Retropinna Semoni Research Articles

Wild fish as reservoirs of parasites on Australian Murray cod farms

Controlling pathogens within the aquaculture systems is important to prevent their impact on both human health and wild native populations. It is not uncommon for aquaculture ponds to contain undesirable species of fish which can compete for food or space with the farmed fish or play a role as a reservoir of diseases. The aims of this study were to determine the occurrence of parasites in undesirable species of fish commonly found in Australian Murray cod farms in New South Wales to determine if they can act as a reservoir for important parasites. In the present study, a total of 106 Australian smelt (Retropinna semoni), and 144 gudgeons (Hypseleotris spp.) were sampled from Murray cod ponds for parasitic infections. These fish are among the most common undesirable fish found on Murray cod farms in the region. No metazoan parasites were found in Australian smelt. One hundred and nine gudgeons were used for parasitology and 35 for histology. Parasites were examined morphologically followed by analyses of their DNA sequence data. Seven percent of infected gudgeons (out of total 109 examined) had externally visible signs of parasitic infection. The gudgeons had high parasitic burdens with 89.9% containing encysted metacercaria, identified as Apatemon hypseleotris, belonging to the family Strigeidae, 41.3% non-encysted metacercaria identified as Clinostomum sp., belonging to the family Clinostomidae, 6.4% (larval) cestodes identified as the metacestode of the genus Parvitaenia, belonging to the family Gryporhynchidae and a single Camallanus larval nematode (0.9%), all known to be of low host specificity and potentially transmissible to Murray cod. Histopathology results showed the presence of an additional parasite, a monogenean parasite in the gill. Some of the parasites found in the present study can be pathogenic for infected fish. This study provides critical baseline information on the occurrence of parasites in wild reservoir fish on Murray cod farms. Further research is needed to understand the potential risks to Murray cod populations in farm systems as well as to other fish in natural water resources where Murray cod are released.

Morphological predictors of swimming speed performance in river and reservoir populations of Australian smelt Retropinna semoni.

Dam construction is a major driver of ecological change in freshwater ecosystems. Fish populations have been shown to diverge in response to different flow velocity habitats, yet adaptations of fish populations to river and reservoir habitats created by dams remains poorly understood. We aimed to evaluate divergence of morphological traits and prolonged swimming speed performance between lotic and lentic populations of Australian smelt Retropinna semoni and quantify the relationship between prolonged swimming speed performance and morphology. Prolonged swimming speed performance was assessed for 15 individuals from each of three river and two reservoir populations of R. semoni using the critical swimming speed test (Ucrit ). Body shape was characterized using geometric morphometrics, which was combined with fin aspect ratios and standard length to assess morphological divergence among the five populations. Best subsets model-selection was used to identify the morphological traits that best explain Ucrit variation among individuals. Our results indicate R. semoni from river populations had significantly higher prolonged swimming speed performance (Ucrit = 46.61 ± 0.98 cm s-1 ) than reservoir conspecifics (Ucrit = 35.57 ± 0.83 cm s-1 ; F1,74 = 58.624, Z = 35.938, P < 0.001). Similarly, R. semoni sampled from river populations had significantly higher fin aspect ratios (ARcaudal = 1.71 ± 0.04 and 1.29 ± 0.02 respectively; F(1,74) = 56.247, Z = 40.107, P < 0.001; ARpectoral = 1.85 ± 0.03 and 1.33 ± 0.02 respectively; F(1,74) = 7.156, Z = 4.055, P < 0.01). Best-subset analyses revealed Ucrit was most strongly correlated with pectoral and caudal fin aspect ratios (R2 adj = 0.973, AICc = 41.54). Body shape, however, was subject to a three-way interaction among population, habitat and sex effects (F3,74 = 1.038. Z = 1.982; P < 0.05). Thus sexual dimorphism formed a significant component of unique and complex variation in body shape among populations from different habitat types. This study revealed profound effects of human-altered flow environments on locomotor morphology and its functional link to changes in swimming performance of a common freshwater fish. While past studies have indicated body shape may be an important axis for divergence between lotic and lentic populations of several freshwater fishes, fin aspect ratios were the most important predictor of swimming speed in our study. Differences in body morphology here were inconsistent between river and reservoir populations, suggesting this aspect of phenotype may be more strongly influenced by other factors such as predation and sexual dimorphism.

