Fish-Friendly Flood Pumps Compared: Bosman Vision vs. Axial Flow vs. Archimedes Screw Pumps

Written by Prime Fluid Management | 14 May 2025

With fish passage now a legal, ecological, and cultural requirement for New Zealand’s water infrastructure, selecting the right stormwater pump means thinking beyond flow rates. Engineers, asset managers, and civil contractors are increasingly being asked to balance hydraulic performance with fish safety, whole-of-life cost, recyclability, and compliance with NES-F and iwi values.

Here’s how three leading fish-friendly pumps stack up: Bosman Vision vs. Axial Flow vs. Archimedes Screw Pumps.

1. Hydraulic Efficiency & Performance ⚙️

Pump	Hydraulic Efficiency	Head (Lift Capacity)	Operating Range	Scalability
Bosman Vision Series (MC & CV)	Up to 85% efficiency	Up to 8m head	Wide, cavitation-free operating range, no pilot pump required	Easily retrofittable with higher-powered motor for future flow needs
Axial Flow	~75% efficiency	~5m head	Suitable for traditional low-head, high-volume flood control	Limited scalability; fish-friendly impeller in development
Archimedes Screw	Unknown	Limited to low-head lift (<2–3m)	Constant-speed operation; not designed for head pressure	Seismic constraints. Suited to lifting water within catchments as discharge over stop-banks not ideal.

Bosman Vision pumps are designed in direct response to fish-friendly requirements, with verified mortality rates under 2% and fish-safe configurations that align with regulatory and cultural expectations. This is supported by field studies in Europe, conducted independently and without Bosman’s knowledge and made available after the studies’ completion.

2. Fish Passage, Guidelines & Impeller Design 🐟

Pump	Fish Mortality Rate	Impeller Design	Species & Sizes Tested
Bosman Vision Series (MC & CV)	<2% (independently verified)	Mixed-flow screw impeller with one leading-edge blade	European eel (Anguilla anguilla) 200–1030 mm, roach, bream, perch, pike
Axial Flow	In-market (NZ). Low mortality reported	Dual leading-edge axial impeller	NZ longfin/shortfin eel testing scheduled for 2025
Archimedes Screw	Low mortality rates	Slow-turning screw impeller	In-house trials on eel, smelt, whitebait-sized species up to 300mm, longfin eel up to 1600mm

Fish-Friendly Pumps NZ - Parameters Affecting Fish Mortality

Fish mortality when passing through pumps is influenced by factors related to both pump design and the species of fish. The main factors include:

Pump Passage: The open passage for objects to pass through without contacting any moving or stationary parts of the pump.
Pump Speed: Higher speeds can create greater barometric pressure differentials and increase the velocity of the impeller blades, leading to impeller strikes, the most common cause of fish mortality.
Number of Blades: Fewer blades reduce the likelihood of impeller strikes, making the pump more fish-friendly.
Capacity: High-capacity pumps (>500L/s) may be more fish-friendly than smaller ones.
Head: The pump’s designed manometric pressure impacts the barometric pressure within the pump.
Species: Fish species vary in size and shape, with certain species (e.g., eels) more vulnerable to injury.

Fish-friendly pumps have significantly reduced mortality rates by implementing slower rotation speeds, larger passages, and specialised blade designs that minimise physical contact with aquatic life while maintaining efficient water movement.

What the NZ Fish Passage Guidelines Say:

From the NZ Fish Passage Guidelines v2.0 (2024):

All instream structures must maintain or improve fish passage (Section 2.3, NPS-FM).
Flood pump stations should avoid creating barriers for migratory species such as tuna (eels), īnanga, and smelt (Section 4.9.1).
A recommended fish mortality rate is <2% for taonga species (Section 3.2).
Design considerations include:
- Impeller strike reduction: Fewer leading-edge blades preferred.
- Safe swimming velocities: 0.6–1.2 m/s for adult tuna.
- Lubrication: Oil-free/water-lubricated bearings preferred to avoid contamination (Section 5.5.8).

3. Environmental & Cultural Fit 🌱

Pump	Lubrication	Construction Material	Recyclability	Waterway Risk
Bosman Vision MC & CV	Water-lubricated	Cast steel	Fully recyclable	None
Axial Flow	Grease-lubricated	Cast steel	Fully recyclable	Grease may contaminate waterways
Archimedes Screw	Grease-lubricated	GRP (Glass Reinforced Plastic)	Not recyclable	Moderate contamination risk

Seismic Safety: Screw pumps require heavy concrete works, posing risk in liquefaction-prone sites. Bosman’s lightweight, containerised system is better suited for seismic regions compared to FishFlow Innovations or MacEwans pumps.

4. Construction, Transport & Maintenance

Pump	Footprint	Construction Cost	Transport	Maintenance
Bosman Vision Series	Small, modular	Minimal groundwork; no forebay required	Containerised; easy to freight	Low OPEX; fewer moving parts
MacEwans Axial Flow	Traditional layout	Moderate	Local NZ support	Medium OPEX; grease servicing required
Archimedes Screw	Large civil footprint	High	Bulky site-dependent	Unknown

Summary: Choosing the Right Fish-Friendly Pumps NZ

Scenario	Best Option
Retrofit with future-proofing	Bosman Vision MC
New-build station	Bosman Vision MC &CV
Low-head ecological lift	Archimedes Screw
Budget upgrade, basic compliance	Axial Flow
High cultural/legislative scrutiny	Bosman Vision MC or CV

About Prime Fluid Management

Prime Fluid Management has worked alongside New Zealand’s civil and municipal sectors for more than 20 years, providing reliable fluid pumping and treatment systems both in a temporary (emergency response) and permanent capacity.

Our experience spans both new-build and retrofit installations, including sites with challenging environmental, cultural, or seismic considerations. We support engineers, councils, and contractors with practical, fit-for-purpose advice and technologies that meet today’s ecological expectations - around fish passage, energy efficiency, and whole-of-life value.

We supply and support a wide range of fish-friendly and flood-resilient pumping systems, including the Bosman Vision Series and other future-ready solutions.

Learn more about our stormwater work

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