My research asks the question: do predatory fishes influence benthic communities?
To get there, we first had to determine exactly what those predatory fishes were eating. The predators include lingcod, copper and quillback rockfish, and kelp greeling. Anne Beaudreau, a former FHL graduate student, has already done a wonderful job of describing lingcod diets. We chose to focus on rockfish diet.
Importantly, we wanted to sample rockfish diets without killing the fish. Excision of the stomach is a common practice for examining stomach contents of fishes, but since rockfish populations are at risk in the Salish Sea, we didn’t want to contribute to their decline. Gastric lavage, or stomach pumping, is a viable alternative, but had not been attempted with rockfish prior to our attempts.
We caught rockfish from less than 20m depth so as to reduce the risks of barotrauma. This was both to reduce the risk of injury to the fish, and to ensure that the expanding gas bladder didn’t press on the stomach and cause the fish to vomit their stomach contents before we could catch them. We also used barbless hooks to keep capture injuries to a minimum.
Once on board the boat, the fish were anesthetized in 100mg/L buffered tricaine methanosulfonate. We had previously determined that this was a sufficient dosage to knock the fish out in about 5-10 minutes, keep them under for the 5-10 minutes of handling time, and then recover quickly in clean seawater.
After anesthetization, the fish were measured for mass, total length, body depth, and gape height and width. We placed the fish upside down in a foam cradle and inserted a small plastic tube through the mouth and esophagus. The hose end of a hand-pumped garden sprayer filled with clean seawater was inserted through the tube and into the stomach. The sprayer hose size was stepped down in size and made smooth with surgical and Tygon tubing.
Because the garden sprayer was hand-pumped, we were able to easily adjust the amount of water pressure coming out of the hose. We used this pressure to flush stomach contents out onto a collection screen. Stomach contents were preserved in ethanol for later identification, and the fish were placed in a cooler of clean seawater to recover from anesthesia.
After the fish recovered from anesthesia we needed to return them to their capture depth, but their swim bladders were still inflated. There are several methods for recompressing rockfish (see this video from NOAA SWFSC); we used an inverted weighted basket to return the fish.
We initially tested three individuals in the lab to establish an anesthetic dose and to learn the lavage methods. All three individuals were kept for several days and appeared to recover without issues. In our field work we captured and lavaged 29 fish, without any mortality or indication of significant injuries. In the lab testing we offered shrimp to the rockfish, but failed to monitor how many shrimp were consumed, and we did not directly observe the fish feeding on the shrimp. It is possible that the lavage procedure damaged or irritated the throat or stomach, preventing the fish from feeding. If we were to repeat this work, we would want to monitor fish in the lab to determine when they start feeding again.
One of the projects that lavage could be used for is to monitor individual diets through time: fish could be tagged and recaptured and resampled periodically. Copper rockfish are known to change their diets seasonally, with fish size, and at different locations. Resampling individual rockfish would help determine if diet preferences are individual- or population-specific. However, this would only be possible if the lavage procedure does not significantly alter feeding behavior.