Harmful algal blooms (HABs) are natural phenomena that occur when colonies of microalgae grow out of control and produce toxic or harmful effects on the surrounding fauna. In August 2022, an HAB, dominated by the diatom, Cylindrotheca closterium (Ehrenberg) Reimann et J.C. Lewin, 1964, occurred on the Yucatan coast, southern Gulf of Mexico. In the presently reported study, two photo transects were established along the coastline, one at the onset of the bloom, at Telchac port, and the other at the final phase of the event, at Chixchulub port. The affected fish species were documented photographically and a taxonomic list, with their abundance, density, and biomass is presented, as well as a summary of the affected ecosystems according to the affinity of these species. A total of 54 species were recorded; 48 in Telchac and 21 in Chicxulub, with 15 species occurring at both sites. The affected species have a greater affinity to reef systems, beaches, and estuaries, in that order, between 3 m and 113 m depth. In the International Union for the Conservation of Nature (IUCN) nomenclature, the majority of the species are in the “least concern” category, however, species were also recorded in the “near threatened”, “vulnerable”, and “endangered” categories. Therefore, it is extremely important to monitor these events and report the affected species, since the increase in the frequency of these phenomena due to local and global factors can have significant repercussions on species endemic to the coastal zone.

Cylindrotheca closterium, dead fishes, harmful algal bloom, red tide, Yucatan Peninsula

Harmful algal blooms (HABs), colloquially known as red tides, are a natural phenomenon that occurs when colonies of microalgae (in the sea or in freshwater) grow out of control and produce toxic or harmful effects on the surrounding fauna (Hallegraeff et al. 1995). There are toxic and non-toxic HABs; in the toxic ones the algae produce toxins that affect or kill organisms, while the non-toxic ones can also cause the death of organisms, but this is because of the low concentration of oxygen in the environment due to their high density (Sidabutar et al. 2021). Although HABs are a natural phenomenon, they are magnified by changes in the concentration of nutrients (eutrophication) and in the temperature of the ecosystem, which is why these phenomena are expected to become more frequent and intense due to global warming and the increase in contamination (Anderson 1989; Smayda 1990; Hallegraeff 1993; Hallegraeff 2023).

When HABs appear on the coasts, they cause not only ecological impacts but also strong economic impacts on the local community, directly affecting fishing, as well as the restaurant and tourist industry. On 3 August 2022, fishermen noticed occurrence of live octopuses on the shore, and on 4 August dead and intoxicated fish started to deposit on the beach due to the HAB effects, dominated by Cylindrotheca closterium (Ehrenberg) Reimann et J.C. Lewin, 1964, a non-toxic diatom species (Herrera-Silveira et al. 2022). The stranding of dead and intoxicated fishes began at San Felipe port on the east of the Yucatan coast and gradually moved to the west until reaching the coasts of Chelem, Yucatan. On the night of 4 August, the communities at the coast began cleaning and burying the dead fish; the stranding of dead and moribund fish lasted approximately one week. HABs dominated by C. closterium have been documented in the area because of increased concentrations of nutrients such as nitrite, nitrate, phosphate, and urea (Poot-Delgado and Okolodkov 2020). During this phenomenon, the stranding of great diversity and abundance of fishes was documented photographically in order to present a taxonomic list of the affected species, as well as their abundance, density, and biomass, in addition to determining the affected ecosystems according to the affinity of these species.

The shore was monitored based on two photo transects, the first, at the early site of the stranding, on 5 August, covering 1 km of the shore (Telchac) and the second one at the final phase of the event, on 12 August, covering 10 m (Chixchulub) (Fig. 1A). On shore, we randomly established a starting point and walked east–west, taking photographs of the organisms already deposited on the beach by using a PVC square frame of 1 m² and a camera held at an approximate height of 1 m (Fig. 1B). No specimens were collected.

Figure 1. 

Diagrams showing the study area on the Yucatan coast, Mexico. (A) map of the study area (Sampling sites: 1 = Chicxulub, 2 = Telchac port). The green line marks the stretch of the shore affected by the harmful algal bloom in summer 2022. (B) Diagrammatic representation of a photo transect on the coastline. (C) Diagrammatic representation of the habitats where the affected species have been recorded.

The stranded organisms were identified up to the species level based on photographs. The identification was facilitated by the relatively fresh coloration of the specimens. Once we had a list of species, using FishBase (Froese and Pauly 2023) and Robertson and Van Tasell (2023), we collected information (1) on the size and weight of the species to make an approximation of the biomass; and (2) in order to determine the main habitat associated with the affected species, the depth range and the habitats (Fig. 1C) where the species have been recorded such as reef, estuary, and beach, among others, were recorded and used as a binary variable indicating the presence as (1) if the species have been recorded in that habitat or absence (0) if the species have not been reported in that habitat. Finally, the conservation status of the recorded species, according to the categories of the Red List of the International Union for the Conservation of Nature (IUCN), was recorded.

