Abstract
The creation and deployment of plastic structures made out of pipes and panels in freshwater ecosystems to enhance fish habitat or restore freshwater systems have become popularized in some regions. Here, we outline concerns with these activities, examine the associated evidence base for using plastic materials for restoration, and provide some suggestions for a path forward. The evidence base supporting the use of plastic structures in freshwater systems is limited in terms of ecological benefit and assurances that the use of plastics does not contribute to pollution via plastic degradation or leaching. Rarely was a cradle-to-grave approach (i.e. the full life cycle of restoration as well as the full suite of environmental consequences arising from plastic creation to disposal) considered nor were decommissioning plans required for deployment of plastic habitats. We suggest that there is a need to embrace natural materials when engaging in habitat restoration and provide more opportunities for relevant actors to have a voice regarding the types of materials used. It is clear that restoration of freshwater ecosystems is critically important, but those efforts need to be guided by science and not result in potential long-term harm. We conclude that based on the current evidence base, the use of plastic for habitat enhancement or restoration in freshwater systems is nothing short of littering.
Introduction
Aquatic ecosystems around the globe have suffered from habitat alteration and loss for decades (Arthington et al. 2016). This is particularly evident in freshwater systems and has contributed to dramatic declines in freshwater biodiversity (Dudgeon et al. 2006; Reid et al. 2019). Indeed, losses have been so extreme that protection and restoration of freshwater systems are included in the so-called ‘emergency recovery plan’ for freshwater biodiversity (Tickner et al. 2020). Habitat alteration and loss come in many forms including the loss of structural habitat features from lotic and lentic systems. For decades, research has focused on how to restore such environments to benefit aquatic biodiversity (e.g. Søndergaard et al. 2007; Wohl et al. 2015). Even created environments such as reservoirs have benefited from such habitat enhancement initiatives (Moore and Thornton 1998; Cooke et al. 2016). Freshwater restoration and habitat enhancement can include many techniques (e.g. biomanipulation, reducing nutrient inputs, riparian/shoreline plantings) although one of the most common methods is the addition of physical habitat structure in the form of woody materials (e.g. logs, root wads, stumps; also sometimes referred to as coarse woody debris).
Structural aspects of freshwater habitat are particularly important for freshwater fishes (Sass et al. 2022) and provide a range of ecological functions (Harmon et al. 1986). Freshwater fishes use these structures for a variety of purposes such as feeding and predator avoidance (Sass et al. 2022). Much of what we know about the value of structural aspects of fish habitat comes from experiments where various structural elements are removed and (or) added (e.g. Bryant 1983; Lehane et al. 2002; Helmus and Sass 2008; Sass et al. 2012; Sass et al. 2019) and it is clear that in most systems the addition of woody structural habitat (particularly where it is absent or minimally present such as in some reservoirs, lakes and rivers) derives some benefit to fish (Sass et al. 2022). However, there are also instances where such structural additions simply serve to attract and aggregate organisms rather than improving fish abundance or condition (Lindberg 1997). Given the current freshwater biodiversity crisis, efforts to restore or enhance freshwater systems may include the addition of structural elements of habitat. Ensuring that such efforts benefit freshwater and in no way harm aquatic life should be paramount to those engaging in such restoration and habitat enhancement efforts (Cooke et al. 2018).
About 40 years ago, there was a big push to evaluate various anthropogenic materials for use in artificial reefs in the marine realm. From used tires (Campos and Gamboa 1989) to war ships (Johnston et al. 2003), many different artificial materials and structures were sunk in coastal marine waters in an attempt to enhance or restore marine ecosystems (Bohnsack and Sutherland 1985; Ramm et al. 2021). The nature of the marine environment is such that many of these anthropogenic structures were often quickly colonized and encrusted by sessile organisms. Fish (and other organisms) of various sizes would take up residency and provide opportunities for anglers, divers and spearfishers to connect with wild marine life (e.g. Stolk et al. 2007; Shani et al. 2012). Over the last few decades, there has been vigorous debate about whether these anthropogenic materials were actually enhancing production or were simply serving to redistribute organisms (i.e. the attraction–production controversy; Bohnsack and Sutherland 1985; Grossman et al. 1997; Osenberg et al. 2002; Sass et al. 2022). There was also discourse about the extent to which such efforts could be considered restoration or a form of pollution (MacDonald 1994; Chou 1997; Eggen 1997; Baine 2001). Over time, research revealed that artificial materials such as tires and ships led to the release of a number of contaminants (e.g. Collins et al. 1995; Devault et al. 2017). Although artificial reef development continues to this day, it is being done more cautiously given an expanded evidence base revealing uncertainty about ecological benefits and a growing understanding of the negative environmental consequences of placing structures made of anthropogenic materials in the ocean. In fact, some efforts have been undertaken to remove tire reefs (see Sherman and Spieler 2006). Concurrently, there has been increasing public awareness about the plight of oceans and plastic pollution which is also contributing to more scrutiny about the materials used in artificial reefs. There are some interesting parallel developments in the freshwater realm, which is the focus of our essay.
The concept of freshwater reefs or other artificial habitats is not entirely new with examples extending back to the 1970s using tires (Prince and Maughan 1978). However, it was not embraced to the extent it was in the marine realm. More typically, large wood structures or placement of rocks are commonly used approaches for the restoration or enhancement of lotic and lentic freshwater environments (Thompson et al. 2018; Rytwinski et al. 2019; Sass et al. 2022; Theis et al. 2022). Over the last decade or so, there has been growing popularity in the use of various structures composed largely of plastics (Sass et al. 2022). For example, polyvinyl chloride (PVC) or acrylonitrile butadiene styrene (ABS) piping or panels (as well as other plastic materials) in a range of sizes and configurations such as crib-like structures or those that mimic a tree (see Fig. 1) are being added to lentic freshwaters. Such projects are usually led by government agencies or local angling clubs, but individual anglers or waterfront property owners may also undertake such activities themselves. Many of the projects are ‘do-it-yourself’ where various plans can be found online (e.g. youtube.com/watch?v=r4J_HDl8oxg; wired2fish.com/fishing-tips/building-fish-habitat-to-enhance-older-lakes/; texasfishingforum.com/forums/ubbthreads.php/topics/10191294/DIY_Fish_habitats/attractors_o) and used to source and then construct plastic habitats. There are also bespoke products available from commercial manufacturers that can be purchased. Deployment typically happens from a large boat or barge with structures weighted down using heavy materials such as concrete blocks. These projects are often conducted with great fanfare including press releases and even politicians on site to celebrate the ‘conservation’ work. To authors (i.e. several scholars who work and teach in the realm of ecological restoration, freshwater ecology, and freshwater conservation and management), this is greatly troubling in that the addition of plastic structures to freshwater bodies is akin to littering. Ecological restoration remains imperfect (Suding 2011), but we know the value of using natural materials such as wood (i.e. logs, trees), rocks and native vegetation so are perplexed by the emphasis on plastics. Here, we outline a number of issues with these activities and the associated evidence base and then provide some suggestions for a path forward.
