EarthLabs > Fisheries > Lab 1: Plenty of Fish in the Sea? > 1C: Interpreting Species Density Graphs

Plenty of Fish in the Sea?

Part C: Interpreting Species Density Graphs

Experts have found that large numbers of predator fish like tuna and billfish (large fish such as swordfish and marlins, that are characterized by long sword-like bills) are commonly found in biodiversity hotspots. This means that the abundances (or species density) of tuna and billfish are good indicators of overall marine biodiversity.

As you saw in Part B, there is a striking contrast between the level of marine biodiversity 50 years ago and that seen today. Environmental changes are often responsible for fluctuations in the the sizes of fish communities from year-to-year. However, global declines in fish abundances over longer timescales are believed to be primarily driven by overfishing.

  1. Examine the graphs below comparing species density and total long-line catch of tuna and billfish by ocean. Here are some tips that will help you better understand the information presented in each graph:
    • Look carefully at what is being plotted on each axis of the graphs.
    • Note that the y-axis scales are not the same for total catch in each ocean.
    • 1 tonne (also known as a metric ton) = 1,000 kilograms



    Checking In

    Answer the following questions to check your understanding of the information provided in the species density graphs above.
    • Which ocean experienced the largest decline in species density between 1960 and 2000?
      To determine how much species density declined in each ocean between 1960 and 2000, use the following formula:
      decline in species density (%) = species density (%) in 1960 - species density (%) in 2000

      Example: The decline in species density in the Indian Ocean between 1960 and 2000 was 30% (80% - 50%).
    • Which ocean experienced the largest percentage increase in total catch of tuna and billfish between 1960 and 2000?
      To determine the percentage increase in total catch for each ocean between 1960 and 2000, use the following formula:
      % increase = 100% x (catch in 2000 - catch in 1960)/catch in 1960

      Example: The percentage increase in total catch in the Pacific Ocean between 1960 and 2000 was 233% (100% x (500 - 150)/150).

  2. Read the MSNBC article So Long Seafood? (Acrobat (PDF) 112kB Jan15 08) to learn more about the potential consequences of biodiversity loss in the world's oceans.

    Stop and Think

    1: Why is it important to have species diversity in the oceans?

    2: Why do you think large predatory fish are important for maintaining the balance of marine ecosystems? Why are they important to the sustainability of the fishing industry?


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