Wednesday, June 12, 2013

Where are all the fish going? How climate change is changing the fisheries

Fish comprise as much as 20 percent of people’s animal protein in their diets. This is significantly higher in poor and island regions, where they rely on fish as a main part of their diet. Currently, about 90 percent of the fisheries around the world are being fully exploited or overexploited (, 2013). Overfishing is certainly a problem that needs to be addressed quickly, but through policy, planning and regulation, overfishing can be fixed. Climate change is not as easy to deal with. Unlike overfishing, which is a problem that for the most part only involves those in the fishing industry, climate change is a problem caused by every single industry in the world, making it much harder to control.

            Since the industrial revolution, an increased amount of greenhouse gases, such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and volatile organic compounds, have been introduced into the atmosphere, causing profound environmental effects. These gases absorb incoming solar energy and outgoing radiant energy, causing the temperature of the atmosphere to increase and impacts climates. Global atmospheric temperatures and CO2 concentrations have risen throughout the last 50 years. They are now reaching levels that have never been seen in human history. In addition to atmospheric temperatures increasing, this has also caused the world’s oceans to experience a net warming. A major consequence of this would be the likely disruption of the oceanic circulations. Currents are driven by winds, the fluxes of heat and freshwater (thermohaline circulation), or by the gravitational pull of the sun and moon. The thermohaline circulation (Figure 1) is driven by global density gradients created by surface heat and freshwater fluxes. The wind surface currents direct water towards the poles, which cools the water along the way. That cold dense water then sinks at high latitudes and upwells in the Pacific and Southern Oceans. Many global climate change models predict that this circulation will weaken, and possibly break down, particularly in the Atlantic Ocean (Roessig et al., 2004). This might be exacerbated with the rising sea levels, because it would decrease vertical mixing due and overturning of nutrients due to increased stratification (Brander, 2007; Roessig et al., 2004). In the Pacific, increased stratification could also lead to more El Nino/Southern Oscillation (ENSO) events. ENSO events are when warm water from the western equatorial Pacific goes over to the eastern equatorial Pacific and causes sea levels to rise, an increase in sea surface temperatures, and a weakened thermohaline circulation, reducing primary productivity (Brander, 2007). The thermohaline circulation is also important for carbon sinking. Increasing water temperatures decreases the solubility of CO2, which results in less CO2 uptake from the atmosphere, leading to more CO2 in the atmosphere (Roessig et al., 2004.
Figure 1. Diagram of the thermohaline circulation. This circulation is responsible for the large-scale exchange of water mass in the ocean, which provides oxygen to the deep ocean and brings nutrients back to the surface (

            Fish can be very sensitive to their surrounding environments. As ectotherms, they rely on their environment for heat. Changes in the temperatures of marine and estuarine waters will most likely have both direct and indirect effects on individual fish, their populations and their communities (Roessig et al., 2004). When fish live in less than favorable conditions, they experience decreased foraging, growth, and reproduction, altered metamorphosis, and it affects their endocrine homeostasis and migratory behavior (Perry, 2005). These changes influence population and community structure from their associated affects on performance, patterns of resource use, and survival. For example, variations in the North Atlantic Oscillation have caused sea temperature changes, which have been linked to fluctuations in cod recruitment and habitat shifts off the coast of northeastern Canada (Roessig et al., 2004).
Many of the commercial fisheries are found in the temperate regions. In the Ocean Conservancy’s The Law That’s Saving American Fisheries, it states that “the greatest challenge facing fishery managers is adapting to a changing global climate.” Many marine fishes exhibit behavioral thermoregulation, which means that they seek preferred temperatures. This means that if the waters in the temperate regions become too warm for some fish, they will move to colder waters, where the temperatures are closer to what their optimum temperatures are (, 2013; Cheung et al., 2013). This could mean just deeper waters, which might not have any impact on commercial fishing. This could also mean that the fish move closer to the poles, which could have an impact on commercial fishing (, 2013; Cheung et al., 2013). Sudden shifts in temperatures could have disastrous effects on fish populations if they are unable to find refuge in cooler places (Brander, 2007; Perry, 2005). Another thing that could cause a shift in fish populations to colder regions would be from restricted food supplies. When fish do not have enough to eat, they move to colder regions because it lowers their metabolic demands. This decrease in food supply could be caused from an ENSO event or decreased circulations in the water (Brander, 2007). An example of fish shifting regions due to reduced food supply is the horse mackerel. They feed on zooplankton, which shifted northward after an increase in sea surface temperatures in the North Sea that caused phytoplankton to shift northward. Zooplankton feed on phytoplankton. This caused the mackerel to change their migratory patterns (Roessig et al., 2004). This shows how changes in ocean temperature due to climate change can affect a food web, ultimately affecting fisheries (, 2013). Another example of how climate change has affected fisheries in the temperate region is in the temperate reef habitats off the coast of North Carolina. Researchers have tracked a decrease in temperate fishes, but they have observed 29 new species of tropical fishes moving in, due to the increasing water temperatures (Roessig et al., 2004).
Figure 2. The white coral is bleached. It also has nothing growing on it, in contrast to the healthy coral right next to it (

