Capture area and macroplastic ingestion as factors of body condition in a potential deep-sea indicator species

Due to the longnose lancetfish’s high trophic status, broad depth range, and slow digestion, the stomach contents of this species have been used for studying trophic relationships of deep-sea fish and for tracking their plastic ingestion. In addition, this species is commonly caught as bycatch in longlines, gillnets and trawl nets. Access to bycatch data, coupled with the species’ physio-ecology, could establish the longnose lancetfish as a mesopelagic indicator species for plastic ingestion. Here, we aim to assess the inter-populational morphological variability of this global species, and the association of ingested plastics and its body condition.

Based on length–weight data from four lancetfish populations, we calculated and compared two body condition indices: Froese’s relative weight and Fulton's condition index. In addition, we compared the indices between individuals which were found with plastic in their stomachs and those without. Finally, we statistically tested whether the geographical area or presence of ingested plastics could explain potential differences in body condition. 

Overall, Froese’s relative weight of individuals with ingested plastic was significantly greater than in those without for all studied populations. The area in which the fish were caught and the presence of plastic in their stomachs were both associated with an increase in relative weight, while the interaction of these two factors did not exhibit a significant effect.

In this study we demonstrate the use of easily obtainable indices to set a (population-specific) baseline for body condition of a deep-sea predator species, and to explore relationships between the presence of ingested plastics and the species’ body condition. We plan to expand our findings by applying dynamic energy budget models, which can shed more light on the specific metabolic impacts of plastic ingestion and estimate its broader effects on under-studied mesopelagic communities.