Shellfishers’ Knowledge about Cockle fisheries
We conducted 60 interviews (N = 5; 8.3% women and N = 55; 91.7% men; Figure 2) in the local communities of Aveiro - Murtosa (N = 13; 21.6%), Costa Nova (N = 15; 25%), Torreira (N = 16; 26.7%), and the Port of Aveiro (N = 16; 26.7%). The average age of the shellfishers was 51.07 years, with an average experience time of cockle harvesting of 29.33 years. Schooling was generally basic, with most respondents (N = 56; 93.3%) having elementary school (up to 9 years of study). The profile of shellfishers in Aveiro corresponds to the shellfishers’ profile in Portugal [35, 67–69]. Portuguese fishers have already shown themselves to be characterized by a middle-aged workforce with low levels of education (concentrated in primary or preparatory education) [68]. Furthermore, according to other studies documented in Portugal [69], Aveiro’s shellfishers also had extensive experience in the fisheries sector.
The cockle shellfish gatherers in the coastal lagoon of Aveiro were carried out in an artisanal and traditional way. Fifty-three (90%) harvesters used small fishing boats to capture cockles. The boats used were the bateira (N = 32; 54%) and the fiber boats (also called “chata”, N =19; 32%). The average fishing boat size was 6.56 meters (m) x 1.84 m. The average crew per trip was 3.04 shellfishers, and about 29 (52%) interviewees harvested the cockle alone or with a companion on board. The bateira has already been recognized as the most traditional fishing boat in Aveiro, being called the berbigoeira, destined to catch cockles that typically had dimensions that could reach 13.8 m in length [70]. However, there was a tendency to use smaller boats up to 7 meters in length (small bateiras) in the current fisheries in the Ria de Aveiro. The number of crew on fishing boats has not changed in recent decades, with an average of 3 fishers per boat in cockle fishing.
Shellfishers mentioned five tools that were used to harvest cockles in the Ria de Aveiro (hand rake or ancinho - N = 52; 86,7%, nassa - N = 45; 75%, joeira or ciranda - N = 49; 81.7%, cabrita - N = 50; 83.3%, ganchorra - N = 11; 18.3%). Three fishers (3%) picked cockles only manually. The most cited utensil in the Ria de Aveiro for the collection of cockles (hand rake) consists of a wooden or metal bar, with teeth (straight or curved and of variable size, number, and spacing) fixed to a wooden or metal handle being used on foot and at low tide [71]. This fishing gear from Aveiro is a type of hand dredge [72]. The nassa (Figure 2) has a conical or cylindrical shape and is dismountable, consisting of a small mesh net mounted on hoops or other rigid structures [72]. Joeira or ciranda is a type of sieve used to separate the permissible-sized cockles from small cockles. The cabrita or berbigoeiro is a hand dredger designed to capture cockles consisting of a metallic structure connected by a wooden handle with teeth whose size, spacing, and number are variable [71]. This fishing gear can be small cabrita (short handle) and large cabrita (long handle). Ganchorra, in this context, possibly referred to a type of towed dredger operated by trawlers that operate on bottoms that are not discovered at low tide on the ocean coast [72]. In the Ria de Aveiro, the hand dredger (ganchorra de mão), also called a berbigoeiro, and hand and rake picking has already been reported as the main fishing gear for catching bivalves in the Ria de Aveiro [73].
The frequency of cockle harvest was daily (N = 45; 76%) when this practice was open. All shellfishers said that the harvest was done at any time and was carried out according to the tides. Six (10%) respondents indicated a preference for the morning period, and three (5%) respondents for the afternoon time. All harvests highlighted low tide as the preferred tide, and 36 (60%) respondents additionally mentioned mid-tide. The harvest time per tide ranged from 1 to 8 hours (average of 4.16 ± 1.05 hours). The minimum harvest size ranged from 16 millimeters (mm) to 60 mm. The mean minimum harvest size was 23.4 ± 7.2 mm. The minimum allowable capture size is 25 mm [74]. However, there were also shellfishers reporting the capture of individuals below 25 mm. The average size of this capture may also indicate that the cockle maybe being harvested before the first year of age [18]. In an estuary area of Portugal, visible disturbances of the population structure of C. edule have been reported due to human overharvesting [75]. These bivalves caught below the allowed size may be accompanied by inadequate collection tools that can reduce the sustainability of cockle stocks and surrounding biodiversity in the Ria de Aveiro. [76].
