Results
Plastic debris and non-plastic debris were found in 96 out of 1,447 pellets analyzed (%FO = 6.63) during both the pre-pandemic and pandemic phases. The total number of debris items identified was 179, of which 150 were classified as plastic debris. Threadlike items (micro and mesoplastics) were the most common plastic debris category compared to sheetlike and fragment items ( Table 2 ). The occurrence of threadlike items in pellets with debris items showed no significant difference before (%FO = 39%) and during (%FO = 56%) the pandemic period (Chi-square, χ 2 = 0.953, P = 0.329, df = 1).

In addition to plastic, various other types of anthropogenic materials found in 23 out of 1,447 pellets were identified and classified as non-plastic debris. The most encountered category was fibers, followed by metal, glass, wood, paint chips and balloons ( Table 2 ).

During the pre-pandemic phase, 57 pellets (%FO = 7.13, N = 800) contained plastic, whereas the content of 13 pellets (%FO = 1.62, N = 800) was identified as non-plastic debris. In the pandemic phase, 16 pellets (%FO = 2.47, N = 647) contained plastic debris, and no pellets contained non-plastic debris. This difference was significant (Chi-squared = 16.47, df = 1, p -value <0.0001, Figure 2 ). There was a clear fluctuation in the percentage occurrence of plastic in the cormorant pellets both prior and during the COVID-19 pandemic ( Figure 2 ), but when data is grouped within seasons of the year (spring, summer and winter, no data for autumn), no significant differences in the percentage of occurrence of plastic in cormorant pellets were found during pre-pandemic (Chi-square = 0.052, df = 1, p -value = 0.974) nor during pandemic phases (Chi-square = 1.38, df = 1, p -value = 0.5).

During the pre-pandemic phase, there was a significantly higher overall litter load per pellet (Mann–Whitney U -test, U = 71, p -value = 0.028, Figure 3 ). In February 2021, there was a peak of litter load attributed to the presence of one exceptional single pellet containing 57 litter items; apart from this anomaly, the litter load during this period was consistently low.

The diet of neotropic cormorants primarily consisted of five fish species, which accounted for 99% and 97% of the total prey species by number before and during the pandemic, respectively. There was no significant difference in the type of prey and the percentage numerical abundance (%NU) between the two phases (Paired t -test, t = 0.0762, df = 4, P = 0.24). The main fish prey included big nose anchovy ( Anchoa nasus ), which comprised 56% before the pandemic and 35% during the pandemic, minor stardrum ( Stellifer minor ) at 20% vs. 23%, Peruvian anchovy ( Engraulis ringens ) at 10% vs. 17%, lorna drum ( Sciaena deliciosa ) at 9% vs 16%, and Peruvian silverside ( Odontesthes regia ) at 4% vs. 6%.

In terms of the classification of plastics by size during the analyzed periods, it was observed that during the pre-pandemic phase, 17.19% were macroplastics, 48.44% were mesoplastics, and 34.38% were microplastics. However, during the pandemic, these percentages shifted to 17.33% for macroplastics, 73.33% for mesoplastics, and 9.33% for microplastics. As mentioned previously, regarding the classification by type of plastic, it is important to clarify that no significant differences were found between the evaluated periods. However, the most common type of plastic observed throughout the study was threadlike ( Fig. S4 ).