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The Science Behind Felt Sole Bans

I have to admit that I was pretty skeptical about felt sole bans, as you might be too. And if I do my job here, that first sentence will be entirely in past tense. I was skeptical that anglers could make much of an impact on moving invasive species - but I am not sure why. I know well that boats transport aquatic invasive species (AIS) all the time. You can literally follow zebra mussels (Dreissena polymorpha), Eurasian watermilfoil (Myriophyllum spicatum), and other AIS spread along the highways from popular waterbodies to other popular waterbodies. So quite logically, why can't this happen by wading anglers' equipment? As I dug deeper into the literature, it quickly became apparent that wading anglers have spread AIS and that some materials - felt - are thousands of times more likely to spread AIS.

I wrote an earlier article for another publication that has not been published (yet?) but I will update this post if/when it does get published. That article is supported with peer reviewed literature, this will be mostly supported with internet links. Each media has it's own strengths - usually my goal here is to support more scientific articles with peer reviewed literature but since I have already done that, here we go...

Black Earth Creek, pre-NZ mudsnails
Bug sampling on Black Earth Creek - before the NZ mudsnails got there.

For streams, a few of the most significant AIS have been a diatom - "Didymo" or the oh, so accurately named "rock snot" (Didymosphenia geminata); a gastropod, New Zealand mud snails (Potamopyrgus antipodarum); and particularly out west, whirling disease which is caused by a cnidarian (Myxobolus cerebralis) of the class Myxosporea. Viruses are another potential threat to coldwater streams. Viral hemorrhagic septicemia (VHS) has garnered the most attention recently but there are other potential viruses that can be considered AIS, despite the fact that viruses are not alive (or are they non-cellular life?).

New Zealand Mudsnails; Photo by Robyn Draheim (Creative Commons)

New Zealand mudsnails (NZMS) are a small mollusc that is rather obviously native to New Zealand (in fairness, we gave them rock snot!). The snail is small - but interestingly much smaller in the Great Lakes (4-6 mm) than in New Zealand (up to 12 mm). Since its accidental introduction to North America, it has spread over a pretty significant portion of coldwater streams and lakes. In the Midwest, it is known from the Great Lakes, 7 watersheds in Wisconsin, 5 watersheds in Minnesota, and 4 Michigan watersheds, including 3 of Michigan's best known trout fisheries - the Au Sable, Pere Marquette, and Boardman Rivers (USGS NAS). Additionally, NZMS have been naturalized in Australia and Europe. They can reach densities of a half a million individuals per square meter and can have significant food chain effects as high densities which they tend to maintain due to a lack of predators and diseases in their non-native range.

Black Earth Creek trout populations
In 2001, the average catch as the site was about 700 adult trout per mile.

From the Wisconsin DNR report, "Trout Stream Management and Status Report of Black Earth Creek Watershed Dane County, Wisconsin 2019"

New Zealand mudsnails were first recorded in Wisconsin from Black Earth Creek. Fortunately, their spread has been slow but they have spread to a few nearby watersheds. It is rather difficult to assess the effects of NZMS on Black Earth Creek due to the other stressors in the watershed and fish kills associated with these stressors but the fishery has been in decline. It is certainly a case of what scientists would call "multiple anthropogenic stressors" - a way of saying that there are many human-caused stressors that combine to have negative effects. In the case of Black Earth Creek, it is on the edge of Wisconsin's second largest city and Cross Plains population has nearly doubled in last three decades, the watershed has significant amounts of agriculture, and NZMS exist with these other stressors.

From the Wisconsin DNR report, "Trout Stream Management and Status Report of Black Earth Creek Watershed Dane County, Wisconsin 2019"

The Black Earth Creek report shows the decline is most significant for young-of-the-year Brown Trout suggesting a decline in spawning success. The report only mentions NZMS once telling the reader that there is ongoing research on their effects which are to date, largely unknown. They put most of the blame on physical habitat declines and potentially the impacts of water temperatures - likely the result of groundwater withdrawals and stormwater runoff (anthrogogenic effects).

