The occurrence of problematic pest organisms is an increasing global phenomenon, adversely affecting a range of environments and associated values. In marine systems, the efficacy of pest control has to date been constrained by a lack of tools that are not only highly effective, but also applicable across broad spatial scales. Here we consider the extent to which biological control (biocontrol) has the potential to fulfil these needs. We describe different biocontrol approaches and potential ecological mechanisms (e.g. consumption, space competition, habitat modification) through which problematic species could be supressed. We also discuss the ideal traits of marine control agents within the context of the selection criteria commonly applied in terrestrial systems. Classical biocontrol based on the deliberate introduction of non-indigenous agents has a high risk of leading to adverse non-target effects in marine environments, and cannot be justified. By contrast, approaches that use indigenous species have a low risk of unacceptable non-target effects, and could be used as part of pest eradication, as a means of containing spread, or for the control of established pest populations to mitigate adverse effects. While biocontrol based on indigenous species can be highly effective for such purposes, it is unlikely that it could be feasibly applied at broad spatial scales, except in specific circumstances (e.g. in some types of aquaculture). There is clearly a need to develop new approaches to manage marine pests. Biocontrol when used in conjunction with traditional approaches can provide a valuable tool for pest eradication, containment and mitigation of adverse effects.

The trade in marine and estuarine ornamental species has resulted in the introductions of some of the world’s worst invasive species, including the seaweed Caulerpa taxifolia and the lionfish Pterois volitans. We conducted an analysis of the historical introductions and establishments of marine and estuarine ornamental species in California using a database (‘NEMESIS’) and the contemporary fluxes (quantities, taxa) based on government records, direct observations of aquarium-bound shipments, and internet commerce. California is the major port of entry of marine ornamental species in the United States, which is the major global importing country. The vector was considered possibly responsible for twelve species introduced between 1853–2011, nine of which successfully established, including Caulerpa taxifolia (Mediterranean invasive strain). The flux of imported ornamental species was over 11 million individual animals in 2009 and included 37 taxa from at least six temperate countries, although the majority originated from the Indo-Pacific region. Almost 4,000 individuals representing at least 149 species were imported into San Francisco on a single day in 2012. Estimates of imported quantities were probably accurate within an order of magnitude but could be improved with data on interstate shipments and internet commerce. Importations of high concern included P. volitans, live rock, Chromis viridis (green chromis), and Cromileptes altivelis (panther or humpback grouper). The low historical establishment rate for ornamental species hypothetically could be explained in part by a low release rate or temperature mismatch, but the flux remains high and is a growing concern that could be addressed by heightened public education.

Most harmful diatoms belong to the marine, planktonic genus Pseudo-nitzschia and are responsible for amnesic shellfish poisoning through the production of domoic acid. Fifteen Pseudo-nitzschia species, nine of them toxigenic (approximately 60% of the species found in our samples) were recovered from 185 ship ballast tanks (water and sediment) destined for Canadian ports. Our results demonstrate that the three Canadian coastal regions receive considerable total propagule pressure (1.2 × 1013, 2.6 × 1013, 1.5 × 1010 cells from water ballast; 4.5 × 109, 3.7 × 1011, 5.3 × 106 cells from sediments), and colonization pressure (15, 11 and 3 species) from these diatoms for Atlantic, Pacific and Great Lakes ports, respectively. Neither ballast water exchange method (intercoastal unexchanged, intercoastal exchanged, transoceanic exchanged), or ship port-of-destination seemed to affect sample groupings recovered by hierarchical clustering and multidimensional scaling. Only weak separations of samples by coastal region, propagule pressure/colonization pressure, and the number of days in the ballast since mid ocean water exchange were recovered. The Canadian Atlantic coast is under particularly high individual propagule pressure from P. turgidula, and the Canadian Pacific from P. seriata. Both species are toxigenic and not yet reported from either region. Alien to the Atlantic Ocean, highly toxigenic P. australis and P. turgidula have been recently found in Scottish waters, but not yet in Atlantic Canada, with the former relatively common in our ballast water samples. A greater number of species may be dispersed by ballast waters than by sediments because lightly silicified and narrow-valve species were absent in our sediment samples. However, these hardy survivors in the ballast sediments may be better adapted to tolerate suboptimal growth conditions when introduced to non-native regions and/or environments. Pseudo-nitzschia delicatissima, found in Great Lakes ship ballast waters, is known for its tolerance of brackish waters and may be one of the potentially toxigenic coastal species well suited for establishment in the Great Lakes, as has been the case for some other coastal marine organisms.

