The head section of this table, defining habitats, is straightforward, and consists mainly of yes/no choice fields indicating the environmental preferences of a species. The categorical breakdown of possible habitats summarizes the otherwise lengthy text descriptions for species found in different types of environment, and makes such descriptive data accessible to rigorous queries.
Three major habitat types are considered here, i.e., freshwater, brackish water and saltwater. Freshwater bodies are represented through yes/no fields, i.e., streams, lakes and caves. The last field in this row is appended to caves and is ticked ‘yes’ when the species in question is an exclusive cave dweller. Brackish water bodies are lumped together under the field estuaries/lagoons/brackish seas, which include (estuarine) river mouths. The final category, saltwater, is divided between the inshore (intertidal) and offshore (marine) zones and is further categorized by a choice field indicating latitudinal temperature zones. Further subdivisions refer to the type of substrate in the intertidal zone, i.e., soft (sandy, muddy, silty) and rocky shores. Saltwater bodies are categorized, with respect to the continental shelf, into oceanic, neritic and coral reefs with substrates specified as soft bottom (sandy, muddy, silty), hard bottom (rocky), sea grass and macrophyte beds.
We are not very satisfied with these classifications, which seem simple enough, but are still complex enough to have precluded clear choices for many species. We would appreciate suggestions for simpler, yet more rigorous approaches for classifying aquatic habitats.
The next section presents general information on the feeding habits of fish.
Feeding Type is a choice field whose three categories give a general idea of the trophic level occupied by a species within a food web (see also Box 22). Thus, a primary consumer which consumes ‘mainly plant/detritus’ (herbivores) may have values of trophic level between 2.0 and 2.19; secondary, tertiary, etc. consumers which consume ‘mainly animals’ (carnivores) may have trophic levels equal to or greater than 2.8; and fish which are partly herbivore and partly carnivore, i.e., omnivores which consume ‘plants/detritus + animals’ may have trophic levels between 2.2 and 2.79.
Feeding habit is a choice field which describes the feeding habits of fish occupying various zones along the water column. Most pelagic species are either predators ‘hunting macrofauna’ throughout the water column, ‘filtering plankton’ as they swim near the water surface, or selectively grazing on plankton (‘selective plankton feeding’).
The ECOLOGY table uses a multiple-choice field to define broadly the trophic niche of fishes, with herbivory being equated to one of the choices, i.e., for fishes consuming ‘mainly plants/detritus’. Similarly, a value of near two (i.e., troph - 2 s.e.
This allowed construction of a FishBase plot of % herbivorous fishes vs. latitude (Fig. 34), i.e., to make accessible in visual form the fact that herbivorous fish species tend to be far more frequent in low than in high latitudes, although their overall percentage among all fishes is small (>1.1%). Both of these phenomena can be explained by the difficulties most fish have in establishing and maintaining, throughout and subsequent to a feeding bout, the low pH levels required for digestion of plant material, especially at low temperatures.
The ‘>’ symbol used above refers to the fact that: (1) not all species have Ecology records; (2) 4% of the more than 4,000 species with Ecology records do not have feeding type information; and (3) that non-herbivorous feeding habits are used as default for species without records.. Still, we expect, when this field is completed for all species, that the overall number of herbivorous species will remain under 2%, and the shape of the graph unchanged, i.e., with a bulge at low latitudes.
Daniel Pauly
Another important attribute of fish, included in the ECOLOGY table is their trophic levels (here abbreviated ‘Troph’, which defines their position within a food web (see Box 23). Trophs can be estimated using various methods. The ECOLOGY table accounts for this by having two fields for entries of trophs and their standard errors (s.e.): one from the DIET COMPOSITION table and the other from the FOOD ITEMS table (see Box 25). In both cases, the troph estimates are either the single value that is currently available or the median number of values available from several studies or localities. The troph estimates in the ECOLOGY table pertain to juvenile/adults or adults unless otherwise noted. A graph (Fig. 35) can be called to show the relationship, among fish species, of their median troph vs. their maximum length.
Trophic levels (here abbreviated to ‘troph’, to avoid overlap with ‘TL’, used for total length), express where fish and other organisms tend to operate in their respective food webs.
Unlike counts of dorsal fin rays, trophs are not attributes of the organisms for which feeding is being categorized, but of their interactions with other organisms. Thus, to estimate the trophs of fish, we must consider both their diet composition, and the trophs of their food item(s). The troph of a given group of fish (individuals, population, species) is then estimated from
where the mean is weighted by the contribution of the different food items.
