Three rivers and a fish: body shape variation in adult inanga (Galaxias maculatus) and implications for reproductive output
Authors:   , Wood Vincent 
Published: November 2021

<p>Inanga (Galaxias maculatus) are the major component of New Zealand’s whitebait fisheries. Monitoring of freshwater-based adult populations in both North and South Island river systems suggests a general decline in numbers of fish, and specific streams and watersheds are of particular concern for some regional councils. Given this context, improved information about the environmental conditions that influence inanga’s reproductive output may help to inform appropriate management actions and improve the long term viability of these populations. The breeding biology of adult inanga has been the focus of considerable research effort, with restoration of spawning grounds further extending our knowledge of spatio-temporal patterns of spawning. However, the behaviour patterns and fates of adult fish outside of the spawning locations and seasons are poorly known, as are the factors that may influence the survival and development of pre-spawning stages of inanga. Variation in body shape and size influences multiple performance and fitness attributes, and has major implications for reproduction. My thesis aims to quantify relationships between morphology and reproductive output for adult inanga, and to investigate environmental factors that may influence morphological development. I collected inanga at two discrete stages of adult development: (i) pre-spawning stage adults at upstream habitats and (ii) spawning-stage adults collected at known breeding grounds during the spawning season. I photographed inanga in the field using a purpose-built aquarium and measured a set of morphological characteristics related to reproductive output and swimming ability. Specifically, I measured standard length, head depth, body depth and caudal peduncle depth. A subsample of adult inanga collected during the spawning season were returned to the laboratory and euthanized to measure aspects of their reproductive biology. Specifically, I measured reproductive output using gonad weight, I estimated maturity using the gonadosomatic index (GSI: weight of the gonad relative to total body weight), and I estimated energetic reserves using the hepatosomatic index (HSI: weight of the liver relative to total body weight). Pre-spawning stage inanga in the Waiwhetu Stream displayed deeper bodies than fish in either the Hutt or Wainuiomata Rivers, potentially due to higher concentrations of food. Inanga in the Wainuiomata River displayed slightly larger sizes prior to the spawning season and substantially larger bodies during the spawning season. Although the Waiwhetu Stream may have had a greater supply of food, high densities of fish in smaller stream systems could constrain adult growth. Spawning-stage adults collected from the Waiwhetu Stream had greater reproductive output than fish collected from the Hutt River, with a peak in spawning activity during May. Fish collected from spawning grounds in the Hutt River had lower reproductive output and two peaks in spawning activity during March and May. Inanga in the Hutt River also displayed greater within-month variation in the maturity of fish. My results suggest that spawning grounds in larger, more complex river systems (e.g., the Hutt River) may be supplied by inanga from a diverse range of main stem habitats and smaller tributaries further inland. Smaller systems (e.g., the Waiwhetu Stream) may be comprised of a more homogeneous population of inanga, and reproductive output of the system as a whole may be greater, but concentrated over a shorter time period. I hypothesise that the reproductive output from larger river systems may be more resilient to disturbance events (e.g., stock trampling of spawning grounds) because these systems (by virtue of their greater diversity of habitats and phenotypes of fish) may enable multiple opportunities for spawning. I suggest that larger rivers, such as the Hutt River, may be of disproportionate importance (independent of their total reproductive output) for the replenishment of inanga stocks.</p>