depends on species

It is a good idea to approach these questions with several experiments and methods and focus on a few 'representative' species. Characterizing predation risk between habitats (bivalve vs. non-bivalve stratum) is complicated, as there are several confounding aspects related to predator-prey interactions, species function and abundance in relation to habitat, etc. Once you have established some relationships with field observations (video is an accepted method but takes many many many hours and several sites to evaluate), you can set up a series of mesocosms or much more limited controlled experiments. BRUVs are ok for examining predator activity in relation to a biophysical variable, but is not the strongest method to see how prey activity is related to habitat or physical alterations in relation to predation. If the hypothesis is akin to higher predation risk with increased thermal stress ('stress' would need to be demonstrated per species-interaction unit on its own), you will need to decide how to evaluate prey activity on stimulus, predator preference and strategy, density dependence, and perhaps even if habitat structure is manipulated by a temperature gradient. So in essence, a series of limited experiments (lab or mesocosm) and observational study to tell the whole story. I look forward to seeing your methods!

Some relevant references:

Humphries AT, La Peyre MK, Decossas GA (2011) The Effect of Structural Complexity, Prey Density, and “Predator-Free Space” on Prey Survivorship at Created Oyster Reef Mesocosms. PLOS ONE 6(12): e28339. https://doi.org/10.1371/journal.pone.0028339

Zamzow JP, Amsler CD, McClintock JB, Baker BJ (2010) Habitat choice and predator avoidance by Antarctic amphipods: the roles of algal chemistry and morphology. Mar Ecol Prog Ser 400:155-163. https://doi.org/10.3354/meps08399

Johnson, D. (2006). Predation, Habitat Complexity, and Variation in Density-Dependent Mortality of Temperate Reef Fishes. Ecology, 87(5), 1179-1188.

[N.C. Ory, D. Dudgeon, C.P. Dumont, L. Miranda, M. Thiel, Effects of predation and habitat structure on the abundance and population structure of the rock shrimp Rhynchocinetes typus (Caridea) on temperate rocky reefs, Mar. Biol. 159 (2012) 2075–2089. doi:10.1007/s00227-012-1994-6.

Hello Alexia,

I agree with Daniel Taylor that examining predation in-situ will be complicated as there are numerous confounding factors, and that field experiments would be well complimented with laboratory or mesocosm experiments to tease apart particular aspects of predator-prey interactions, depending on your hypotheses.

I would also add that, in terms of using underwater video on a bivalve bed, there are a number of factors to consider when determining the efficacy of this method to assess predation in the field. Firstly, would be the species of interest. Many of the small mobile invertebrates, including predators, will exist within the three-dimensional spaces created by the bio-engineering species. This heterogeneous topograpy will reduce the ability of the camera to capture predation events on these small organisms whose interactions may remain completely hidden from the field of view. Secondly would be to consider the species of interest. If they are small mobile invertebrates, the cameras will require a narrow field of view at relatively close distances to the lens, limiting your sampling and likely reducing your chances of observing predatory interaction. If however, your interest is in predation by larger mobile predators, you can use a wider field of view. And lastly would be the local water conditions with respect to clarity. When the water clarity is low, due to organic matter, tannins, suspended sediments, etc... you will have a reduced depth in your field of view. Many bed-forming bivalves are located in areas with high current and low visibility. If the visibility for instance is 1 metre and you are trying to observe predation by diving ducks, you will have a reduced chance of capturing that predatory interaction with only a 1 metre visibility. This is less of an issue if you are observing predation by a crab species that is only a few centimetres carapace width. Depending on your research question, it may be feasible to overcome some of these limitations by increasing your sampling effort with greater number of cameras, but that option may be limited by available equipment. It will be important for you to identify exactly what you will be observing in terms of species and desired metrics (predatory activities).

That being said, underwater video is a viable method. You may want to consider either cameras that are capable of or can be modified to detect infrared light, to capture predatory activities occurring at night. Some infrared torches can provide the lighting you need to make these observations. Battery life, however, might require frequent redeployment. I have included a reference where the authors utilised a lobster literally in a bottle attached in front of a camera to observe predatory activity by fish and invertebrates both within kelp beds and on urchin barrens. Depending on your desired metrics and species, a similar methodology may be usable in your study.

Hopefully that helps as you develop your experimental design and I look forward to hearing more on your study in the future!

Jan Hesse A B , Jenni A. Stanley A and Andrew G. Jeffs (2015) Lobster in a bottle: a novel technique for observing the predation of juvenile spiny lobster (Jasus edwardsii). Marine and Freshwater Research 67(11) 1625-1633 https://doi.org/10.1071/MF15098