Survey method for locating īnanga spawning sites at a catchment scale
1 Waterways Centre for Freshwater Management (WCFM), University of Canterbury and Lincoln University.
2 Marine Ecology Research Group (MERG), University of Canterbury.
Waterways and MERG have recently published details of a survey methodology for locating and
mapping īnanga spawning sites near coastal rivermouths.
The full paper in NZ Journal of Marine and Freshwater Research is available online.
Īnanga (Galaxias maculatus) is one of five galaxiid species that provide New Zealand’s ‘whitebait’
fishery. Four of the five species are also threatened to various degrees. The management of whitebait
species is therefore important for conservation objectives as well as for maintaining the fishery. Aside
from regulations on fishing activities (which target the juvenile fish on their migration from the ocean
into freshwater systems) the degradation of habitat is a particular focus. Although all life stages are
important, the protection of spawning habitat may be particularly important for īnanga. It is found in
very specific places on the riparian margins of coastal waterways. These areas have become degraded
by historical activities associated with urbanisation and other intensive land uses near waterways.
These changes have the potential to dramatically impact on fish populations since the inanga life cycle
is relatively short (1 -2 years). Poor spawning success in one season can be expected to reduce the
stock of whitebait and adult populations in the next season. Repeat cycles of this may very quickly
diminish fish stocks - especially if it occurs at many spawning sites simultaneously. In reality, some
rivers have become ‘sinks’ that contribute little to maintaining stocks each year1. The fish population is
reliant on other rivers (the ‘sources’) where spawning success is higher.
Protecting and enhancing spawning habitat is therefore a very practical activity that can assist with
whitebait conservation, and tools such as spatial planning and the use of protected areas offer
promising ways to address the problem. However, locating where the spawning sites are is not
straightforward and this has become a barrier to practical action. The location and extent of the
spawning habitat requirements in different rivers is essential information for the design of effective
strategies. In a nutshell, we need to know which areas are important to protect, and whether additional
protection (or restoration) would be beneficial. However, because rivermouth systems are dynamic, the
location and extent of spawning sites can change over time. This introduces some complexities for
‘nailing down’ the answers that are needed to inform effective conservation strategies.
Overview of the census survey method
To help address the above problem, we developed a survey methodology to detect all of the spawning in
a catchment. Reliably capturing this information over time is very useful for identifying trends. Different
forms of īnanga spawning surveys have been carried out in New Zealand for several decades and our
approach was to adapt the existing methods to achieve the ‘catchment scale census’ we were after.
The main components of our methodology are:
an initial investigation to characterise the big picture and set the survey limits. This uses a
different approach to existing methods which typically focus on the location of the ‘salt water
a habitat quality assessment step similar to that used in an Australian study2, but guided by set
criteria. The objective of this step is to identify all areas that could potentially support spawning
in the catchment.
intensive searches of riparian vegetation in the areas identified via the above steps to detect
spawning sites3, but using standardised time frames that are repeated over several months.
The latter is important to address spatiotemporal variation that arises at several scales and can
potentially confound the interpretation of ‘snapshot’ surveys. Examples of major confounding
factors that we have addressed are differences in spawning activity between months, and the
effect of egg mortality on the riverbank (meaning that actual spawning sites may be difficult to
discover in searches that are looking for eggs).
Example of the method in action
In the paper we include details of how the census survey was
applied to locate spawning sites in the Heathcote/Ōpāwaho
catchment after the Canterbury earthquakes. We were
especially interested in whether they might have shifted
location in the catchment. At the start of the study the post-
quake pattern was initially unknown (similar to a river had never
been surveyed) but in this case the river has been well-studied
over several decades. By comparing our results with earlier
īnanga spawning surveys we were able to show that the
downstream limit shifted considerably following the
earthquakes. Because many current riparian management
activities are not compatible with protecting spawning habitat
this is important information for conservation4. Other aspects
such as trends in areas that are the most used or produce the
most eggs could be resolved by repeat surveys. Another
interesting finding was the discovery of high egg numbers at
sites where reed canary grass (Phalaris arundinaceae) was the dominant vegetation. Although this
invasive species has previously been regarded as a threat to spawning habitat it currently supports the
majority of spawning in the Heathcote/Ōpāwaho. Together, these examples show some of the useful
outcomes that may be generated by a comprehensive survey methodology to reliably quantify īnanga
spawning habitat at a catchment scale. The census-survey approach is applicable to rivers nationwide
and will help to address the current information gaps around where spawning occurs and what is
needed to protect it.
Link to article in NZ Journal of Marine and Freshwater Research here
Thanks to the Ngāi Tahu Research Centre, IPENZ Rivers Group, MBIE grant C01X1002 in conjunction with NIWA,
and the many volunteers that helped to support this work.
1 Hickford MJH, Schiel DR. (2011). Population sinks resulting from degraded habitats of an obligate life-history
pathway. Oecologia 166: 131-140.
2 Hicks A, Barbee NC, Swearer SE, Downes BJ. (2010). Estuarine geomorphology and low salinity requirement for
fertilisation influence spawning site location in the diadromous fish, Galaxias maculatus. Marine and
Freshwater Research 61: 1252-1258.
3 Mitchell CP, Eldon GA. (1991). How to locate and protect whitebait spawning grounds. Rotorua: Freshwater
Fisheries Centre. 49pp.
4 Orchard S, Hickford M. (2016). Spatial effects of the Canterbury earthquakes on īnanga spawning habitat and
implications for waterways management. Report prepared for IPENZ Rivers Group and Ngāi Tahu Research
Centre Waterways Centre for Freshwater Management and Marine Ecology Research Group. Christchurch:
University of Canterbury. 37pp.