Lake and species specific patterns of non-diadromous recruitment in amphidromous fish: The importance of local recruitment and habitat requirements

Understanding migratory life histories is critical for the effective management and conservation of migratory species. However, amphidromous migrations (fish hatch in streams, immediately migrate to the sea for a feeding period and return to fresh water as juveniles) remain understudied owing to the difficulties of tracking tiny larval fish. Despite this, it has widely been assumed that amphidromous fish have open, resilient populations, with marine-rearing larvae dispersing widely during their pelagic phase. In the present study we tested the hypothesis that when an alternative freshwater pelagic habitat is available, non-diadromous recruitment will be the dominant process in sustaining amphidromous fish populations, with implications for their connectivity and resilience. Otolith microchemical analyses of five species (three Galaxias (Galaxiidae), two Gobiomorphus (Eleotridae)) from paired systems on the South Island of New Zealand indicated that when a suitable freshwater pelagic habitat existed downstream, non-diadromous recruitment was the primary population-sustaining process, typically contributing >90% of recruits. In addition, not all species recruited from all lakes, indicating the importance of the largely unstudied role of species-specific amphidromous larval requirements. The results of the present study emphasise the need to better understand the dynamics of individual populations of amphidromous fish, and highlight the importance of understanding species-specific early life history requirements to fully understand their distributions and management needs

Recruitment sources of brown trout identified by otolith trace element signatures

This study examined whether element:Ca ratios within the otoliths of juvenile brown trout could provide accurate trace element signatures for specific natal tributaries, and attempted to match these to trace element natal signatures found within the otoliths of adult trout caught in the main stem rivers of the same catchment. The trace element signatures of juvenile trout otoliths were analysed from a sample of eight tributaries representing the main sub-catchments of the Motueka River catchment, New Zealand. Trace element signatures were determined using laser ablation inductively coupled plasma mass spectrometry, and differentiated using linear discriminant function analysis with an overall cross-validated classification success of 96.8%. Temporal stability in element:Ca ratios was investigated by repeat collections of juvenile fish over two years. Natal signatures from 11 of 23 adult trout sampled from the catchment main stems were matched to one of the eight tributary signatures showing recruitment sources to be spread relatively evenly throughout the catchment. This study demonstrates the potential of using otolith trace element analysis to determine the natal origins of freshwater fish within a catchment

Life history plasticity affects the population structure and distribution of the widespread migratory fish Galaxias brevipinnis

Partial migration, or variation in migratory propensity within populations, has been reported across a range of taxa, including fish. Otolith microchemistry has revealed a high degree of life history plasticity in many amphidromous species, with diadromous and non-diadromous recruitment occurring. We examined this plasticity and its effect on population structure, dispersal and recruitment in Galaxias brevipinnis, an amphidromous fish widespread around New Zealand. We used otolith microchemistry analyses to examine recruitment sources and fish surveys to assess abundance and size structure in two large river systems, each containing naturally formed lakes and no obvious physical barriers to migration. Otolith analyses revealed discrete recruitment sources for stream-resident populations, with marine recruitment supporting populations downstream from lakes and exclusively lake-derived recruitment for populations upstream of lakes. Although diadromous G. brevipinnis were abundant within 10 km of the sea, the abundance and relative proportion of younger fish declined as distance upstream increased, until a lake was reached, at which point abundance and the proportion of small individuals increased. The results provide a strong indication that supply is limiting recruitment for G. brevipinnis as distance from pelagic larval habitat increases, and that discrete recruitment sources and population structuring exist even within drainages.This research was funded by the Zoology Department, University of Otago, and the Australia & Pacific Science Foundation (grant number APSF 07/2)