The harvest per tide ranged from 0.5 kilograms (kg) to 700 kg, with an average of 137.12 ± 107.49 kg. The maximum daily catch limit for cockles per fishing boat duly licensed for fishing is 200 kg in the Ria de Aveiro [77]. A maximum daily catch limit of 50 kg of cockles per licensed fisher is also established [77]. Despite being within the normalized average of daily catch, there were still fishers who probably collected cockles in larger quantities than established for the region. The areas with the highest cockle harvesting effort in the Ria de Aveiro were RIAV1 (N = 53; 88.3%), RIAV2 (N = 43; 71.7%), RIAV3 (N = 30; 50%) and RIAV4 (N = 4; 6.7%). Fishers’ LEK is in line with data from the collective effort to distribute the bivalves target species of the Ria de Aveiro in 2012 [78]. Cale da Moacha and Cale do Ouro (RIAV1), and the Mira channel (RIAV2) were also the areas with the highest relative abundance of cockles in the Ria de Aveiro with about 70% of the total cockle biomass captured in 2013 [79].
Shellfishers predominantly harvest year-round (N = 47, 78.3%), except when there are mandatory stops due to the presence of toxin contamination (N = 45, 75%). The cockle harvest throughout the year showed how easy it is to catch this bivalve, with solid commercial demand and little investment in equipment and workforce [18]. The main period of the harvest year was from September to March (Figure 2). Bivalves’ capture in affected production zones (RIAV) is temporarily prohibited when confirmation of marine toxin-producing phytoplankton or microbiological contamination above regulatory values in the Aveiro lagoon [80]. Cockles harvested were destined for export (N = 52; 86,7%), factories (N = 51; 85%), own consumption (N = 43; 71,7%), trade and food industry (N = 41; 68, 3%), and bait for fishing (N = 6; 10%). The main export destination for cockles was Spain (N = 48; 80%). This Iberian country is one of the main target markets for shellfish exports from Portugal [81]. In Spain, these mollusks are destined for large and strong seafood canning industries [18]. This product sometimes returns to Portugal in frozen form and to be sold in supermarket chains.
Respondents also mentioned that along the entire length of the Ria de Aveiro, there were on average around 683.67 ± 557.21 shellfishers. This figure is much higher than the fishing associations in Aveiro estimated, which reaches the 413 shellfishers (multispecific gatherer) registered (Data provided by the Fishing Associations in Aveiro). However, the number of registered cockle collectors may be even less than the 413 officially registered. This finding shows how local authorities present obstacles in applying measures to control fishing efforts [18]. Some shellfish gatherers mentioned in the interviews may be related to illegal, retired, and not registered shellfishers with the local associations. The inadequate control of some areas of the Ria de Aveiro where bivalves are diverted and harvested without considering the minimum landing size [78] may be facilitating the continuation of unsustainable exploitation practices in this aquatic ecosystem. Most shellfish gatherers were registered with some fishing association in Aveiro (N = 44; 73.3%). The Association of Artisanal Fishing of the Region of Aveiro - APARA (N = 29; 65.9%) had the highest number of respondents. The Producers Organization of VianaPesca (N = 16; 26.7%) and Docapesca - Ports and Auctions, SA (N = 1; 2.3%) were also mentioned by shellfishers.
