Why is overfishing so detrimental?

Figure 10. Fishing down the web (fish food chain)
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Overfishing leads to a loss of species from disruption of natural food chains and ecosystems. Specifically, fisheries have been "fishing down the web". They focus on catching large predatory fish and when they have been depleted, moving to the next biggest fish down the food chain. Figure 10 shows this trend.

We are now seeing proof of this overfishing tendency consistently. Top predators in the food chain are disappearing (tuna, swordfish, marlin, cod, etc.), leading to a replacement of commercial fish with smaller fish that naturally occur lower on the food chain (13). It is thought that soon we will be turning to jelly-fish as a main seafood dish! Figure 11 depict this chain of predation. 

Figure 11. Marine trophic cascade (yalikedag.southernfriedscience.com)

One example of this concept actually occurring is seen in Tasmania. Recent climate change, resulting in warmer waters, has caused a range expansion of sea urchin. These sea urchin feed heavily on kelp beds. The only predator that keeps the urchin/kelp bed  ratio balanced is the spiny lobster who is extremely overfished. Because of being over fished, the lobster is failing at keeping the ecosystem balanced and urchins are decreasing the resilience of kelp beds (16). This example highlights the negative affects of man-made stresses (climate change and overfishing) on marine life and how important it is that we change our ways to protect species and biodiversity (see Figure 12).  

Figure 12. My diagram depicting the situation in Tasmania. a) The lobster is overfished, b) urchin accumulation occurs, leading to overfeeding on kelp thus lowering kelp resilience to stress c) my own hypothesis for future projections: kelp is totally depleted and urchins don't have anything to feed on and go extinct. Background information for diagram taken from article 16 (see sources). More research is needed to verify my future predictions.

Fisheries are starting to collapse because they are having a hard time catching their normal products. As a result, they attempt to “fish down”, going further out into the oceans and deeper into previously untouched marine territory. Bottom trawling is one method used by fisheries that is incredibly harmful to the ocean bottoms (see Figure 7 on "Types of Overfishing" page). It involves dragging a large net on the sea floor, often times destroying sea beds and disrupting delicate marine ecosystems. This is just one example of collateral damage that comes with our overfishing practices.

Overall, models indicate that if trends continue we will see a total collapse of fisheries within 50 years (3).

Figure 13. The future! www.classbrain.com