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  1. #1

    Catch and Keep Makes Fish Smaller

    Fishing typically removes the large individuals from the population. Over time, this not only leads to evolutionary shrinking of fish but it also erodes natural trait variability in fish stocks. This was demonstrated in a study partly funded by the Academy of Finland where researchers experimentally harvested zebrafish. The loss of natural trait variability is an alarming observation, as variability facilitates population viability in changing environments.

    Intensive and selective removal of large individuals by fishers can act as a pronounced directional selection force, leading to evolution towards smaller body sizes. In addition to evolutionary downsizing, fishing can also reduce size and growth variation among individuals. Preserving variability is important: a population harbouring variation is less susceptible to changing environments and catastrophic disturbances. For example, when food availability is low, individuals that grow slowly will survive while fast-growing individuals will starve. Therefore, populations that consist of individuals growing at different rates will be better off in fluctuating environments.

    The experimental study, conducted by an international team of researchers, focused on the effect of size-selective harvesting on body size variation.

    "We've studied the evolutionary effects of size-selective harvesting for quite some time," says Postdoctoral Researcher Silva Uusi-Heikkilä from the University of Turku.

    The experimental zebrafish populations were harvested size-selectively for five generations after which the harvesting was halted for six generations. The researchers found consistent differences in body size variation among differentially harvested zebrafish populations.

    "Harvesting large fish led to lower size variation compared to a harvesting strategy where the largest fish were protected," says Uusi-Heikkilä. "This result highlights the role of variability in biological dynamics," says Academy Research Fellow Anna Kuparinen.

    Decreased size variation caused by fishing can have serious consequences for the resilience and recovery of exploited fish stocks. It can negatively affect the population's ability to buffer environmental changes, decrease the rate of evolutionary rebound and, ultimately slow down recovery from overfishing.

    "Sustainable fisheries management should not only manage fish stock abundances but also variability within a stock," says Uusi-Heikkilä.

  2. #2
    GBO Member
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    May 2011
    Thanks for the post. I've been saying that for years.... Take the little ones, release the big ones, because their genetic makeup will produce bigger ones again once they've spawned.

    And maybe this should be published in a document that the general angling public sees, not a "fisherman" board. We all know this information from being around this site. It's John Q. Public that needs to release a few, and not fill up the cooler every time they go out.

    My two cents


  3. #3
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    Mar 2008
    I saw this first-hand during the late 70's when I lived in Wawa, Ontario. I left my canoe at an isolated Lake Superior Park brookie lake and for a year me and my friends regularly fished that lake catching our limits of 1 - 3 pound brookies with very little outside competition. After one year, a good brookie was 10 inches. The big ones were gone. The happy conclusion to the story is that after I pulled my canoe out and it was left alone for a few years, it came back to its past glory.

  4. #4
    GBO Member JackFrank's Avatar
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    Nov 2004
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    Good read Lee! I agree too, let the big wild ones swim to do their thing.
    Jack Frank

  5. #5
    For me, it was never rocket science. If you remove the genetics of the large healthy fish from the population, you will not have genetically disposed fish for future generations. I don't fish perch on Simcoe, but from what I understand this is exactly what has happened there. Too many yellowbellies on the ice and too many runts thrown back.

    I cringe when I see stringers of 3+lb smallmouth, dead 20lb pike for a photo shoot on the dock, so on and so forth....... but if someone wants to keep a feed of the younger and more abundant population of whatever fish they target, I don't see anything wrong with that.

  6. #6
    Senior Member
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    Nov 2004
    Actually on Simcoe perch is only getting bigger lately. But maybe because they started eating gobies.

  7. #7
    Quote Originally Posted by Alex Konov View Post
    Actually on Simcoe perch is only getting bigger lately. But maybe because they started eating gobies.
    If that's the case, then I stand corrected. I was only going off what I have heard. Cheers.

  8. #8
    GBO Member Dan Hasson's Avatar
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    Jan 2013
    Kincardine/Paisley, ON
    It makes sense... The problem is unless everyone is educated or there is a slot size limit, all the bigger fish will keep ending up on someone's dinner plate.

  9. #9
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    Nov 2004
    When I started fishing a small stream some 45 yrs ago the gentleman who allowed me to tag along always told me to let the big ones go so they can spawn. His reasoning wasn't so much in genetics rather big females laid more eggs so more would hatch and survive. Seems either everyone has been keeping those big ones or our streams and rivers have lost so much flow that the fish just don't grow because the ecosystem cant support them. I suggest this because I just don't see the numbers of or size of fish caught as in the days gone by, of course most of the fish I catch are of the ghost species

  10. #10
    This is sort of along the same lines:

    Stone Age hunters didn’t need to tell fish tales—the fish they caught really were whoppers, according to a new study.

