Trophic interactions and mercury accumulation in batoids (Chondrichthyes: Batoidea) of California coast.

Batoids are fishes of the Chondrichthyes group comprising stingrays, skates and guitarfishes. These long-lived predators can feed on a variety of benthic prey species allowing these animals to occupy multiple trophic levels throughout its lifetime. Knowledge regarding feeding pattern and how it varies over lifetime is crucial to understand habitat use and its ecological role in the food webs but information regarding trophic ecology is limited for many batoid species. As top predators of benthic habitats and common inhabitants of bays and estuaries, Batoids may be good candidates to serve as indicator species to better understand mercury (Hg) cycling in these systems. Our main objective is to describe diet breadth and trophic niche of batoid species occurring in Californian coastal waters, specifically in San Diego Bay (SDB) and Tomales Bay (TB), and how such information may relate to Hg accumulation. Preliminary results from 27 specimens of three species (Urobatis halleri n = 10, Myliobatis californica n = 13, Rhinobatos productus n = 4) in SDB suggest overlapping trophic niches with co-occurring species occupying multiple trophic levels. Large variation in δ¹⁵N ratios was observed for U. halleri ranging from 14.9‰ to 19.9‰ with a mean 16.5 ± 1.7‰. Lower δ¹⁵N values were observed in M. californica with a mean of 15.5 ± 1.0‰ (14.2‰ to 16.7‰). The highest mean δ¹⁵N value was observed in R. productus with a mean of 17.6 ± 0.6‰ (16.7‰ to 18.2‰). These preliminary data suggest U. halleri has the widest isotopic niche among co-occurring species in SDB. Preliminary Hg assessments in M. californica (0.43 ± 0.06 μg.g^-1 w.w.) showed no correlation with δ¹⁵N in muscle tissues, and no difference in average total Hg compared to R. productus (0.48 ± 0.14 μg.g^-1 w.w.). We found no clear trends between muscle tissue isotopic ratios (δ¹⁵N and δ¹³C) and Hg levels with animal size (juvenile v.s. sub-adult specimens). In general, SDB present moderate levels of Hg contamination in these top benthic predators. Currently, additional sampling and analysis are underway to include to our models δ¹⁵N and δ¹³C ratios and Hg levels from prey items which should help to better describe our system and fully address the objectives of this ongoing study.