Complementary methods assessing short and long-term prey of a marine top predator ‒ Application to the grey seal-fishery conflict in the Baltic Sea

Abstract

The growing grey seal (Halichoerus grypus) population in the Baltic Sea has created conflicts with local fisheries, comparable to similar emerging problems worldwide. Adequate information on the foraging habits is a requirement for responsible management of the seal population. We investigated the applicability of available dietary assessment methods by comparing morphological analysis and DNA metabarcoding of gut contents (short-term diet; n = 129/125 seals, respectively), and tissue chemical markers i.e. fatty acid (FA) profiles of blubber and stable isotopes (SIs) of liver and muscle (mid- or long-term diet; n = 108 seals for the FA and SI markers). The methods provided complementary information. Short-term methods indicated prey species and revealed dietary differences between age groups and areas but for limited time period. In the central Baltic, herring was the main prey, while in the Gulf of Finland percid and cyprinid species together comprised the largest part of the diet. Perch was also an important prey in the western Baltic Proper. The DNA analysis provided firm identification of many prey species, which were neglected or identified only at species group level by morphological analysis. Liver SIs distinguished spatial foraging patterns and identified potentially migrated individuals, whereas blubber FAs distinguished individuals frequently utilizing certain types of prey. Tissue chemical markers of adult males suggested specialized feeding to certain areas and prey, which suggest that these individuals are especially prone to cause economic losses for fisheries. We recommend combined analyses of gut contents and tissue chemical markers as dietary monitoring methodology of aquatic top predators to support an optimal ecosystem-based management.

Population structure, life cycle, and trophic niche of the glacial relict amphipod, Gammaracanthus lacustris, in a large boreal lake

“…Fatty acids are rather conservatively assimilated, and can thus be used as biomarkers indicative of dietary origin (Dalsgaard, St. John, Kattner, Müller-Navarra, & Hagen, 2003;Galloway et al, 2014;Tverin et al, 2019). Polyunsaturated fatty acids generally originate from phytoplankton, while detritus and bacteria are rich in SAFA, MUFA, and branched fatty acids (Dalsgaard et al, 2003).…”

DNA metabarcoding in diet studies: Unveiling ecological aspects in aquatic and terrestrial ecosystems

“…Moreover, other studies have inspected the spatial differences in diet composition (Granquist et al, ; Thomas, Nelson, Lance, Deagle, & Trites, ) and investigated the seasonal variation in this seals' dietary habits (Voelker, ). The red lionfish, Adelie penguins, and the gray seal H. grypus were used in three different studies each, mostly for methodological purposes (Harms‐Tuohy et al, ; Jarman et al, , ; Parsons et al, ; Tverin et al, ), but also to investigate the species' foraging habits (Côté et al, ; Jarman et al, ; Méheust, Alfonsi, Le Ménec, Hassani, & Jung, ; Valdez‐Moreno et al, ). As such, all these species are good candidate models for future barcoding‐based dietary studies.…”

“…On the other hand, among the 13 studies on interactions between humans and aquatic taxa, the majority focused on the pressure top predators, mostly seals, can exert on marine fisheries and/or on endangered fish (Granquist et al, ; Méheust et al, ; Purcell et al, ; Schwarz et al, ; Thomas et al, ; Tverin et al, ), or on the impacts of two invasive species, the lionfish on Caribbean coral reef ecosystems (Côté et al, ; Harms‐Tuohy et al, ; Valdez‐Moreno et al, ) and the catfish in Chesapeake Bay, USA (Aguilar et al, ). One study measured the consumption of fishery discards by birds, revealing that, although breeding success was higher in populations with access to discarded fish, mortality could also be enhanced by the increased interacting with fishing vessels (McInnes, Jarman, et al, ).…”

“…Despite being broadly consensual that DNA-based methodologies allow a more accurate taxonomic understanding of prey items in digested material (Pickett, Bergey, & Di Fiore, 2012;Zeale, Butlin, Barker, Lees, & Jones, 2011), several authors still advocate the use of both nonmolecular (morphology) and molecular (barcoding) techniques to describe vertebrates' diets, particularly in terrestrial ecosystems (e.g., Groom, White, Mitchell, Roberts, & Mawson, 2017;Pereira, Xavier, Perera, Salvi, & Harris, 2019;Yang, Zhan, Cao, Meng, & Xu, 2016) and to study ancient dietary habits in humans (Sawafuji, Saso, Suda, Hattori, & Ueda, 2018). Stable isotope analysis was also recommended by some authors to complement the study of diet habits of aquatic organisms (Syväranta, Harrod, Kubicek, Cappanera, & Houghton, 2012;Tverin et al, 2019). Ultimately, however, DNA metabarcoding is foreseen as the most powerful tool to reconstruct both trophic networks (Roslin & Majaneva, 2016) and networks of ecological interactions (Berry et al, 2019;Bohan et al, 2017;Evans, Kitson, Lunt, Straw, & Pocock, 2016).…”

Factors affecting the degree of vertical stratification of fatty acids in grey seal blubber

“…Studying blubber fatty acid (FA) composition is a frequently used method to monitor the diet of marine mammals (e.g. Bradshaw et al 2003;Walton and Pomeroy 2003;Budge et al 2006;Guerrero et al 2016;Meier et al 2016, Bourque et al 2018Tverin et al 2019). The tissue biochemistry of a top predator is a proxy of its diet, and at the same time reflects the structure and biochemistry of the whole food web that can be influenced by global climate change, local human impact and natural environmental changes (Iverson et al 2007;Engelhard et al 2014;Lind et al 2017).…”