Daily age determination and growth rates of freshwater fish throughout a regulated lotic system of the Murray‐Darling Basin Australia

To facilitate future research in freshwater fish recruitment response to environmental flow delivery, size-at-age and growth models are presented for eight fish species occurring in south-eastern Australia; three small-bodied species and the juvenile 0+ age classes of five large-bodied species. Otolith increments were used to estimate the daily age of golden perch Macquaria ambigua, bony bream Nematalosa erebi, common carp Cyprinus carpio; Murray cod Maccullochella peelii, freshwater eel-tailed catfish Tandanus tandanus, Australian smelt Retropinna semoni, un-specked hardyhead Craterocephalus stercusmuscarum fulvus and Murray–Darling rainbowfish Melanotaenia fluviatilis. Linear growth models provided the best fit for length-at-age data of juvenile 0+ age large-bodied species; whereas von Bertalanffy growth functions provided the best fit to length-at-age data of small-bodied species. The results provide novel baseline data for future research in this area.

Non‐diadromous life history and limited movement in northern lineages of Australian smelt: Evidence from otolith chemistry analysis

AbstractA comprehensive knowledge of migratory behaviour is paramount for the effective management and conservation of fishes in coastal drainages. The Australian smelt (Retropinna semoni) is a common fish distributed throughout coastal and inland drainages in south‐eastern Australia. Multiple life history patterns (potamodromy, facultative diadromy, estuarine residence) in the southern regions of its geographic distribution have been reported, and it is possible that some of these differences reflect characteristic behaviours of cryptic species within the group. To elucidate the life history patterns of smelt at the northern extent of their range and to identify any differences in migratory behaviour between two genetically distinct lineages (“SEQ‐N”, “SEQ‐S”) in this region, the movement patterns of Australian smelt were assessed using otolith chemistry. No evidence of marine residence was found based on otolith Sr:Ca and Ba:Ca transects for either group, suggesting that both are non‐diadromous. Significant differences in multi‐elemental otolith chemistry signatures were found among rivers and between paired sites within some rivers, suggesting no connectivity among drainages and limited dispersal of individuals over large spatial scales within rivers. These findings provide key insights into the migratory patterns of northern Australian smelt populations and, in combination with previous research, suggest that life history variation in the Australian smelt complex may occur primarily at the intra‐population level rather than among genetically distinct lineages. Conservation and management strategies for Australian smelt should take limited dispersal and individual life history variation into consideration in order to protect the components of this important species complex.

Occurrence of Contracaecum bancrofti larvae in fish in the Murray-Darling Basin.

There is a paucity of information on the diversity and occurrence of freshwater fish parasites in Australia. This study investigates the distribution and occurrence of a parasitic nematode of the genus Contracaecum in freshwater fish from south-eastern Australia. Fish (n = 508) belonging to nine species and eight families were collected from eight wetlands associated with the Murrumbidgee River floodplain in the southern Murray-Darling Basin and subjected to laboratory examination. Third-stage nematode larvae were found in eight of the nine fish species. The exception was the fly-specked hardeyhead (Craterocephalus stercusmuscarum), although only one specimen of this species was examined. Nematode larvae were identified as Contracaecum bancrofti using a combined morphological and molecular approach. The parasite was most prevalent in weather loach (Misgurnus anguillicaudatus; 34.8%) and Australian smelt (Retropinna semoni; 21.4%), followed by carp (Cyprinus carpio; 9.5%), of which the former and latter are non-native species. Contracaecum prevalence differed between locations, with Two Bridges having the highest number of infected fish. This may be due to the higher number of suitable host species collected at these localities. Contracaecum spp. are parasites with low host specificity that have also been reported in Australian marine fish, humans and piscivorous birds. The high parasite prevalence in the two abundant non-native fish species in the region suggests that they act as a suitable host for this endemic parasite, resulting in the increase in the parasite population. It would be interesting to study host-parasite interactions in this area, especially if introduced fish populations declined dramatically in the attempt to eradicate them or control their population.

