Behind the Scenes: Gathering Data on Nursery Habitat and collecting Blood and Muscle Samples

Month after month, we are continuing to find juvenile sevengill sharks residing in San Francisco Bay. What this demonstrates is that we are finding juvenile sevengills here more than other areas, and we find them repeatedly across years, two criteria that must be filled to demonstrate that a habitat is a nursery area for a shark species. But how do we demonstrate that individuals are staying in locations for extended periods of time?

Ideally, you would have acoustic tags to help track individual movement to demonstrate residential behaviour. But those tags are often very cost prohibitive. So what’s the solution? For our project, we decided to utilise stable isotope analysis to demonstrate longterm residency of juvenile sevengill sharks.

Meghan is washing muscle biopsies of broadnose sevengill sharks in preparation for stable isotope analysis. Photo © Meghan Holst

Stable isotope analysis is a process in which we can evaluate what an animal is eating and where it has been. Everything an animal consumes creates a chemical signature in the body. By looking at carbon and nitrogen atoms (called isotopes) in the muscle tissue, we can start to figure out 1) what the animal has been eating, because each animal will have a unique carbon and nitrogen signal and 2) where the animal has been, because different habitats have isotope signals, particularly for carbon. Muscle tissue is a great way to investigate these questions in sharks, because muscle tissue can hold a long-term history of isotopes.

So how are we doing it? Well, we take a small muscle biopsy from the sharks, and then we can go through the process of analysing the isotopes within the shark muscle biopsy. If the signal looks close to what the baseline signal is in San Francisco Bay, then this would indicate long-term presence of juveniles in San Francisco Bay!

Meghan is training colleague Kimberly Stauffer how to pull blood on a juvenile broadnose sevengill shark, while Sarah Detmering and Manny Ezcurra safely restrain the animal. Photo © Gregory Urquiaga

We are also looking at how fisheries interactions may be impacting juvenile sevengill sharks in this theorised nursery habitat. How do we do that? By looking at properties of the blood! Many parameters about an animals health, including how physiologically stressed they may be, can be evaluated in the blood. We have been replicating 30-minute fisheries interactions and taking blood draws throughout the process to see how their physiology is impacted during these events. Results will help us understand how much these juvenile sharks may be impacted by fisheries interactions!