Background
Everyday hundreds of people, mainly men, depart from home to participate in a vibrant artisanal capture fishing industry in Kenya’s Lake Victoria fishery [
1‐
3]. This industry forms the primary income source for the locals [
1,
4‐
6]. Landed fish is hardly consumed within the fishers’ households [
7] due to the small stock sizes [
8]. The fish is sold off to generate cash income that is used to buy food, pay for medical care and other basic needs [
2,
7]. The proceeds are often supplemented with agricultural produce [
1,
9,
10]. Fishing crew must increase effort to find, catch and obtain sizeable stocks from the declining fishery. They cope by migrating to adjacent fisheries perceived to harbor larger fish stocks [
1,
11], migrating to fisheries near large economic markets [
11,
12], using more extractive fishing gears [
1,
13] and use of more effective fishing baits [
14].
The evolving and current threat of outdoor transmitted malaria [
15,
16], especially in outdoor groups engaging in compelling social, cultural and economic activities at night [
17] e.g. capture fishers [
18], can be viewed as an ecological disaster [
19]. Artisanal capture fishers exert big pressure on the environment [
20] through relentless exploitation of fishery resources [
3,
21]. As noted elsewhere ‘
poor people are forced to overuse environmental resources to survive from day to day, and their impoverishment of the environment further impoverishes them, making their survival ever more difficult and uncertain’ [
22]. Persistent pressure by poverty on ecosystems also has negative repercussions on human health [
19,
23‐
25]. Thus, artisanal capture fishing, as practiced in the Lake Victoria fishery, is not a sustainable livelihood source [
1].
In this study, an ecosystem approach [
19,
23‐
25] was employed to understand the association between artisanal fishing and the problem of malaria on Mageta Island in western Kenya. The central goal was to establish whether actions of artisanal fishers, in their unrelenting quest for existence, surpass ecosystems’ sustainability thresholds with potentially negative repercussions on human health. This was achieved through a cross-sectional survey seeking to (a) determine if artisanal capture fishing leads to creation, hence occurrence, of
Anopheles breeding habitats, and (b) establish the potential correlation between artisanal capture fishing and
Anopheles larval productivity. In the context of this study artisanal capture fishing is defined as a small-scale activity in which fish are caught in the wild using rudimentary methods. Although some authors have pointed out at the increased risk of malaria in the context of artisanal fishing [
18,
26‐
30], no one has specifically assessed the link between artisanal fishing, habitat degradation and larval ecology of malaria vectors. This is the remit of this article.
Discussion
This study applied an ecosystem approach to find out if artisanal capture fishing facilitates breeding of Anopheles larvae. Although 74% of ‘fishing habitats’ and only 38% of ‘non-fishing habits’ contained Anopheles larvae, there was a significant negative association between artisanal capture fishing and the probability of finding Anopheles larvae in the habitats. Interestingly, 83% of the fishing boats, which formed the majority of ‘fishing habitats, contained Anopheles larvae. Although the total numbers of Anopheles larvae collected were about equal, the mean density in ‘fishing habitats’ was twice that in ‘non-fishing habitats’. Forty-eight percent of the larvae were recovered from fishing boats and 49% from rock pools. Despite being the most common habitat type, the mean number of Anopheles larvae present in rock pools was significantly less than those found inside the wooden fishing boats. These data underscore the importance of artisanal capture fishing on the epidemiology of malaria on Mageta Island.
The significant negative association between artisanal capture fishing and the probability of finding
Anopheles larvae in habitats is puzzling on initial thought. However, this relationship is not infinite. The fitted logistic regression equation predicts that if 100% of breeding habitats on Mageta Island were to be created through artisanal capture fishing then only 38% of them would contain
Anopheles larvae. On the contrary over half (78%) of stagnant water bodies would contain
Anopheles larvae if no single breeding habitat on Mageta Island was to be created through artisanal capture fishing. This analysis implies that although artisanal capture fishing is an important facet of malaria epidemiology on Mageta Island, other drivers of endemicity do exit. Thus, malaria control efforts need to be informed by holistic approaches that recognize the interdependent nature of health and other societal, developmental and ecosystem factors [
25].
Peer reviewed literature about the breeding of
Anopheles larvae in boats (or any wooden containers) is scarce. However, this was one of the most fascinating findings of this study. Traces of available data relate to the role of boats and other transport vessels as agents for the worldwide dispersal of arthropod vectors [
47,
48]. What is more is that these data largely derive from observations on
Aedes, and to a lesser extent
Culex, species outside Africa [
48‐
52]. Two recent studies document utilization of boats for breeding by
Anopheles coluzzii (initially the M form of
An. gambiae sensu lato) in two fishing communities within the Wouri river estuary near the port of Duala in Cameroon [
53,
54]. Data in this article corroborate these findings,
albeit with respect to
An. gambiae s.s., which was the only
Anopheles species identified on Mageta Island. Mbida et al. [
54] explain the phenomenon of
An. coluzzii breeding in boats, among other man-made habitats, as an adaptation to utilizing artificial habitats when natural ones become rare.
