Ecosystem-based management of seaweed harvesting

Abstract

Harvesting wild seaweeds has a long history and is still relevant today, even though aquaculture now supplies >96% of global seaweed production. Current wild harvests mostly target canopy-forming kelp, rockweed and red macroalgae that provide important ecosystem roles, including primary production, carbon storage, nutrient cycling, habitat provision, biodiversity and fisheries support. Harvest methods range from selective hand-cutting to bottom trawling. Resulting ecosystem impacts depend on extraction method and scale, ranging from changes in primary production to habitat disruption, fragmentation, food-web alterations and bycatch of non-target species. Current management often aims for sustainable harvesting in a single-species context, although some agencies acknowledge the wider ecosystem structure, functions and services seaweeds provide. We outline potential ecosystem-based management approaches that would help sustain productive and diverse seaweed-based ecosystems. These include maintaining high canopy biomass, recovery potential, habitat structure and connectivity, limiting bycatch and discards, while incorporating seasonal closures and harvest-exclusion zones into spatial management plans. Other sustainability considerations concern monitoring, enforcement and certification standards, a shift to aquaculture, and addressing cumulative human impacts, invasive species and climate change. Our review provides a concise overview on how to define and operationalize ecosystem-based management of seaweed harvesting that can inform ongoing management and conservation efforts.

Biomass and height of Ascophyllum nodosum after two decades of continuous commercial harvesting in eastern Canada

“…While our sampling sites covered a broad geographical range, our results may not apply to all A. nodosum beds in eastern Canada especially those in regions with much lower growth rates. Lotze et al (2019) have reviewed the canopy and community impact of A. nodosum harvesting based on short-term studies and found that the type of harvest played a major role in mitigating the impacts. While a broad range of species has been found in A. nodosum beds (Seeley and Schlesinger 2012), little is known about the specific requirements of these species as to the A. nodosum beds.…”

“…While cultivation accounts for most of the global production (96%; FAO 2018), harvesting of wild stocks continues to play a vital role in the sustainable livelihood of many coastal communities (Rebours et al 2014) despite the fact that only a fraction of the 10,000 algal species known are being commercially harvested (Mac Monagail et al 2017). Given the ecological importance of seaweed in primary production, carbon cycling, detrital production, and carbon sequestration as well as habitat and food for a whole community of organisms, a proper management framework is essential to ensure their sustainable harvest (Lotze et al 2019). Without considered management, some seaweed resources could become overexploited (Vásquez 2008), and their harvest can have an impact on multiple trophic levels within a food web (Lorentsen et al 2010).…”

The intensity of kelp harvesting shapes the population structure of the foundation species Lessonia trabeculata along the Chilean coastline

“…40 m (Dayton et al 1985;Harrold and Pearse 1987;Vásquez 1997;Graham et al 2007) coastline (Krumhansl et al 2016;. They form three-dimensional biogenic structures which offer habitat, shelter and food to a wide diversity of associated species, including species of commercial importance (Teagle et al 2017;Lotze et al 2019). As such, kelp species serve as foundation organisms that support high levels of biodiversity and provide a wide range of ecological goods (e.g.…”

“…For example, kelp harvesting can cause changes in population dynamics and loss of standing biomass at regional scales (Geange 2014), affect predator-prey interactions (Lorentsen et al 2010), as well as impact kelp-associated species, including fish (O'Connor and Anderson 2010) and macroalgae (Vasquez and Santelices 1990). Impacts of harvesting at the population, community and ecosystem level can consequently impinge on the provision of ecological goods and services, with the magnitude of impacts dependent on harvesting methods, target species, and the scale, duration and frequency of harvesting (Waage-Nielsen et al 2003;Steen et al 2016;Lotze et al 2019).…”

“…In recent decades, due to the high demand for kelpderived products such as alginates, commercial kelp harvesting has increased, leading to declines in kelp biomass and abundance in some regions (Wernberg et al 2018). Currently, Chile accounts for approximately 40% of wild kelp harvesting globally (Lotze et al 2019), where this fishery is one of the most important benthic fisheries in the country, directly or indirectly employing 13,000 people and landing up to 350,000 dry tonnes per year (valued at US$90 million) (Vásquez et al 2014). Prior to the twenty-first century, the fishery was mainly sustained by natural mortality, where plants cast ashore were collected by artisanal fishermen, with little-to-no impact on wild populations or ecological processes (Vásquez et al 2012).…”