Southern Portugal (south-western Europe), which coincides with the Algarve region, is a Mediterranean climate region and the only arid or semiarid region in Portugal since annual rainfall is lower than 400 mm (Santos et al., 2010). Mean air temperatures range between 11°C in January and 27°C in August (World Weather Online 2021). The Algarve has four estuarine ecosystems, the Guadiana estuary and Ria Formosa lagoon in the eastern Algarve and the Arade estuary and Ria de Alvor lagoon in the western Algarve (Figure 1). The eastern zone of the south coast is mostly sandy, only interrupted by the Ria Formosa lagoon and its barrier islands, while the central and western zones of the south coast are characterized by limestone and sandstone rocky shores along with pocket sandy beaches, and cliffs towards the west coast (Moura et al., 2006). The continental platform also follows this typology, displaying a wider shelf (>40 km) and gentle slope to the east of Cape Santa Maria, while from here and towards Cape Saint Vincent (Sagres) the shelf is narrower (<15 km), the slope is steeper, and depths of 100 m can be reached within 10 km from the coast (Relvas and Barton, 2002; Garel et al., 2016). Mean sea surface temperature in the southern coast of the Algarve range between 15°C in January and February and above 20°C during summer, and it is increasing at a rate of +0.2°C decade⁻¹ (Baptista et al., 2018).

NEMA is a citizen science campaign launched in April 2019 that focuses on the new aquatic species found along the Algarve coast, including estuaries and lagoons. We created a logo design that matches with the institutional image of the research centre (CCMAR—Centre of Marine Sciences) to increase credibility, facilitate outreach, and ultimately the number of submitted records (Figure 2A). The logo was designed for free by Dr. Sarita Camacho, as part of her graduation internship in Communication Design at the University of Algarve. The taxa chosen for the logo include some of the most emblematic new species in the Algarve and encompassing different taxonomic groups, as the invasive Atlantic blue crab Callinectes sapidus and bloom-forming jellyfish Catostylus tagi, or subtropical species like the ornate wrasse Thalassoma pavo, or the bearded fireworm Hermodice carunculate which may pose public health risks (Verdes et al., 2017; Encarnação et al., 2019). The website of NEMA (www.NEMAlgarve.com) was launched in May 2020, so it did not influence the outreach and outcomes of the first year of NEMA.

During NEMA’s first year, we only used free web tools to promote the campaign and increase communication with citizen scientists. So, we created accounts on the main social media platforms—Facebook (link), Instagram (link), Twitter (link)—to promote NEMA and reach a high number of citizens in the shortest period possible. Additional communication channels were created, as a dedicated email account (nemalgarve@gmail.com) and a project page on BioDiversity4All (link)—a free biodiversity citizen science platform which is the Portuguese version of iNaturalist. NEMA’s account on BioDiversity4All gathers the validated records received across all communication channels and are publicly available for consultation.

We actively promoted NEMA on social media platforms with information about its objectives, species of interest, and how citizens could participate in the campaign. We also made regular publications with the observations submitted by citizen scientists to acknowledge their contribution. To increase outreach, publications were often shared by our research centre (CCMAR) on their social media accounts. A poster with the species of interest (Figure 2B) was created and shared regularly on NEMA’s social media accounts. Every month, from June to October 2019 and also January 2020, this poster was used as an outreach tool to engage with the public on several Facebook groups related to fishing and general ocean activities. On average, we reached out to 23 ± 6.3 Facebook groups per month in this period. In most instances, we used the Portuguese version of the poster.

In May 2019, we issued a press release to local and national media about NEMA and its objectives. The visibility of NEMA on social media, in tandem with the press release, led to a growing interest from traditional media on the NEMA campaign, biological invasions, and species reaching southern Portugal owing to climate change (Figure 2C).

We asked citizen scientists to provide information about the species of interest and for five details about their observations: 1) a photograph of the specimen(s), 2) date, 3) location, 4) method of capture or observation, and, whenever possible, 5) the inclusion of an object to serve as a scale in the photograph. Only observations that included, at least, one photograph to allow the species identification, date of observation and a detailed location were considered valid and included in NEMA’s database. The direct communication channels provided the opportunity to obtain all the details to validate observations and permission to add the observation to the database. Observations classified as “personal communications” refer to direct messages sent by friends or colleagues about an observation, or with a link or contact to the citizen scientist that made the observation.