Risk assessment tools for non-native species can avert ecological and economic harm when they inform regulatory or voluntary management actions that seek to reduce the probability of introducing high-risk species. The Laurentian Great Lakes region contains many aquatic invasive plants, non-native species whose introduction causes economic or environmental harm or harm to human health. Additionally, new species continue to arrive, including through trade (e.g., aquarium, water garden). Currently, each Great Lakes jurisdiction manages a separate risk assessment program, leading to a regional situation with diverse assessment methods and large discrepancies in assessed and regulated species. Because the Great Lakes ecosystem crosses state and national borders, each jurisdiction will be best protected when all jurisdictions prevent the import of potentially harmful species. We have modified an existing risk assessment tool for use across the Great Lakes Basin to assess the invasion risks posed by aquatic plants. The tool comprises 38 questions, with points associated with each response that are summed to give a total score. We assessed all known established aquatic plant species in the Great Lakes (n=40) and found this score to be positively correlated with invasiveness, allowing thresholds to be identified that distinguish between invasive and non-invasive species with 84% to 90% accuracy. Assessing species proposed for introduction with this tool, and using these thresholds to determine acceptable risk, could reduce the number of future invasions. If widely adopted, this risk assessment tool would enable a common suite of species to be regulated and thus a more effective approach to reducing the risk of future invasions. Regional risk assessment approaches should reduce the threat of invasive species where environmental and climate conditions are relatively consistent across jurisdictional boundaries.

Species introduced to the Great Lakes region through shipping, pet and nursery trade, and as biological control have caused significant environmental damages and have increased the direct and indirect costs to boat owners and various water-dependent industries. Once established, recreational boating becomes the primary vector of spread for some of these species, such as zebra mussels (Dreissena polymorpha Pallas, 1771). Prevention and mitigation efforts in the past have focused on boater education, boat washing stations, and inspections; yet these management actions can be expensive with limited or largely unknown effectiveness. In this study, we used a gravity model framework to accurately simulate the spread of an aquatic invasive species. After parameterization, the constructed model effectively simulated the human-mediated movements of the historical dreissenid spread patterns, correctly predicting an average accuracy of 78.2% (standard deviation = 0.01%) lakes invaded per model run. We then used the model to determine the effectiveness of three different invasive species management scenarios in Michigan: deterring boaters from lakes with a high likelihood of invasion, targeted education at high-risk lakes, and a large-scale education effort. Results indicated that deterring boaters from high-risk lakes is effective in the first five years of an invasion, targeted education is more effective at late stages of an invasion, and large-scale education is effective at all stages of an invasion. Our results indicate that managers should be flexible in their management actions and that different strategies are likely more effective at different stages of an invasion.

An evaluation of Zequanox® (a naturally derived biopesticide that is non-toxic to humans and other aquatic life and selectively kills dreissenid mussels) for controlling zebra mussel infestations in shallow-water habitats in lakes was conducted at Deep Quarry Lake in DuPage County, Illinois during summer 2012 and 2013. During the 2012 trial, three sets of paired 24-m² treatment and control sites were established within the lake, while a single 324-m² treatment site was established for the 2013 trial. Zequanox was applied to treatment plots, contained using PVC barrier curtains, and barriers were removed during the morning following application. Zebra mussel mortality and size distributions on natural substrates were assessed one day and one week post-treatment for 2012 trials and one day and two weeks post-treatment for 2013 trials; percent mortality of zebra mussels in mesh containers in treatment and control sites was also monitored up to 14 days and 48 days post-treatment in 2012 and 2013, respectively. Several water quality parameters were measured in control and treatment plots before and during application and up to 14 d post-treatment. Mean percent mortality for adult zebra mussels on natural and artificial substrates in treatment locations 7–48 d following Zequanox application ranged from 92–98% during both years, while mortality was consistently ≤10% in control locations. Mean percent mortality ranged from 15–76% in locations > 5 m from and in water shallower than Zequanox application points (<0.6 m depth) during the 2013 trial likely due to limited product dispersal into these areas. There was no significant difference in the size distribution of live and dead zebra mussels in treatment plots. Mean veliger mortality was 94.4% 20-h after the start of the 6-h Zequanox treatment period in the treatment area compared to 15.1% in untreated locations during the 2013 trial. Temporary but substantial reductions in dissolved oxygen were observed in treatment locations during the morning following Zequanox treatment in both 2012 and 2013 trials, likely due to the presence of the barriers that prevented well-oxygenated water from circulating into treatment zones from adjacent areas in the lake. Dissolved oxygen concentrations quickly rebounded to levels consistent with control sites upon removal of barriers. No effects of Zequanox treatment on ammonia, total nitrogen, total phosphorus, biochemical oxygen demand, chlorophyll a, pH, or conductivity were observed. Results suggest that Zequanox has potential as a tool for controlling zebra mussels in shallow-water habitats in lakes without significant long-term effects on water quality.