Patterns of genetic structuring at the northern limits of the Australian smelt (Retropinna semoni) cryptic species complex.

Freshwater fishes often exhibit high genetic population structure due to the prevalence of dispersal barriers (e.g., waterfalls) whereas population structure in diadromous fishes tends to be weaker and driven by natal homing behaviour and/or isolation by distance. The Australian smelt (Retropinnidae: Retropinna semoni) is a native fish with a broad distribution spanning inland and coastal drainages of south-eastern Australia. Previous studies have demonstrated variability in population genetic structure and movement behaviour (potamodromy, facultative diadromy, estuarine residence) across the southern part of its geographic range. Some of this variability may be explained by the existence of multiple cryptic species. Here, we examined genetic structure of populations towards the northern extent of the species’ distribution, using ten microsatellite loci and sequences of the mitochondrial cyt b gene. We tested the hypothesis that genetic connectivity among rivers should be low due to a lack of dispersal via the marine environment, but high within rivers due to dispersal. We investigated populations corresponding with two putative cryptic species, SEQ-North (SEQ-N), and SEQ-South (SEQ-S) lineages occurring in south east Queensland drainages. These two groups formed monophyletic clades in the mtDNA gene tree and among river phylogeographic structure was also evident within each clade. In agreement with our hypothesis, highly significant overall FST values suggested that both groups exhibit very low dispersal among rivers (SEQ-S FST = 0.13; SEQ-N FST= 0.27). Microsatellite data indicated that connectivity among sites within rivers was also limited, suggesting dispersal may not homogenise populations at the within-river scale. Northern groups in the Australian smelt cryptic species complex exhibit comparatively higher among-river population structure and smaller geographic ranges than southern groups. These properties make northern Australian smelt populations potentially susceptible to future conservation threats, and we define eight genetically distinct management units along south east Queensland to guide future conservation management. The present findings at least can assist managers to plan for effective conservation and management of different fish species along coastal drainages of south east Queensland, Australia.

Development and characterization of 21 novel microsatellite markers for the Australian smeltRetropinna semoni(Weber, 1895)

Development and characterization of 21 novel microsatellite markers for the Australian smelt<i>Retropinna semoni</i>(Weber, 1895)

Low light inhibits native fish movement through a vertical‐slot fishway: Implications for engineering design

AbstractLight intensity within a vertical‐slot fishway was manipulated to determine the effect on fish movement. Three treatments (darkness, low light, artificial light) were tested with natural daylight used as a control. Light intensity varied from 0 to 1,692 lux for the three treatments and from 1 to 4,550 lux for the control. Light intensity outside the fishway ranged from 31 to 80 900 lux. A total of 64 385 fish were collected from six species. The abundance of Australian smelt Retropinna semoni (Webber), unspecked hardyhead Craterocephalus stercusmuscarum fulvus Ivantsoff, Crowley and Allen, bony herring Nematalosa erebi (Günther), carp gudgeon Hypseleotris spp. and Eastern gambusia Gambusia holbrooki (Girard) moving upstream reduced significantly under low‐light conditions. Conversely, movement of macroinvertebrates (freshwater shrimp Macrobrachium australiense Holthuis and freshwater prawn Paratya australiensis Kemp) increased at low‐light intensities. The number of fish moving under artificial light (28 617) was similar to that under natural daylight (33 919). Movements of Australian freshwater fish and macroinvertebrates were found to be influenced by changes in light intensity. Instream structures that alter light conditions, such as road culverts, may thus act as behavioural barriers to fish movement, and this could be mitigated by the provision of natural or artificial light.

