The Shrinking Fish

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My grandpa has always been an avid fisherman, and like most grandparents, he loves sharing his stories. His favourites usually centre around the biggest, strongest fish… the ones he used to catch. There’s a fair amount of fish-related grumbling these days as well. The most common complaint: “damn fish just ain’t as big as they used to be.” My grandpa isn’t the only one making this complaint. In fact, it became a common enough sentiment that in the mid 90’s a number of research groups began to study the phenomenon of the mysterious shrinking fish.

The thing is, fish really are getting smaller. By using international catch data, researchers have shown that roughly 96% of all human-targeted fish species have not only reduced in numbers, but also in size. Not just by a small amount either – many of the highly fished species are an average of 20% smaller than they were a century ago. This is in contrast with fish with no commercial or recreational value that have stayed a size consistent with that of their predecessors.

So, what is it about the fishing process that has caused this shrinkage? Essentially it comes down to the fact that humans are remarkably strange predators. A typical, non-human predator will often target small, weak prey (something anyone who has watched a BBC nature special will already know). Modern humans, on the other hand, are unique in that they harvest large, attractive, reproductive-aged adults. The bigger, the better, right?

Over time, as large fish are removed from the gene pool, only the small individuals (the ones that are thrown back or are small enough to escape from commercial nets) are able to reproduce and spread their genes on to the next generation. The more heavily a fish population is harvested, the faster the average size of the fish will decrease. This process has been termed “harvest selection” and has been studied increasingly over the last few decades due to its extreme effects.

Harvest selection has been shown experimentally as well. Trials on salmonid species have shown that removing the largest adults can cause an 18% size reduction within five generations (their generations, not ours). Conversely, removing the smallest adults caused an increase in overall size over that same generation time.

Interestingly, size isn’t the only trait being affected by harvest selection. Evidence shows that wild populations of marine fish are now reproducing at smaller sizes and at younger ages than they used to. By removing the reproductive-age fish from the population, the few that can reproduce at a younger-than-normal age will have a better chance spreading their genes. In other words, we are penalizing the fish that reproduce at larger sizes/older ages. Unfortunately, fish that reproduce younger and at smaller sizes are less likely to produce healthy offspring.

Unlike humans, fish become better reproducers as they grow. A 30” rockfish, for example, may lay ten thousand eggs whereas a 110” rockfish can lay upwards of three million. Not only do larger, older fish produce more eggs, their eggs are also more likely to hatch and survive. Thus, by removing large fish from the population, humans are also reducing the fish most capable of replenishing the stock. (Incidentally this same biological characteristic is true of crabs, shrimp and most other marine invertebrates).

In summary, by targeting large fish we have directly caused a reduction of average fish size, reproductive age, and egg survival. The results of harvest selection can be seen in roughly 96% of all commercial and recreational fish species internationally.

Current fishing regulations dictate the minimum catch size, but there are no maximum size restrictions. This discrepancy further fuels the sentiment that when it comes to fishing, “bigger is better.” However, next time you pick up that hook and rod consider going for the nice medium-sized one. Let the big mama go forth and spread her big mama genes.

For more information about harvest selection, shrinking fish, and other trends in the world of fisheries science, check out the work of Dr. Daniel Pauly (University of British Columbia), Dr. David O. Conover (University of Massachusetts), Dr. Chris Dairmont (University of Victoria). There are of course many, many more amazing researchers in the field, but this is a good place to start!

Emily Beeson is the Senior Biologist at the Ucluelet Aquarium

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