Reproduction

Adult fish typically reach sexual maturity at one year and spawning is triggered by changes in day length and temperature. Unless landlocked within a lake, the common galaxias spawns mainly in autumn during spring tides in the tidally influenced reaches of rivers and streams ^[11] but spawning in winter and spring has occurred.^[12] The eggs are laid en masse amongst flooded riparian vegetation by females. Male fish then release sperm into the water and the eggs are fertilised externally.

This type of spawning is called polygynandry. Eggs remain attached to the vegetation as the tide recedes.^[11] Two types of reproductive strategy occur: the most common is a 'boom bust' strategy whereby spawning occurs in one event and is followed by death (semelparity), or much more rarely spawning occurs in multiple years before death iteroparity.^[13]

Egg development

Eggs (about 1 mm diameter) develop at the base of vegetation for 2–4 weeks. Environmental conditions in the vegetation (particularly temperature and humidity) are critical for successful egg development. Egg mortality occurs from excess exposure to sunlight, predation from mice and spiders, grazing and trampling by livestock, mowing of bankside vegetation in urban areas, and flooding.^[14] The following spring tide floods the eggs stimulating them to hatch.^[11]

Marine life

After hatching, the 7-mm-long larvae are swept out to sea and spend 3–6 months in the marine environment. This phase of their lifecycle is little understood, as the larvae are small, transparent, and difficult to locate.^[15] The speed and direction of ocean currents play an important role in their dispersal; temperature and food availability are also important in determining how long they spend at sea. This marine dispersal phase is a critical part of the common galaxias's lifecycle, because it gives larvae from different populations or rivers the opportunity to 'connect'.

When the juveniles are sufficiently grown, about 30–55 mm in length, they migrate back into fresh water. The juveniles form large shoals as they move through estuaries. Some of their life is spent in the lower reaches of rivers, where they metamorphose, before spending their adult life in suitable freshwater habitat. Some individuals return to the river they were born in (natal homing), but most return to rivers other than their birth site.^[16][17]

Freshwater life

Following metamorphosis, adult spend around 6 months in fresh water, where they gain sufficient growth and energy to begin investing in reproduction. Males generally reach sexual maturity earlier and at a smaller size than females.

Fishing

The juveniles are caught as whitebait while moving upstream and are much valued as a delicacy, leading to their protection with controlled fishing seasons to preserve adult populations. They are fished commercially in New Zealand, Chile, and Argentina, but the last Australian commercial fishery closed in Tasmania in the 1970s.^[19]

Some jurisdictions permit fishing of the adults, but again under regulation or licence to preserve the adult population, but others ban it altogether unless the fisher belongs to an indigenous people (e.g., New Zealand Māori). For instance, in Tasmania, the adult common galaxias may only be caught using a pole of a specified maximum size (1 m).

Restoration

In parts of New Zealand, this species spawning habitat has become degraded due to activities related to agriculture, urbanisation, and land use change. This creates sink populations in rivers as adult fish have nowhere suitable to lay their eggs and the majority of eggs die. Because these sink rivers produce no eggs or larvae, a gap is created during marine dispersal. No opportunities exist for the exchange of larvae from these sink populations with other populations. However, these sink populations can receive larvae that were born in different rivers. They will not be able to successfully reproduce and the sink cycle continues.

Innovative methods to restore the riparian spawning habitat include using straw bales as a temporary replacement for vegetation.^[20] Straw bales provide the same conditions and physical structure as natural vegetation, enabling the eggs to develop successfully. This method ensures that eggs and larvae are produced and that each river is a source of larvae. Exclusion of livestock and fencing of the bank-side vegetation is also an effective method to encourage regrowth of suitable vegetation. Restoration of the spawning habitat helps to maintain connectivity between larvae from different rivers during marine dispersal.