From the Wisconsin DNR report, "Trout Stream Management and Status Report of Black Earth Creek Watershed Dane County, Wisconsin 2019"

While NZMS are a non-native AIS, the story of didymo or "rock snot" is a more interesting AIS tale. Diatoms are incredibly common and important species. It is estimated that diatoms generate 20 to 50% of the oxygen on Earth and are nearly half of the biomass of the oceans. Didymosphenia geminata is a holoarctic species meaning it is native to much of the northern hemisphere. Didymo is a diatom, a Stramenopila, that are photosynthetic and one of many taxa collectively referred to as algae (a terribly useless common name). While didymo is single-cellular, it grows in large colonies or blooms under certain conditions but the species has historically been rare in its native range. The question, of course, is what has lead to its increased importance and "invasiveness"?

Didymo - rock snot - in a stream.
Didymo - rock snot - in a stream.

© David Pérez (DPC), Wikimedia Commons, License cc-by-sa-4.0

Some of the first concern about didymo occurred because of nuisance blooms on Vancouver Island, British Columbia, Canada in the 1980's. Didymo blooms in New Zealand in 2004 created a new found interest in the species. Since then, research has shown them to be native to much of the northern hemisphere and has linked their blooms to low phosphorus and stable water flows. While didymo in New Zealand and Chile are non-native introductions, in the northern hemisphere, it is uncertain what role transport has played in these blooms. However, there is no support for the aggressive genotype hypothesis (keep reading...).

Some invasive species are more accurately invasive genotypes of native species. This is the story for Giant Reed (Phagmites australis) which is generally thought to be native to North America but the American genotype has largely been replaced by a heartier non-native European genotype (Saltonstall 2002). In fact, we know Phragmites was introduced from Europe several times before one particularly invasive genotype became established (Saltonstall 2002). A similar story exists with reed canary grass (Phalaris arundinacea) though it is thought that reed canary grass is often a mixture of native and European genotypes much as many (most?) cattails in Wisconsin are hybrids. There is an unsubstantiated hypothesis about didymo having more invasive genotypes but to date, no genetic evidence supports this hypothesis. However, I do wonder if simply the movement of didymo to other parts of its range has the same effect of transferring other non-native AIS.

With didymo, there is a lot we do not know but we do have good evidence that there are some factors that make it more likely for blooms to occur. While most algal blooms are associated with elevated phosphorus, didymo blooms occur in phosphorus poor streams. Here in the Midwest, blooms are more likely in places like Minnesota's North Shore or other nutrient poor streams. Here in the Driftless, we are less likely to experience didymo blooms. Additionally, didymo appears to be negatively affected by floods and blooms are more common below dams and stream with stable water levels. There is some evidence that climate change has increased didymo blooms (Lavery et al. 2014).

List of other potential AIS in the Midwest:

Evidence that Wading Spreads AIS

A significant change, like restricting felt soled wading boots, requires significant evidence that wading boots spreads AIS.

4 pairs of wading boots
My recent history of wading boots - the Orvis Pros - bright green - are my current pair.

There are basically two lines of evidence for angler spread of AIS. The first is circumstantial but quite damning - AIS are MUCH more common and likely to exist in high angler traffic areas. The simplest and most effective piece of evidence is that didymo have been introduced to Chile and New Zealand and NZMS to Australia, Europe, and North America. More detailed studies have tracked angler effort and movement and linked them to AIS introductions. Again, not a smoking gun but these introductions are centered around access points and do not move upstream of these access points. We know boats and their trailers and livewells spread AIS and AIS have largely followed major highways - waders and wading boots are like to do the same. Secondly, there have been studies looking at the amount of interstitial spaces in felt soled boots and how well they hold AIS. Gates et al. (2008) found that felt soles had the largest interstitial spaces of any wading gear (and it wasn't even close) and that the spores of the species that causes whirling disease were held by felt soles by several thousand orders of magnitude greater than did any other surface.

Anglers are generally not very good at preventing AIS spread for a variety of reasons. But it is a lot to ask of waders as preventing AIS only required "one bad apple" and an AIS is spread to a new location. In fact, I am quite surprised that NZMS have not spread further in Wisconsin from Black Earth Creek (though it has spread to a few other Dane County streams). The decline in the Black Earth Creek fishery likely has something to do with that as there are likely fewer anglers. However, as a scientist, I feel compelled to remind readers that correlation does not necessarily equate to causation.

