Discussion
This study revealed that the occurrence of plastic in neotropic cormorant pellets from CPCV beaches, Lima’s most recreational coastal areas, was three times lower and had a higher litter load per pellet during a period of COVID-19 lockdowns in Peru compared to pre-pandemic conditions. The reduction in plastic ingestion is likely attributed to restrictions on beach access for beachgoers. Neotropic cormorants did not shift the type or relative abundance of prey consumed between the two phases. Therefore, any differences observed in the occurrence of plastic cannot be attributed to a change in diet if secondary plastic ingestion may have occurred. A lower volume of microplastic in the marine ecosystem seems to be an immediate response to a reduction of littering on beaches during the mandatory lockdowns ( Zambrano-Monserrate, Ruano & Sanchez-Alcalde, 2020 ; Okuku et al., 2021 ; Orzama-González, Castro-Rodas & Statham, 2021 ; Soto et al., 2021 ), but microplastic found in cormorant pellets probably needed years to decades to breakdown from their original source ( Gregory, 2009 ; Andrady, 2011 ), remaining in the marine environment even after the onset of lockdowns. However, the response by cormorants in reducing plastic ingestion was almost immediate, which suggests that other processes may be involved. Large amounts of plastic particles had been already detected on the CPCV ( Blondet, Plaza-Salazar & Barona, 2023 ) and microplastic debris in other beaches in Lima ( Purca & Henostroza, 2017 ; De-la Torre et al., 2020 ) prior to the COVID-19 pandemic.

Two possible explanations for the quick turnover observed could be attributed to the introduction of microplastics into the sea due to the high presence of people engaging in recreational activities. These activities, such as swimming and surfing, have been shown to generate microplastic fibers through the shedding of clothing fibers ( Retama et al., 2016 ; Akkajit et al., 2021 ; Luo et al., 2022 ), categorized as “threadlike” in this study, which were the most common type of microplastic and mesoplastic identified throughout the course of the study. Alternatively, it is suggested that microplastics were no longer displaced into the sea by beachgoers during their recreational activities when access was restricted. It is worth noting that sandy beaches may experience a greater reintroduction of microplastics, as sand can easily contain this type of residue ( Vermeiren et al., 2021 ), which can then be transported to the sea, particularly critical in beaches adjacent to the collection site. These factors may also explain the progressive increase in plastic presence in pellets when restrictions on beach access were relaxed months after the onset of the pandemic. Additionally, oceanographic processes such as local currents and upwelling further influence the dynamics of plastic distribution in coastal areas. Region-specific oceanographic processes, as highlighted by Manay et al. (2021) , contribute to the accelerated presence of microplastics in coastal environments ( Lebreton, Greer & Borrero, 2012 ; Eriksen et al., 2014 ).

Neotropic cormorants are considerably affected by entanglement in ghost nylon nets and the use of plastic for their nests ( Ayala et al., 2023 ). Plastic has already permeated the marine trophic web in Peru, with reports in fish ( De-la Torre et al., 2019 ; Fernández & Anastasopoulou, 2019 ), seabirds ( Thiel et al., 2018 ; Díaz-Santibañez, Clark & Zavalaga, 2023 ), and marine mammals ( Perez-Venegas et al., 2020 ; Santillán, Saldaña Serrano & De-la Torre, 2020 ). Given the common occurrence of threadlike plastic in marine sediment ( Cisneros, Montero & Guevara, 2021 ), it is crucial to recognize its impact on the marine ecosystem. Being benthic divers ( Quintana et al., 2004 ), neotropic cormorants consume benthic and mesopelagic fish ( Galarza, 1968 ; Casaux et al., 2009 ; Petracci et al., 2009 ; Muñoz Gil et al., 2012 ). As a result, they are likely to ingest this type of plastic through trophic transfer from their prey. This finding is consistent with Franco et al. (2019) and Álvarez, Barros & Velando (2018) , who noted that threadlike plastic was the most frequently encountered category of plastic debris among seven seabird species. Nevertheless, the possibility of direct ingestion of plastic particles during underwater prey pursuit cannot be ruled out, considering their potential persistence within the water column—whether suspended, floating at the sea surface, or buried in the bottom sediment. This is particularly relevant as neotropic cormorants exhibit a foraging behavior that involves the exploitation of mid-water and bottom-dwelling fish ( Casaux et al., 2009 ; Petracci et al., 2009 ). The observed high density of plastic fragments and microplastics on the beaches of the CPCV ( Blondet, Plaza-Salazar & Barona, 2023 ) and neighboring coastal areas ( Purca & Henostroza, 2017 ; Gambini et al., 2019 ) implies that the transport of such materials to the sea is likely facilitated by tidal movements, ocean swells, and human activities along the beaches.

Overall, plastic items were found in 7.13% of neotropic cormorant pellets during the pre-pandemic phase. These results are very similar to those reported by the Guanay cormorant in the coast of Peru (7% frequency of occurrence) ( Díaz-Santibañez, Clark & Zavalaga, 2023 ) and within the range reported in other cormorant species ( Robards, Piatt & Wohl, 1995 ; Acampora, Newton & O’Connor, 2017 ; O’Hanlon et al., 2017 ; Brookson et al., 2019 ; Franco et al., 2019 ; Baak et al., 2020 ). While our study provides insights into the immediate effects of lockdown measures on plastic ingestion by neotropic cormorants, it is crucial to acknowledge the complexity of plastic pollution dynamics, influenced by various factors. These factors include but are not limited to, human activities unrelated to the pandemic, such as the impact of wastewater treatment plants (WWTPs). Currently, only one WWTP, La Chira, is responsible for wastewater treatment in southern Lima, yet there is insufficient evidence to confirm its efficacy in waste eradication (ATUC, 2023). Moreover, WWTPs do not fully eliminate microplastics, with removal rates sometimes falling below 30% ( Lyare & Bond, 2020 ; Xu, Bai & Ye, 2021 ). Thus, the significant influence of WWTPs on the occurrence of microplastics and mesoplastics in the sea near CPCV cannot be overlooked. Future research endeavors should delve deeper into this aspect to comprehensively understand how human activities and environmental factors collectively contribute to plastic pollution and its uptake by marine organisms.