There have been a number of scientific reports on zander in England, the best I've found is this one:
http://www.clw.csiro.au/publications/controlling_carp.pdf
Although the title refers to Controlling Carp in Australia, pages 82 to 92 consider the effect of zander on English fish populations and the methods used to control them.
Here's the summary at the end:
"The establishment of zander populations in some canals has resulted in a
number of problems for fishery managers. At present, the addition of zander
does not improve the fishery value of waters in the UK as anglers
do not want to catch them. Furthermore, anglers claim that zander reduce
the abundance of other fish and advocate removal programs.
A review of the UK literature found that the impact of zander is not
known. Culling zander has been attempted but the evaluation of 18 years
data of harvesting a canal population suggests that culling at a low level
has little effect on population dynamics. Increasing culling intensity to the
1988–1991 level altered the population dynamics. It resulted in a decrease
in the average size of the zander, but did not affect total biomass.
However, as the size of prey eaten by zander is related to its size (Figure
6.5) this will have shifted predation pressure towards smaller prey. As the
type of prey consumed also depends on the size of zander (Figure 6.4), a
population of small zander would consume more cyprinids than a
population of larger ones.
Consequently, the reduction of the average size of zander by culling may
have exacerbated its impact on native cyprinid fish. As cyprinids form the
basis for the recreational fishery, culling zander may actually increase its impact."