Fish populations do not live by themselves. Rather, they are embedded in
ecosystems where they perform their roles as consumers and prey of other
organisms, including larger fishes.
The role of fishes within ecosystems is largely a function of their size: small fish are more likely to have a vast array of predators than very large ones. On the other hand, various anatomical and physiological adaptations may lead to dietary specialization, enabling different fish species to function as herbivores, with a trophic level of 2.0, or as carnivores, with trophic levels typically ranging from 3.0 to about 4.5.
Moreover, trophic levels change during ontogeny
of fishes. Larvae, which usually feed on herbivorous zooplankton (TL= 2.0)
consequently have a trophic level of about 3.0. Subsequent growth enables the
juveniles to consume larger, predatory zooplankton and small fishes or benthic
invertebrates; this leads to an increase in trophic level, often culminating in
values around 4.5 in purely piscivorous, large fishes.
For formal descriptions of the role of fish in ecosystems and their
responses to changes in fishing, and other changes, see the Ecopath modeling
tool at www.ecopath.org.
There is a strong link between Ecopath and FishBase, i.e., FishBase has
a special routine to assemble and print out information on the fish of a given
area or ecosystem, such that ecosystem models can be straightforwardly
constructed.
Task for the student:
· Use the diet composition data previously standardized (see section 3.7 Diversity of food and feeding habits, above) and calculate the trophic level implied by the diet, given the prey trophic levels (Table 1) of FishBase.
· Assemble diet composition studies for different sizes of the same species of fish, preferably in the same population, and show trophic level changes with ontogeny.
· Identify a marine ecosystem that interests you and run the FishBase routine which extracts information on the fish of that and similar (or adjacent) ecosystems. Use this information to draw a food web incorporating the diet information on major fish and invertebrates in that ecosystem.
Food web-related topics covered in FishBase:
| Food webs: | see Box 21 in http://www.fishbase.org/manual/FishbaseTROPHIC_ECOLOGY00002693.htm; see also Box 23 in www.fishbase.org/manual/FishbaseThe_ECOLOGY_Table.htm |
| Ecopath parameters: | Use the Information by topic search in www.fishbase.org/search.cfm and select the Ecopath parameters button to get a list of species ordered by habitat type and size. The list indicates for which of these species Ecopath-related parameters, i.e., growth parameters, Q/B, diet and predator information, are available. It is also possible to output the list as an Excel file as described in Chapter 3. |
| Trophic levels & catches: | From www.fishbase.org/search.cfm under Tools, click on link to Catch analysis. |
| Diets: | In the Species Summary page, click on links to Diet and Predators. [Note: links to Diet and Food consumption are also available in the Life-history tool page.] |
| To list species with available diet data in FishBase, use the Information by topic search and click on the Diet option. |