Following a convention established in the 1960s by the International Biological Program, we attribute primary producers and detritus (including associated bacteria) a definitional troph of 1 (Mathews 1993).
Thus, for example, an anchovy whose diet would consist of 50% phytoplankton (troph = 1) and 50% herbivorous zooplankton (troph = 2) would have a troph of 2.5. The last value is an estimated, fractional troph, differing conceptually and numerically from the integer values that are often assumed for higher trophs, and which we think are too imprecise and inaccurate to be useful in any kind of analyses.
An omnivore is a "species which feeds on more than one trophic level" (Pimm 1982). Thus, an omnivory index (O.I.) can be derived from the variance of the trophs of a consumer’s food groups. The O.I. takes values of zero when all feeding occurs at the same troph, and increases with the variety of food items’ trophs.
Routines for estimation of trophs and O.I. values are incorporated in the Ecopath software, which has been applied to a large number of ecosystems (see Pauly and Christensen 1995; Pauly et al. 1998 and Box 21). Troph estimates from Ecopath have been found to correlate closely with troph estimates based on stable isotope ratios (Kline and Pauly 1998).
This has led to numerous troph estimates for a wide range of taxa becoming available, notably for the invertebrates, fish, marine mammals and other groups covered by FAO statistics, and now included in FishBase.
The diet compositions given, within FishBase, for many species of fishes, also allow the estimation of trophs. The trophs of the preys required for such computation are given in a sub-table of the FOOD ITEMS table.
It is anticipated that analyses based on the trophs incorporated in FishBase will tend to combine estimates from a number of groups (as e.g., in the analyses which led to Fig. 4), so that inaccuracies on some estimates will be compensated for by inaccuracies with opposite signs, related to other groups. For more rigorous approaches to uncertainties, standard errors are also attached to most estimates of trophs, based on s.e. = SQR (O.I.), where O.I. is the omnivory index presented above.
References
Kline, T. and D. Pauly. 1998. Cross-validation of trophic level estimates from a mass-balance model of Prince William Sound using 15N
/14N
data. In Fishery stock assessment models. Alaska Sea Grant College Program. AK-SG-
Mathews, C.P. 1993. Productivity and energy flows at all trophic levels in the River Thames, England: Mark 2, p. 161-171. In V. Christensen and D. Pauly (eds.) Trophic models of aquatic ecosystems. ICLARM Conf. Proc. 26. 390 p.
Pauly, D. and V. Christensen. 1995. Primary production required to sustain global fisheries. Nature 374:255-257.
Pauly, D., V. Christensen, J. Dalsgaard, R. Froese and F. Torres, Jr. 1998. Fishing down the food webs. Science 279:860-863.
2
Pimm, S. 1982. Food webs. Chapman and Hall, London and New York. 219 p.
Daniel Pauly and Villy Christensen
You get to the ECOLOGY table by clicking on the Ecology button in the SPECIES window. You get to the graph of troph vs. length among species from (1) the ECOLOGY window; or (2) by clicking on the Reports button in the FishBase Main Menu, on the Graphs button in the PREDEFINED REPORTS Menu, and on the Trophic ecology button in the GRAPHS Menu.
You get to the graph of herbivory vs. latitude by clicking on the Reports button in the FishBase Main Menu, on the Graphs button in the PREDEFINED REPORTS Menu, and on the Trophic ecology button in the GRAPHS Menu.
On the Internet, you find the ECOLOGY table if you click on the respective link in the ‘More information’ section of the ‘Species Summary’ page. Main food and Trophic level are also shown in the ‘Key Facts’ page available from the ‘More information’ section. You can create a list of all species with ecology data if you select the respective radio button in the ‘Information by Topic’ section of the ‘Search FishBase’ page.
Maria Lourdes D. Palomares
Box 22. Herbivory as a low-latitude phenomenon.
Fig. 34. Percentage of herbivorous species of Cichlidae and of other fish, by latitude. See Box 22 for a
discussion of this graph.
Box 23. Trophic levels of fishes.
98-01.
Fig. 35. Relationship between trophic levels and maximum length of fish species. Note positive slope, indicating that larger species tend to be more piscivorous than smaller species.
How to get there
Internet