Complete mitogenomes for two lineages of the Australian smelt, Retropinna semoni (Osmeriformes: Retropinnidae)

Complete mitochondrial genome sequences were determined for two lineages (“CEQ” and “SEQ”) of the Australian smelt, Retropinna semoni. Both mitogenomes contain the typical vertebrate arrangement of 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes and control region. A conventional start codon for ND2 was not present in either lineage; instead CTG (Leucine) was present at this position. These sequences will be a useful resource for evolutionary studies of a significant species complex in the Australian freshwater fish fauna.

Does localized control of invasive eastern gambusia (Poeciliidae:Gambusia holbrooki) increase population growth of generalist wetland fishes?

While invasive fish management is heavily focussed on containment measures when introductions occur, examples from invasive species management in terrestrial systems suggest that there may also be considerable conservation benefits in implementing localized control programmes. We conducted a field-based experiment to assess the effectiveness of removing a globally significant invasive fish, eastern gambusia Gambusia holbrooki, from natural wetland habitats of south-eastern Australia. With recent work suggesting the impacts of eastern gambusia may be minimal for species with generalist life-history strategies, we hypothesized that the removal of eastern gambusia will reduce localized population growth of the invasive species, but will have little influence on the population growth of more generalist sympatric wetland fish species. We used a predictive modelling approach to investigate changes in eastern gambusia populations following removal activities, and how sympatric fish species responded to such changes. Although eastern gambusia rapidly populated habitats, we demonstrated that control actions substantially reduced the rate of population increase over the four-month study period. This suggests that control may be an effective localized strategy to suppress eastern gambusia densities. There was however, no evidence of any response to the removal actions by any of the three sympatric fish species investigated – carp gudgeon (Hypseleotris spp.), Australian smelt (Retropinna semoni) and the invasive common carp (Cyprinus carpio). These results support previous work which suggests that the flexible life-history strategies and behavioural traits of all three species allow co-existence with eastern gambusia. The study highlights the importance of understanding the potential outcomes of control options which is particularly pertinent for established aquatic invasive species where information on control effectiveness, population dynamics and/or ecosystem response is currently lacking.

River regulation and recruitment in a protracted‐spawning riverine fish

We present and test an extension of the "match/mismatch" hypothesis that attempts to explain the persistence, under conditions of flow alteration, of small, short-lived, native, riverine, fish species. The premise is that flow alteration typically changes environmental conditions, such as temperature and prey abundance, which may affect survival during the larval period of fishes. This "window-of-opportunity hypothesis" states that, if optimal conditions for recruitment vary temporally within a year, the probability that a proportion of the larvae of protracted-spawning species will encounter a period of optimal conditions is greater than for larvae with only a brief spawning period, and so the former will have a recruitment advantage. We determined whether all hatching events contributed equally to juvenile recruitment of the protracted-spawning Australian smelt (Retropinna semoni) during one breeding season in three pairs of heavily regulated and largely free-flowing unregulated rivers in the Murray-Darling Basin, Australia, and related patterns in the hatch dates of recruits to temperature or prey biomass for one pair. For all rivers, heavily regulated or not, recruits present at the end of the breeding season most commonly hatched in the latter part of the breeding season. Mortality of those fish hatched in the first part of the season likely explains this trend. Furthermore, while hatching times were similar for all rivers, each river showed a distinct pattern of hatching and recruitment, which may relate to the temperature range within which epigenetic processes are aligned. Patterns of zooplankton biomass differed between the largely free-flowing ovens and regulated Goulburn rivers and likely had different sources: within the channel and within the storage lake, respectively. For the Ovens River, recruits hatched subsequent to the period when the first significant increase in zooplankton biomass occurred. We hypothesize that temperature may largely influence the "window" during which recruitment can take place but that prey density, responding to river-specific interactions between temperature and discharge, plays a role in the timing and magnitude of recruitment of Australian smelt. We conclude that the match/mismatch hypothesis may be applicable to rivers, that the window-of-opportunity hypothesis has some currency and deserves further investigation, and that river regulation may have significant impacts on fish recruitment.

