Leopold MF & Overmaat W (2023) Common Guillemots Uria aalge drowned in bottom-set nets and washed ashore in the Netherlands: new opportunities for diet studies?. LIMOSA 96 (4): 174-186.

While bycatches in bottom-set nets are a major cause of death in seabirds world-wide, such bycatches have long been considered a minor problem in the Netherlands. However, beached bird surveys have shown that drowning incidents do occur. One such incident is reported here: 15 freshly drowned Common Guillemots were collected, examined for clues that might suggest drowning, and their stomach contents were studied. All birds were unoiled, but had wet and sandy plumages, and many had damaged feathers, or broken bones. The birds were in good body condition, except for the lungs that were all dark red and wet, from hyperaemia. The stomach contents showed that most birds had been feeding until shortly before death, mostly on Sprats. All these features point at drowning as the common cause of death. Prey remains in the stomachs varied from nearly intact fish to otoliths and pro-otic bullae, small bony spheres present in the skulls of Herring and Sprat. We showed that their size, like the size of otoliths, can be used to reconstruct the sizes of the fishes consumed by the birds. Average sizes of consumed Sprats were 11.9 ± 1.45 cm (based on 117 nearly pristine otoliths) or 11.5 ± 1.43 cm (based on 336 bullae). An at-sea seabirds survey conducted immediately after the incident showed that Guillemot plumages of birds in nearshore waters matched those of the drowned birds, that were all juveniles and immatures in winter plumage, rather than birds further offshore, that were mostly in summer plumage. In Dutch waters, most nets are set in nearshore waters. All evidence taken together suggests that these Guillemots succumbed in a bottom-set net deployed in nearshore waters close to the stranding site, an area apparently used by wintering first and second winter Common Guillemots that had Sprats as their staple food. Such dietary information cannot be collected easily on wintering, diving seabirds, and opportunities such as the incident described here, should be seized to learn about seabird diet.

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