Several observations pre-dating NEMA—before April 2019 and hereafter named pre-NEMA—were included in the present database. These were mostly made by two informed citizens, Mr. Gonçalves and Mr. Fernandes, that continued to provide records after the ones published by Morais et al. (2019). With the launch of NEMA, we were also able to reach several citizen scientists that already had older observations stored in digital devices and such records are also labelled as pre-NEMA.

Facebook was our most popular social media account, so we retrieved several metrics to assess the impact of social media outreach on the number of records. These metrics include the number of daily new Facebook followers, daily total impressions, and daily total consumers. Facebook defines daily total impressions as the number of times any content from the page or about the page entered a person’s screen (e.g., posts, stories, check-ins, ads, and social information from people who interact with the page), while daily total consumers are the number of people who clicked on any of the account’s content. We used linear regressions to assess the relationship between these metrics and the number of submitted observations.

During NEMA’s first year, we focused mainly on social media outreach which resulted in traditional media becoming interested in the subject (Figure 2C). Between April 2019 and March 2020, two interviews were broadcasted on national television, fifteen online articles were published, two articles published on printed newspapers with one making cover page, and two interviews given to radio stations. The content of these news pieces included the discovery of NIS in Algarve and NEMA’s citizen science campaign. The full list of articles and details about each one is available in Supplementary Table S1.

Facebook was the social media platform most used by citizen scientists to contact NEMA—68% of all 84 validated observations of the Atlantic blue crab. NEMA’s Facebook account received 57% of these observations (n = 48) and the other 11% (n = 9) were made through Facebook groups, or as a direct response to our explanatory publications in these groups (Figure 3A). No observations were reported through Instagram or Twitter. Observations uploaded on BioDiversity4All accounted for 12% of the records (n = 10), despite that most first contacts were also made through social media, followed by the upload of the observations on this platform by citizen scientists. NEMA’s email received 7% of the observations (n = 6) and the remaining observations (13%, n = 11) were personal communications sent to us (Figure 3A).

The significant interest in NEMA’s Facebook publications is shown by six sudden increases in the number of impressions (Figure 3B). The peak occurred on October 14, 2019, when these publications reached 40,905 people (daily total impressions) and generated 825 interactions (daily total consumers) (Figure 3B). The maximum number of interactions with NEMA’s Facebook account was registered on December 21, 2019—2,839 interactions and 17,414 people reached (Figure 3B). During NEMA’s first year, publications in Facebook reached a total of 669,417 people (impressions) and 31,565 interactions (consumers). We registered a positive relationship between the number of observations received with the daily total impressions (R ² = 0.976), daily total consumers (R ² = 0.973), and also with the number of Facebook followers (R ² = 0.968). The number of observations reported on Facebook followed the increase in Facebook followers—2,163 by the end of March 2020 (Figure 3C). The main sources of new Facebook followers occurred after the publication of monthly explanatory posts on Facebook groups (Figure 3D), the coverage made by traditional media (dark arrows on Figure 3B), and publication of regular posts in NEMA’s Facebook account.

Most Atlantic blue crab records were collected with a fishing gear (48.0%, n = 59), mostly with fishing nets (32.5%, n = 40), but also by hand (14.6%, n = 18) or found dead (18.7%, n = 23) (Figure 4). By the end of March 2020, NEMA’s database, and therefore our sample size, included 166 valid observations from 1747 Atlantic blue crabs, submitted by 60 citizen scientists and two informed citizens (Figure 5A). Citizen scientists recruited by NEMA submitted 84 valid observations of 117 Atlantic blue crabs, while informed citizens contributed with 82 observations of 1,630 specimens. Observations registered before the launch of NEMA in April 2019, included observations mostly provided by the two informed citizens already mentioned (n = 23), while six citizen scientists provided seven observations (blue symbols in Figure 5A). These pre-NEMA observations were made in locations where the species had not been recorded before: one female in Ria de Alvor (May 15, 2018, record #20); one male in the eastern sector of the Ria Formosa near Tavira (March 4, 2019; record #12); one male in the coastal area off “Barrinha”, an inlet of the Ria Formosa (March 27, 2019, record #4). The complete list of observations and NEMA’s references are available in Supplementary Table S2.

