Microplastic ingestion and diet composition of humpback whales from the Northeast Pacific Ocean

The rapid recovery of humpback whale populations in the North Pacific, following decades of intense commercial whaling, represents a conservation success story. However, the future population growth and stability of this enigmatic species is now threatened by a growing list of anthropogenic stressors, including plastic pollution. My thesis aimed to quantify the level of microplastic (MP) ingestion among humpback whales foraging near the Juan de Fuca Strait, via opportunistically collected fecal samples. A total of 20 humpback whale fecal samples were collected during the summer of 2024, yielding an average of 24.4 ± 16.4 MP/g (dry weight), the majority of which were regenerated cellulose fibres. Stable isotope analysis and whole genome sequencing indicated that the sampled whales were primarily feeding on krill, enabling estimation of consumption rates which, combined with fecal MP concentrations, resulted in an average ingestion of 418,227 MP particles per day (95% CI: 402,191 - 436,172). This corresponds to an ingested mass of 0.37 g of MP per day (95% CI: 0.36 - 0.39), representing approximately 1.3×10^−6 % of the body mass of a 30 tonne humpback whale. Given the low estimated mass of ingested MP and scarce toxicological effects of regenerated cellulose, the risk to whales in this study appears relatively low. However, this finding is context-specific, and smaller particle classes such as nanoplastics, which were not assessed here, may have different bioavailability and toxicological implications. Nevertheless, these findings provide insight into the toxicological landscape of coastal food webs and can help inform future species risk assessments in Canadian Pacific waters.