Discussion
In this study, a MCDA risk ranking framework integrating empirical data
and expert opinion was used to rank pathogens in the live baitfish
pathway. Applying the framework as a case study to the problem of
pathogen introduction via the Minnesota bait pathway resulted in
distinct risk scores for each of the 15 pathogens assessed. The
highest-risk pathogen group included the Asian fish tapeworm, O.
ovariae, VHSV, FHMNV, IPNV, A. salmonicida and Y.
ruckeri . To our knowledge, this is the first study that has employed
both semi-quantitative scores and expert opinions to evaluate and rank
pathogens in the live baitfish pathway. The inclusion of expert
judgement in the risk ranking exercise allowed a more detailed ranking
analysis with distinct risk scores, avoiding the risk score clustering
observed in the unweighted system. The weighted framework also made
explicit the impact of subjective beliefs about which criteria were most
important, emphasizing the importance of considering value judgements
when making decisions about which pathogens to manage.
The Asian fish tapeworm, O. ovariae, and VHSV were the top-ranked
pathogens in both the unweighted and weighted risk scoring systems,
confirming the relevance of these three fish pathogens to the bait
supply pathway. The highest ranked pathogen was the non-native Asian
fish tapeworm, a generalist fish parasite that can infect hundreds of
fish species and known to be present in the live baitfish supply in the
region (Boonthai et al. 2017; Kuchta et al. 2018). Ovipleistophora
ovariae is an obligate intracellular and vertically transmitted
parasite, infecting the ovarian tissue of golden shiners, leading to
significant declines in fecundity by age-2 (Phelps & Goodwin, 2008).
Although O. ovariae is believed to be widely distributed and
highly prevalent in the golden shiner supply chain, surveys of wild
populations to confirm establishment have not been completed (McEachran
et al. in review), and the parasite remains of concern. Indeed, a
previous qualitative risk assessment for golden shiners imported from
Arkansas bait producers identified both Asian fish tapeworm and O.
ovariae as high-risk (J. L. Gunderson, 2018)(J. Gunderson, 2004).VHSV
is a broadly recognized risk to fish health globally (Escobar,
Escobar-Dodero, & Phelps, 2018), and following its invasion in the
Great Lakes in 2003 (Elsayed et al., 2006), has been identified as a
concern in previous evaluations of the Minnesota bait industry (Boersen
et al., 2017; Phelps, Craft, Travis, Pelican, & Goyal, 2014).
The results of the risk ranking framework highlight the paradoxes of
risk management efforts that focus on the host species, rather than the
pathogen of interest. For example, the ranking framework identified the
Minnesota certifiable diseases IPNV, A. salmonicida , and Y.
ruckeri as high-risk hazards for the bait pathway. These pathogens can
have serious fish health implications for salmonid species (Furones,
Rodgers, & Munn, 1993; Roberts & Pearson, 2005; Wiklund & Dalsgaard,
1998) and are consequently regulated in Minnesota to limit introduction
and spread (MN Statute 17.4982). However, these regulations only apply
to salmonid species, despite known susceptibility and evidence of at
least A. salmonicida and Y. ruckeri in the local retail
baitfish supply (McEachran et al, in review). In contrast, VHSV is
another state-certifiable pathogen identified as high-risk in this
study, but it is managed at the pathogen level, with all susceptible
species (including legal bait species) subject to regulatory conditions
(MN Statute 17.4991). These paradoxes highlight the importance of
managing specific invasive pathogens of known risk, rather than host
species, when attempting to reduce the risk of pathogen spread via any
live animal movement pathway.
Estimates of evidence uncertainty varied across pathogens, with some
pathogens having higher or lower uncertainty than average (Figure 3a).
Some pathogens in the high-risk group (e.g. FHMNV and IPNV) and low-risk
pathogens (e.g. WSBV, FHMPV) obtained high uncertainty scores,
suggesting that as more information becomes available in the future, the
risk ranking may change for these less well-described pathogens. Because
of the high number of fish species and increasing rates of pathogen
reporting and surveillance, pathogens of fish account for a large number
of emerging diseases of wildlife (Tompkins et al., 2015), and so
invasion management tools must be equipped to dealing with both emergent
and well-documented pathogens. Fish health managers could apply the risk
ranking to evaluate potential risk and determine what type of action, if
any, is warranted, based on their own tolerance for uncertainty and risk
(Figure 3b). If new evidence emerges in the future, the risk ranking
framework can be updated and risk ranking scores recalculated, providing
support for risk-based disease management.
It is important to note that while the risk ranking framework identifies
pathogens of importance (‘high risk’) in the live baitfish supply, this
does not directly translate to an inevitable impact on wild fish
populations. Like all invasion scenarios, many factors must align to
result in the successful establishment and negative outcome of a hazard
(e.g. baitfish pathogen) to a new environment (e.g. naïve wild
population of concern) (Simberloff, 2009; Stohlgren & Schnase, 2006;
Wang & Jackson, 2011). Examples of failed introductions are impossible
to quantify given the limited information for the disease status of
baitfish and their movement patterns, and the disease status of wild
populations. For VHSV, a pathogen where significant surveillance has
occurred (i.e. Phelps et al. 2014), no detections have occurred in the
Minnesota baitfish supply and therefore transmission via this pathway is
presumed to be nonexistent. Evaluating the current distribution and
potential for establishment of high-risk pathogens known to be in the
baitfish supply (e.g. O. ovariae , A. salmonicida , Y.
ruckeri ) is warranted to better inform future risk assessments.
Regardless, the risk ranking framework is a useful tool to identify and
prioritize pathogens for further management consideration and provide
justification for proactive prevention efforts.
Incorporating variability and uncertainty from multiple different
stakeholder groups (managers, academia, and industry), and not just a
single sector, is increasingly recognized as a critical part of managing
invasive species (Shackleton et al., 2019). The expert opinion-based
risk ranking framework developed in this study incorporates expert
opinion with empirical evidence, and improves on previous qualitative
evaluations and unweighted rankings to distinguish between high-,
medium-, and low-risk pathogens in the live baitfish supply. Where
uncertainty exists, the precautionary principle is often employed,
whereby novel and highly uncertain pathogens are automatically assigned
a high-priority ranking and allocated resources and risk management
efforts (Larson et al., 2013; Sainsbury & Vaughan-Higgins, 2012) This
approach risks obfuscating management plans and creating burdensome
regulations for producers (van Senten & Engle, 2017). Conversely,
failure to systematically assess all possible hazards may indeed
overlook important pathogens, leaving fish populations at risk (Gaughan,
2001). Although the framework discussed in this study provides a
relative, rather than an absolute determination of risk, disease
management in the absence of a structured and inclusive process for
identifying hazards could result in the over or under-inclusion of
important pathogens. This framework has broad applicability for
understanding risks and will provide support for difficult conservation
decisions to balance invasion risks with the economic, cultural, and
societal benefits associated with live baitfish use. In addition, its
flexibility in application means it could be modified to assess risks in
other pathways, jurisdictions, or other taxa of invasive species as
invasion prevention needs emerge.