Investigating Galaxias postvectis (Shortjaw Kōkopu) area of occupancy, mesohabitat selection, and spawning behaviour in the Waipoua River catchment, western Northland, New Zealand

Abstract

<p dir="ltr"><b><i>Galaxias postvectis</i></b><b> is a threatened freshwater fish species endemic to New Zealand. Numerous factors such as habitat modification, deforestation, and invasive species threaten their persistence. Cryptic, nocturnal behaviors and their rarity means ecological and biological knowledge is limited. Identifying areas of high species density and important habitat for spawning and adult growth is crucial for informing conservation management. Previous research has identified numerous habitat variables that characterise </b><b><i>G. postvectis</i></b><b> habitat. However, these studies are limited to river systems in the South Island and Southern regions of the North Island. Furthermore, spawning habitat for adult fish has been identified only once. In 2002, 12 spawning sites were found in the Katikara stream in Taranaki. Since then, no further sites have been identified and spawning is yet to be observed. Given current knowledge of spawning is based on a paucity of data it is possible that variation in the timing, cues and site selection could exist across New Zealand. This study contributes to understanding of the spawning behaviour and habitat preferences for </b><b><i>G. postvectis</i></b><b> through population monitoring and habitat analysis in the Waipoua Catchment in Northland.</b></p><p dir="ltr">In chapter two, I examined meso- and macro-habitat preferences of <i>G. postvectis</i>. From 2019 to 2023, 148 sites were surveyed. Data on the species’ occupancy, distribution and habitat were collected. At each site, fish were identified by spotlight survey methods. A total of 247 adult <i>G. postvectis</i> were recorded. Other native fish such as <i>G. brevipinnis</i>, <i>G. fasciatus</i> and <i>Anguilla dieffenbachii</i> were also frequently observed in reaches where <i>G. postvectis</i> were present. Macro-scale habitat variables were obtained from the High-Resolution Digital River Network for Northland, a GIS-based river network model derived from regional LiDAR. 12 of the survey sites were analysed further to investigate meso-habitat relationships. These 12 sites were 100 m in length and divided into five evenly spaced transects. Meso-habitat characteristics were recorded at each transect. A single benthic macroinvertebrate sample was also collected from each of these 12 sites. Statistical analyses revealed the stream’s mean slope, mean elevation and distance downstream to the ocean to have a significant effect on the presence of <i>G. postvectis</i> in the Waipoua river catchment at the macro-scale. Substrate size, conductivity, elevation, in-stream moss, interstitial refuge space availability and flow type were identified to be important habitat variables in determining <i>G. postvectis</i> presence at the meso-scale. The substrate size index was found to be the most significant variable with large substrate size demonstrating a strong correlation to <i>G. postvectis</i> presence. Chapter two also establishes the Extent of Occurrence (EOO) and Area of Occupancy (AOO) of G. postvectis in the Waipoua Catchment.</p><p dir="ltr">In chapter three, the timing, environmental cues, and spawning site selection of <i>G. postvectis</i> in a 3rd order stream within the catchment area was investigated. Adult fish in three 100 m survey reaches were tagged using Visual Implant Elastomer (VIE) tags and monitored from March to August 2023. Rainfall, stream flow and in-stream temperature were monitored using data loggers during this period. Fish were found to spawn following a peak in rainfall and subsequent rise in water flow. Two separate spawning events were recorded from the middle of May to the end of June. A total of three spawning sites were found. Two sites were found above the wetted edge amongst bedrock, cobbles, gravel, and vegetation. The other site was found mainly submerged upon bedrock substrate. All eggs were in the stage of early development when first found. One site was washed away within two days following a subsequent rainfall event and another site was predated on by a rodent species with all eggs gone within 10 days. These are the first spawning sites to be found in over 20 years. These data should contribute to more informed and targeted species management.</p>