Project Summary

The western distinct population segment of Steller sea lions (SSL) is listed as endangered due to steep population declines in the 1970s and 1980s with subsequent failure to recover for some metapopulations. While the reasons for this are unknown, methylmercury has been identified as a possible stressor affecting SSL. This is particularly relevant for SSL from the central (CAI) and western (WAI) Aleutian Islands, where relatively high total mercury concentrations ([THg]), an often-used proxy for [MMHg], have been found in SSL tissues and in their prey. A high-resolution understanding of trophic relationships is essential for deciphering the food web pathways that deliver MMHg to upper trophic level organisms in these regions.

To infer routes of primary production and trophic positions, we are applying compound-specific stable isotope analysis (CSIA) of carbon and nitrogen in amino acids (AAs) to muscle samples of archived fishes and cephalopods (n= 30 samples/region from 9 species) from the east and west of Amchitka Pass, a discrete boundary within the SSL management zones. Previous research has shown that [THg] levels in fishes, cephalopods, and SSL transition are higher to the west of Amchitka Pass., and this study will provide a deeper understanding of the food web linkages that allow for MMHg transfer through food webs to the west and east of this boundary.

Stable carbon isotope compositions of essential AAs fingerprint primary production sources, as essential AAs are synthesized by primary producers and transferred unchanged to consumers (Figure 1). We are coupling this information with analyses of the stable nitrogen isotope ratios of AAs to determine the trophic positions of the same fish and cephalopod samples (Figure 2). In addition, muscle samples will be analyzed for MMHg, as it is the form of Hg that biomagnifies in food webs and is of most toxicological concern for upper trophic levels. The novel coupling of these isotopic approaches to analyze relevant diet items of SSL provides a more nuanced perspective of the pathways of MMHg transfer in these food webs. This is of great interest to multiple stakeholders, as some SSL from the CAI and WAI have [THg] that exceed levels of concern.

The goals of this research are:

1

To describe and better understand species-specific and regional trends of [MeHg] in several prey species of Steller sea lions from the Central and Western Aleutian Islands.

2

To constrain food web linkages that allow for MeHg transfer through marine food webs by conducting CSIA of carbon (C¹³/C¹², expressed as δ¹³C values) and nitrogen (N¹⁵/N¹⁴, expressed as δ¹⁵N values) in AAs from fish and cephalopods that serve as prey items for SSL.

Figure 1: Linear discriminant function analyses based on δ¹³C_EAA (isoleucine, leucine, phenylalanine, threonine, valine) of bacteria (red), macroalgae (green), microalgae (blue), and terrestrial plants (orange). Due to variation in amino biosynthetic pathways, essential amino acids of each primary producer group have consistent, distinct patterns of carbon isotope ratios, termed “carbon isotope fingerprints”. These can be used to decipher the basal carbon source fueling the food web of an organism of interest.

Figure 2: Schematic demonstrating relationship between source amino acid (phenylalanine) and trophic amino acid (glutamic acid) in a hypothetical aquatic food web. (Ohkouchi et al. 2017). The δ¹⁵N value of phenylalanine does not increase significantly (~0.4‰) relative to diet during trophic transfer, providing an indicator of the δ¹⁵N value of the base of the food web. Conversely, the δ¹⁵N value of glutamic acid increases significantly (~8‰) relative to diet during trophic transfer. The offset between the two can be used to calculate an organism’s trophic position.