Early life history of the amphidromous galaxiid inanga: disentangling the consequences for their migratory dynamics, population structure and adult growth

Abstract

Amphidromy is a type of diadromous migration involving pelagic larval development (mostly in the marine environment) with adult growth and maturity occurring in freshwater environments. Pelagic development and dispersal is a “black box” for many amphidromous species preventing a holistic understanding of their ecology and population dynamics. Factors driving early life history variability and relationships between pelagic and adult life are imperative to further our understanding of their demographics and population dynamics. In this thesis, I examined the life histories of inanga (Galaxias maculatus) within and among populations in New Zealand. Inanga are the most important component of New Zealand’s culturally, recreationally and commercially important “whitebait” fishery. Populations are declining throughout the country and there is increasing pressure for tighter fishery management controls. My thesis set out to fill in some of the critical knowledge gaps pertaining to the life histories of inanga to better understand their migrations, population structure and demographics.

Studies of size, age, condition, and hatch dates of post-larvae at inward migration showed larger-scale spatial and temporal processes were driving most of the variation in the migratory characteristics of inanga. Significant differences in hatch date distributions were found among regions and were likely related to the timing of spawning within the wider area. Inanga were significantly younger and smaller at inward migration in the northern region and older and larger in the southern region. Hatch date was significantly associated with timing of inward migration, with autumn-hatched fish mostly migrating during September and winter-hatched fish during November. Although inanga were variable ages at migration, size was relatively consistent within regions showing that migration was largely size dependent.

Mixed effects modelling was used to partition the pelagic growth histories of inanga into intrinsic and extrinsic sources. Pelagic growth of inanga showed considerable variation among regions, however, growth was more consistent within regions. Seasonal variation in temperature and productivity were identified as extrinsic factors that influence growth. Inanga showed distinct developmental transitions from a pelagic-larval stages to a more competent post-larval stage. Growth during the pelagic larval stages was identified as a significant driver of age at inward migration and signifies that intrinsic growth thresholds underlie their migrations.

Current fishery management practices consider inanga as one biological unit. Otolith morphology was used to investigate the existence of putative stocks of inanga among regions with varying abiotic and oceanographic conditions. Spatial structuring of inanga was found that largely reflected the direction of the dominant ocean currents in New Zealand. Regions with high levels of mixing from other natal sources were also identified. The existence of distinct stocks shows that dispersal among some regions is restricted and inanga should be considered as multiple regional stocks for management of the fishery.

Reconstructions of the life time growth histories of adult inanga were done to investigate relationships between pelagic and adult growth. Temporal variation in abiotic conditions (as defined by hatching time) had a significant effect on growth. Winter- and spring-hatched fish with faster pelagic growth rates were slower growing for their age in rivers. On the contrary, autumn-hatched fish with slower pelagic growth rates maintained higher growth rates for their age as adults in the freshwater habitat. Age at sexual maturity and reproductive investment showed substantial variation among hatching times, however, no differences in body size at sexual maturity were found. Inanga can utilise pelagic and freshwater habitats to maximise growth and ultimately body size at maturity.

The life histories of inanga are shaped by a suite of factors acting at various spatial and temporal scales that affect their inward migrations, population structure and adult population demographics.