    Remains of prehistoric fish dinners from caves in northern Spain suggest that Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) have shrunk significantly in size over the past 20,000 years—from a combined average of up to 5 pounds (2.2 kilograms) to about a pound (0.4 kilogram).

    What’s more, it’s because of our ancient ancestors’ fishing skills that rivers in the region today are populated with much smaller fish, according to a team of scientists from the University of Oviedo in Spain. (See “Hot Stew in the Ice Age? Evidence Shows Neanderthals Boiled Food.”)

    That’s because the scientists suspect that the Stone Age fishers deliberately targeted larger specimens.

    “Bigger fish will be more valuable as food,” said archaeologist Pablo Turrero, who led the study. “They would only have caught the small fish if they absolutely had to because there was nothing else.”

    Over time, prehistoric hunters’ preference for catching big ones caused salmon and trout to downsize, because proportionally more small fish survived to breed and pass on their genes.

    A trend for shrinking sizes starts to show up in the cave fish fossils about 10,000 years ago, “but size selection had been going on all the time,” said Turrero.

    Big Fish, Little Fish

    Published October 22 in the journal Royal Society Open Science, the study compared the backbones of about 30 Atlantic salmon and brown trout specimens unearthed at ten prehistoric sites in the northern province of Asturias (map) with those of their living descendants. (See “Salmon Farming Gets Leaner and Greener.”)

    These calculations revealed the ancient fish averaged 6 to 10 inches (16 to 26 centimeters) longer than those types of salmon and trout in the same rivers today.

    Turrero said this equates to a typical weight of 2.9 to 5 pounds (1.3 to 2.2 kilograms), whereas the modern-day Asturias angler can expect an average catch that weighs less than a pound (0.4 kilogram).

    The fossils also indicate that the modern humans who replaced the Neanderthals some 40,000 years ago were serious about fishing “pretty much as soon as they started settling in the region,” according to Turrero.

    Fish meal leftovers found in the caves aren’t as abundant as those of prey animals such as deer, “but you always get them,” Turrero added.

    Prehistoric Fish Fry

    Often burnt, the fish fossils show that hunters “would catch the fish in the nearest stream, then bring them to the cave to cook by the fire,” he said.

    Past excavations at the fishbone sites have found evidence for these fires, as well as the human remains of these prehistoric salmon-eaters. (See “Cooking Gave Humans Edge Over Apes?“)

    Though no fishing gear remains have been identified to date, such artifacts have been found at similarly aged sites elsewhere in Europe.

    The ancient salmon hunters may have used hooks made from mollusk shells, traps made of branches, spears, or even their bare hands, Turrero added.

    The importance of salmon and trout to these cavemen can also be seen in two Asturias caves that boast extremely rare Ice Age examples of ancient fish paintings, Turrero noted.

    Human Influence

    In addition to human preference, climate change and other factors also likely played a role in shrinking fish size over time, the study authors said.

    Karin Limburg, a fish expert at the State University of New York’s College of Environmental Science and Forestry, agreed.

    However, “given that humans have been in that region for a very, very long time, and salmon running into rivers would have been a great source of protein,” the new finding is “not so surprising,” Limburg said. (See “Eating Crocodile Helped Boost Early Human Brains?”)

    “What strikes me about the study is that it shows, yet again, size declines in a fish that is exploited by humans,” she added. “The more we look, the more we see this has occurred.”

  11. #11
    (Fishing) - (Ego) = (C&R)... or at least a more sensible harvest approach.

    In the above equation, you could also substitute Greed for Ego.


  12. #12
    GBO Member Nick Toth's Avatar
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    Mar 2007
    Yes, every organism maximizes effort for nutrition including us - we shop a lot at NoFrills!

    No dates were provided in your post, but it is safe to state that predation was scant because of scant numbers of fishermen.

    Without having read your references, how did the size and distribution of the anglers come into play regarding fish predation? Were there remote fish populations that remained unaffected?

    Your comment is interesting and poses more thought. I'll sift your links tomorrow.


  13. #13
    Dates are on the links Nick. Sept. 20, 2016 for the University of Finland study and the other is an article from Oct. 21, 2014 in the National Geographic which sites a study from the journal Royal Society Open Science (didn't dig further to find the date when the study was published).

    I obviously didn't write these, I only shared the information. If you wish to dig further, by all means do so and let us know if there is any pertinent information to be found or if you refute any of the information provided.

  14. #14
    Quote Originally Posted by Niel Palmer View Post
    (Fishing) - (Ego) = (C&R)... or at least a more sensible harvest approach.

    In the above equation, you could also substitute Greed for Ego.

    I ran the math too and came up with the same result

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