DBQ: Fishing Down Marine Food Webs

DBQ: Fishing down marine food webs

Here are the answers for the DBQ on page 180:

1. A possible way to deduce the trophic level of a fish once it is captured is to examine its stomach contents. By determining the trophic levels of the fish's prey, one could potentially determine the tropic level of the captured fish.

2. a) Both marine and freshwater fisheries have been declining over the study period. The rate of decline is faster in fresh water populations than in marine populations. The rate of decline is more constant in marine/accelerating in freshwater.

b) Freshwater fisheries have been overfished for longer; freshwater populations smaller/more susceptible to disruption.

3. Generally, increasing in age means increasing size. Therefore increasing size means broader range of prey including larger fish/higher trophic level fish. (note for students: just because an animal is bigger doesn't necessarily mean that its tropic level is higher. For example the Northern Right Whale has a trophic level of 3.2, while the Pantropical Spotted Dolphin has a trophic level of 4.3). I didn't make those numbers up, they came from the paper "Diet composition and trophic levels of marine mammals" by D. Pauly, A.W. Trites, E. Capuli and V. Christensen (ICES Journal of Marine Science, 55, 467-481)

4. Age is correlated with trophic level; lowering of the tropic level means lowering the mean age.

5. Greater biomass of lower tropic level means a higher sustainable yield which allows higher trophic levels to recover. In addition, less energy is lost when there are fewer tropic levels. As the trophic levels increase, less of the initial energy is available.