The Guadiana estuary and the contributions made by Mr. Gonçalves represent a particular sub-set of records. This informed citizen alone reported 1,624 Atlantic blue crab specimens, all captured along a 12 km stretch of the middle Guadiana estuary, close to the village of Odeleite (Figures 1, 5A). Most specimens were males (58.8%, n = 955) and females only accounted for 6.0% (n = 97), while the remaining specimens were not sexed (35.2%, n = 572). Two months stood out—September 2019 (125 males, 6 females, 508 unsexed specimens) and March 2020 (456 males, 0 females). In 2019, the maximum daily catch was 105 specimens (September 17), and it reached 110 and 130 specimens in 2020 during two consecutive days, March 4 and March 5, respectively. No similar amount of daily catches were ever reported anywhere in Portugal. An additional 16 observations were made in this estuary by 11 citizen scientists about 21 specimens (Supplementary Table S2). Observations were mostly done in the middle and lower Guadiana estuary, but one dead specimen was found in Mértola at 70 km from the river mouth on October 6, 2019 (Figure 5A).

Citizen scientists alone contributed with 77 valid observations about 109 Atlantic blue crabs, observed between April 2019 and March 2020—44.0% males (n = 48), 43.1% females (n = 47), 12.8% unsexed (n = 14) (Figure 5A). Observations made in estuarine ecosystems (Ria de Alvor, Arade estuary, Ria Formosa, and Guadiana estuary) accounted for 50.6% (n = 39) of the observations—61.7% males (n = 29), 34.0% females (n = 16), 4.3% unsexed (n = 2) (Figures 5A, 6). Observations made in coastal areas represented 49.4% (n = 38) of all records—30.6% males (n = 19), 50.0% females (n = 31), 19.4% unsexed (n = 12) (Figures 5A, 6). There were differences in the proportion of sexes between coastal and estuarine areas during the non-reproductive (p = 0.044) and reproductive periods (p = 0.065) (Table 1). In both cases, females were more frequent in coastal areas (31 specimens) than in estuarine ecosystems (16 specimens) (Figure 6).

In the first 3 months of NEMA (April-June 2019), only one Atlantic blue crab specimen was reported. Nonetheless, five specimens captured during July 2019 extended the known distribution westwards by over 50 km, from Faro to the Arade estuary in Portimão (Figure 5A). Two specimens captured in Ria de Alvor (one male, one female) further extended the distribution westwards by 8 km in August 2019. On September 19, 2019, one male specimen captured near the beach of Zavial further extended the western distribution limit by 23 km (Figure 5A).

Between August and November 2019, 16 observations (20 specimens: 7 males, 7 females, and 6 unsexed) confirmed the establishment of the Atlantic blue crab in the area between Albufeira and Alvor (green and orange symbols in Figure 5A). In the same period, between Faro and Vila Real de Santo António, 19 observations were made (21 specimens: 3 males, 13 females 4 unsexed) of which nine females have washed ashore in the beaches close to the mouth of the Guadiana estuary (Figure 5A). In August and September 2019, ovigerous females were reported (Figure 5B), one found dead in a beach close to the mouth of the Guadiana estuary (observation #34), two inside the Ria Formosa lagoon (observations #26 and #41), one in the Arade estuary (observation #17), and another one in the coastal zone of Portimão (observation #31). It is worth mentioning that two non-ovigerous females were captured at night while swimming at the surface on August 27, 2019, and October 2, 2019 (observations #22 and #42). In December 2019, a single observation (observation #66) reported one male and six females in the lower Guadiana estuary near Vila Real de Santo António, and one additional female was captured in the Sagres’ harbor (observation #63). This last record extended the western distribution limit by another 4.5 km (Figure 5).

In January 2020, one fisherman made three observations on subtidal areas off Alvor and Lagos and mentioned that the Atlantic blue crab was a “frequent” bycatch. Two of these observations narrowed the gap of records made between Alvor and Sagres (Figure 5A). One of such observations reported 8 males and 3 females, all captured at night with a fishing net set near the Porto de Mós beach (Lagos, January 17, 2020) (Figure 5C). During the first 3 months of 2020, 10 observations confirmed the presence of the species in vicinities of Ria de Alvor and the Arade estuary (red symbols in Figure 5A). The entire south coast of the Algarve was formally colonized by the Atlantic blue crab when a female specimen was recorded in the Mareta beach (Sagres) on March 3, 2020 (observation #86, westernmost red symbol in Figure 5A).

