Future halibut are slow-growing aerobic champions

Fish have adapted to survive at most temperatures that the planet can throw at them, from −2°C at the poles to 44°C in the tropics, but what is less clear is how fish stocks will respond as temperatures rise and ocean pH drops. ‘There is great concern over the ability of fish to acclimate and adapt to the current ocean warming and acidification’, says Fredrik Jutfelt from the University of Gothenburg, Sweden. Explaining that the fish's ability to transport oxygen and their peak metabolic rate are thought to decline as temperature and the fish's standard metabolic rate rise, Jutfelt says that this difference between the fish's standard and peak metabolic rates – the aerobic scope – is thought to be correlated with a wide range of physiological traits. ‘But it has long been unclear if reduced aerobic scope is the cause of poor growth’, says Jutfelt. Realising that there was no large-scale study investigating the impact of temperature and CO₂ on fish growth and aerobic scope, Jutfelt and his colleague Erik Sandblom decided to attempt the most comprehensive study to date on the impact of temperature and elevated CO₂ on fish (p.711).

‘We wanted to use a stenohaline marine fish that can tolerate a large temperature range’, recalls Jutfielt, so he and his colleagues opted for juvenile halibut. Setting up 24 tanks ready for the youngsters to arrive from Iceland, Albin Gräns then painstakingly adjusted the conditions in each tank by varying the temperature between 5 and 18°C and the pH by 0.4 units (from 8.0 to 7.6 on average) until they had simulated a wide range of conditions. ‘Keeping 500 halibut healthy at six temperatures and two P_CO2 levels for 4 months was a massive challenge for Albin’, recalls Jutfelt.

Then Jutfelt and Gräns teamed up with Michael Axelsson and Henrik Seth to begin the mammoth task of measuring the standard and peak metabolic rates of each fish, as well as recording a whole suite of other physiological characteristics, including the fish's growth rate and their heart function.

However, after weeks of analysis, when the team pulled all of the results together, they were in for a surprise. They had thought that the fish's aerobic scope would be reduced at the higher temperatures, and were therefore astonished to find that the aerobic scope continued increasing, even up to 18°C – which is close to the halibut's lethal temperature. However, despite the fish's apparent ability to sustain oxygen delivery at near-fatal temperatures, their growth was dramatically impaired. ‘That shows that decrease in aerobic scope at higher temperatures is not the cause for the decline in other performance measures’, says Jutfelt.

Also, when the team compared the coldest (5°C) fish's growth rates, they were alarmed to see the growth rate of the coldwater fish at pH 7.7 – which simulates the predicted environment in 2100 – plummet by 24% compared with that of their modern-day counterparts. Jutfelt says this is very worrying as most halibut are found at 5°C where their growth is likely to be impaired.

Having found that oxygen transport capacity and aerobic scope do not appear to restrict fish growth or other aspects of their performance, Jutfelt and his colleagues are keen to find out which physiological systems are most at risk from elevated temperature and reduced pH. ‘If the 24% decrease in growth rate at the cold temperature by CO₂ addition is confirmed, then ocean acidification could be more detrimental for fish than previously thought’, he warns.