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Live slow, decline fast: ‘speed of life’ may explain tuna population declines

It is well understood that large-bodied species of mammals, fish, and birds are especially susceptible to steep population declines and even extinction due to environmental pressures and overhunting. But new research shows that time-related traits, such as growth rate and longevity, might be even more important, at least for fish.

A team led by researchers at Simon Fraser University in British Columbia, Canada, examined 26 tuna populations in a wide range of environments around the globe.

By combining datasets on population biomass trajectories over the past 50 years with demographic data of the various tuna species, they tested whether size or time-related life history traits was the better predictor of how well populations have responded to fishing.

They found that time-related traits, not body size, best correlate to several measures of vulnerability, from population declines and collapses to threat status and recoveries. The results of the study were recently published in the journal Proceedings of the Royal Society B.

Yellowfin tuna (Thunnus albacares), a species of tuna that lives mainly in the tropics. Tropical members of the tuna family tend to grow faster and have shorter lives than temperate species, such as albacore tuna (T. alalunga). They also have undergone slower and less severe population declines and have a lower probability of being overfished, according to new research. Photo credit: SEFSC Pascagoula Laboratory; Collection of Brandi Noble, NOAA/NMFS/SEFSC.

The researchers chose to study tuna and related species in the family Scombridae because they are widely distributed throughout tropical and temperate oceans and “have some of the highest quality marine population trend and life-history data known,” according to the study.

In fact, good data on the age of maturity, longevity, growth rate, and mortality rate of a variety of marine species is becoming increasingly available, the authors explain, and it has helped scientists better understand the wide variation in life histories. This has allowed them to rank species along a slow-to-fast continuum, where “slow” means slower growth, longer time to maturity, and longer lifespans.

Maximum body size is still a “broadly reliable” predictor of population and species vulnerability, the authors note. Because they tend to have lower rates of reproduction and take longer to reach maturity, large-bodied species are prone to steep population declines and are under greater threat of extinction than smaller-bodied species.

But temperature also plays an important role in shaping the life histories of tuna and their relatives, mainly because of how much it controls metabolic rates. The authors write that rates of growth and reproduction both scale with body size and temperature. In other words, species that live in warmer climates live faster and die younger.

That means, for instance, that because of the way temperature varies by latitude, tuna in tropical waters are more likely to grow faster, reach maturity earlier, and live shorter lives than tuna in temperate oceans. Tropical tuna compensate for their shorter lifespans with faster population growth rates than their cousins in colder climes, and that may be the key to their resilience.

The team found that the higher-latitude tuna populations with slower life histories suffered steeper declines over the past half century and were at greater risk of being overfished than tropical populations with faster life histories.

“[T]he strong, temperature-driven, latitudinal gradients in life-history traits may underlie the global patterning of population declines, fisheries collapses and local extinctions,” the authors write.

As a result, the researchers argue in the report, time-related traits might be “more suitable” for evaluating and ranking species vulnerability to fishing exploitation than body size when designing conservation strategies for large geographical areas.

“Yellowfin tuna is a good example,” they write. “Although it is relatively large — up to 232 cm in length — it is a fast-growing and short-lived tropical species, and consequently it can cope with relatively high fishing mortality rates compared with the similar-sized temperate bluefin tunas.”

The authors acknowledge that their findings appear to contrast sharply with the large majority of previous studies that found that large-bodied fish species tend to have declined more steeply and are under greater threat of extinction than small-bodied species. But they argue that their research is actually reconcilable with the conventional wisdom.

“Instead, it is likely that our finding complements, rather than contradicts, these studies,” they write.