Based on the number of hours we invested in launching and handling NEMA, the corresponding cost during its first year would have summed up to 3,751.47 € (Figure 7). Hiring the services of professionals to develop and handle all the digital platforms plus gathering the same number of records of the Atlantic blue crab in the field, would have cost between 29,815.58 € and 153,485.58 € (Figure 7).

The service quotes from three software developers to build a website and a smartphone app with basic features (i.e., submission of a photograph, location, date, and contact of the citizen scientist) were quite distinct—12,115 €, 55,350 €, and 81,180 €. The cost associated with the time invested in creating NEMA’s social media accounts, e-mail, and project page on Biodiversity4All was only 358.40 € (Figure 7).

Between April 2019 and the end of March 2020, we made a total of 335 publications on Facebook—198 publications on NEMA’s Facebook account and 137 explanatory publications on Facebook groups (Figure 3C). Considering a value of 9.60 € per publication, the cost of media handling associated with these publications would correspond to a total of 3,216 € for this first year of NEMA (Figures 3C, 7). For the same 355 publications, service quotes provided by professional social media managers were at 20 €, 50 €, and 183 € per publication, which would result in a total of 6,700 €, 16,750 €, and 61,305 € respectively (Figure 7).

The total cost for a NEMA scientist to go to the field and make the same 166 observations (1747 Atlantic blue crabs) would have reached 11,000.58€. The observations made before NEMA would sum up to a minimum of 2,015.61 €, while during the first year of NEMA, the total minimum cost would have been 8,984.96€—4,965.06 € for records made by citizen scientists and 4,019.90 € for records made by informed citizens. This represents an average minimum savings of 748.75 ± 505.77 € month⁻¹ during NEMA’s first year. The maximum cost per individual was 75.73 € for the westernmost observation (record #86, Mareta beach, Sagres) and averaged 36.59 ± 28.05 € individual⁻¹ (Figure 8). The cost per individual was on average higher for observations provided by citizen scientists (6.59–75.73 € individual⁻¹, 58.99 ± 16.20 € individual⁻¹) than informed citizens (0.52–68.06 € individual⁻¹, 13.64 ± 16.73 € individual⁻¹) because observations made by informed citizens were mostly made in the Guadiana estuary and many individuals were reported in most observations (Figure 8). The minimum average cost per trip for a NEMA scientist to obtain the same record (one individual or several) as those made by citizen scientists was 64.35 ± 6.42 € trip⁻¹. This value was similar to the cost to obtain the same record as of informed citizens (68.23 ± 0.56 € trip⁻¹) since all these observations were done in the middle and lower Guadiana estuary (Figure 8). By investing our time in handling the digital communication channels to retrieve the 166 observations submitted by citizens scientists and informed citizens, we saved 177.07 € (Figure 7).

Detecting the presence of aquatic non-indigenous species after their introduction is extremely challenging and, in most cases, they only become noticed when an invasive status is reached (Mehta et al., 2007; Pyšek and Richardson, 2010). This has been tackled across the globe with rapid assessment surveys in artificial structures (Collin et al., 2015) or systematic surveys with fishing gears (Yamada et al., 2015; Poirier et al., 2017), but also using new technologies (e.g., eDNA analyses) that enhance the success of detecting NIS with low abundances (Rees et al., 2014). However, implementing eDNA monitoring programs is unfeasible in most regions due to the financial costs associated with this technology. In some cases, citizen sciences campaigns may mitigate the lack of intensive monitoring programs. For example, several successful citizen science campaigns have focused on crustaceans (e.g., Asian shore crab Hemigrapsus sanguineus and the European green crab Carcinus maenas (Delaney et al., 2008; Grason et al., 2018)), algae (e.g., Caulerpa taxifolia (Ellul et al., 2019)), or fish (e.g., lionfish Pterois miles (Azzurro et al., 2017; Giovos et al., 2018)). However, the running costs of citizen science campaigns are unavailable to analyze and validate the cost-benefit of this approach.

