Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 437

The rate at which food must be consumed to allow for growth and re-
production depends on both the requirements imposed by physiological de-
mands of the environment and the actual energy investment in obtaining
food. Under stressful conditions, a greater portion of the food ingested
must be used to maintain basic metabolism, and less must be used for
growth. Determining actual energy requirements can be very complex. For
example, in a heterogeneous environment, metabolic costs may vary over
space and time, as can the amount of energy required to locate and cap-
ture prey. Consider, for instance, the idea that foraging is more difficult
when a predator is present. Ample food may be available in the environ-
ment, but when a predator is nearby, a fish must use energy to avoid be-
ing eaten (Milinski, 1993) and balance that against its need to forage for
food (Fig. 21.8).

POPULATION DYNAMICS OF FISHES

Fish population dynamics can be determined by several key character-
istics, including the size of the population (stock) and the population
growth (production). The population can be divided into a number of age
classes or size classes. The number of new fish entering each size or age
class (recruitment) and the number of fish that are lost between each class
(mortality) are also related to population dynamics. Also, the number of
offspring that reproductive females produce (fecundity) can influence pop-
ulation size.

1.8

No predator
With predator

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

14 16 18 20 22 24 26 28

Distance from predator enclosure (cm)

FIGURE 21.8 Effect of predator (Oreochromis mariae) on the rate at which a stickleback
(Gasterosteus aculeatus) consumes prey (Tubifex). This experiment included an adjacent fish
cage that could hold the predator, and worms were placed in tubes at various distances from
the adjacent cage. The stickleback had to enter the tube and lose sight of the predator to take
a worm. With no predator, the worms were taken from all tubes equally. With the predator,
the sticklebacks preferred to feed as far away from the predator as possible (modified from
Milinski, 1993).