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Mutualisms are ubiquitous in nature but are understudied in freshwater ecosystems. Mutualisms can be unstable, shifting to commensal or even negative outcomes with context. Quantifying context dependency in mutualisms is critical for understanding how biotic interactions will shift along disturbance gradients in freshwater systems. A common reproductive interaction among stream fishes, nest association occurs when individuals of one species spawn in nests constructed by a host fish. Hosts benefit from a dilution effect: high proportions of associate eggs decrease the odds of host brood predation. Thus, partner abundance can be an important source of biotic context influencing the outcome of an association. We conducted a large in situ experiment manipulating abundance of partner yellowfin shiner (Leuciscidae: Notropis lutipinnis) (absent, low, high) at constant abundance of host bluehead chub (Leuciscidae: Nocomis leptocephalus), and quantified chub reproductive success using genetic tools. Evidence suggests that the nest association switched from mutualistic to parasitic outcomes as shiner abundance decreased. Chub reproductive success was highest at high shiner abundances. However, chub reproductive success was actually higher in the complete absence of shiners than at low shiner densities. This study shows that outcomes of biotic interactions in freshwater systems are context‐dependent, and that partner abundance can be a key source of context‐dependency in nest associations. We encourage future studies on freshwater mutualisms, which are thus far largely overlooked, relative to competition and predation.

The American shad Alosa sapidissima is an anadromous clupeid with once‐prolific stocks that have experienced major coastwide declines in abundance over the past century. The American shad spawning run in the Edisto River (South Carolina) has been exhibiting the same decreases as spawning runs in other coastal rivers, and stocking is now being considered as a restoration option for this river system. We utilized a suite of 13 microsatellite loci to provide a baseline genetic characterization of the Edisto River spawning run prior to supplementation and to evaluate the initial success of an experimental stocking program enacted from 2008 to 2010. No significant temporal genetic differentiation was found between sampling years, indicating that the genetic composition of the Edisto River spawning run is temporally stable over short time frames. Estimates of genetic diversity for Edisto River American shad were high (observed heterozygosity = 0.82–0.85) and similar to those observed in other river systems. Estimates of effective population size (3,505–8,379) resembled those reported for other diadromous species and were within the levels recommended for maintaining evolutionary potential. Hatchery‐produced individuals were detected within the 2010 year‐class of juvenile American shad prior to out‐migration (11/314 fish, or 3.5%), demonstrating initial success of the stocking effort (i.e., contribution of hatchery fish to the wild stock). Our results provide valuable information that can be incorporated into management plans for aiding the recovery of American shad in the Edisto River.

Quahog Parasite Unknown (QPX) is a significant cause of hard clam Mercenaria mercenaria mortality along the northeast coast of the United States. It infects both wild and cultured clams, often annually in plots that are heavily farmed. Subclinically infected clams can be identified by histological examination of the mantle tissue, but there is currently no method available to monitor the presence of QPX in the environment. Here, we report on a polymerase chain reaction (PCR)-based method that will facilitate the detection of QPX in natural samples and seed clams. With our method, between 10 and 100 QPX cells can be detected in 1 l of water, 1 g of sediment and 100 mg of clam tissue. Denaturing gradient gel electrophoresis (DGGE) is used to establish whether the PCR products are the same as those in the control QPX culture. We used the method to screen 100 seed clams of 15 mm, and found that 10 to 12% of the clams were positive for the presence of the QPX organism. This method represents a reliable and sensitive procedure for screening both environmental samples and potentially contaminated small clams.

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As pressure from anthropomorphic stressors continues to negatively affect the abundance of wild fish populations, aquaculture-based fisheries enhancement (i.e., sea ranching, stock enhancement, restocking) is being used increasingly as a management option to augment and restore wild stocks. However, as supplementation with cultured individuals may have unintended impacts on the genetic fitness of a target species, emphasis is placed on scientifically based protocols for fisheries enhancement, which involves continuous observation of genetic measures within and among populations. American Shad Alosa sapidissima, which had once-abundant spawning runs that experienced major declines throughout the early 20th century, are currently stocked in several rivers in North Carolina and South Carolina. A

Gag (Mycteroperca microlepis) from the eastern Gulf of Mexico and northwestern Atlantic are managed as separate stocks, although evidence for their demographic isolation remains equivocal. Several hundred individuals were genotyped at 11 microsatellite loci and it was found that gag across these regions were genetically indistinguishable (FST < 0.001). A coalescent-based computer simulation was employed to quantitatively assess the relative importance of dispersal rate, population size, and time since divergence on gag genetic homogeneity. Using empirical estimates of long-term effective population size (16 500), a range of dispersal rates and divergence times (500 to 500 000 generations) was modeled, and it was concluded that present-day genetic homogeneity must be maintained by dispersal rates of at least 20-30 migrants per generation and up to hundreds or thousands per generation. This study also documents the absence of significant temporal genetic structure and inbreeding in the Atlantic when comparing cohorts separated in time from weeks to 20 years. This suggests that the long-standing practice of overfishing gag has yet to manifest as an inbreeding effect. Overall, gag dispersal rates remain uncertain, and consequently, this study can neither support nor refute management schemes that independently regulate gag in the eastern Gulf of Mexico and northwestern Atlantic.Résumé : Les badèches baillou (Mycteroperca microlepis) de l'est du golfe du Mexique et du nord-ouest de l'Atlantique sont gérées comme des stocks distincts, bien que les données sur leur isolement démographique restent équivoques. Nous avons déterminé le génotype à 11 locus microsatellites de plusieurs centaines d'individus et nous n'arrivons pas à distinguer génétiquement les badèches baillou dans l'ensemble de ces régions (F ST < 0,001). Une simulation sur ordinateur basée sur la coalescence nous à servi à évaluer quantitativement l'importance relative du taux de dispersion, de la taille de la population et du temps depuis la divergence pour l'homogénéité génétique de la badèche. En utilisant des estimations empiriques de la taille effective de la population à long terme (16 500), nous avons modélisé une gamme de taux de dispersion et de temps depuis la divergence (500 à 500 000 générations); il en ressort que l'homogénéité génétique actuelle doit être maintenue par des taux de dispersion d'au moins 20-30 migrants par génération jusqu'à des centaines ou des milliers de migrants par génération. Notre étude note aussi l'absence de structure génétique temporelle significative et de consanguinité dans l'Atlantique dans des comparaisons de cohortes séparées dans le temps par des périodes allant de semaines à 20 années. Cela indique que la pratique de longue date de surpêche de la badèche baillou ne s'est pas encore manifestée comme un effet de consanguinité. Globalement, les taux de dispersion de la badèche demeurent incertains et, en conséquence, notre étude ne peut ni appuyer, ni rejeter les propositions de gestion qui contrôlent de façon indé...

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