This review describes recent molecular biological research on olfactory chemoreception in fishes. The recent rapid development of molecular biological techniques has provided new valuable information on the main and vomeronasal olfactory receptor (OR) genes, the axonal projection from ciliated, microvillous and crypt-olfactory receptor cells to the olfactory bulb, properties of odorant substances and olfactory imprinting and homing in salmon. Many important questions, however, remain unanswered on functional differences among OR genes, on ligand binding to each OR and on the molecular biological mechanisms underlying olfactory imprinting and homing in salmon. Olfactory chemoreception is believed to be the oldest sensory cue for both animal survival and adaptation to various different environments. Further intensive molecular biological research on olfactory memory formation and remembrance should be carried out to clarify the fundamental process of olfactory chemoreception in fishes.

Abstract. Diadromous fishes are a frequent but poorly understood component of coastal riverine fish communities in Australia. There are ,33 diadromous fishes found in Australian waters, mainly catadromous and amphidromous species. An extensive review of the literature identified major information gaps about the lifecycles and ecology of many of these species, with information on facultative diadromy, navigation, marine and early life stages being particularly limited. In many cases, this lack of information has led to poor management decisions and consequently many of the Australian diadromous species are under increasing threat from a range of environmental impacts. Much of the required information is difficult to obtain with traditional field surveys and, as a result, new and improved research tools and technologies, including telemetry, otolith chemistry, stable-isotope analysis (SIA) and functional magnetic resonance imaging (fMRI) are increasingly being applied. Key areas for research on Australian diadromous fishes should involve: (1) use of telemetry and otolith chemistry to determine the level of facultative diadromy and variation in diadromous movements across a species range; (2) use of otolith chemistry and SIA to gain a greater understanding of larval and juvenile marine life stages of catadromous and amphidromous species; and (3) use of fMRI or traditional techniques such as electroolfactogram (EOG) to determine the role of olfaction in spawning and migration, and the impact of impoundments and agricultural run-off on these critical life history stages.

Diadromy is an important characteristic of the lifecycle of many Australian coastal fishes, but many of these species remain poorly studied. The migratory patterns of five riverine fish species from south-eastern Australia were examined using otolith chemistry. Analyses of individual otoliths from wild-caught fishes revealed distinctive lateral variation in otolith Sr : Ca values that provide good evidence for an amphidromous lifecycle for two species: Myxus petardi and Gobiomorphus australis. Gobiomorphus coxii, Potamalosa richmondia and Notesthes robusta displayed Sr : Ca patterns that indicated that these species may have more complex movements between marine and fresh water. Overall, these results provided quantitative data that supported the lifecycles previously hypothesised for most of the studied fish species. However, M. petardi, which was thought to be catadromous, displayed Sr : Ca variations that suggested an amphidromous lifecycle, at least for the specimens examined. These results also provided further evidence to demonstrate that otolith chemistry is a useful tool for studying the movement patterns of diadromous species and this technique will be especially valuable in identifying species that are most at risk from river regulation and barriers to migration.