The current study revealed rich fisheries and ecology-related knowledge about plants across the fishing communities of LB, Philippines, including positive plant-fisheries (with the beneficial roles of plants) interactions as well as negative ones. Local fishers in the studied areas shared abundant novel information on the uses of plants and related algae and their interconnectivity with the local environmental challenges affecting the entire LB ecosystem.

Positive plant-fisheries interactions

The important ecological functions of plants, which mainly focus on their benefits to humans and animals and as ecosystem and natural climate buffers, were highlighted by interviewees. In the previous studies on plant usage data from around the world, uses such as catching fish, habitat, and hiding places have been reported [9, 45,46,47]. Moreover, an earlier study found that Actinoscirpus grossus is used as fish habitat in the Philippines [48], which is directly in line with the findings of this study. A similar observation was reported in India [49]. In Brazil, Pontederia crassipes serves as a hiding place for fish, while Pontederia rotundifolia helps maintain a steady temperature [9]. Numerous fishers underlined the value of aquatic plants like Vallisneria natans as fish hideouts in severe weather conditions [48].

Our fishers shared that the plants of LB produce a variety of microhabitats that provide organisms, such as fish and birds, with habitat, hiding places, and nesting and spawning areas, making such places ideal for fish conservation as well. The reported uses of tree branches for fish conservation are consistent with the observation of Buen et al. [50] who reported that Laguna Lake’s traditional fish aggregating device, the “yangkaw,” is constructed from Tamarindus indica or Pithecellobium dulce branches to shelter and protect fish, which is a part of their community-based fisheries resource management. One explanation for this finding is that the tannins and flavonoids from traditional fishing gear, such as the "yangkaw", as per Concepcion [51], increase water quality for aquatic plants like Hydrilla verticillata, creating an area for fish reproduction.

According to LB fishers in Mabato-Azufre and Talim Island, plants serve as a food source for fish, demonstrating their important role in the food web dynamics of LB. The present finding corresponds well with previous studies in that fish feed on Hydrilla verticillata in India [1], and thus it is used to catch fish in both the Philippines and India [47, 50]. Similarly, Pontederia crassipes and Pontederia rotundifolia are used to feed fish in Brazil [9] as well as harvest fish in India [52]. Another fish food source found in a previous study of Laguna Lake is the cyanobacteria Microcystis aeruginosa [48]. One possible explanation for this finding is that microalgae and cyanobacteria linked with aquatic macrophytes feed tiny fish and invertebrates even close to the shore of the LB ecosystem. Aquatic animals also reproduce well on aquatic macrophytes because of the complex coexistence of algal periphyton [53]. This result is also due to the fact that in aquatic environments algae are widely distributed and are in charge of primary production and lay the groundwork for aquatic food webs [54]. Thus, because of its global spread and quick growth, sea lettuce (Ulva spp.), which is still currently underutilized, can become a source of food, medicine, and biofuel, and can be exploited by bioremediation industries [55]. Ulva biomass is mostly fed to abalone, shrimp, and aquaculture fish [56,57,58,59,60] Therefore, integrating Ulva spp. into livestock monocultures, including that of shrimp, urchin, and abalone, provides several benefits, such as reducing effluent nutrient loads, decreasing feed requirements, and potentially increasing the economic value of the final product [61,62,63].

Furthermore, according to interviewees, plants filter the water naturally and reduce the action of waves. These reveal the ecological significance of aquatic plants and algae in preserving water quality in the LB ecosystem.

Negative plant-fisheries interactions

Pontederia crassipes, together with Vallisneria natans, Ipomoea aquatica, and Microcystis aeruginosa, caused the Laguna Lake aquaculture fish kills. According to fishers, fish kills in this region were a problem for many fish cage operators and caused difficulties in local fishing activities. The former also damages fishing gear, as do Hydrilla verticillata and Ulva spp., and causes navigation problems in the LB region. In the Philippines, local fishers say that Pontederia crassipes and Microcystis aeruginosa overgrowth, shallow water depth, a decrease in oxygen, solid waste, polluted water from agriculture, and chemical substances from hydroelectric power plants cause fish kills [48]. Additionally, eutrophication, which is caused by nutrient pollution from wastewater discharges and agricultural runoff, can result in hazardous algal blooms that lower oxygen levels and endanger aquatic life [64].

In the present study, fishers reported issues related to the presence of water hyacinths and Ipomoea aquatica in Laguna Lake. Similarly, in Africa, Pontederia crassipes is invasive, increasing other plant taxa such as Pistia stratiotes, and its overgrowth may hinder transportation and mobility in the Lake Victoria Basin (see [46]). Semi-aquatic tropical Ipomoea aquatica grows abundantly as floating herbs in canals, rivers, lakes, and ponds, or as creepers in marshy plains in India and Asia [65]. The plant grows quickly and encroaches over most of a water body, causing irrigation, navigation, fisheries management, and other ecological issues in aquatic environments in Texas in the United States [66].

