13.1. Introduction
259
agricultural sectors have all failed in their respective responsibilities to
steward a healthy ecosystem. To this account, both the European Union
and the World Bank have restoration and assistance programs in place (to
the toll of approximately 20 and 80 million dollars US, respectively) .
13.1.2. The Past Twenty Years
Despite the great losses in functional and species diversity at least partially
attributable to Nile perch introduction, the Nile perch fishery is still considered a savior by many . Without a doubt some of the people and the economy of East Africa have benefited throughout the past twenty years from
the establishment of the fishery. But the boom days of past are now gone.
The initial exponential growth of Nile perch population was built upon the
process of converting haplochromine biomass into Nile perch flesh. During
the explosion, L. niloticus grew considerably faster in Lake Victoria than in
its natural habitat (Hughes 1992). Now that the haplochromines have fallen
to levels that are nearly insignificant bioenergetically, growth rates and
length-weight relationships of Lates have decreased to more closely resemble their values in its indigenous habitat (Hughes 1992; Ogutu-Ohwayo
1994; Ogutu-Ohwayo 1999). Nile perch, now feeding primarily on R. argentea, invertebrates, juvenile Nile perch, and some O. niloticus (Hughes
1986; Ogari and Dadzie 1988; Ogutu-Ohwayo 1990), are far less productive
in Lake Victoria today than they were years ago.
Fish bioenergetics theory requires that a decrease in growth rates be explained by 1) an increase in either excretion, egestion, and /or respiration,
or 2) a decrease in consumption rates (Hewett and Johnson 1992; Jobling
1994; Kitchell et al. 1974; Kitchell et al. 1997; Kitchell et al. 1977). At a constant temperature for a given weight fish exposed to a specific prey community, consumption rates are a function of species and individual-specific
physiological and behavioral parameters (Hyatt 1979; Jobling 1994; Peter
1979), and are thus not likely to vary significantly so long as the environment remains more or less constant. Instead, feeding rates can be considered a function of predator-prey ratios (Leslie 1948), and various system
specific factors such as prey refugia (Huffaker 1958), search images (Tinbergen 1969), and some set of constants (Holling 1959). These driving variables, coupled with the prey's response to predation, will determine the
functional response of a predator, and thus the free energy for growth for a
given community structure . So we believe that the decrease in productivity
of Nile perch in Lake Victoria has been caused by a relative increase in active metabolism due to prey community changes.
In other words , it is unlikely that Nile perch in Lake Victoria have suddenly
lost their appetite. Mass balance theory would suggest that Nile perch now
expend more energy per unit of energy gained capturing R. argentea,
C. niloticus, and juvenile L. niloticus in Lake Victoria today than they did when
feeding on haplochromine cichlids. Energy that should be accumulating in
259
agricultural sectors have all failed in their respective responsibilities to
steward a healthy ecosystem. To this account, both the European Union
and the World Bank have restoration and assistance programs in place (to
the toll of approximately 20 and 80 million dollars US, respectively) .
13.1.2. The Past Twenty Years
Despite the great losses in functional and species diversity at least partially
attributable to Nile perch introduction, the Nile perch fishery is still considered a savior by many . Without a doubt some of the people and the economy of East Africa have benefited throughout the past twenty years from
the establishment of the fishery. But the boom days of past are now gone.
The initial exponential growth of Nile perch population was built upon the
process of converting haplochromine biomass into Nile perch flesh. During
the explosion, L. niloticus grew considerably faster in Lake Victoria than in
its natural habitat (Hughes 1992). Now that the haplochromines have fallen
to levels that are nearly insignificant bioenergetically, growth rates and
length-weight relationships of Lates have decreased to more closely resemble their values in its indigenous habitat (Hughes 1992; Ogutu-Ohwayo
1994; Ogutu-Ohwayo 1999). Nile perch, now feeding primarily on R. argentea, invertebrates, juvenile Nile perch, and some O. niloticus (Hughes
1986; Ogari and Dadzie 1988; Ogutu-Ohwayo 1990), are far less productive
in Lake Victoria today than they were years ago.
Fish bioenergetics theory requires that a decrease in growth rates be explained by 1) an increase in either excretion, egestion, and /or respiration,
or 2) a decrease in consumption rates (Hewett and Johnson 1992; Jobling
1994; Kitchell et al. 1974; Kitchell et al. 1997; Kitchell et al. 1977). At a constant temperature for a given weight fish exposed to a specific prey community, consumption rates are a function of species and individual-specific
physiological and behavioral parameters (Hyatt 1979; Jobling 1994; Peter
1979), and are thus not likely to vary significantly so long as the environment remains more or less constant. Instead, feeding rates can be considered a function of predator-prey ratios (Leslie 1948), and various system
specific factors such as prey refugia (Huffaker 1958), search images (Tinbergen 1969), and some set of constants (Holling 1959). These driving variables, coupled with the prey's response to predation, will determine the
functional response of a predator, and thus the free energy for growth for a
given community structure . So we believe that the decrease in productivity
of Nile perch in Lake Victoria has been caused by a relative increase in active metabolism due to prey community changes.
In other words , it is unlikely that Nile perch in Lake Victoria have suddenly
lost their appetite. Mass balance theory would suggest that Nile perch now
expend more energy per unit of energy gained capturing R. argentea,
C. niloticus, and juvenile L. niloticus in Lake Victoria today than they did when
feeding on haplochromine cichlids. Energy that should be accumulating in