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March 17, 2014

Ocean-Based Habitat Compression

Oxygen Minimum Zones may affect billfish in
 ways not previously understood.

In fisheries management, we sometimes get so focused on the proverbial tree that we completely forget about the forest and miss a large piece of the puzzle. Engrossed in fish stocks, fishing effort and the availability of quota, we often overlook the factors that directly influence the fish — one being the water itself.

We are very familiar with habitat compression on land. Whether it is polar bears causing havoc in dumpsters in Alaska or deer falling victim to trucks on the highway, the impacts of habitat compression are easy to understand. Changes in terrestrial habitats create simple cause-and-effect relationships, resulting in changes in animal behavior. But habitat compression in the ocean is much more difficult to grasp because of the many invisible factors contributing to the shrinking habitat of prized sport fish, such as marlin and tuna. Similar to their terrestrial counterparts, important fish populations are becoming threatened by habitat compression in the ocean, presenting a significant challenge for fisheries managers.

Here is a quick crash course in Oceanography 101 regarding hypoxia, dead zones and their impacts on large pelagic fish. Dissolved oxygen in the water is vital for marine life; when minimized, mortality or changes in behavior can result. If areas become void of oxygen to the point where fish cannot survive over the long term, hypoxic areas called dead zones are formed. More formally called oxygen minimum zones, or OMZs, these areas often form natural habitat boundaries for fish.

Dead zones are found in all of the major oceans and are a naturally occurring phenomenon caused by the normal cycling of nutrients in the ocean. Basically, after an organism dies it sinks to the bottom of the ocean where it continues to consume oxygen as it decomposes. On a large scale and with a minimum amount of turnover and lack of currents to move oxygenated water around in the equatorial waters of the world’s oceans, large dead zones are created.

Compounding this process is the fact that warm ocean water is not able to hold as much dissolved oxygen as colder water. As the temperature of ocean waters continues to increase due to global climate change, the amount of dissolved oxygen held within the water will inversely decrease, contributing to increasingly larger OMZs. It is estimated that since 1960, the Atlantic OMZ has increased in size by 15 percent. Consequently, the expanding OMZ reduces the available viable habitat for fish by 15 percent.

Rather than thinking of the OMZ as only expanding horizontally, it’s important to realize an OMZ can also grow vertically by 1 meter per year, causing the overall volume of the OMZ to increase. To put the size of the OMZ into context, the entire continental United States would fit inside of the current OMZ found within the tropical Atlantic Ocean. The most recent models even show that the dead zone reaches across the entire Atlantic, from the coast of Africa to parts of South America.