Social, economic, and ecological reasons for differences in plant-related fishery knowledge in the three fishing communities

Of the three study areas, fishers in Mabato-Azufre, which is situated in the least industrialized part of the region, reported the majority of the uses related to positive and negative plant-fisheries interactions. This could be due to several interconnected social, economic, and ecological factors. Mabato-Azufre is located in the East part of the lake where water quality is the highest, but the area scored the lowest in fisheries indicators, with a high concentration of fishers, invasive species, and smaller fishing grounds ([27] The Laguna de Bay Ecosystem Health Report Card, 2016). However, taxa associated with negative plant-fisheries interactions were cited most in the island fishing village of Buhangin. This could be due to the fact that this island fishing village has been vulnerable to cyclonic storms, and it has issues with population growth, waste management, and erosion [67]. Moreover, all of the study areas share nearly half of the reported plant uses, which is probably the result of similar fishing activities and local environmental challenges. Given the proximity of the entire LB ecosystem to Metro Manila and the highly populated communities in nearby Rizal and Laguna provinces, the lake is being used as a waste repository because of the temporary retention of floodwaters to alleviate inundation in the lower areas of Manila, to supply irrigation water, potable water, and electricity, for water transport to and from Talim Island and neighboring lakeshore communities, as well as for recreational and tourism activities [68]. Sampiruhan, on the other hand, is a barangay in the city of Calamba, which is a premier industrial hub near Metro Manila, boasting more than ten industrial parks and the highest income in Region 4A [69]. Another reason for the recorded differences in knowledge is that the study of the utilization of plants associated with traditional practices, as noted by Savo et al. [13] in Italy, indicates that a significant number of plants and their applications have disappeared, complicating the assessment of whether the reported uses within a specific fishing community are exclusive to that region or have diminished elsewhere.

The study of these communities, which range in their level of industrialization from the least to the most industrialized in the area, reveals the understanding of local resource users including fishers regarding the effects of waste from domestic and agricultural discharges and effluents from the chemical and tourism industries, such as the decline of submerged macrophytes occurring in different parts of this region, thus affecting their crucial ecological functions and how they impact fishing activities in LB, Philippines. This finding contributes to advancing our understanding of crucial positive and negative interactions concerning plants, humans, fishes, and other species threatened by interconnected social, economic, and ecological factors. This demonstrates the importance of fishers’ perspectives for freshwater resource conservation, which is essential to fisheries management in the LB ecosystem.

Factors affecting plants in the Laguna Lake environment

Fishers explained that the aquatic plants are impacted by three main environmental factors in different parts of the LB region namely hydropower plant pipe cleaning activities, sedimentation, and less saltwater intrusion reducing the quality of water. The decline in fish in the region is connected to the decrease in submerged macrophytes resulting from the above-mentioned issues. In this regard, fishers linked the loss of aquatic plant species in Laguna Lake, including Pontederia crassipes, Actinoscirpus grossus, Scirpus grossus, Vallisneria natans, and Hydrilla verticillata, to hydropower plant operations, such as chemically cleaning pipes of moss using chlorine [48], as chlorine can have an impact on aquatic ecosystems [70, 71]. For example, Watkins and Hammerschlag [72] found that “high-level chlorine discharges from wastewater facilities and electric generating plants could be a contributing factor impacting nearby submerged aquatic vegetation.”

In the present study, fishers also noted that while plants in LB provide fish with spawning areas and places to live and hide from predators, they are also negatively impacted by the low quality of the water in the region and invasive species like knifefish (Chitala ornata). Given these local environmental challenges, they are also directly tied to the decline in native fish populations such as ayungin (Leiopotherapon plumbeus). This is concerning because aquatic plants are vital to the survival of other aquatic species and provide a variety of ecosystem services including habitat, spawning sites, and nursery areas for several important fish species [73].

In regard to the sedimentation problems of LB waters, in the present study, fishers from the east side of LB stated that putting tree branches in the lake is not advisable in that area because the water is shallow and muddy, unlike in the Talim Island area (or Central Bay area) where the lake is deeper. In relation to this, the LB ecosystem, and specifically the Central Bay area, is impacted by natural disasters, pollution, and anthropogenic constructions. Central Bay, a municipal water body, is used by industries for waste discharge, which exceeds acceptable limits for the preservation of aquatic life. Waste from domestic activities, fertilizers, pesticides, and animal industries also contribute to the pollution. The use of insecticides has increased by 20–25% since 1976, and the lake's sedimentation and siltation resulting from forest denudation contribute to the lake's shallowness, raising turbidity levels and reducing production [67]. Furthermore, the erosion in LB is a result of improper agricultural practices, quarrying, deforestation, landfills, and land conversion [32].

In addition to the above issues, fishers also reported less saltwater coming into LB. A possible explanation for this might be that the lake environment is affected by man-made structures, as in Napindan Channel, which block entry of saltwater from Manila Bay. Therefore, biotic-abiotic connections could be slightly affected by thermal pollution resulting from the more than 1000 companies that use lake water as a cooling agent and then release it as heated water [67]. Human-caused pressures such as pollution (e.g., Häder et al. [74]), invasive species [75], and overfishing [76] are crucial and might affect people who depend on aquatic ecosystems.