MNF are known to be proliferating around the tropical seas (Short et al. 2018) and their use has been argued to be unsustainable and environmentally destructive. In the present study, we provide the first empirical investigation into the sustainability of MNF by quantitatively analysing catch composition to provide evidence of the ecological cost of these fisheries. We illustrate how social and economic policies (distribution of free mosquito nets and lack of diverse livelihoods) intersect in unexpected and perverse ways that negative consequences for marine ecosystems and human well-being. Landings from these MNF may be considered unsustainable as a result of very high juvenile catch and removal of species with critical ecological functions, more so with the traditional indicators of Malthusian principles being met (Pauly 1988, 1990; Pauly et al. 1989).

The use of MNF is now accepted as a global threat (Short et al. 2018). Recent evidence of this extends throughout Asia and Africa (Srivastava et al. 2002; Abbott and Campbell 2009; Lover et al. 2011) and its use is extensive in both marine and freshwater environments. (Jiddawi and Öhman 2002; Malleret 2004; Barr 2010; Hamerlynck et al. 2011). As with other small-scale and artisanal fisheries, landings data on these fisheries are poor (Berkes 2001) and in the academic literature, we are not aware of any previous examples of the quantitative analysis of MNF landings. Due to the fine mesh size (≤ 3 mm) needed for the exclusion of mosquitos, these nets are entirely unselective if used as fishing gear. As a result, our findings provide evidence in support of previous qualitative generalisations that these MNF are of major conservation concern due to the high numbers of juveniles they catch (Short et al. 2018).

Our study revealed that across a range of sites, 56% of the species and 61% of the total biomass of catches were comprised of juvenile fish. Also, we find very low fish density when the catch is quantified per-unit-area, relative to similar regional studies with much greater diversity which we discuss further below (Gell and Whittington 2002). The levels of juvenile catch recorded for MNF are high and are comparable to other fishing gears that are being used in an unsustainable manner. A recent examination of static fish fence gears found a 40% juvenile fish count (Exton et al. 2019), whilst beach seine net gears have been recorded in Kenya to have juvenile proportions at up to 68% (Mangi and Roberts 2006).

Our study was restricted diurnally, and nocturnal fishes, particularly some of the larger predatory species that migrate into seagrass meadows during night, may not have been recorded in catches (Unsworth et al. 2007), these functionally important species were found to be in very low abundance throughout Palma Bay more broadly, indicative of a fishery in decline (Unsworth et al. 2015). Only one obligate piscivore (Pickhandle barracuda) was recorded across catches and while individuals from predatory groups such as the Snappers, Emperors and Groupers were present, these were in very low abundance in terms of frequency and biomass compared with similar and more selective regional seagrass fishery catches (Gell and Whittington 2002; de la Torre-Castro et al. 2014). From a sustainability perspective, the patterns presented here of low catch yield, lack of key functional groups and low trophic diversity likely indicate a fishery in a state of overexploitation and possibly near collapse.

The catch composition of this MNF of Mozambique indicates that this technique is far more effective in catching broad species assemblages than previously reported (Short et al. 2018). Our data record the presence of individuals from 29 families from just 25 MNF catch landings. Additionally, the average biomass for a single drag, at 1.4 kg, is high compared with the national small-scale fishery catch rate of 2.47 kg fisher−1 day−1 (Jacquet et al. 2010), especially when considering that multiple drags can be and are conducted in a single day (only 3 drags in 45 min in this study). The presence of only 39 species of marine fish and invertebrates throws into question the sustainability of MNF when compared with census data from protected seagrass meadows within the Quirimba Archipelago (< 100 km away). Using seine net catches (with similar mesh size to this study), Gell and Whittington (2002) recorded 249 species of fish in 62 families from seagrass meadows evidencing a stark difference between a more natural assemblage and the study assemblage, likely a result of intensive overfishing in the study area.

The use of mosquito nets for fishing represents a growing change demographics of the fishing community. Women (and children) contribute significantly to fisheries both directly (e.g. gleaning) and indirectly (e.g. sorting, de-scaling and selling catch), but with women and children now entering finfish fisheries (generally associated to men) and greater access to marine resources for the poor and unskilled, there is higher pressure on the marine environment. With such high juvenile catch, fishers using mosquito nets, who may already be the lowest earners of society, are putting greater pressure on their future food security. Removing fish biomass in such intensity, as reported here, can significantly alter the trophic structure of seagrass and adjacent habitats, especially when slow‐developing and economically important species are removed (Unsworth and Cullen 2010). Given that seagrass communities are defined by top-down predator control (Eklof et al. 2009; Burkholder et al. 2013), a significant loss of these predatory species can result in higher intensity (and frequency) urchin grazing events, resulting in a loss of seagrass structure and function (Eklof et al. 2008b). Coral reef fisheries provide substantial support for communities within Palma Bay. Seagrass meadows directly support coral reef productivity due to their role as feeding habitat for predatory fish and as a nursery habitat for other important reef species (Unsworth et al. 2008; Guannel et al. 2016). The removal of these species, as well as important herbivores from the Siganidae and Scaridae families, place reefs and their associated fisheries at risk (Mumby et al. 2006).

Small-scale and artisanal fisheries are dynamic (Finkbeiner 2015). They are influenced by changing drivers that present social and ecological challenges, as well as new opportunities for fishers. These opportunities are important for fishers that depend solely on such fisheries for livelihood and subsistence. The use of free or cheap mosquito nets for fishing is one such opportunity that is being harnessed globally. Here we present a unique case in which seagrass meadows and mosquito net use exists as contrasting sides of poverty alleviation (greater opportunity and empowerment yet reduced future security). Managing fisheries is intrinsically difficult when considering these fluctuating drivers (Mahon et al. 2008), and even more so with a limited understanding of how fishers and stakeholders respond to such opportunities like the distribution of mosquito nets.

In conclusion, our study presents the first quantitative analysis of marine MNF that the authors are aware of. While a small snapshot, it provides evidence of the extent of biomass removal, much of which is of juvenile fish. While mosquito nets have been distributed to improve healthcare, their use in fishing puts communities at much higher risk of future poverty due to the potential impact these gears have on the sustainability of natural resources upon which they depend. Our research highlights the need for a multi-level and cross-disciplinary approach to the management of this issue. Top-down approaches are in force that make use of mosquito nets for fishing illegal in some localities (Blythe et al. 2013), however, such mechanisms are evidently insufficient and need re-thinking. We believe that bottom-up approaches may be more beneficial to understanding the drivers that result in the use of mosquito nets in fishing, and for communities to help develop solutions to these challenges. Generally, greater effort is needed to ensure that mosquito nets are being used for the purpose intended. Such measures need to also support fisheries management initiatives in the regions in which they are used and focus on education schemes that present the issues of using mosquito nets in fisheries alongside the health issues of malaria.