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Summary Landscape‐scale clines in environmental parameters can lead to life‐history gradients. Study of interspecific life‐history differences can provide evidence of evolutionary adaptations in response to environmental cues, whereas examination of intraspecific life‐history variation can identify the ecological mechanisms driving trait evolution. We examined egg size and fecundity variation in three species of closely related freshwater fish (Galaxiidae) which occur across a habitat gradient in South Island, New Zealand. Inter‐population trait variation was assessed in the species with the widest distribution to investigate the ecological mechanisms driving egg size evolution. Substantial interspecific egg size and fecundity differences were observed. Galaxias paucispondylus, which generally occurs in high altitude headwater streams, showed an average egg size an order of magnitude larger, and fecundity an order of magnitude lower than G. brevipinnis, a species which rears in pelagic environments. Galaxias vulgaris, which rears fluvial environments but is generally distributed in lower catchment reaches than G. paucispondylus, showed intermediate fecundity and egg size traits. This interspecific trait variation is likely to be driven by differences in the larval rearing habitat, i.e. rearing in cold high altitude locations requires large well‐developed larvae (and therefore large eggs) but these come at the cost of maternal fecundity, whereas in rearing in warmer lower catchment watercourses, and pelagic habitats, does not require such large well‐developed larvae. Hence, fitness is maximised by progressively increasing fecundity by reducing egg size across this habitat cline. Life history‐habitat trends were repeated when intraspecific patterns in the widely distributed G. vulgaris were assessed, with population egg size increasing, and fecundity decreasing, in relation to a suite of interrelated environmental variables up an approximate altitudinal cline. The results highlight how inter‐ and intraspecific variation in egg size–fecundity trade‐offs in closely related species can interact to drive the evolution of landscape‐scale life‐history gradients. Egg size differences are likely to be driven by differences in the quality of the larval rearing habitat, i.e. rearing in a challenging habitat requires a larger egg whereas in relatively benign larval rearing habitat, fecundity can be maximised by reducing egg size. The interspecific variation observed provides evidence of trait adaptation in response to an environmental cue, while the intraspecific inter‐population differences indicate the potential ecological mechanisms driving trait evolution.

1. Early life history is widely recognized as the stage of fish life history where mortality is the highest. Incorrect adaptation of early life-history characteristics to environmental conditions will result in high larval mortality and therefore recruitment failure. As a result, understanding early life-history requirements is critical if conservation techniques such as translocations are to be successful. For amphidromous fishes, landlocked and marine rearing environments presentfundamentally different challenges, and thus early life history must be adapted to allow the establishment of landlocked populations. This study investigated gonadal development to determine spawning time, egg size, and fecundity in landlocked and diadromous populations of the threatened fish Galaxias brevipinnis to assess early life-history adaptation to facultative amphidromy in southern New Zealand.3. Ripening and spawning times of landlocked populations were protracted compared with diadromous populations. Diadromous populations simultaneously spawned during a March flood, whereas ovulating landlocked individuals were found from August to December, with a peak in October. Oocyte size in landlocked individuals was approximately 25% smaller than that in diadromous populations, and the fecundity of landlocked individuals was higher than that for diadromous individuals. 4. Translocations of landlocked individuals to diadromous stocks are therefore unlikely to be successful, whereas the large egg size of diadromous populations would allow the translocation of diadromous stocks to landlocked environments.

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Landlocking is a process whereby a population of normally diadromous fish becomes limited to freshwater, potentially leading to behavioural, morphological, and genetic changes, and occasionally speciation. The study of recently landlocked populations can shed light on how populations adapt to environmental change, and how such life-history shifts affect population-genetic structure. Kōaro (Galaxias brevipinnis) is a facultatively diadromous Southern Hemisphere galaxiid fish that frequently becomes landlocked in inland lakes. This study compares seven landlocked kōaro populations to diadromous populations from main and offshore islands of New Zealand. Genotyping-by-sequencing was used to obtain genotypes at 18,813 single nucleotide polymorphism sites for each population. Analyses of population structure revealed that most landlocked populations were genetically highly distinct from one another, as well as from diadromous populations. A few particularly isolated island and lake populations were particularly strongly genetically differentiated. Landscape characteristics were measured to test whether lake elevation, size, or distance from the sea predicted genetic diversity or differentiation from diadromous kōaro. While there were no significant relationships indicating isolation-by-distance or isolation-by-environment, we detected a trend toward lower genetic diversity in lakes at higher elevations. Our findings illustrate the critical role that landlocking can play in the structure of intraspecific genetic diversity within and between populations.