Cockle Ethnoecological Knowledge
Habitat - The Aveiro coastal lagoon presents varied natural values with numerous habitats for bivalves [53], such as the cockle. This mollusk can predominantly inhabit the first few centimeters of sediment zones [40]. Fishers’ LEK showed that the edible cockle C. edule could be found buried at a depth of 10 centimeters (cm). The average depth was 2.71 ± 1.68 cm. The most frequent value of the LEK on cockle depth was 1 cm. Cockle sampling studies usually limit up to 10 cm in the sediment [82], which is within the range considered by shellfishers from Aveiro. Cockles’ preferred habitats (Figure 3A) in the Aveiro lagoon were sand (N = 53; 88.3%), mud (N = 42; 70%), sludge or sand muddy (N = 8; 13.3%), and dry bottoms (N = 8; 13,3%). Ecological studies also indicated that this bivalve lives in sediment surface, sand muddy, sandbank, mud gravel bottom and is found mostly in intertidal and subtidal areas [5, 18, 83, 84].
Food items – This study shared a range of potential cockle food items (Figure 3B). These food items were typical of bivalves suspension filter feeders with a fundamental role in purifying the water column, organic filtration, and energy flow in the biological community [5, 75, 84]. The main ethnobiological data about the foods consumed by cockles were: small particles present in water (N = 30; 50%), plankton (N =13; 21.7%), mud (N = 9; 15%), algae (N = 4; 6.7%), water impurities (N = 46; 7%), bacteria (N = 3; 5%), fish remains and pieces (N = 3; 5%), roe (N = 2; 3.3%), krill (N = 2; 3.3%), and zooplankton (N = 1; 1.7%). Some respondents did not present any ethnobiological data on this topic (N = 10; 16.7%). Cockles can feed on zooplankton, phytoplankton, organic particulate matter, juveniles of their own species, and eggs and larvae [75]. These bivalves still consume small particles suspended in the water column, including non-living materials such as suspended soil particles and plant debris [8].
Predators - LEK provided twenty folk names of potential cockle predators in the Ria de Aveiro (Table 1). This finding reinforces the importance of cockles in the food chain as a link between primary producers and consumers [84]. The wide spectrum of cockle predators shared through the LEK highlights this critical potential in the ecological function of the ecosystem, especially regarding the influence of this bivalve at higher trophic levels [85]. This study highlighted the crabs (N = 36; 60%), European plaice (N = 16; 27%), European eel (N = 15; 25%), seagulls (N = 14; 23%), European seabass (N = 11; 18%), Great Cormorant (N = 11; 18%) and Greater flamingo (N = 11; 18%). Some shellfishers specifically cited green crab (N = 10) within the general crab group.
Table 1
Main likely predators of the edible cockle C. edule in Ria de Aveiro, Portugal.
Folk name (Portuguese)
|
Common name (English)
|
Scientific names (Linnaean)
|
Shellfishers’ citations and frequency (%)
|
Caranguejos
|
Crabs in general
|
(N = 36; 60%)
|
|
Solha
|
European plaice
|
Pleuronectes platessa Linnaeus, 1758
|
(N = 16; 27%)
|
Enguia
|
European eel
|
Anguilla anguilla (Linnaeus, 1758)
|
(N = 15; 25%)
|
Gaivotas
|
Seagulls
|
Seagulls in general - Probably: Larus fuscus Linnaeus, 1758 and Larus michahellis J.F. Naumann, 1840 and Larus melanocephalus Temminck, 1820
|
(N = 14; 23%)
|
Robalo
|
European Seabass
|
Dicentrarchus labrax (Linnaeus, 1758)
|
(N = 11; 18%)
|
Corvo Marinho
|
Great Cormorant
|
Phalacrocorax carbo (Linnaeus, 1758)
|
(N = 11; 18%)
|
Flamingo
|
Greater Flamingo
|
Phoenicopterus roseus Pallas, 1811
|
(N = 11; 18%)
|
Linguado
|
Common sole
|
Solea solea (Linnaeus, 1758)
|
(N = 8; 13%)
|
Aves
|
Birds
|
Birds in general
|
(N = 5, 8%)
|
Peixes
|
Fish
|
Fish in general
|
(N = 5, 8%)
|
Garça
|
Egret
|
Egret in general - Probably: Egretta garzetta (Linnaeus, 1766) and Ardea cinerea Linnaeus, 1758
|
(N = 4; 7%)
|
Tainha
|
Mullet
|
Mugil spp.