Update to the catalogue of South Australian freshwater fishes (Petromyzontida &amp;amp; Actinopterygii)

South Australia is a large Australian state (~1,000,000 km2) with diverse aquatic habitats spread across temperate to arid environments. The knowledge of freshwater fishes in this jurisdiction has advanced considerably since the last detailed catalogue of native and alien species was published in 2004 owing to significant survey and research effort, spatial analysis of museum data, and incidental records. The updated list includes 60 native and 35 alien species. New additions to the native fauna include cryptic species of Retropinna semoni s.l. (Weber) and Galaxias olidus s.l. (Günther). Two others have been rediscovered after long absences, namely Neochanna cleaveri (Scott) and Mogurnda adspersa (Castelnau). Range extensions are reported for native populations of Galaxias brevipinnis Günther, Leiopotherapon unicolour (Günther), Hypseleotris spp. (hybridogenetic forms) and Philypnodon macrostomus Hoese and Reader. There are five new alien species records (all aquarium species) including Phalloceros caudimaculatus (Hensel), Poecilia reticulata Peters, Xiphophorus hellerii Heckel, Astronotus ocellatus (Agassiz) and Paratilapia polleni Bleeker, with confirmation of Misgurnus anguillicaudatus (Cantor). Other range extensions for alien (exotic or translocated native) species in different drainage divisions (various modes of human-mediated dispersal) include Nematalosa erebi (Günther), Cyprinus carpio Linnaeus, Oncorhynchus mykiss (Walbaum), Salmo salar Linnaeus, Salvelinus fontinalis (Mitchell), Melanotaenia fluviatilis (Castelnau), Atherinosoma microstoma (Günther), Macquaria novemaculeata (Steindachner), Nannoperca australis Günther, Pseudaphritis urvillii (Valenciennes), and Hypseleotris spp. (hybridogenetic forms). New records are a combination of greater available information and new incursions, highlighting the need for ongoing detailed surveys and reporting to detect rare native and alien species.

Model development of a Bayesian Belief Network for managing inundation events for wetland fish

Wetlands are essential components of floodplain–river ecosystems that often suffer degradation due to river regulation. To this end, the application of environmental water is increasingly being seen as an important amelioration strategy. However, decisions regarding the delivery of water to maximise environmental benefits, including native fish population health, are complex and difficult. This paper describes the development of a Bayesian Belief Network (BBN) model as part of a Decision Support Tool for assessing inundation strategies to benefit native wetland fish. Separate, albeit closely related, BBNs were developed for three native (golden perch Macquaria ambigua, carp gudgeon Hypseleotris spp., Australian smelt Retropinna semoni) and one alien fish species (common carp Cyprinus carpio carpio). The model structure was based on a conceptualisation of the relationships between wetland habitats, hydrology and fish responses, with emphasis on the types of inundation activities undertaken by managers. Conditional probability tables for fish responses were constructed from expert opinion and the model was validated against field data. The predictive ability and sensitivity of the model reflected the inherent high variability in relationships between wetland characteristics, hydrology and fish responses, but was nonetheless able to address satisfactorily such complexities within a holistic framework. As the model was designed in conjunction with managers and evaluated by them, its application will be enhanced by on-going engagement between managers and scientists.

