Old, Fat Fish Have the Most Offspring, Sustainability Study Finds
By Annie Roth
It might seem smart to eat the big fish and throw the little ones back. But a recent study in the journal Science says just the opposite. Big fish are the ones to throw back, especially if they're female.
That's because bigger females have disproportionately more babies than their smaller counterparts.
Biologists from Monash University in Australia and the Smithsonian Tropical Research Institute in Panama came to this conclusion after comparing size and fecundity across 342 fish species. In 95 percent of the species, larger mothers produced far more offspring, pound for pound, than smaller mothers. The authors point to Atlantic cod, for instance, where one 30-kilogram (approximately 66-pound) female can lay more eggs per spawning event than 28 smaller, 2-kilogram females.
Scientists already knew this for some species, "but we didn't know it was so widespread," said ecologist Diego Barneche of the University of Sydney, who co-authored the study. Until recently, it was widely believed that for most fish, fecundity scaled linearly with size, meaning that 15 2-kilogram cod could lay the same number of eggs as one 30-kilogram cod. But this assumption is not only false, it may also severely underestimate the contributions of larger moms.
The new study looked at 342 fish species, including mackerel. Here a school swims in Bimini Island, Bahamas. OCEANA / Houssine Kaddachi
The Bigger, the Better
The study also found that hefty females lay bigger, more nutrient-rich eggs than small females. Previous research has shown that hatchling survival increases with both egg size and nutrient content. So, even if several small cod lay the same number of eggs as one large one, more of the larger cod's eggs are likely to survive to adulthood.
When calculating the amount of fish that can be sustainably harvested, fishery managers rarely factor the importance of big mothers into their equations, "and that may explain why some populations have declined," said Barneche.
However, some scientists are skeptical of that conclusion. Ray Hilborn at the University of Washington in Seattle, and Michael Sissenwine, a Massachusetts-based member of the New England Fishery Management Council, agree that large females disproportionately contribute to the reproductive pool, but hesitate to blame overfishing of large females for past population declines.
Populations of many commercially harvested species, including tuna, mackerel and bonito, did decline by as much as 74 percent between 1970 and 2010, according to a report published by the World Wildlife Fund and the Zoological Society of London. In roughly one-third of all fish populations, the largest, and therefore most valuable, fish have declined by 90 percent or more, one recent study found. That's even true for so-called sustainable fisheries, said Trevor Branch, a fishery scientist with the University of Washington in Seattle, who co-authored the study.
"We should think about how we can change our fishing to increase the big, old fish," Branch said. The Pacific rockfish fishery, and a few others, are already protecting big moms, he said, but more fisheries need to get on board—and fast.
Overfishing big females could be a factor in population declines, the authors say. But not all researchers agree. OCEANA / Enrique Talledo
Bringing Big Moms Back
Protecting important habitats is one way to make sure female fish get bigger.
One study found fish living within marine protected areas, or MPAs, are on average 28 percent longer than those living in unprotected areas. For the West Coast's formerly overfished populations of widow rockfish, a size increase of this magnitude would bolster a mother's reproductive output by 74 percent, according to Barneche's calculations.
For fishery managers, the science is clear. Global fish stocks in decline will have a better chance of bouncing back if we leave more big fish in the ocean.
New Seafood Sustainability Report Ranks 15 Foodservice Companies https://t.co/O4dT39sriL @BusinessGreen @GreenCollarGuy— EcoWatch (@EcoWatch)1508880009.0
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By Katherine Kornei
Clear-cutting a forest is relatively easy—just pick a tree and start chopping. But there are benefits to more sophisticated forest management. One technique—which involves repeatedly harvesting smaller trees every 30 or so years but leaving an upper story of larger trees for longer periods (60, 90, or 120 years)—ensures a steady supply of both firewood and construction timber.
A Pattern in the Rings<p>The <a href="https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/coppice-standards-0" target="_blank">coppice-with-standards</a> management practice produces a two-story forest, said <a href="https://www.researchgate.net/profile/Bernhard_Muigg" target="_blank">Bernhard Muigg</a>, a dendrochronologist at the University of Freiburg in Germany. "You have an upper story of single trees that are allowed to grow for several understory generations."</p><p>That arrangement imprints a characteristic tree ring pattern in a forest's upper story trees (the "standards"): thick rings indicative of heavy growth, which show up at regular intervals as the surrounding smaller trees are cut down. "The trees are growing faster," said Muigg. "You can really see it with your naked eye."</p><p>Muigg and his collaborators characterized that <a href="https://ltrr.arizona.edu/about/treerings" target="_blank">dendrochronological pattern</a> in 161 oak trees growing in central Germany, one of the few remaining sites in Europe with actively managed coppice-with-standards forests. They found up to nine cycles of heavy growth in the trees, the oldest of which was planted in 1761. The researchers then turned to a historical data set — more than 2,000 oak <a href="https://eos.org/articles/podcast-discovering-europes-history-through-its-timbers" target="_blank" rel="noopener noreferrer">timbers from buildings and archaeological sites</a> in Germany and France dating from between 300 and 2015 — to look for a similar pattern.</p>
A Gap of 500 Years<p>The team found wood with the characteristic coppice-with-standards tree ring pattern dating to as early as the 6th century. That was a surprise, Muigg and his colleagues concluded, because the first mention of this forest management practice in historical documents occurred only roughly 500 years later, in the 13th century.</p><p>It's probable that forest management practices were not well documented prior to the High Middle Ages (1000–1250), the researchers suggested. "Forests are mainly mentioned in the context of royal hunting interests or donations," said Muigg. Dendrochronological studies are particularly important because they can reveal information not captured by a sparse historical record, he added.</p><p>These results were <a href="https://www.nature.com/articles/s41598-020-78933-8" target="_blank">published in December in <em>Scientific Reports</em></a>.</p><p>"It's nice to see the longevity and the history of coppice-with-standards," said <a href="https://www.teagasc.ie/contact/staff-directory/s/ian-short/" target="_blank">Ian Short</a>, a forestry researcher at Teagasc, the Agriculture and Food Development Authority in Ireland, not involved in the research. This technique is valuable because it promotes conservation and habitat biodiversity, Short said. "In the next 10 or 20 years, I think we'll see more coppice-with-standards coming back into production."</p><p>In the future, Muigg and his collaborators hope to analyze a larger sample of historic timbers to trace how the coppice-with-standards practice spread throughout Europe. It will be interesting to understand where this technique originated and how it propagated, said Muigg, and there are plenty of old pieces of wood waiting to be analyzed. "There [are] tons of dendrochronological data."</p><p><em><a href="mailto:email@example.com" target="_blank" rel="noopener noreferrer">Katherine Kornei</a> is a freelance science journalist covering Earth and space science. Her bylines frequently appear in Eos, Science, and The New York Times. Katherine holds a Ph.D. in astronomy from the University of California, Los Angeles.</em></p><p><em>This story originally appeared in <a href="https://eos.org/articles/tree-rings-reveal-how-ancient-forests-were-managed" target="_blank">Eos</a></em> <em>and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.</em></p>
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