|
(N = 4; 7%)
|
Cegonha-branca
|
White stork
|
Ciconia ciconia (Linnaeus, 1758)
|
(N = 3; 5%)
|
Cockle predation is also characterized by being very specific as it varies according to the size of this bivalve [8]. C. edule is a crucial prey for demersal fish, birds, shrimps, and crabs [84, 85]. In our findings, there are reports of cockle consumption by fish species such as the European plaice and the crab Carcinus maenas [86]. Many wading birds with protection status are also cockle consumers [8]. The common eider (Somateria mollissima), oystercatcher (Haematopus ostralegus), and herring gull (Larus argentatus) are potential consumers of cockles [87]. A variety of gastropods also present this bivalve as a food supply [88]. Forty-five (75%) shellfish gatherers said there were many cockle predators in this lagoon ecosystem, eleven (18.3%) said there were few predators, and eight (13.3%) said there was a moderate number of predators. The most abundant predator in terms of quantity in the Ria de Aveiro was the crab (N = 8; 18.3%), and the least abundant was the European eel (N = 11. 18.3%).
Spawning - The cockle spawning event took place throughout the year (Figure 4A). The results mainly highlighted the end of spring (May and June) and the summer (July, August, and September). Gametogenesis of the main cockle species generally occurs between February and March, development of the gonads in April and May, and spawning between May and August [4]. Maia and collaborators (2021) reported that the spawning season for cockles in the Ria de Aveiro lagoon could occur from March to October, predominantly in the summer months (July to September) [18]. The spawning period observed in the present study is like that recorded in this last biology investigation at Aveiro. On the European coast, findings equivalent to those in our shellfish LEK study have also been reported [4, 11, 89].
Ethnoconservation of the cockle - Informants shared that the most recent decades were the greatest decrease in cockle stocks in the Ria de Aveiro lagoon (2000-2009: N = 7; 11.7%, 2010-2019: N = 20; 33.3%, 2020-2021: N = 7; 11.7%). In the late 2000s, published data for this bivalve species already registered a decrease in the biomass and abundance of cockles, highlighting the urgent need to improve the management of this fishery in Aveiro [78]. The main factors behind the decrease in cockles in the region, according to the interviews, were overharvesting (N = 13; 22%), increased rainfall in the lagoon (N = 6, 10%), land modification (suction dredging, and the construction of a canal for tourism - N = 4; 7%). The high number of fishing boats, the increase in the variety of fishing gear (especially the trawl gear), the presence of the Pacific oyster - Crassostrea gigas and the Japanese carpet shell - Ruditapes philippinarum nurseries, and the pollution (agriculture and factories) were also remembered (N = 3; 5% each other). There were 45 quotes from shellfishers in which they indicated a perception of decreased cockles’ productivity (Figure 4B) in RIAV1 (N = 17), RIAV2 (N = 15), RIAV3 (N = 9), and RIAV4 (N = 4). There were 16 quotes from shellfishers sharing information that there was an increase in cockle productivity in the RIAV1 (N = 8), RIAV2 (N = 3), RIAV3 (N = 4), and RIAV4 (N = 1) zones in the last years.
Burdon et al. (2014) identified some factors that cause mortality in C. edule, such as food limitation, temperature and salinity, changes in sediments, suspended solids, topography and bathymetry, oxygen depletion, persistent depletion, toxic pollutants and organic loads, pathogens, parasites, and commensals [90]. The cockle is still the most abundant bivalve species in the Aveiro lagoon, but it is widely harvested and traded [73]. The authorities in the Ria de Aveiro also do not have an ideal control of this fishing effort in the bivalve production zones (RIAV) [18], which could generate a future problem. This fishery’s economic relevance and diffusion in the Iberian Peninsula, combined with illegal fishing, may also raise concerns about bivalve stocks and associated ecosystems [91], especially in Ria de Aveiro. Variation of salinity gradients can affect estuarine organisms, especially in cases of abrupt changes [5]. Different responses of estuarine organisms to salt stress may be related to structures and differences in habitat at each stage of development [14]. Fishers’ LEK showed that the increase in freshwater in estuarine environments due to rainfall events might be related to the decrease in cockle productivity in some productivity zones of the Aveiro lagoon. Verdelhos and collaborators (2015) also showed that C. edule in the face of extreme climatic events such as floods could severely alter the population structure of this species due to drastic changes in salinity [5].