Species richness was determined as the number of species recorded on both photo transects, abundance as the total number of individuals recorded per species, density as the number of organisms per species per 1 m², and biomass as the weight [g] per species per 1 m².

Because the photo transect in Telchac was established at the beginning of the HAB, the beach cleaning campaigns had not started and a greater number of species could be observed, while in Chicxulub the photo transect was smaller and recorded less extensive diversity because the cleaning campaigns were already ongoing, however, it was possible to list species not recorded in Telchac.

Richness. A total of 54 species were recorded (Table 1), 48 in Telchac and 21 in Chicxulub. Six species not recorded in Telchac were recorded in Chicxulub: Abudefduf saxatilis, Caranx hippos, Chilomycterus schoepfii, Gymnothorax saxicola, Ogcocephalus parvus, and Sphoeroides testudineus. Of the total, five species were elasmobranchs and 49 actinopterygians. The families with the highest number of affected species were Haemulidae, Sciaenidae, and Serranidae with three species each.

Abundance. Telchac presented the mean abundance” of 6.4 individuals per species (range 1–36 ind./spec.) and Chicxulub 16.95 individuals per species (range 1–245 ind./spec.), the overall mean value was 12.4 ind./spec. (range 1–281 ind./spec.) (Table 2). Among the ten most abundant species, both localities share Opsanus beta, Sphoeroides spengleri, and Haemulon plumieri. In both locations, Opsanus beta presented the highest values of abundance and thus was the most abundant species with a total of 281 individuals recorded, followed by Acanthostracion quadricornis with 61 individuals, Hypoplectrus puella with 27, Harengula jaguana 26, and Eques lanceolatus 25 (Fig. 2).

Density. In Telchac a mean density of 0.2142 ind. · m^–2 (range 0.03–1.2 ind. · m^–2) was recorded, while in Chicxulub 0.4241 ind. · m^–2 (range 0.025–6.13 ind. · m^–2), the overall mean value was 0.25 ind. · m^–2 (range 0.025–3.66 ind. · m^–2) (Table 2). In both locations, O. beta presented the highest values of density. Among the ten species with the highest density, both sites share Opsanus beta, Sphoeroides spengleri, and Haemulon plumieri. The species with the highest density were Opsanus beta 3.66 ind. · m^–2, Hypoplectrus puella 0.90 ind. · m^–2, Harengula jaguana 0.87 ind. · m^–2, Eques lanceolatus 0.83 ind. · m^–2, and Acanthostracion quadricornis 0.79 ind. · m^–2 (Fig. 2).

Biomass. In Telchac the mean biomass was 593.69 g · m^–2 and the total biomass (range 0.39–20 000 g · m^–2) and in Chicxulub it was 3750.46 g · m^–2 (range 35.4–55 370 g · m^–2), the overall mean value was 1249.51 g · m^–2 (range 0.39–27 820.6 g · m^–2) (Table 2). Among the ten species with the highest density, both sites only share Opsanus beta. The species with the highest biomass in Telchac were Ginglymostoma cirratum reaching 20 000 g · m^–2, Hypanus say 5847 g · m^–2, and Pomacanthus arcuatus 1110 g · m^–2, while in Chicxulub—Opsanus beta with 55 370 g · m^–2, Acanthostracion quadricornis 7920 g · m^–2, and Diodon holocanthus 4867.20 g · m^–2. In total, the species with the highest biomass were Opsanus beta (27 820 g · m^–2), Ginglymostoma cirratum (20 000 g · m^–2), and Hypanus say (5847 g · m^–2) (Fig. 2).

Habitat. The species affected by the HAB presented a range of sizes between 8 and 300 cm, the majority, 42 species, with sizes between 8 and 62 cm and an overall mean value of 47.69 cm (Table 2). They were associated with seven general habitats, reefs, beaches, estuaries, submerged vegetation, water column, freshwater, and flotsam. A total of 28.6% of the affected species had an affinity for reefs, 26.3% for beaches, and 18.8% for estuaries, adding up to 74% of the affected species, another small percentage was divided between submerged vegetation, water column, freshwater, and flotsam (Fig. 3A). These species occur not deeper than 611 m, although, the majority had a minimum depth range between 0 and 5 m and a maximum between 5 m and 150 m, so the mean range was between 3 and 113 m of depth (Fig. 3B). The majority of the species (46 species) are in the category “least concern”, Ginglymostoma cirratum and Ocyurus chrysurus in the category “data deficient”, Pseudobatos lentiginosus “near threatened” Lachnolaimus maximus “vulnerable”, Sanopus reticulatus “endangered” (Fig. 3C). A total of four individuals of Pseudobatos lentiginosus, five individuals of Lachnolaimus maximus, and three individuals of Sanopus reticulatus were recorded.

Figure 2. 

Community descriptors. Abundance, density, and biomass of fish species affected by the harmful algal bloom off the coast of Yucatan, Mexico in summer 2022.

Figure 3. 

