Hydraulic burden and swimming behaviour of benthic fish in a vertical slot fish pass

The majority of the world’s rivers is fragmented and fish passes are frequently installed to enable fish
passage across river obstacles. Beside native, also invasive species, such as round goby, can use fish passes
to disperse upstream in uninvaded ecosystems. The present study aimed at evaluating a prototype hydraulic
barrier in a vertical slot fish pass that impedes passage of the invasive round goby and enables passage of
native, comparable species, such as gudgeon and bullhead. In addition, the study was designed to provide
basic insight in the differences between the hydraulic forces experienced by the fish species in the flow and
their individual behavioural response to different flow conditions.
The barrier was designed to homogenize the flow field over an extended distance to exceed the swimming
capability of the round goby and impede resting by station holding due to the smooth bottom. The selective
effect was created by the flow field that induced hydraulic burden, varying between species due to the
individual body shapes of the fish.
The performance of the hydraulic barrier was assessed using a three step-approach: Flow measurements
(Step 1), measurements of the hydraulic forces experienced by preserved fish (Step 2) and live fish
swimming behaviour observations in the same flow field (Step 3). This approach was applied in a flow
channel study under homogenised flow conditions and in a nearly full-scale vertical slot fish pass model.
The purpose of this approach was to evaluate the prototype hydraulic barrier (i), create basic evidence about
how flow affects the fish species individually and whether the hydraulic burden experienced in the flow
field differ between species (ii), and to assess how the hydraulic burden experienced by benthic fish affect
the passage behaviour across the prototype hydraulic barrier and at an unaffected vertical slot (iii).
The results showed: There was a species selective effect of the prototype hydraulic barrier. No round goby
passed the barrier at the highest water discharge tested (130 L/s), while six gudgeon passed the barrier and
four bullhead attempted passage but immediately returned. This passage behaviour agreed with the
hydraulic forces measured as gudgeon experienced significantly lower forces at 130 L/s water discharge
compared to the other species. Because the hydraulic forces differed between species and corresponded to
the live fish swimming behaviour, and the live fish passage behaviour differed between water discharges,
it is probable that the selective effect of the barrier was mediated by the created flow field. Beside this
selective effect, the prototype barrier had a general impact on the fish migration behaviour. There were
significantly less passages recorded at the prototype hydraulic barrier compared to the downstream
untreated slot.
The fish indeed experience hydraulic burden differing between species and these hydraulic burden
correspond to the individual swimming styles of the corresponding species. The fish respond to these
hydraulic burden individually, depending on their own species biology. Nevertheless, there was a general
adaptation of the passage behaviour to increased water discharge: All species swam faster with more speed
variation and with straighter paths upstream the barrier.
With providing basic evidence about the individual hydraulic burden an invasive species experiences in
comparison to two native, comparable species, this thesis is a further step towards species selective
fragmentation of rivers for ecosystem conservation purposes. These findings open an avenue to fish pass
design adapted to the fish community of specific ecosystems and will inform fish pass engineers, decision
makers and researchers who work about the behavioural response of fish to flowing water.