Book Review Conservation Human impacts

There are plenty of fish in the sea! ….or are there?

The Paper: Mccauley, D. J., Pinsky, M. L., Palumbi, S. R., Estes, J. A., Joyce, F. H., & Warner, R. R. (2015). Marine defaunation: Animal loss in the global ocean. Science, 347(6219), 247–253. doi:10.1126/science.1255641

The Background: Tigers and orangutans are losing their homes due to deforestation. Elephants and rhinos are threatened by poachers. The list goes on an on. Unless you live under a rock, you have undoubtedly heard about how many animals are imperiled, and most of them are at risk due to human causes. Humans have been aiding in the endangerment and extinction of numerous species for centuries: The dodo bird, the passenger pigeon, and the Tasmanian tiger are just several of the more than 300 species that have been declared extinct over the past 500 years. We are now in what many scientists are declaring the earth’s 6th mass extinction.

Interestingly, the majority of these extinctions have occurred in terrestrial ecosystems and the marine environments have been lucky enough to avoid mass destruction. That is until recently at least. We are now beginning to highly impact marine environments in a way that is much reminiscent of the terrestrial destruction that occurred around the time of the industrial revolution (Fig. 1). But there may still be some good news out of all of this doom and gloom. The terrestrial extinction events can be used as a map into the future of marine ecosystems if we are not careful. If we are smart about how we read the map and follow the right directions, we may be able to avoid an ocean catastrophe!

Figure 1: Caribbean monk seal (Monachus tropicalis) was highly harvested by humans and the species. The last sighting was reported in 1952 and the species was officially declared extinct in 2008 (; image from
Figure 1: Caribbean monk seal (Monachus tropicalis) was highly harvested by humans and the species. The last sighting was reported in 1952 and the species was officially declared extinct in 2008 (; image from

The Study: The authors compiled data on all of human- associated extinctions that have occurred on both land and in the sea over the past 55,000 years. Additional information on species at risk was compiled based on the IUCN red list of endangered species. From here, the scientists were able to make comparisons between the history of terrestrial extinctions and the more recent extinctions in marine systems.

What they found: In the past 514 years, 15 marine species have gone extinct. In this same time, however, over 500 terrestrial species have been declared extinct. Although it appears that there is a lower degree of extinction in the marine environment, this is likely due to the lag in human impact affecting the marine species. The levels of extinction seen in the ocean over the past 514 years are very similar to the rates of terrestrial extinction that occurred right before the industrial revolution (Fig. 2). Large scale marine defaunation (extinction and large scale population depletion) has only occurred within the past century, coinciding with expansion of coastal populations and the commercial fishing industries.

Figure 2: Time scale comparing marine and terrestrial defaunation events coinciding with human activity. Marine environments are expected to be more heavily impacted with climate change (indicated by the colored bar changing from grey to red; image from McCauley et al., 2015).

The study sites three different types of extinction to be aware of in addition to the traditional “everything is gone, done, kaput!” definition. 1) Local extinction– occurs when a species disappears from a certain area. This can reduce a species’ native range and be detrimental for the health of the species and the local ecosystem alike. Currently ~90% of open water fishes (think your tuna, mahi mahi, and many sharks) have already become locally extinct from parts of their ranges. 2) Ecological extinction– occurs when a species is not completely extinct but is no longer abundant enough to carry out its function in the environment (like when you only have one dollar but need three to buy that coffee from Starbucks!) With the extensive declines in marine animals, we are beginning to see the effects of ecological extinctions taking place. 3) Commercial extinction- occurs when species drop below a level that can still be harvested on a commercial scale. When some species, like the grey whale for example, reach this level humans stop catching the species, allowing their populations to rebound a bit. However, there has been an unfortunate trend in many species (again, think about your tuna!) where no matter how few fish are left in the sea, some species will never be too rare for humans to demand the continued harvest of endangered animals.

Who’s at risk?: Can we predict which species are more at risk of becoming extinct? Unlike the historical terrestrial extinctions, it appears that smaller marine species are at risk (i.e. anchovies) because they are easily harvested in mass quantities. According to the list of endangered species, it also appears that animals that have some sort of terrestrial contact (sea turtles, sea birds, seals, etc.) are more vulnerable to extinction. Likewise, species living in coastal environments (coral reefs, mangroves, and sea grass beds to name a few) are also at risk due to their proximity to human settlements which makes them easily harvested, more likely to be damaged or removed to support the development of new coastal infrastructure (hello Malibu beach home! First we just need to clear away this swampy sea grass…), and also collect pollutants running off from of surrounding cities.

Unfortunately, humans are beginning to expand their range of influence on marine species. Areas used as travel routes tend to have a higher degree of marine defaunation, for example. As humans branch out and begin to explore the vast expanses of our ocean, more and more species become at risk. Furthermore, technological advances make it easier to exploit marine species. New technologies like sonar allows for the location of large aggregations of fish in the middle of the open ocean while advances in fishing gear enables the exploitation of species that have previously been too difficult to harvest (Fig. 3).

Figure 3: Species which have been historically difficult to capture, like deep dwelling orange roughy (Hoplostethus atlanticus), are now easily exploited given advances in fishing gear (Monterey Bay Aquarium Seafood Watch; image credit:
Figure 3: Species which have been historically difficult to capture, like deep dwelling orange roughy (Hoplostethus atlanticus), are now easily exploited given advances in fishing gear (Monterey Bay Aquarium Seafood Watch; image credit:

Why should we care? Well besides general concerns for preserving life on our planet, there are actually some far-reaching consequences of large-scale marine defaunation. For one, the extinction of a species can be detrimental to the ecosystem, throwing off the entire balance of the environment. For example, when we remove small bait fish (like anchovies) from the ecosystem, larger predators (fish, sharks, and seabirds for example) may suffer from depletions in their food source which can result in alterations to their population structures as well. Conversely, if we remove all of the large predators in an ecosystem (like sharks), smaller fish populations may skyrocket, causing detrimental effects to the system (think of when all the coyotes are removed from the neighborhood and the deer and rabbits start eating everything in your garden).

But let’s get to the part everyone actually cares about: how will marine defaunation impact humans? Damaging coastal ecosystems makes our coastal cities far more vulnerable to storms. Those mangroves and salt marshes we destroy actually act as natural buffers against large storm events like hurricanes! Marine organisms, both the large and microscopic alike, are also important for removing carbon from the atmosphere- a service we should greatly appreciate with all of this talk about climate change spurred by raising atmospheric carbon dioxide. And of course- Sea food! Marine animals account for a huge portion of the global protein consumed. A collapse in marine harvesting could wreak havoc on the world food supply, and lets face it, we just aren’t ready to give up our fish and chips or sushi!

The Silver Lining: It is not all doom and gloom. Through studying massive terrestrial extinctions the scientists were able to find some avenues to prevent a similar fate in the oceans. By realizing the threats present to the marine systems we can begin to protect important marine environments now, before it is too late. Currently less than 4% of the oceans are protected but the marine species have a much better shot if we take the opportunity now to increase marine protected areas and take new management strategies into consideration. Furthermore, based on the terrestrial extinctions, the scientists project that the detrimental effects of overfishing will be overshadowed by the effects of habitat destruction in the future. If we can step in now, before getting to this stage on the defaunation time line, we have a much better shot at protecting the marine environment. Basically, there are a lot of hurdles to overcome for endangered marine fauna, but if we realize these challenges and begin to act now the projections for mass extinction need not become a reality.

2 thoughts on “There are plenty of fish in the sea! ….or are there?

Leave a Reply

Your email address will not be published.