The effects of human-induced river fragmentation on the migrations of diadromous fish is well
documented in the literature, though research has focused on the effects of large-scale
anthropogenic drainage structures on strong swimming salmonids, whilst the impacts of small-scale
structures on weaker swimming fish is less well known. Despite being the most prevalent and widely
distributed whitebait species of the Galaxiidae family, and having significant recreational,
commercial and cultural importance in New Zealand, populations of the small-bodied inanga
(Galaxias maculatus) are in decline. The amphidromous migrations of juvenile inanga require
unrestricted passage into upstream habitats to feed, sexually mature and spawn, rendering them
highly vulnerable to potential barriers like bridge aprons, culverts, flood gates and pumping stations.

The aims of this study were to describe and evaluate the impacts of a range of small-scale
anthropogenic structures and associated environmental variables on the abundance and the
diversity of sympatric fish communities along 16 lowland waterways across the Hawke’s Bay region
of New Zealand, so that migratory barriers can be identified and remediations can be implemented.

Up to six fine meshed fyke nets were set above (upstream) and below (downstream) various
drainage structures along each waterway, and the abundance of all caught individuals were counted,
taking into consideration any potential ethical and sampling issues. Generalised linear models were
used to statistically analyse the impact of each structure on the abundance of fish, whilst Shannon
Weiner, Pielou’s evenness and species richness indices assessed the impact of each structure on the
diversity of fish communities. Associated environmental measures were also recorded and were
statistically analysed using mixed effect models to determine their effect on fish abundances.

The results of this study found that a bridge apron, a culvert and a ‘Fish-Friendly Flood Gate’ (FFFG)
did not act as barriers to upstream migrating inanga. The structures also did not restrict the
upstream passage of various other migrating and resident fish as a higher total abundance of all
species was found upstream, and biodiversity indices were similar above and below the structures.
In contrast, this study found that flood gates and pumping stations do act as full migratory barriers
to inanga and other native fish, including the catadromous grey and yellow-eye mullet. Although the
total abundance of all species was higher above these structures, the biodiversity was lower, as fish
communities mainly comprised of the invasive resident mosquitofish, or the environmentally
tolerant New Zealand freshwater shrimp.

The abundance of inanga and the total abundance of all species were also impacted by varying
environmental factors, although the differences varied according to the type of structure present
and whether the structure acted as a migratory barrier. Inanga abundances significantly increased
with increasing water temperature, though significantly decreased with increasing dissolved oxygen
(DO) saturation, whilst increasing specific conductivity (SPC) did not have an effect on inanga
abundances. The total abundance of all species significantly increased with increasing temperature
and SPC, whilst abundances were found to decrease with increasing DO saturations.

The findings of this study can be used to prioritise the mitigation or retrofitting of barrier structures
on the site-specific level. They can also be applied on local and national levels so that comparable
small-scale anthropogenic structures can be remediated to allow unrestricted upstream fish
passage, with the aim of increasing native fish abundances and biodiversities, improved upstream
habitat quality and the maintenance of healthy aquatic ecosystems.