During NEMA’s first year, the campaign relied mostly on social media to communicate with potential citizen scientists. Social media provides a dual-communication channel with citizen scientists, i.e., allows promoting the project while providing updates on recent discoveries, increasing scientific literacy, and interact directly with citizen scientists. Direct communication with citizen scientists on Facebook provided valuable records about the Atlantic blue crab in the Algarve. However, asking citizen scientists to independently register their observations on BioDiversity4All citizen science platform was unsuccessful—only 11% of observations were registered on this platform by citizen scientists. Relying on free digital platforms (social media, e-mail, and citizen science platforms) meant running NEMA with no associated costs during its first year and save over 11,000€ for the total of 166 observations received (8,900€ for observations made only in the first year period). This is the minimum amount of money that we would need to obtain the exact same information on the field and with just one scientist. In comparison to the methods applied with NEMA’s digital channels of communication, the corresponding cost in gathering these records would still be much lower (358 €).

Other successful citizen science projects relied on dedicated websites and/or smartphone apps (Gallo and Waitt, 2011; Azzurro et al., 2013, 2019; Zenetos et al., 2013; Marchante et al., 2017; Eritja et al., 2019). Such technologies are extremely costly to produce and maintain. Additionally, NEMA is being implemented as a long-term detection campaign, and such web platforms also require recurring annual fees. NEMA’s approach to engage with citizen scientists mostly through online outreach is still a time-consuming methodology, that requires constant communication with participants and all the tasks associated with social media handling. If the time invested would result in a direct cost to create all the platforms and handling the social media pages ourselves, the correspondent cost during this first year of NEMA would have summed up to 3,574 €, which is still much lower than hiring professionals (18,815 €—142,485 €).

In Portugal, obtaining funding to establish long-term monitoring programs on aquatic invasive species is extremely unlikely. To overcome the idiosyncratic nature of Portuguese science funding, we opted for this low-cost approach which turned out to be extremely successful, while increasing the regional and national scientific and environmental literacy of the population. We will continue promoting NEMA for the foreseen future and we endorse the implementation of similar approaches in other regions of the world where scientific funding is scarce. Finally, biological invasion scientists should establish at least an “open communication channel” with citizens, even if not running a citizen science project, so that they can receive spontaneous contacts about new records while scouting social media and online forums (e.g., naturalists, fishers, hikers) for records of new NIS.

The best strategy to maximize participation and increase the number of records reported by citizen scientists is to establish multiple communication channels with scientists and research institutions (Encarnação et al., 2021). Despite following this recommendation, we acknowledge that our data may be biased since it likely engaged citizen scientists already concerned with environmental issues or with a strong interest in fishing (i.e., fishers and anglers). One informed citizen recorded 93.0% of the total 1747 Atlantic blue crabs reported to NEMA. Yet, the other 117 Atlantic blue crabs allowed to track the fast westward expansion of the species for over 90 km along the coast of the Algarve (Figure 5A). Furthermore, the number of reported individuals represents a 46-fold increase in comparison to data obtained during the 3 years prior to NEMA (Morais et al., 2019; Vasconcelos et al., 2019). NEMA also brought to light a record made in May 2018 at Alvor (Figure 5A, observation #20) which would have extended the known distribution in 65 km by the time the two scientific publications were made in 2019 (Morais et al., 2019; Vasconcelos et al., 2019).

Our study made clear that citizen scientists have different engagement levels, yet equally valuable to monitor biological invasions. Without a wide network of citizen scientists, we could not track the westward expansion of the Atlantic blue crab. Without an informed citizen from the Guadiana estuary (Mr. Gonçalves), we could not obtain precious information about the presence of the species in this estuary for an extended period of time. Therefore, all connections should be nourished. Developing short-training actions with citizen scientists will provide valuable long-term data while giving more autonomy for citizen scientists to gather data with different methodologies.

NEMA also obtained interesting details about the ecology of the Atlantic blue crab. Two females were reported to be swimming at the surface during the night close the coast, which is a typical behavior of ovigerous females that perform vertical migrations at night during the spawning periods (Tankersley et al., 1998; Aguilar et al., 2005; Forward et al., 2005). NEMA’s data also showed that female Atlantic blue crabs were more common in coastal areas throughout the year, and not only during the reproductive period (August–October). Ovigerous females in coastal areas were only recorded once off Portimão, but the other three ovigerous individuals were found in the lower Arade estuary and Ria Formosa. The high mobility of Atlantic blue crabs and its fast adaptation to environmental conditions, namely salinity, are key factors for the selection of spawning areas (Forward et al., 2003; Aguilar et al., 2005), therefore spawning areas in Algarve seem to include both the lower section of estuaries and coastal areas.