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The quagga mussel, Dreissena bugensis, is a harmful aquatic pest that invaded the Southwestern United States in 2007. Challenges with managing this pest have been encountered because the invaded systems are primarily open water sources used for human consumption and/or are connected to freshwater habitats containing threatened and endangered species. Existing chemical and physical control methods are undesirable, with use of some methods restricted or prohibited, because they pose risks to humans and ecosystems more broadly. To address this problem, we investigated the efficacy of using resident fishes as biocontrol agents for managing different life stages of quagga mussels on different spatial scales in a site-specific manner. We conducted field experiments to test whether planktivorous Bluegill, Lepomis macrochirus, reduced mussel infestations on substrates of varying orientations in small and large pens through predation on larval mussels. We also conducted an experiment to evaluate whether the carnivorous Redear Sunfish, Lepomis microlophus, reduced mussel infestations established on substrates of varying orientations in small pens through predation on juvenile and adult mussels. Bluegills significantly reduced mussel infestations on all substrates in the pens through predation on larvae and small, juvenile mussels. Redear Sunfishes reduced existing juvenile and adult mussel populations in some cases, with consumption varying among individuals and substrate orientation. Our results indicate that fishes, specifically Bluegill, may represent effective site-specific biocontrol agents for quagga mussels, reducing impacts on targeted infrastructure (e.g., water towers, docks, pipes) and habitats having different surface orientations by controlling more than one life stage of the pest. Development of an integrated pest management strategy, that considers application of this tactic in combination with others, would undoubtedly improve management of quagga mussels, and potentially congeneric zebra mussels, within lake and reservoir ecosystems.

Biological invasions can dramatically affect the ecology of invaded regions, and globally have resulted in economic damages that total billions of dollars annually. In recent years, an invasive alga, Sargassum horneri, has become established and spread along the coast of southern California (USA). Using field observations and a field experiment, we explored how this non-native alga influences the structure of fish assemblages on temperate reefs in southern California where S. horneri has become prolific. Fish and algal assemblages were quantified along transects on rocky reefs at depths of 3 and 6 m at 6-8 study sites spanning 5 km on 4 occasions over 1.5 yr. Spatiotemporal variation in the fish assemblage was not strongly correlated with the abundance of invasive S. horneri over this period, although it became less variable as native giant kelp Macrocystis pyrifera disappeared from the study sites due to a warm-water event, during which the invasive S. horneri became more dominant. An experiment removing a total of 4.25 t of S. horneri from 6 × 6 m plots (n = 14) revealed that the invasive alga did not affect fish abundance, species richness, species diversity (H’), or multivariate assemblage structure over a 5 mo period. Overall, we found little evidence of negative effects of S. horneri on fishes even though it drastically changed the underwater landscape. Nevertheless, we advise cautionary management actions to limit the movement of this invasive alga because its effects on other community members, such as other algal species, may be detrimental, and longer-term effects on fishes might develop.

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