One of the greatest empirical challenges in studies of marine metapopulations is assessing the role of connectivity in maintaining local populations. The use of natural tags in body structures such as otoliths has led to significant improvements in our understanding of dispersal dynamics; however, the real knowledge gap remains our ability to identify the origins of dispersing larvae. In the present study we show that embryonic otoliths of a diadromous fish native to southeastern Australia, Galaxias maculatus, can be used to quantify unique natal elemental signatures and identify potential source populations spread out along a contiguous coastline. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), we analysed embryonic otoliths from G. maculatus hatchlings from 10 rivers across coastal Victoria, Australia. We found significant differences among rivers in the concentrations of all 8 elements analysed (B, Mg, Mn, Zn, Cu, Sr, Ba, Pb). More importantly, significant differences among rivers in the multi-element signature of hatchling otoliths indicate that sites separated by 10s to 100s of km can be discriminated with a high degree of confidence. Our results show that these methods can be successfully applied to examine questions of population connectivity within metapopulations of diadromous species. KEY WORDS: Otolith chemistry · LA-ICPMS · Galaxias maculatus · Diadromy · Natal signatures Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 343: [273][274][275][276][277][278][279][280][281][282] 2007 the role of self-recruitment (Swearer et al. 2002, Levin 2006. Much of the recent evidence for significant levels of self-recruitment in marine populations comes from reef fish species with relatively short larval durations and highly specialized habitat requirements (e.g. Jones et al. 2005). Although these results are highly compelling for these species, it is unclear whether similar patterns will hold for species with much longer larval durations, and hence greater potential for dispersal (e.g. Warner et al. 2005).Attempts to empirically quantify levels of exchange between local populations within a metapopulation have largely been hindered by the inherent difficulties of tracking small planktonic larvae. In recent years, considerable progress has been made using natural environmental markers to determine the sources of locally recruiting larvae. Otoliths, the balance structures in the inner ear of teleost fishes, have been particularly useful as recorders of the chemistry of the surrounding environment (Campana 1999). Otoliths are composed of calcium carbonate crystals (primarily aragonite) within a protein matrix. They are formed prior to birth and grow daily as new crystal layers are deposited around the existing core. During otolith growth, elements from the surrounding water can substitute for calcium and become permanently incorporated into the crystal (Campana 1999). Thus, otoliths function as daily recorders of the chemical en...

Applying uniform population models and management strategies to widespread species can be ineffective if populations exhibit variable life histories in response to local conditions. Galaxias maculatus, one of the world’s most widely distributed fish species, occurs in a broad range of habitats and is highly adaptable, making it an ideal species for examining variation in life history traits across large geographic scales. Here, we examine the spawning biology and early life history of diadromous G. maculatus in coastal rivers in Victoria, Australia, and compare them to other populations throughout its range. We predicted that traits associated with these critical life stages, especially those that respond to environmental conditions that vary geographically, such as seasonal cues and temperature, are likely to vary across large spatial scales. We found that spawning occurs later in Victoria than in New Zealand (NZ) and South America, but migration back to rivers occurs at the same time in Victoria and NZ, but not South America. G. maculatus returning to rivers are also smaller and younger in Victoria than those in NZ. Other traits, like some attributes of spawning schools and spawning habitats, did not vary across large scales. Researchers and managers should be cautious when making broad assumptions about the biology of widely distributed species.

Avoidance of predator odours by migrating Galaxias maculatus was studied in the laboratory using a two-chamber choice tank. The odours of a native predator (short-finned eel, Anguilla australis) and an introduced predator (rainbow trout, Oncorhynchus mykiss) were tested on both whitebait (i.e., new recruits) and juveniles. Both whitebait and juveniles exhibited avoidance behaviour when exposed to eel odours. There was no avoidance response from either the whitebait or juvenile G. maculatus when exposed to trout odours, which could suggest differences in the ability of G. maculatus to detect native and introduced predators. Observed negative effects of introduced predators on native species may be owing in part to the absence of innate avoidance behaviour in native prey species.

Herbivorous insects have received relatively little attention in studies of consumer resource interactions in tropical compared to temperate streams. In this study I examined the effects of small, highly mobile, herbivorous insects on epilithic algae in riffle habitats of a stream located on the Pacific coast of Costa Rica. By elevating artificial substrates 20-30 cm off the streambed, while holding other abiotic factors constant, I successfully reduced the densities of grazing insects (primarily mayflies and dipterans) throughout an 8 week experimental period. This manipulation resulted in approximately 30% more algal biomass (measured as AFDM and chlorophyll a) on raised (grazer exclusion) than on control substrates placed directly on the streambed. Observations of the one common epibenthic grazing fish species, Sicydium salvini (Gobiidae), indicated that there were no consistent differences in their visitations to raised and lowered substrates, suggesting that fish grazing pressure did not contribute to observed differences in algal biomass between treatments. These results show that insects can be effective in reducing algal biomass in a tropical stream. While previously neglected, the role of grazing insect in contributing to the regulation of algal resources in tropical benthic communities warrants closer examination.