Felt-Soled Wading Boot Bans

I expect more states to ban wading boots with felt or other porous soles. It simply makes too much sense and the impact of AIS are just too great to "risk it". And again, it just takes one time for an AIS to be introduced and once introduced, there are rather few examples of where AIS have been eliminated. This is largely because that by the time an invasive species is known to exist in a stream, its populations are generally at a level that make them difficult to control.

States with felt-soled / porous sole wading boot bans:

The regulations in New Zealand are a little convoluted - IMHO. They state that "Felt soled waders may be brought into the country. They state that "this is a separate issue from restricting the use of felt soles while fishing" which I suppose it is but, yeah, not really. New Zealand is quite serious about biosecurity and I would suggest not bringing them with you. I know many anglers buy or rent wading gear in New Zealand due to their biosecurity laws. I have experienced these laws second hand when a student I travelled with brought in an apple from Australia that created a semi-large scene at the Auckland airport. From what I can find, Chile suggests you do not use felt soles but they do not have a ban (please correct me if I am wrong). Many Patagonian lodges offer guests boots to prevent the spread of AIS which has been a significant issue in Chile.

FWIW, I post the above video but I think he is very wrong - there is evidence from manipulative experiments (Gates et al. 2008) that show felt held several thousand times more whirling disease spore that did neoprene - the 2nd place finisher - and other materials. Could laces spread AIS, yes but felt is several thousand times more likely to be the culprit. Add to this that the only part of the boot in contact with the stream bottom all the time are the soles. Logically, they are by far the most important in spreading AIS.

I understand that there will be people that do not like these regulations and that think that felt can't be replaced. First, I would ague, tough shit! Do the right thing and prevent the spread of AIS. It is a rather selfish point of view that your wading is more important than not spreading AIS. Second, felt soled boots came into popularity relatively recently (late-1980's) so many had fished without felt for decades before it became a common boot sole. Yes, felt soles are great but they are also problematic. They have large interstitial spaces that harbor AIS and they dry slowly. As more states and countries ban porous materials for wading boot soles, companies will come up with better alternatives. There was a time around the early 2010's when we seemed to be making some gains here. Today, the best non-AIS spreading option are some combination of a soft rubber and metal such as studs or traction bars for anglers that need them. I fish the Driftless Area most of the time and see no need for felt soles which only serve to trap snow and cow shit. I get why it is a larger issue for those fishing larger rivers with larger, slicker substrates - I have fished Big Fishing Creek's Narrows (if you know, you know).

I view felt sole bans as rather necessary to prevent AIS spread. Most anglers are unwilling / unable to disinfect their boots between locations. Many of these AIS survive for quite long out of the water and there are only a few proven methods for disinfection - a 20 minute Virkon Aquatic soak is really the only reliable way to prevent AIS spread. It is what fisheries professionals, hatcheries, and fish farms use to prevent the spread of AIS. Sure, be upset about the bans but know they are probably a necessary "evil" (they're not really that evil...). For those that feel the need for felt soled boots, I think the best options are to 1) have boots dedicated to particular waters where they are needed (they are not needed most places), 2) follow decontamination and drying protocols, and 3) you are ultimately responsible - be a decent human being and do all you can to prevent the spread of AIS (yes, it is a moral issue). I know consciousness anglers in Dane County that have boots dedicated to Black Earth Creek so not to spread New Zealand mud snails to other streams.

As always, I like to provide as much information as possible and I am not the first person to write on this topic.

Literature Cited

Gates, K. K., Guy, C. S., Zale, A. V., & Horton, T. B. (2008). Adherence of Myxobolus cerebralis myxospores to waders: implications for disease dissemination. North American Journal of Fisheries Management, 28(5), 1453-1458.

Lavery, J. M., Kurek, J., Rühland, K. M., Gillis, C. A., Pisaric, M. F., & Smol, J. P. (2014). Exploring the environmental context of recent Didymosphenia geminata proliferation in Gaspésie, Quebec, using paleolimnology. Canadian Journal of Fisheries and Aquatic Sciences, 71(4), 616-626.

Saltonstall, K. (2002). Cryptic invasion by a non-native genotype of the common reed, Phragmites australis, into North America. Proceedings of the National Academy of Sciences, 99(4), 2445-2449.

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