It is well known that
An. gambiae uses manmade habitats for larval breeding [
42,
55], but it is puzzling why fishers’ boats constituted a highly profilic
Anopheles larval breeding resource. This study was carried out in the dry season, thus the finding that boats formed an important breeding habitat for malaria mosquitoes is confusing. The boats should have been devoid of water at this time. By iteratively engaging community actors (most of whom were artisanal fishers) it was explained that fishers engage in an active maintenance process where fresh lake water is poured in boats parked ashore between fishing rounds. The water prevents the wood from cracking. Boats not-in-use are normally stationed ashore [
6,
12] during months when fishing is illegal [
4,
56,
57], when fish catches are significantly low, when actors are off duty and during tumultuous party times when fishermen revel after receiving cash bonuses from their cooperative societies. That aside, it is unlikely that the larvae found in boats were introduced through the maintenance process. Strong waves must have killed any mosquito larvae present in lake water around the open beaches where fishing boats were parked. Besides the open lake is not a typical breeding habitat for
Anopheles mosquitoes [
46]. What is more is that
Anopheles eggs, possibly introduced through direct oviposition by gravid females, were found inside the fishers’ boats.
Perhaps the dominance of
Anopheles larvae in fishers’ boats can be best explained borrowing from life history theory [
58‐
61]. A mosquito’s life cycle encompasses four key life history stages namely eggs, larvae, pupae and adults [
62]. Eggs, larvae and pupae are aquatic, and will most likely exist in pools of water in boats for the case of malaria vectors on Mageta Island. Utilization of boats as a breeding resource is a very risky phenomenon because this habitat type is highly ephemeral. Although water is placed in the boats in the morning hours and emptied in the late afternoon on the same day or after a few days, the most common malaria vectors in the area, i.e.
An. gambiae complex mosquitoes [
32], need about 1 week to complete the aquatic cycle [
63]. From an evolutionary standpoint selection pressure should favor traits that promote shorter aquatic developmental periods and production of large numbers of offspring by gravid female malaria mosquitoes. Alternatively, gravid malaria vectors may, through an ecological phenomenon referred to as ‘bet-hedging’ [
60], cope by distributing single egg loads into several fishing boats containing water. Reproduction should also entail a relatively small energy investment in each offspring [
58]. This should result in a sizeable number of young offspring that are capable of evading extrinsic larval mortality [
59] and developing into terrestrial adult beings. The adults should then live for long enough while accessing readily available blood meals from the vast human blood meal reservoir in the fishing hamlets.
Looking further, the larger number of
Anopheles larvae in rock pools relative to mud-bottomed habitats is not surprising. This is because larvae of
An. gambiae s.l. are often found in habitats containing algae [
39] and rock offers a better substrate for algae to grow on than mud substrates [
38]. Besides, rock pools were all found near the shoreline and the water in them was frequently refreshed by spilling waves. This served to oxygenate the water, which may have promoted
Anopheles larval productivity [
64,
65]. However, the fact that most rock pools were found under tree canopies could explain the relatively lower
Anopheles productivity compared to boats. Generally, anopheline larvae prefer open sun-lit waters [
38,
39].
Anopheles gambiae s.l. tolerates relatively high water temperatures [
65], thus the warmer sun-lit water pools in boats may have been an important factor for larval development because warm water accelerates larval development [
64]. In addition, warm water temperatures in boats may have allowed more microorganisms to grow, which provided food sources for mosquito larvae [
64,
66‐
68]. Fishing boats on Mageta Island are made using timber from the Africa teak tree (
Milicia excelsa), commonly known as Mvule among locals. Unsubstantiated reports indicate that timber of this tree contains pores that harbour bacteria. These bacteria probably multiplied rapidly and acted as a mosquito larval food source [
64,
66], so increasing
Anopheles productivity in boats. On the contrary, presence of aged water may have harboured larger numbers of predators that suppressed abundance of
Anopheles larvae [
69] in some rock pools.
The majority of the
Anopheles larval habitats reported in this study were created through human activities fashioned around supporting livelihoods. Artisanal capture fishing was the most notable livelihood source. This result goes in tandem with observations by other researchers in relation to crop cultivation [
70,
71], livestock herding [
71] and brick making [
72]. The findings underscore man’s own contribution towards the viciousness of malaria and affirm the link between malaria and poverty, hence the poverty trap formed by the ecology of infectious diseases [
73,
74]. This implies that the poor of the south (e.g. the artisanal fishers of Mageta Island) whose wealth, by definition, is primarily gained by extracting natural resources [
19] are unable to make enough to lift themselves out of poverty [
75]. They are stuck in a cycle of poverty that is almost impossible to break [
73]. As fishing activities intensify so does the chance of increasing
Anopheles larval densities in breeding habitats. This fuels malarial disease, hence the need to extract more fish to generate income as a coping strategy towards treatment.
The cross-sectional design used in this study presents several shortfalls towards associating cause to effect [
76]. First, although it is generally impossible to infer the temporal sequence between exposure and outcome in cross-sectional studies, it makes biological sense, in this study, to assume that the presence of water in fishing habitats (the exposure) preceded the appearance of
Anopheles larvae in the water (the outcome). This is because stagnant water is a prerequisite for oviposition and larval development. Second, cross-sectional studies tend to identify a high proportion of prevalent (rather than incident) cases/outcomes. By dialoguing with community actors it was learnt that boats containing water were parked ashore just for a few hours or days [
6,
77]. Thus, it is unlikely that most of the breeding occurring in boats resulted from boats overstaying with water. Third, although this study was conducted in an informal occupational setting, it is highly unlikely that the effect of artisanal capture fishing on creation of
Anopheles larval habitats was attenuated by inherent exclusion of ramshackle fishing boats from those sampled. Boats abandoned near the shorelines because of being in conditions of disrepair also contained water received from rainfall and/or spilling waves and were included in the sample. Thus, the study suffered a limited ‘
healthy worker survivor effect’.