Mozambique tilapia Oreochromis mossambicus were recently discovered in ‘Aimakapā Fishpond, a 12-hectare brackish-water wetland complex in Kaloko-Honokōhau National Historical Park, on the Island of Hawai’i. As a possible eradication method, we evaluated rotenone, a natural piscicide used in fish management and the active ingredient in plants traditionally used by indigenous Hawaiians for capturing fish. To assess rotenone’s efficacy in killing tilapia and effects on non-target species, laboratory toxicity tests involved exposing organisms to various concentrations of liquid CFT Legumine (5% rotenone) in static trials of 48-h to 72-h duration. Test organisms included: Mozambique tilapia, non-native guppy Poecilia reticulata, the non-native odonate Rambur’s forktail Ischnura ramburii, native feeble shrimp Palaemon debilis, and native ‘ōpae’ula shrimp Halocaridina rubra. All organisms and water used in tests were obtained from ‘Aimakapā (12.6–12.7 ppt salinity), or, for H. rubra, an anchialine pool (15.0–15.2 ppt salinity). Survival analyses indicated CFT Legumine concentrations >3 ppm (>0.15 mg/L rotenone) achieved 100% mortality of tilapia and 93% of guppies within 24 h, with most tilapia killed by 6 h and most guppies by 2 h. Little or no mortality was observed among invertebrate exposed to 1 to 5 mg/L CFT Legumine: 0% mortality for ‘ōpae’ula shrimp, 4% for feeble shrimp; and 16% for odonate larvae. The 48 h LC50 values for Mozambique tilapia and guppy were 0.06 and 0.11 mg/L rotenone, respectively. Results demonstrate rotenone’s potential for non-native fish eradication in brackish-water habitats, with benefit of low mortality to certain macro-invertebrates. High rotenone tolerance displayed by ‘ōpae’ula shrimp is noteworthy. Invasive fish are common in anchialine pools, threatening existence of shrimp and other invertebrate fauna. Although rotenone’s effects on freshwater organisms have been well studied, our research represents one of only a few controlled laboratory experiments quantitatively assessing rotenone tolerance of brackish or marine fauna.

Natural history museums provide permanent storage for specimen collections, including non-native species. We extracted the records for specimens and photographs of exotic non-native fishes collected in Japan by experts and citizens at the Kanagawa Prefectural Museum of Natural History, Japan. The museum began operation in 1994. The records of alien species known to be established in Japan (FSAK) consisted of 1756 specimens (789 lots) belonging to 29 species and 611 photographs (494 lots) of 25 species. Additionally, there were records of alien species that were introduced to Japan but not known to be established (FSUK) consisting of 23 specimens (23 lots) belonging to 11 species and of 46 photographs (31 lots) of 17 species. The FSAK could be classified as 22 primary freshwater, six diadromous, and one marine species, whereas the FSUK were classified as 12 primary freshwater, one diadromous, and six marine species. We identified a significant difference in the life-cycle types of FSAK and FSUK suggesting that fluvial species are established more easily than marine species, which reflects the biogeography of Japan. In addition, the records of FSUK were probably caused by an increase of aquarium fish introductions due to dereliction of pet fish, ejectment for pleasure, or crime by traders. The museum collections were mostly provided by experts, followed by citizens and other institutions. We also discussed the functions of a public museum of natural history for accumulating information and for citizen participation.

Ireland is one of the few locations in Europe where non-indigenous North American crayfish species have not been introduced, and is a refuge for endangered white-clawed crayfish, Austropotamobius pallipes (Lereboullet, 1858). The parthenogenetic crayfish species Marmorkrebs, Procambarus fallax f. virginalis (Hagen, 1870), is sold in the pet trade in Ireland within the recorded range of A. pallipes. Marmorkrebs risk being introduced into Irish waters, where they could threaten A. pallipes populations, particularly as a vector for crayfish plague.