Effects of Lipid Extraction on δ13C and δ15N Values and Use of Lipid-Correction Models Across Tissues, Taxa and Trophic Groups

1. Lipid-rich animal tissues have low δ13C values, which can lead to inaccurate ecological inferences. Chemical lipid extraction (LE) or correction models account for this depletion, but the need for LE or correction is tissue- and species-specific. Also, LE can alter δ15N values, increasing labour and costs because bulk samples must be analysed for δ15N values separately. 2. We studied the effects of LE on δ13C and δ15N values in liver, muscle and skin of common bottlenose dolphins Tursiops truncatus and West Indian manatees Trichechus manatus, two ecologically important species that occupy different trophic levels. We fit lipid-correction models to each species. We also performed a meta-analysis to more broadly determine the effects of LE across taxa, tissues and trophic groups (carnivores, omnivores and herbivores) and to fit lipid-correction models to different taxonomic and trophic groups. 3. Lipid extraction increased the δ13C values in dolphin tissues but had little effect on manatee tissues and no effect on the δ15N values in either species. A mass balance lipid-correction model best fit the data from all dolphin tissues, and a linear model best fit data for manatee liver while null models best fit data from manatee muscle and skin. Across 128 terrestrial and aquatic species, the effects of LE varied among tissues and were lower for herbivores compared to carnivores. The best-fitting lipid-correction models varied among tissue, taxa and trophic groups. Finally, the δ15N values from muscle and liver were affected by LE. 4. Our results strengthen the growing body of evidence that the need for LE is tissue- and species-specific, without a reliable C:N ratio predictive threshold. The prediction errors of lipid-correction models generally decreased with taxonomic and trophic specificity. The smaller effects of LE in herbivores may be due to differences in diet composition or the physiology of lipid synthesis in members of this trophic group. These results suggest that researchers should use the most species-, tissue- and trophic group-specific information on LE available and, if not available, perform LE on a subset of samples prior to analysis to determine effects

Linking Use of Ship Channels by West Indian Manatees (Trichechus manatus) to Seasonal Migration and Habitat Use

Research on marine mammal occurrence in ship channels often focuses on large cetaceans in offshore shipping routes, while nearshore research largely addresses small vessel strikes. Marine mammals, such as the West Indian manatee, that reside in or migrate through nearshore areas, have potential to travel through a wide range of channel types, encountering a greater diversity of vessels than previously recognized. We tested the extent and conditions of ship channel use by manatees along the north-central Gulf of Mexico (nGoM) coast by combining data from telemetry-tracked individuals, opportunistic citizen-sourced sightings, and environmental attributes linked to manatee movements. Manatees used both nearshore boat channels (130 and 300 m wide) and open water fairways but used nearshore channels much more frequently, consistent with habitat requirements. Satellite-tracked individuals swam faster and moved more directly in all channel types, indicating use of these channels as migratory and travel corridors. Accordingly, generalized additive models revealed that manatees used channels most often during spring/early summer and fall and at temperatures coincidental with entry to and exit from the nGoM during migration. Manatees also occurred in ship channels when freshwater discharges were low, likely because timing of peak manatee occurrence in the nGoM coincides with seasonally low discharge periods. Expanding shipping activity world-wide is likely to increase interactions between marine mammals and a variety of vessel types, and these effects may be particularly impactful to migratory animals like manatees that use nearshore habitats at the interface of recreational boating and commercial shipping. Linking near- and offshore ship channel use to migration and habitat use will better aid risk-assessment for vessel collision and other shipping related activities for migratory marine species globally