Ecological affinity, depth, and conservation status of species affected by the harmful algal bloom off the coast of Yucatan, Mexico in summer 2022; (A) percentage of species with affinity to the recorded habitats; (B) distribution in the water column, minimum and maximum depth; (C) number of species in the Red List categories. Abbreviations: Min = minimum, Max = maximum, LC = “least concern”, DD = “data deficient”, NT = “near threatened”, V = “vulnerable”, E = “endangered”.

Different authors recorded between 14 and 94 fish species on the Yucatan coast (Córdova-Tapia and Zambrano 2016; Palacios-Sánchez et al. 2019; Aguilar-Medrano et al. 2020), 159 in the Campeche Bank (Aguilar-Medrano and Vega-Cendejas 2019), and 183 on reefs (Núñez-Lara et al. 2015). According to the presently reported results, the species affected were most commonly associated with reefs, beaches, and estuaries in a depth range of 3 m to 110 m. We recorded in only two days of sampling, 54 species of bony and cartilaginous fishes, so it can be assumed that a greater number of species were affected during the whole duration of the HAB.

In total, 680 organisms were recorded, 281 representing Opsanus beta. This type of phenomenon occurs mainly in the coastal zone and Opsanus beta is a coastal species, distributed between 0 m and 5 m of depth, which is why it was highly affected, recording the highest abundance, density, and biomass. However, since this species is in the category of “least concern” and it is distributed from the east coast of Florida to Belize, we can assume that the HAB did not cause significant damage to its populations. However, a species with similar habits is Sanopus reticulatus, which is microendemic to the west coast of the Yucatan Peninsula and is in the “endangered” category. Although only three individuals of this species were recorded, these were recorded at the beginning of the HAB, so we can assume that more organisms were affected during the course of the HAB, and thus this could have affected their populations.

Among the species with the highest abundance and density were Acanthostracion quadricornis, Hypoplectrus puella, Eques lanceolatus, and Harengula jaguana. These species have a wide distribution in the Gulf of Mexico and the Caribbean; they are coastal inhabitants of beaches, reefs, and submerged vegetation, except for Harengula jaguana which is found in coastal schools in the water column, all of these species are in the conservation category “least concern”. Lachnolaimus maximus is also widely distributed in the Gulf of Mexico and the Caribbean, it is a coastal inhabitant of beaches, reefs, and submerged vegetation; however, it is in the “vulnerable” category. Only five individuals of this species were recorded, four at the beginning of the HAB and one at the end, so we assume a low impact of the HAB on the species population.

Four cartilaginous species stand out among the species with the highest biomass, Ginglymostoma cirratum, which reaches 60 kg and 300 cm of total length (TL), of which only one individual was recorded, Hypanus say with 19.5 kg and 78 cm of TL, of which nine individuals were recorded, Urobatis jamaicensis with 5 kg and 76 cm TL of which six individuals were recorded, and Pseudobatos lentiginosus with 700 g and 76 cm of TL of which four individuals were recorded. The species Hypanus say and Urobatis jamaicensis are in the conservation category “least concern”, Ginglymostoma cirratum in “data deficient” and Pseudobatos lentiginosus “near threatened.” The latter species is distributed in South and North Carolina, the coast of Florida, and the whole Gulf of Mexico; however, it is an uncommon species (Lieske and Myers 1994), with a very low reproductive resilience rate, where the population doubles in a minimum time of more than 14 years (Froese et al. 2017), therefore, three individuals recorded at the beginning of the HAB and one at the end, indicate its possible effect on a greater number of individuals throughout the time duration of the HAB.

On the coast of Yucatan, tourism, urbanization, sewage discharges, livestock, agriculture, shrimp farming, and atmospheric deposition have been implicated as the most important continental sources of nutrient inputs into the coastal ecosystem (Aranda-Cirerol et al. 2011; Padilla 2015; Castillo-Pavón and Méndez-Ramírez 2017; Aguilar-Medrano et al. 2020; Poot-Delgado and Okolodkov 2020), swine being the greatest single nutrient input (Drucker et al. 2003; Aranda-Cirerol et al. 2011). The increase in the nutrients along with the non-existent or inefficient wastewater treatment plants, are the main causes of water quality problems in the area (Castillo-Pavón and Méndez-Ramírez 2017; Aguilar-Maldonado et al. 2018). As is known, nutrients can stimulate or enhance the impact of HABs (Anderson et al. 2008; Glibert et al. 2010; Aranda-Cirerol et al. 2011) thus, if nothing is done to stop the input of nutrients into the Yucatan coast, we can expect an increase of the HABs in the area. Although the majority of the species recorded are in the IUCN red list category “least concern”, the increase in the frequency of these events due to local and global factors can cause damage to endemic species in coastal areas such as Sanopus reticulatus, which is in the category “endangered” and is microendemic to the Yucatan coastal area.

All the authors are recipients of the National System of Researchers grant (CONAHCYT-SNI).