Search

Contemporary genetic spatial structure in north temperate marine species is likely the culmination of multiple vicariant and dispersive cycles. Here we evaluate spatial genetic struc- ture in an estuarine fish, rainbow smelt Osmerus mordax, from coastal Newfoundland, Canada, us- ing data from both mtDNA (ND5) sequences and nuclear loci (11 microsatellites). Sequence analysis of ND5 identified a previously unrecognized genetic discontinuity between 2 hypo - thesized glacial clades in southeastern Newfoundland. Microsatellite based tests for directional se- lection identified a locus (Omo11, p < 0.001) that mirrored mtDNA clades in the geographic distrib- ution of its 2 common alleles but did not display elevated differentiation following correction for heterozygosity. Bayesian multilocus clustering of the remaining micro satellite loci supported the presence of 2 predominant groups, for which the spatial distribution was also largely consistent with those of the mtDNA and Omo11 clades. Taken together, the similarity in microsatellite and mtDNA clines supports the hypothesis that contemporary spatial structure in smelt reflects histori- cal landscape isolation maintained by low dispersal and selective processes producing reinforce- ment between diverging populations. As genetic structure in northern marine and estuarine spe - cies may be largely determined by historical glacial cycles of vicariance, contemporary estimates of connectivity should be interpreted in the context of both past and present landscape structure.

Dispersal and connectivity are central to stability and persistence in natural populations. The use of otolith composition as geo-referenced tags may provide unparalleled resolution of spatial movements in marine and anadromous fish, although these geochemical signatures remain largely undescribed and the factors influencing otolith composition poorly understood. We examined spatial variation in the otolith geochemistry of juvenile rainbow smelt Osmerus mordax using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotope ratio monitoring mass spec- trometry to evaluate its potential as a natural tag to resolve fine-scale geographic patterns and dis- persal of estuarine early life history stages. Otolith element ratios (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) and isotope ratios (δ 13 C, δ 18 O, 87:86 Sr) varied significantly among locations (n = 9) and provided 83% cross- validated accuracy using a quadratic discriminant function analysis. Assignments based only on the 3 isotope variables resulted in the highest rates of correct assignment (87%), largely driven by a sig- nificant increase (~11%) in correct assignments at nearby locations (

Observations of homing and straying in marine organisms based on traditional Eulerian approaches may fail to resolve dispersal kernels or be unable to differentiate homing from invariant local residency. The roles of spawning site fidelity and straying in structuring populations of anadro- mous smelt Osmerus mordax were examined in coastal Newfoundland through the analysis of otolith elemental composition and a series of adult tagging experiments. Otolith elemental baselines were generated from freshwater residents, estuarine juveniles and laboratory-held individuals (32 psu), using both a single-element (i.e. Sr:Ca and Ba:Ca) and multivariate approach (i.e. discriminant function analysis). Ten spawning fish of unknown dispersal history were sampled from each of 6 spawning locations, and otolith composition was examined using laser ablation inductively coupled plasma mass spectrometry at 12 to 16 locations across each otolith. Single-element and multivariate approaches both indicated estuarine residency predominated, with limited marine (0.7%) signatures indicative of rare marine movements. Multiyear spawning site fidelity was examined through a fin- clipping (2003, n = 7076) and visual implant elastomer (VIE) experiment (2004, n = 13 524), encom- passing each of the 6 spawning locations. We observed limited straying, with 90 to 99% annual spawning site fidelity. Tag returns declined significantly with distance from location of tagging, and declines were linear over small (

In the St. Lawrence middle estuary, the larvae and juveniles of rainbow smelt Osmerus mordax are infected with intestinal parasites (the cestode Proteocephalus tetrastomus). Such para- sitism is suspected to impair growth rate, which may in turn influence the ability of smelt to migrate vertically and to be retained in the estuarine turbidity maximum (ETM), the nursery area of this population. We hypothesised that parasitism contributes to size-selective mortality by altering patterns of vertical migration, thus contributing to transport losses from the nursery area. Parasitism was found to affect growth rate, as determined by otolith microstructure analysis, and to contribute to mortality. Larvae which survived the parasitic infection were found to be, on average, larger than the uninfected survivors at some point in their early life history prior to infection. Significant size- selective mortality was observed among infected larvae relative to uninfected larvae. There was, however, no significant difference in the vertical distribution of parasitised versus non-parasitised larvae, indicating that size-selective mortality was not due to transport losses from the nursery area. Rather, starvation or predation was most likely responsible for observed mortality patterns. We illustrate that variability in the incidence of parasitic infection among years may form the basis for variability in recruitment and the abundance of the adult smelt population.

We investigated the impact of turbidity, food density and parasites on ingestion and growth rates of rainbow smelt larvae Osmerus mordax. These 3 variables were selected because of their potential to substantially influence the feeding success, growth, and the subsequent survival of smelt larvae. A laboratory experiment was first performed to evaluate, in turbulent conditions, the combined effects of turbidity and food density on the ingestion and growth rates of smelt larvae. A field survey of the gut contents of larval smelt was conducted to directly estimate ingestion rates in 2 different regions of the St. Lawrence estuarine turbidity maximum (ETM) exhibiting different levels of turbidity but otherwise sharing similar environmental conditions. This study demonstrated that lower energetic costs are incurred by larvae that exploit similar feeding conditions at higher turbidities. Larval rainbow smelt in the ETM fed during the coincidence of daylight hours and flooding tide. Cestode parasites (genus Protocephalus) were found in the digestive tract of 38% of the larvae collected in the ETM. Parasitised larvae ingested half as much food as non-parasitised larvae. The decrease in feeding due to parasitism was associated with a reduced growth rate as suggested by the significantly lower standard lengths observed in parasitised larvae. Moreover, the size advantage of non-parasitised larvae is expected to be amplified because larger larvae ingest proportionally more food than smaller larvae. We suggest that the impact of parasitism on larval survival and subsequent recruitment in fishes merits far more attention than afforded to date.

We investigated the link between hatch date, growth trajectory, estuarine retention and survival of rainbow smelt larvae Osmerus mordax in the St. Lawrence Middle Estuary. Young smelt were collected over their entire distributional range across the salinity gradient of the Middle Estu- ary (0 to 25 psu) during 2 growing seasons. Individual growth trajectories and hatch-date distribu- tions were reconstructed from otolith microstructure. High abundances of larvae were observed in the upstream region (0 to 10 psu) corresponding to the estuarine turbidity maximum (ETM). Larval rainbow smelt hatched in May during a period of 26 to 27 d, and mortality rates were variable among hatch dates. These variations were related to the feeding conditions experienced by first-feeding lar- vae (3 to 5 d after hatching) that fluctuated from day to day according to a predictable cycle of 15 d associated with photoperiod and tide. Comparison of growth trajectories of larvae at various ages indicated that older individuals, i.e. survivors, grew faster. Furthermore, smelt larvae collected within the ETM exhibited significantly higher growth rates than those sampled in the downstream area of the Middle Estuary, suggesting that larvae collected in the latter region represented dispersal from the ETM. We concluded that inter-annual variations in year-class strength might not be generally affected by early starvation, as larvae will experience recurrently good, medium and bad first-feed- ing conditions each year because of the length of the hatching period. However, any factor that reduces growth rate during the larval stage, including parasitism, has the potential to affect interan- nual variability in year-class strength.