The action of suction dredging in specific fishing grounds can remove the larger cockles from the tidal plains and generate mortality of other fauna, making the habitat unsuitable for some species [92]. Effects of disruption of the bottom may also be collaborating to impact certain productivity zones in Aveiro. The bivalve harvesting method should also be considered a control measure for the possible indirect effects of fishing exploitation [93]. Bivalve harvesters in Ria de Aveiro had already called for more effective control of bivalve dredgers towed from boats, as this method of trawling affects fishery resources and the entire aquatic ecosystem [78] and is illegal. Shellfishers cited a variety of harvesting methods carried out in Ria de Aveiro. However, according to the interviewees, the capture of cockles by boats through trawling stood out as a possible threat to cockle stocks. Improving the management of the capture methods used in the Ria de Aveiro in the intertidal and subtidal zones is essential since it becomes a possible solution for the conservation and more selective and efficient exploitation of this bivalve [18].
According to interviews conducted with shellfishers, the conservation status indicated that the cockle population in the Ria de Aveiro is stable (N = 27; 45%). Other respondents said that the species was threatened (N =14; 23.3%), highly threatened (N = 9; 15%), not threatened (N = 7; 11.7%), and little threatened (N = 3; 5%). C. edule has not yet been evaluated for the International Union for Conservation of Nature (IUCN) Red List, and the European Nature Information System (EUNIS) species database does not provide information on the conservation of this species [94, 95]. Aveiro’s shellfishers also shared some harvesting possibilities to conserve the cockle in their natural territory in the Ria de Aveiro. This LEK on the proper harvesting of the cockle is described in Table 2.
Table 2
Shellfishers’ recommendation for a proper cockle harvest (N = 60).
Shellfishers’ LEK about harvesting in the habitat
|
N of citations
|
“Cockle harvesting must be done with proper fishing gear and tools and the correct sizes.”
|
33
|
“Trawl gear must not be used when harvesting cockles.”
|
29
|
“You must collect and leave the small cockles in the same terrain where they were caught.”
|
25
|
“It will help if you spread out the remaining cockles that are on the ground. It would be best if you did not leave them huddled on the floor”.
|
09
|
07
|
“It should only have manual cockle harvesting.”
|
02
|
Shellfishers reported that the constant presence of researchers in the Ria de Aveiro and the interaction with local fishing villages could favor the conservation of the cockle. About 70% (N = 45) of interviewed said they tended to accept scientific advice from university researchers about cockle harvesting and cockle’ conservation in the Ria de Aveiro. Some respondents (N = 13; 21.7%) remained neutral in the face of this question. Only two (3.3%) informants did not want to give an opinion. These findings showed how part of the Aveiro community of harvesters might be predisposed to work together to conserve the cockles. The collaboration of these fishing villages in collaborative management with all active stakeholders can favor the underlying patterns and make it possible to test monitoring tools to improve results in marine systems [96].
Additionally, some shellfishers (N = 6, 10%) highlighted the veracity of the toxin analyzes carried out in the Ria de Aveiro. The main questions about this point were the lack of transparency in collecting samples, the care taken with transporting these samples to the place where analyzes were carried out, and the distrust about the points in the lagoon where the technicians collect the samples. Promoting alternative environmental education activities with the fishing community to publicize existing procedures and standards can improve trust among all interested parties [97]. Even with this type of action through discussion and education, there may still be some fishers who will still have attitudes contrary to the established norms [98]. However, understanding and analyzing the knowledge of artisanal fishers become crucial for a more flexible approach to the conservation of fisheries resources in these communities dependent on biological resources [34].