Effects of an experimental environmental flow release on the diet of fish in a regulated coastal Australian river

Environmental flow rules are developed to provide a flow regime necessary to maintain healthy river and floodplain ecosystems in rivers regulated for human uses. However, few studies have experimentally assessed potential ecological mechanisms causing declines in the health and productivity of freshwater fish assemblages in regulated rivers to inform the development of appropriate environmental flows. We tested whether an experimental flow release in a regulated tributary of the Hunter River, Australia, altered the diet of two widely distributed fish species (Australian smelt Retropinna semoni and Cox’s gudgeon Gobiomorphus coxii) compared with data from unregulated reference and regulated control tributaries. Neither species had significant differences in the number of prey taxa ingested, gut fullness or composition of gut contents due to the environmental flow release (EFR). The diet of R. semoni did not differ significantly between regulated and unregulated tributaries in either catchment. However, the diet of G. coxii differed in only one of the two pairs of rivers consistently across all sample times. Assuming the EFR was sufficient to alter the composition of prey available for consumption by the fish species studied, our findings imply that functional indicators, such as the diet of generalist higher-order consumers, may be more suitable indicators of long-term flow regime change rather than short-term flow events.

Early fish growth varies in response to components of the flow regime in a temperate floodplain river

Summary 1. The biological productivity of floodplain rivers is intimately related to their flow regimes and it has been proposed that fish production should be linked to components of the flow regime in productivity models. To assess applicability of existing models of productivity in floodplain rivers, we tested predictions about growth during the early life stages of a common, short-lived fish (Australian smelt Retropinna semoni) in a non-flow-altered, temperate Australian floodplain river. 2. The morphometric condition of larval and juvenile fish measured over a five-year period was positively related to annual discharge, but the highest average seasonal growth rates occurred in two years of contrasting hydrology, one with early spring flooding and the other with predominantly low flows and a late season (within channel) flow pulse. 3. Analysis of daily growth measures indicated that timing, river height, the duration of in-channel flow events and antecedent flood events are all significant factors influencing the early growth of Australian smelt. The flexible manner in which fish growth responds to these factors appears to be an effective early life history strategy for a short-lived species occupying a highly variable environment. 4. Growth rates conformed to some predictions of the Flood Pulse Concept (in particular the Extended Flood Pulse Concept), but specific growth responses suggest that the Riverine Productivity Model and tenets of the Low Flow Recruitment Hypothesis best describe the production of Australian smelt in this system. We suggest that none of the existing conceptual models adequately describes fish productivity in temperate Australian floodplain rivers but that aspects of each are likely to be relevant under different flow conditions.

Responses of Fish to Experimental Introduction of Structural Woody Habitat in Riffles and Pools

The declining condition of river systems associated with rapid development of human societies has led to substantial declines in fish diversity. One cause of decline is the loss of in‐stream Structural Woody Habitat (SWH), an important component of stream ecosystems, particularly as fish habitat. As a result there has been an increase in the number of rehabilitation programs that introduce SWH into rivers. This paper assesses fish responses to SWH introduction in riffles and pools in the Hunter River, eastern Australia, using a Multiple‐Before‐After‐Control‐Impact (MBACI) experimental and analytical design. In the riffle experiment, species richness was comparable among all control and treatment riffles across the entire study period. However, there were significant differences in assemblage structure, fish abundance, and biomass between control and treated riffles. The introduction of SWH (bank embedded deflector jams) appeared to create additional habitat which was utilized by one native fish species (Retropinna semoni—Australian smelt) and one alien species (Gambusia holbrooki—mosquito fish). In pools there were no significant changes in fish species richness, abundance, or biomass following introduction of SWH (pool jams). These findings have important practical and cost implications in terms of the design and implementation of rehabilitation strategies using SWH to restore fish assemblages in degraded streams.

