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Ocean-Based Habitat Compression

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

March 18, 2014
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MAR0314_Conservation

MAR0314_Conservation

Vertical expansion of oxygen minimum zones may make billfish more susceptible to commercial gear, such as longlines. (Photo by Raul Boesel Jr.) Raul Boesel Jr.

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.

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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.

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So where does habitat compression come into play as an important factor in billfish conservation? Billfish, and other pelagic predators like tuna, are high-energy fish with a physiology that requires a lot of dissolved oxygen for growth and survival. The vertical movements of these fish were originally thought to be driven solely by water temperature, but satellite tagging studies have demonstrated that the vertical movements of blue marlin actually correspond to available dissolved oxygen. Blue marlin will dive to depths exceeding 800 meters as long as there are sufficient oxygen levels; however, in instances where there are not, they spend more time within the surface mixed layer at shallower depths, where there are adequate levels of oxygen. Within the ever-growing OMZ, vertical and horizontal expansion will continue to encroach upward in the water column and compress the viable habitat of important billfish and tuna stocks.

Combining both the oceanography of OMZs and the biology of billfish, the final pieces of the puzzle fall into place: the impacts of fishing and increased susceptibility to surface fishing gear. It is no coincidence that over the last 50 years the commercial industrial longlining effort for bigeye tuna catches on the high seas directly corresponded to the increasing OMZ. As the OMZ compresses fish upward into the shallower areas of the surface mixed layer where there is sufficient oxygen, fish become easier targets for nondiscriminant surface fishing techniques, such as longlines or purse seines. One would expect that as the OMZ continues to expand, the trend of increased fishing pressure would also continue to increase in these habitat-compressed areas.

In addition to the pressure being put on fish stocks in these areas, the OMZ and habitat compression may be causing inaccurate indicators on stock health, creating an overly optimistic stock assessment for these species. As a result, current management decisions may not reflect the true health of the fishery and unintended overfishing could be occurring, which would in turn further deplete valuable fish stocks and affect the sustainability of the fisheries.

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Dr. Eric Prince of the National Oceanic and Atmospheric Administration’s Southeast Science Fisheries Center, the leader on the impacts of the OMZ on billfish and tuna, says, “It’s incumbent upon the stock assessment community to develop future stock assessment methodologies that take into account the increased catchability that’s associated with hypoxia-based habitat compression.” This might be possible in the assessment during the catch per unit effort standardization process by scaling the model based on the estimated size of the growing OMZ and the shrinking surface mixed layer. If successful, this approach may have the potential to avoid overly optimistic assessment results that might occur if this issue continues to be ignored.

OMZs are not only a problem in the Atlantic Ocean but also in all tropical ocean basins. It is imperative for regional fisheries management organizations, such as the International Commission for the Conservation of Atlantic Tunas and the Inter-American Tropical Tuna Commission, to take a serious look at the impacts on fishing efforts within the OMZ when making future fisheries management recommendations.

As we move forward in an attempt to utilize the best available science for fisheries management decisions, it is important that we remember to look at the entire forest and not just a single tree. We very well could be missing a large piece of the puzzle.

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