What Is Responsible for Smaller Shrimp Catches?
ES2206  How Does the Dead Zone Affect Gulf Shrimp Catches?

Many people whose livelihood comes from the Gulf of Mexico believe that if the dead zone is not controlled, it will damage the fishing and shrimp industries of the Gulf states. Although shrimp catches in the Gulf states have generally declined from peak yields in the mid-1980s, annual catches do not show a continuous downward pattern.

It can be difficult to make accurate estimates of how fish and shrimp populations have changed based only on how many are caught. For instance, when fewer fish are available in an area, fishermen may still show normal harvests because they spend more time and effort in fishing areas farther away to keep yields up. To do this, however, they must spend more money for additional fuel, machinery, and labor costs. In order to compare how the number of available fish has changed over time, a measurement called Catch Per Unit Effort (CPUE) has been devised. CPUE is a measure of how much seafood is caught for the amount of effort (work, time, energy) it takes to catch it. A decrease in CPUE over time would indicate that it was becoming harder to catch fish.

Data: James Nance, National Marine Fisheries Service
The graph shows annual changes of CPUE for brown shrimp in areas of the Gulf of Mexico. The colored bars are "decadal means". They show the average CPUE for a span of ten years.

One species that is affected by hypoxia is the brown shrimp. The dead zone appears to block juvenile brown shrimp from reaching their offshore spawning grounds, where they reproduce. This graph shows CPUE for brown shrimp in the Gulf of Mexico from 1960 to 1997. In this graph, CPUE is measured in kilograms of shrimp per hectare (1 hectare equals 10,000 square meters).

8. Describe the trend of catch per unit effort (CPUE) for brown shrimp from 1960 to 2000.

9. Do the CPUE data support the hypothesis that increased nitrate levels in the Mississippi River Basin reduce the shrimp harvests in the Gulf of Mexico? Why or why not?

10. Briefly explain how the geosphere, hydrosphere, and biosphere interact in this story.


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