Contemporary and historical patterns of connectivity among populations of an inland river fish species inferred from genetics and otolith chemistry

Patterns of dispersal in riverine populations of Australian smelt ( Retropinna semoni ) were examined using otolith chemistry (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) and genetic markers (allozymes, mtDNA, microsatellite DNA). During a period of prolonged low flows, young-of-year smelt were collected from 13 streams within three catchments of the southern Murray–Darling Basin, Australia. Spatial differences in otolith core and edge chemical signatures and high levels of genetic assignment to sampling locations were observed, suggesting that most recruits were retained in natal areas after spawning. Following a subsequent period of hydrological connection, the same cohort was sampled as 1-year-olds. Maximum likelihood estimation using otolith core chemistry data from these fish suggested that retention in natal areas was highly variable between years and a similar, though less pronounced, pattern was evident in genetic assignments. Partitioning of genetic variation among catchments was not significant (FCT &lt; 0.004) and probably reflects disequilibrium between migration and genetic drift due to an historical population expansion (~270 000 years ago). Taken together, otolith chemistry and genetic analyses suggest that contemporary dispersal of smelt within these catchments is relatively restricted and may be mediated by changes in hydrological connectivity.

Effects of sample size on numerical estimates of diel prey consumption in a fish population

Numerical data based on stomach content analysis of the zooplanktivorous freshwater fish, Retropinna semoni, were used to examine the effect of fish sample size on mean counts of dominant prey items. Fifty adult R. semoni were collected from throughout the open‐water of Lake Benanee, Australia at each of five times over a diel period. Bootstrapping was used to generate confidence intervals around sample means, and markedly more accurate means were obtained from samples collected in the day than the night. High variation in night samples was the by‐product of a diurnal feeding regime. Traditional sample sizes of 10 to 15 stomachs resulted in reasonable confidence intervals of sample means derived from collections on the first day, corresponding to uniform feeding patterns in the population. However, increased sample sizes were required to describe more complex feeding behaviour on the second day, when a proportion of the population switched to an alternative prey source.

A Method for Improving the Estimation of Natal Chemical Signatures in Otoliths

AbstractLaser ablation‐inductively coupled plasma mass spectrometry has been widely used in studies aimed at identifying the natal sites of fish through analysis of the otolith core region. It is critical, then, that measurements of chemistry at the core accurately represent the natal chemical environment, rather than reflecting potential physiological and ontogenetic influences on elemental incorporation during the embryonic and early larval phases. We analyzed sagittal otoliths from 840 age‐0 and age‐1 Australian smelt Retropinna semoni using a 193‐nm Ar‐F excimer laser system with a fast response two‐volume sample cell to demonstrate a rapid depth‐profiling method that enables more accurate estimations of the natal chemistry by sampling a target region of the otolith that excludes the primordium. The method involves (1) examination of validated daily growth increments to identify the target region; (2) measurement of the amount of otolith material removed by each laser pulse; and (3) use of elevated 55Mn concentrations at the primordium to provide a marker to facilitate data selection. The target region in this study was material accreted onto the otolith 2‐10 d posthatch. We sampled this region by excluding data between the maximum value of the Mn peak and the otolith material representing the first 2 d of life. The laser pulses in the acquisition sequence that represented the target were then retained and all other data excluded. This method illustrates the advantages of the uniform energy distribution produced by 193‐nm excimer laser systems for depth profile analysis of otoliths and may provide the basis for a standardized approach to sampling at the otolith core that more accurately reflects the natal chemical environment.

Otolith increment width responses of juvenile Australian smelt Retropinna semoni to sudden changes in food levels: the importance of feeding history

To aid the interpretation of otolith microstructure in wild fishes, the present study assessed responses in daily otolith increment widths of early juvenile Australian smelt Retropinna semoni to sudden changes in feeding conditions. There was an almost immediate response in otolith increment widths (which can be interpreted as growth in length) to sudden changes in feeding conditions, with such changes being detected statistically after c. 4 days. Fish displayed compensatory growth when food supplies were increased following a period of limited food, indicating the magnitude of the response in growth appears highly dependant on feeding history. Additionally, fish size also appeared to influence increment widths suggesting that both fish size and feeding history are important factors that must be considered when interpreting otolith microstructure for the species.