Environmental Engineering Reference
In-Depth Information
with 500 fish comprising roach, rudd (Scardinius erythrophthalmus), bream
(Abramis brama) and carp (Cyprinus carpio) of varying age classes being intro-
duced. The fish were monitored over a 12-month period for growth, diet,
condition and contamination with heavy metals. Contamination by heavy
metals released from the sediments was not marked in the stocked fish
(Hendry
1991
), though concentrations of zinc reached 138mg kg
1
in indigen-
ous stickleback (Hendry et al.
1997
). This finding corroborates the view that
maintaining an oxidised surface sediment layer segregates contaminants from
the overlying water, reducing availability to biota. High growth rates were
recorded, particularly of carp, roach and rudd. Due to the success of the pilot
stocking, 12 000 coarse fish were stocked into the Quays in November 1989.
Species added were similar to those in the trial, with the addition of chub
(Leuciscus cephalus) and dace (Leuciscus leuciscus) to increase diversity, and also
perch (Perca fluviatilis) principally as a predator. Stocking was deliberately
restricted to the comparatively low density of 100 kg ha
1
compared with a
typical carrying capacity of northern temperate lakes (300 kg ha
1
) to allow for
expansion of the population. Fish were recaptured during subsequent years to
enable assessment of their growth and condition. Growth of all fish was rapid
during residence in the Quays due to the abundance of food, and roach growth
rates were amongst the highest in the UK (Hendry et al.
1997
). A Fulton Condi-
tion Factor (Fulton
1911
) of 1.65 was calculated for fish in the enclosed basins,
which is considered to compare favourably with normal populations elsewhere
(White et al.
1993
).
The prevailing homogeneity at the Quays, originating from its use as a port
(deep basins, vertical walls and featureless sediment), was considered to
severely limit the overall ecological stability of the system (Hendry et al.
1997
). A programme of habitat diversification was therefore devised, with the
aim of creating a more complex food web, promote colonisation and provide
refuges and spawning substrates. Artificial structures intended to provide
additional habitat and spawning substrates included tyre reefs, frayed ropes
and discarded Christmas trees; the latter to mimic fronds of macrophytes.
Submerged raised platforms were installed containing a number of aquatic
macrophytes including bullrush (Scirpus albescens) and the common water lily
(Nymphaea alba). The success of the introduction of macrophytes inspired fur-
ther initiatives, including the use of submerged gabions to provide a base for
additional macrophytes. In subsequent years, floating islands were introduced,
planted with a combination of species including yellow iris (Iris pseudocorus),
common reed (Phragmites australis) and purple-loosestrife (Lythrum salicaria).
These efforts to increase heterogeneity were necessary at a time when the
prospect of natural colonisation by macrophytes was not considered a possibil-
ity, principally due to the basin depth. In recent years however water clarity
within the Quays has increased rapidly and to a degree that has facilitated the
