Old fish matter too

While it’s no surprise that commercial fishing has reduced the number of fish in the sea, but it may surprise you to know that there are far less ‘old’ fish – and we should be concerned about it.

All of our oceans have been heavily fished and there are very few remaining unutilised resources. Around 75% of important commercial fish species are classified as fully or over-exploited. Commercial fishing has had a number of evolutionary effects on fish populations, with fishing affecting the age at which they reach maturity as well as their length at certain ages.

Now, a study has found that the population of ageing fish in the seas around the US and Europe has reduced by an average of 72%.

Old fish are just going to die anyway, so they’re not that important, are they? Wrong. While older fish may be on their last legs (or should I say fins?), they have an important impact on the remaining populations. An author, Lewis Barnett from the University of Washington, explains that having age complexity in a population creates more stability in that population. He states that “if you trim away that diversity [of age], you’re probably reducing the marine food web’s ability to buffer against change”. The pacific cod, pacific hake, red snapper and Atlantic cod are just some examples of species that have been affected, with the population of older individuals reduced by 95%.

Red snapper: a species that has been significantly affected by commercial fishing. Source: Flickr

In 2010, a study done on 10 fisheries found that there was only a 25% decline in older populations, but these results may not necessarily show the full picture. Barnett and his team created models demonstrating what populations were like before commercial fishing and compared this to the current state of fisheries populations. These models provided a more accurate representation of how populations have changed and explained the massive difference in decline of older fish compared to the 2010 study.

Older fish may be unable to reproduce anymore, however, they are often the largest individuals and have produced the most offspring in their lifetime. Not only this, but they tend to have the greatest ability to adapt, responding to environmental change better than other members of the population. Older fish spawn in different areas and at different times, so changes in the environment may not affect them as much as fish who always spawn in one location at a specific time; being less rigid in behaviour is important in a constantly changing environment.

Is it possible to keep fisheries functioning while maintaining a diverse age structure? Yes, there are options. Firstly, fisheries need to be given a break. Every so often, fishing should cease in certain areas so the population of fish can recover properly, allowing them to return to their natural state. Additionally, minimum and maximum size restrictions should be monitored. Removing all individuals in a certain size category can significantly alter the population’s age structure, as age often correlates with size.

Fish populations are struggling with the increased pressure of commercial fishing. Source: Flickr

Having marine protected areas can also help fish populations. They are able to exist and thrive without intense fishing pressures. These protected populations are generally more diverse in age structure and the average lifespan of individuals is longer, which is beneficial for the entire population.

While older fish may not seem as important, they play a critical role in maintaining a healthy population. If you found yourself thinking ‘surely old fish can’t reproduce so they can’t be that important in maintaining a population’, you aren’t alone. I too believed this. I guess the saying “respect your elders” applies to animals as well as humans.


3 Responses to “Old fish matter too”

  1. Jennifer Feinstein says:

    Interesting post! This is something I’ve never thought about before… thanks!

  2. Rob Dabal says:

    Nice article. I can’t help but think of parallels between large old fish and large old trees. Some vegetation communities have similar rates of reduction of old growth, Box Ironbark Forests for example have had well over 95% of old growth stands removed with pretty much all remaining coverage having been harvested at some stage in the past century. Large old trees are the biggest producers of nectar and resources like seed, so their removal has a significant short term effect but can also play out over decadal time frames.

  3. Gabriel Cornell says:

    Old fish are possibly the most important in some populations! The Big Old Fat Fecund Female Fish (BOFFFF hypothesis) says that the big, old female fish often produce disproportionately more, healthier larvae than younger females. It’s certainly alarming that fish populations are getting younger, as the oldies are probably best equipped to maintain those populations, especially with such huge fishing pressure.

    https://www.huffingtonpost.com/2014/10/23/boffffs-big-fat-fish-research_n_6039252.html