Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 433

40

30

20

10

0

0 100 200 300 400 500

Distance from headwaters (km)

FIGURE 21.3 Number of fish species as a function of distance from headwaters in the Kali
Gandaki River, Nepal. Statistical analysis of the data suggests that the number of fish species
increases with river distance up to about 300 km (data from David Edds).

The diversity of prey species may also alter the diversity of piscivorous
species. For example, it is unlikely that the number of predatory species
will exceed that of prey species (Fig. 21.4). However, this is a weak rela-
tionship and water quality, habitat structure, and large-scale patterns of
species dispersal are stronger determinants of fish diversity.

PHYSIOLOGICAL ASPECTS INFLUENCING GROWTH, SURVIVAL,
AND REPRODUCTION

Energetics can be thought of as the overriding factor controlling growth,
survival, and reproduction of fishes. Environmental extremes can cause ex-
cessive energy drains affecting the success of fish. Survival requires a set
amount of energy that varies depending on the external environment.
Growth requires additional energy, and successful reproduction requires
the most energy (Fig. 21.5).
Osmoregulation requires energy and is necessary for survival. Salinity
must be an important determinant of fish survival because a significant
portion of the world's fishes are restricted to marine waters. Osmoregula-
tion mechanisms vary among freshwater fishes. The lower limit of a fish's
distribution in an estuary may be determined by its salt tolerance, as may
the response to runoff from road salt additions and hypersalinity in closed-
basin lakes.
All fishes that enter freshwater must osmoregulate because their inter-
nal salt concentrations exceed those found in most freshwaters. In general,
excess water is excreted by well-developed kidneys. Because water is not
limiting, up to one-third of the fish's body weight may be excreted each day
(Moyle and Cech, 1996). Diadromous species must have versatile os-
moregulation strategies to deal with the transitions from one salinity to an-
other. Fishes living in highly dilute waters may have ion-specific active trans-