Overfishing Explained Part 1: How We Deplete The Ocean

Updated: May 6



If you have access to Netflix, you've probably heard about the new film Seaspiracy. Filmmaker and main character Ali Tabrizi sets out on a mission to discover why the ocean he loves is in so much trouble. To his surprise, there's a threat far worse than plastic pollution that's decimating our world ocean - overfishing.


Industrial fishing is arguably the greatest direct threat faced by our oceans, and most people know very little about it. The global industry is fraught with human rights abuses, wasted life (bycatch), corruption, and the outright destruction of habitats.


Seaspiracy has brought these issues directly into public spotlight, but the film is far from perfect. While watching, I implore you to keep 2 things in mind:

  1. There really are such things as sustainable fisheries, and sustainable aquaculture.

  2. In the end, the filmmakers offer a completely unrealistic solution that the best action anyone can take to save our oceans is to stop eating fish.

As a former fisheries observer, and current marine mammal rescuer, I've decided to weigh in with a series of blogs that explain the VERY complex issue of overfishing in our world's oceans.


To begin, I'll dive into the issue itself: how do we deplete the ocean?


The Ocean's Perspective


I want to start by saying that our oceans face five serious threats from mankind: Overfishing, Climate Change, Pollution, Habitat Loss, and Invasive Species.


Overfishing directly impacts marine environments by physically killing and removing marine life, but it isn't the only force impacting these ecosystems. Ocean life is very resilient, and is most-often able to recover from human impact, given enough time and space. However when multiple of these threats are acting on an ecosystem at once, we begin to see serious consequences.


To protect our oceans, we must look at these issues all together, especially considering how interconnected they are by nature. Keep this in mind as we dive further into the world of fisheries.


Maximum Sustainable Yield, Total Allowable Catch, and Overfishing


Before we explore all of the different ways that fishing can impact marine environments, it's important to understand how fishing works.


Unlike most land-based food production, fishing vessels (both industrial and small-scale) chase and capture wild populations of marine species. Without any regulations, fishers would try to capture as many fish as possible to make the greatest profits possible. This would obviously lead to a major problem: we would fish these animals into oblivion pretty quickly. This phenomenon is commonly referred to as The Tragedy of the Commons.


Instead, fishers and scientists use data to figure out the best number of fish to catch each year, so that they can collectively invest in a healthy future of the fishery. So how do they figure this out?

Photo credit: NOAA via Unsplash

There's no more difficult question to answer than "how many fish are in the ocean". To do so, scientists examine catch data, abundance data, and biological data on fish stocks. The process involves lots of statistics and modeling, but in the end it gives us a good picture of how many fish are in a population.


Taking a trip down memory lane to high school science class, you might remember learning about something called the population growth curve. It's an s-shaped (logistic) curve that shows how a population changes through time. When a population size is low, growth is slow for a variety of reasons - called Allee Effects - such as difficulty to find a mate, the inability to cooperatively hunt/defend, or more recently the fact that rare fish are worth more money. As a population becomes larger, it experiences more rapid growth without Allee Effects holding it back. But then resources become limited. After a population reaches its Maximum Growth Rate, competition between members within the same population intensifies (intra-specific competition), and eventually growth levels-out at the population's Carrying Capacity.

Photo by: Nchisick Creative Commons (CC BY-SA 4.0)

Now, let's apply this concept to the world of fisheries. Since fish populations grow most rapidly around 1/2 of their carrying capacity, this is the most efficient level to be fishing at. The population is growing rapidly, and as predators, we are enjoying high catches while removing fish at a sustainable rate. Fisheries scientists refer to this "just right" level of fishing effort as the Maximum Sustainable Yield (MSY).


Today, many fisheries scientists realize that the MSY of any fishery can't be a fixed number, since fisheries are constantly changing and perpetually complex. Instead, they prefer to use Total Allowable Catch, which is updated annually and determines the "just right" level of fishing while accounting for natural fluctuations in the environment.


Wild-caught fisheries can be sustainable, as long as they are well-managed and fishers work together with their local governments to ensure fish populations don't become overfished.


Breaking Down Bycatch


Now that you have an understanding of how fish populations are managed in a perfect world, I'll introduce the real-world complexities that make managing wild fish so complex.


One of the most important unintended consequences of fishing is Bycatch. NOAA Fisheries defines bycatch as "discarded catch of marine species due to a direct encounter with fishing vessels and gear". The vast majority of these unintentionally-caught animals die, tossed back to sea as wasted life. Fish are discarded for a variety of reasons, including that they're too small, inedible, damaged, not marketable, or can't be kept because of quota restrictions.


To put the scale of this waste into perspective, 10 percent of all fish caught around the world are discarded back into the sea as bycatch - that's 10.5 million tons each year. The majority of this bycatch comes from industrial fishing methods including: bottom trawling (4.6 million tons), purse seining (1.8 million tons), and gill-net fishing (0.9 million tons).


Spatially, the vast majority of discarded fish occur within the EEZs (exclusive economic zones) of countries. From the 1950's-1980's, the majority of discarded fish were documented in Northern Atlantic waters, but more recently the Northwest Pacific and Western Central Pacific have become hot spots for fisheries bycatch. Ultimately, high levels of bycatch are the result of poor fishing practices and management on a country-level.


Thankfully, scientists and fishers are working together to reduce bycatch through innovative new approaches. Some fixes are as simple as re-designing traditional fishing gears in a way that prevents unwanted animals from being caught, while ensuring the fishers are still successful at catching their target species. Examples include:

  1. Excluder Devices

  2. Separator Trawls

  3. Circle Hooks

  4. Ropeless Fishing Gears

  5. Pingers (acoustic deterrent devices)

  6. Streamers

The truth is that bycatch of fish, marine mammals, seabirds, sea turtles, and countless other marine species can be avoided when fishers and scientists work closely together.

A loggerhead sea turtle escapes becoming bycatch via a turtle excluder device. Photo credit: NOAA

Shifting Baselines Syndrome


In order to properly manage anything, it's crucial that we have a goal to aim for. What's a 'healthy' ocean supposed to look like?


One fisheries-related phenomenon that Seaspiracy fails to mention is called 'Shifting-Baselines Syndrome'. The idea, coined by Daniel Pauly in 1995, explains that as our marine ecosystems decline through time, people tend to forget what a healthy baseline looks like. Gradually, we accept overfished ecosystems as being 'healthy', when in reality they're severely impacted.


It's impossible to rationally manage and restore marine ecosystems if we are blind to what 'healthy' looks like.


Here's an example of shifting baseline syndrome in the Gulf of Mexico.


And another example from the Florida Keys - documenting the loss of trophy fish through historical photographs.

Photo: Wildlife By Yuri

If We Fail The Ocean, We Fail Ourselves


I'll be straight here, the ocean is in grave danger. There's a reason that today's geological epoch is named the Anthropocene - humans have become the dominating force acting on Earth's climate and ecosystems, and fishing is a major part of that force. Today, 90 % of all fisheries are either fully-exploited, or facing collapse.


Yet at the same time, 3 BILLION people around the world rely on fish as their primary source of protein.


Which leads to the conclusion that if we fail our ocean, we fail ourselves.

Photo credit: NOAA via Unsplash

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Stay tuned for part two of Overfishing Explained, where I will shed light on the dark side of the global fishing industry - Illegal, Unreported, and Unregulated fishing.


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