Influence of Approach Velocity and Mesh Size on the Entrainment and Contact of a Lowland River Fish Assemblage at a Screened Irrigation Pump

Fish screens can help prevent the entrainment or injury of fish at irrigation diversions, but only when designed appropriately. Design criteria cannot simply be transferred between sites or pump systems and need to be developed using an evidence-based approach with the needs of local species in mind. Laboratory testing is typically used to quantify fish responses at intake screens, but often limits the number of species that can studied and creates artificial conditions not directly applicable to screens in the wild. In this study a field-based approach was used to assess the appropriateness of different screen design attributes for the protection of a lowland river fish assemblage at an experimental irrigation pump. Direct netting of entrained fish was used along with sonar technology to quantify the probability of screen contact for a Murray-Darling Basin (Australia) fish species. Two approach velocities (0.1 and 0.5 m.sec(−1)) and different sizes of woven mesh (5, 10 and 20 mm) were evaluated. Smaller fish (<150 mm) in the assemblage were significantly more susceptible to entrainment and screen contact, especially at higher approach velocities. Mesh size appeared to have little impact on screen contact and entrainment, suggesting that approach velocity rather than mesh size is likely to be the primary consideration when developing screens. Until the effects of screen contacts on injury and survival of these species are better understood, it is recommended that approach velocities not exceed 0.1 m.sec(−1) when the desire is to protect the largest range of species and size classes for lowland river fish assemblages in the Murray-Darling Basin. The field method tested proved to be a useful approach that could compliment laboratory studies to refine fish screen design and facilitate field validation

Mean (± S.E.) Cath per unit effort (number of fish/Megalitre) across different mesh and velocity treatments.

<p>Mean (± S.E.) Cath per unit effort (number of fish/Megalitre) across different mesh and velocity treatments.</p

Number of fish entrained within the experimental pump system.

<p>Catches are pooled within each velocity and mesh combination. The electrofishing/seine column demonstrates the composition and relative abundance of fish captured at all the experimental site using electrofishing and seine netting.</p

Predicted relationship of probability of screen contact with length and rheotaxis of fish (see methods for description of rheotactic behaviour).

<p>Solid lines are at 0.1 m.sec⁻¹ and broken lines are at 0.5 m.sec^−1.</p

Length frequency plot showing the size range of fish observed by sonar to make a) contact or b) avoid contact with the experimental screen.

<p>Length frequency plot showing the size range of fish observed by sonar to make a) contact or b) avoid contact with the experimental screen.</p

Length frequency plot for fish sampled a) by electrofishing and seine netting at pumping sites, and b) those collected after being entrained by the pump (all treatments pooled).

<p>Length frequency plot for fish sampled a) by electrofishing and seine netting at pumping sites, and b) those collected after being entrained by the pump (all treatments pooled).</p

Tabular representation of the two Latin square experimental designs showing the order of allocation of mesh treatments within replicate runs for each of two approach velocities.

<p>Tabular representation of the two Latin square experimental designs showing the order of allocation of mesh treatments within replicate runs for each of two approach velocities.</p

Odds Ratios of rheotactic categories compared to random orientation.

*<p>The odds ratio is the increase in the probability of contact when compared to random orientation. For example, fish showing positive rheotaxis at the 0.1 m/s velocity are 14×less likely to make contact (1÷0.07 = 14).</p>†<p>Significant (sig) or non-significant (ns) at the p = 0.05 level.</p

Location of the four study sites on the Namoi River showing the Murray-Darling Basin (grey thatched).

<p>Location of the four study sites on the Namoi River showing the Murray-Darling Basin (grey thatched).</p

Odds ratios for probability of screen contact for different mesh sizes when compared to the no mesh treatment at 0.1 m.sec^−1.

*<p>The odds ratio is the increase or decrease in the probability of contact when compared to the ‘no mesh’ treatment. For example, at 10 mm mesh size the probability of screen contact is 1÷0.62 = 1.61. Therefore contact is 61% less likely using the 10 mm mesh than the ‘no mesh’ treatment, however this was non-significantly (ns) different than a 1∶1 ratio at the p = 0.05 level.</p