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Big fish make better mothers. Fishing selectively removes those individuals that are best able to maintain fish populations. So what can we do?

Big Fish

The demand for fish is growing as our population grows.  We are taking more and more fish and fish numbers are decreasing.  Fishing removes the biggest fish from the ocean, but big female fish produce much more young than smaller females. How do you preserve fish populations if the most fertile individuals are constantly being taken? The answer may be to protect areas where fish can grow and reproduce uninterrupted.

 Why are bigger fish harder to find?

When we fish, we usually take the biggest fish and throw small ones back.  This removes the largest fish from the population. The average size of a fish species can drop if the fishing pressure is great enough, because smaller fish are left behind to breed and small fish tend to have small babies. Big fish become harder and harder to find.

This creates a problem.  The largest and most fertile fish are taken. These are the fish that are best able to maintain the size of fish populations.

 Bigger fish are better mothers

Larger females lay many times more eggs than smaller fish. And often, their young are more likely to survive.  In many species, the offspring of big fish swim better, grow faster and live longer than ones from smaller mothers.

 How can we maintain fish populations?

One of the best ways of preserving fish numbers is to protect some of these big fish. Setting aside portions of the reef where fishing is prohibited provides a refuge where fish can grow.  Big fish can reproduce in peace producing large numbers of offspring in these secure areas.

Reserves also help repopulate the surrounding area because once the numbers of fish build up they spill into other areas where fishing is allowed. The creation of marine reserves helps maintain fisheries. Another benefit of marine parks is that the big fish within them can attract tourists, and bring income into the local economy.

 Big fish and sustainability

Overfishing is a disturbance which lowers diversity. By removing big fish the rest of the population is made less productive.

Creating marine protected areas preserves diversity and protects the fish best able to maintain the population size.


Baskett, M. et al. (2005). Marine reserve design and the evolution of size at maturation in harvested fish. Ecological Applications 15(3): 882–901.

Brown, J. (2005, February 19). New science sheds light on rebuilding fisheries. Seaweb. Retrieved 16 July 2008 from http://www.eurekalert.org/pub_releases/2005-02/s-nss021605.php

Coltman, D. (2008). Molecular ecological approaches to studying the evolutionary impact of selective harvesting in wildlife. Molecular ecology 17: 221-235.

Fenberg, P. & Roy, K. (2008). Ecological and evolutionary consequences of size-selective harvesting: how much do we know? Molecular ecology 17: 209-220.

Jørgensen,C. et al.(2007, November 23). Managing evolving fish stocks. Science Magazine.

Understanding evolution. (2007). The case of the shrinking fish. University of California Museum of Paleontology. Retrieved 16 July 2008 from http://evolution.berkeley.edu/evolibrary/article/side_0_0/shrinkingfish_01

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