body weight they can reach could decline by 14-20 per cent between years 2000 and 2050, with the tropics being one of the most impacted regions.

“We were surprised to see such a large decrease in fish size,” says the study’s lead author William Cheung, an assistant professor at the UBC Fisheries Centre. “Marine fish are generally known to respond to climate change through changing distribution and seasonality. But the unexpectedly big effect that climate change could have on body size suggests that we may be missing a big piece of the puzzle of understanding climate change effects in the ocean.”

This is the first global-scale application of the idea that fish growth is limited by oxygen supply, which was pioneered more than 30 years ago by Daniel Pauly, principal investigator with UBC’s Sea Around Us Project and the study’s co-author.

“It’s a constant challenge for fish to get enough oxygen from water to grow, and the situation gets worse as fish get bigger,” explains Pauly. “A warmer and less-oxygenated ocean, as predicted under climate change, would make it more difficult for bigger fish to get enough oxygen, which means they will stop growing sooner.”

This study highlights the need to curb greenhouse gas emissions and develop strategies to monitor and adapt to changes that we are already seeing, or we risk disruption of fisheries, food security and the way ocean ecosystems work.

Note the press release headline: Fish getting smaller as the oceans warm: UBC research – they tout that as if it were measured, it isn’t.

Of course actual field experiments with real data trump models every day of the week and twice on Sunday. For example here’s a graph from the paper The effect of temperature and fish size on growth, feed intake, food conversion efficiency and stomach evacuation rate of Atlantic salmon post-smolts by Handeland et al published in the journal Aquaculture in June 2008:

Fig. 1. Mean weight in Atlantic salmon smolts (±SE, n=23) transferred to seawater at 6 (□), 10 (Δ), 14 (⋄) and 18 (○) °C. The first point (week 0) refers to the freshwater group (control). Different letters indicates significant differences (Student–Newman–Keuls, pb0.05) between temperature groups at same time of sampling, n.s., non significant.

The authors conclude:

In conclusion, the present study shows ontogenetic variation in optimum temperature for growth in juvenile Atlantic salmon smolts, with increased temperature optimum for growth and decreased temperature for feed conversion efficiency as the fish grow bigger.

Temperature tolerance increases with size, but Atlantic salmon smolts are eurythermal (Able to tolerate a wide range of temperatures.) in the size range investigated. 

Full paper here (PDF)

Now as Willis would point out, clearly this is tank studies, and not the open ocean, and you can’t duplicate the complexity of the ocean in a tank. But the fish don’t seem to have a temperature issue, in fact they seem to thrive at warmer temperatures. The claim is that as oceans warm, less oxygen will be available, and that will stunt the growth of fish. This claim in the modeling paper comes from the elemental saturation curve for dissolved oxygen (DO) in water, which is much like that of CO2. From a lecture on water chemistry at keystone.edu:

I found this part of the lecture interesting, and was something I didn’t know:

Nota bene:  100% saturation does not mean that no more O₂ can be held in solution.  I have measured DO >200%.  Does this mean that bubbles should be forming?  No, not necessarily. Saturation here means that 10.92 mg/l can be held at equilibrium; if 200% is produced by intense photosynthetic activity, the extra amount will be lost (diffused) at the air/water interface.

• a nomogram can be used to determine degree of saturation; use a straightedge to connect the water temperature and DO.  Read the % saturation at the intersection of this line with the middle line.

 ·         at 10 meters, with a temperature of 10°C, at surface pressure would hold (at 100% saturation) 10.92 mg, but you may find 15 mg/l.; compared to the surface it would be supersaturated, but at the depth and pressure it’s at, it may be less than saturated.

o       How can water be supersaturated?

§         intense photosynthesis

§         entrainment of air falling over a dam or spillway; high pressure of impact drives gases into solution; may lead to gas bubble disease, a problem in TVA dams

§         affects fish if subjected for a few hours to >115% saturation; bubbles form in tissues; emboli collect in gills causing anoxia and death; also affects cladocerans.  Other biota, e.g., crayfish and stoneflies are hardier.

So, too much oxygen is also a problem. But what really piqued my interest wad the statement of “intense photosynthesis” as a cause. That made me wonder if photosynthetic algae and diatoms would respond to increased temperature, so I went looking and found this paper:  Production and fate of extracellular polymeric substances produced by benthic diatoms and bacteria: A laboratory study by Lundkvist et al.

And the graph showing how photosynthetic oxygen production changes with temperature, again hard data from observation:

Fig. 7. Dose-response curve on light intensity and photosynthesis measured as oxygen production by benthic algae population.

So, it seems to me that the ocean already has this worked out. If O2 can be supersaturated, and “intense photosynthesis” can be a cause, it would seem that warmer water that normally would get oxygen from air-sea interaction and entrainment might be supplemented from increased algal photosynthesis.

Besides, broad differences in oxygen content by latitude are well known:

Image: Wikipedia

And fish aren’t static entities…they move. So I suppose I’m not too worried about global warming shrinking fish. Overfishing is likely a far greater problem for reduced fish size, as are oxygen deprived dead zones due to fertilizer runoff as we’ve seen in the Gulf of Mexico:

Dead zones occur throughout the world and are caused primarily from excess fertilizer and animal manure run-off, as well as, emissions from sewage treatment plants, urban and suburban run-off, and air emissions from vehicles. The largest dead zone in the country occurs at the mouth of the Mississippi River in the Gulf of Mexico each spring. In past years, the dead zone (pictured in the satellite image as the red coastal areas around Texas, Louisiana, Mississippi, Alabama, and Florida) has encompassed some 5,000 square miles. – ewg.org