Our findings suggest that the effectiveness of a binder to retain Saccharina latissima gametophytes onto cultivation rope, measured by the sporophyte density, was comparable in the high flow velocity (15 cm s− 1) across all binder treatments, including the non-binder control treatment. The highest sporophyte densities were observed in relatively low flow velocities (0 and 5 cm s− 1) in the absence of a binder.
Our results confirm previous studies that found relatively poor growth of S. latissima after using the binder technique in situ compared to traditional hatchery methods (Boderskov et al. 2021; Forbord et al. 2020). Adherence of sporophytes onto substrate was found problematic both in situ (Boderskov et al. 2021; Forbord et al. 2020) as well as under controlled laboratory conditions (Umanzor et al. 2020). Interestingly, a Danish study found the binder method to be only successful at the exposed site compared to more sheltered sites, potentially confounded by fouling on the rope rather than a lack of adherence at the sheltered sites (Boderskov et al. 2021). Finally, Kerrison et al. (2018) found no difference between the traditional hatchery seeding method and using a binder to adhere juvenile sporophytes in situ. Taken together, these results highlight that the successful use of a binder to aid adherence of propagules in kelp aquaculture is context dependent and partially determined by the environmental conditions at the cultivation site.
A limitation of our study may be the transferability of a controlled laboratory study into a real-world scenario where binders are used in situ. However, Kerrison et al. (2020) found a positive correlation between density of S. latissima sporophytes on the cultivation line and growth performance (i.e., yield) when gametophytes were applied using a binder in situ. Thus, confirming the assumption that higher plant densities typically lead to higher yield for this species. However, the inversed relationship (i.e., high density leads to reduced harvested biomass) was noted for the kelp species Alaria esculenta (Kerrison et al. 2020). In addition, we did not test the effect of substrate that may accompany and further assist adherence of kelp propagules.
Thus far, most studies have focussed on a limited number of Atlantic kelps (i.e., S. latissima and A. esculenta). Future work could focus on other commercially interesting species, that have different modes of adherence/settlement depending on the life-stage compared to kelp or are not traditionally cultured. For many of these species or life-stages, adherence onto substate is a major challenge and assisting them may proof beneficial for subsequent large-scale cultivation at-sea. For example, the green species Ulva (Steinhagen et al. 2021), Chaetomorpha (Gao et al. 2018), Cladophora (de Paula Silva et al. 2008), the red species Asparagopsis (Zhu et al. 2021) and Palmaria (Grote 2019), or species of the order Fucales, such as Sargassum (Xie et al. 2013), Phyllospora (Cumming et al. 2020), and Durvillaea (Velásquez et al. 2020). Furthermore, the use of a binder can be helpful in seaweed restoration projects, where propagules or seeded substrates may need assistance with adherence when they are transplanted into areas with relatively high flow rates or wave action (Morris et al. 2020).
In conclusion, our results highlight that the effectiveness of binder assisted seeding of kelp propagules did not differ between the binder and non-binder treatments and was not affected by flow velocities in a controlled laboratory study. These findings are important in the development of novel methods and further optimisation of existing binder-based methods aimed at retaining seaweed propagules onto cultivation rope.