Climate change is not only affecting the fisheries, it is affecting every single organism in the ocean. A prime example of this is in the coral reefs. Coral depends upon a symbiotic relationship with zooxanthellae. Coral bleaching is happening more often now, because the increasing temperatures of the oceans affect the coral’s ability to supply the zooxanthellae with nutrients for photosynthesis. This leads to the zooxanthellae abandoning the coral, resulting in coral bleaching. Many tropical fish rely on the coral reefs for food and protection. They are home to some of the most biodiverse ecosystems in the world. Over 4,000 species of fish are found in coral reefs. Damage to these reefs drastically affects the amount of fish that are able to live off of that reef (Brander, 2007; Roessig et al., 2004). In coral reef communities where intense bleaching has occurred, significant changes in the abundance of some fishes were observed. Coral reefs are especially important for island countries that rely on those ecosystems for food and for their economy through tourism (, 2013). Jeremy Jackson’s TED talk goes more in depth in how the oceans are changing and how that affects the fisheries and coral reefs.

To try to deal with the effects of climate change on fisheries, as well as overfishing, the U.S. government has set catching limits for the 2013-2015 seasons well under the limits that were set in 2012. The catching limit for cod is down 78 percent in some areas. Yellowtail, witch flounder and other species also have lowered catching limits. Besides lowering catching limits, more monitoring and accounting for the impacts of climate change need to happen to fully understand what is happening to the fisheries (, 2013).

Climate change is not just affecting the oceans. It is affecting all organisms in the world, including humans. This is a very important issue that we need to address immediately. More research needs to be done to determine how climate change will affect the fisheries. Many people rely on fish for their diet and for their livelihood. We need to make sure that those people will still be able to do that well into the future.


Brander, K. M. "Climate Change and Food Security Special Feature: Global Fish Production and Climate Change." Proceedings of the National Academy of Sciences 104.50 (2007): 19709-9714. Print.

Cheung, William WL, Reg Watson, and Daniel Pauly. "Signature of Ocean Warming in Global Fisheries Catch." Nature 497 (2013): 365-69. Print.

Jr., Tom Zeller. "Climate Change Impacts Ripple Through Fishing Industry While Ocean Science Lags Behind." The Huffington Post., 17 May 2013. Web. 12 June 2013.

Perry, A. L. "Climate Change and Distribution Shifts in Marine Fishes." Science308.5730 (2005): 1912-915. Print.

Roessig, Julie M., Christa M. Woodley, Joseph J. Cech, and Lara J. Hansen. "Effects of Global Climate Change on Marine and Estuarine Fishes and Fisheries." Reviews in Fish Biology and Fisheries 14.2 (2004): 251-75. Print.

"The Law That's Saving American Fisheries: Magnuson-Stevens Fishery Conservation and Management Act." Ocean Conservancy, n.d. Web. 12 June 2013.

Walsh, Bryan. "Why Warming Oceans Could Mean Dwindling Fish." Science Space Why Warming Oceans Could Mean Dwindling Fish Comments. Time Magazine, 16 May 2013. Web. 12 June 2013.