A while ago, I read this paper by Marc Weissburg and coauthors , and came across the idea that in flowing water environments, like streams, turbulence could create an area of flow where predators are unable to detect (or less able to detect) odour cues from their prey (this has been termed a “hydrodynamic refuge” or “sensory refuge”). I was (and still am) interested in how predators that search using olfactory, rather than visual cues, respond to changes in the grouping behaviour of their prey, and consequently, how olfactory predation influences the evolution of animal aggregation. We know quite a lot about how grouping behaviour affects predation risk when predators hunt using vision (generally speaking, there are lots of benefits to grouping), but not so much when predators use olfaction.
One failed NERC standard grant application later, and I was wondering how I might follow up on some of my ideas when I was contacted out of the blue by Ása Johannesen (who, because no British person can successfully pronounce her first name, became Freya). Freya suggested that we apply to Granskingarráðið, the Faroe Islands Research Council (where she is from) for a PhD studentship. One grant application and one lovely positive set of reviewers’ comments later, we were in possession of a three year studentship to investigate the link between olfactory predators and grouping.
In her first paper [3; paywalled, but OA version here], Freya investigated whether three-spine sticklebacks could use olfactory cues to detect and locate prey (bloodworm). So the predators wouldn’t be able to see well, she reduced the availability of visual cues by adding clay to the water to increase turbidity, making it murky. To hide olfactory cues, she added a masking “extract of bloodworm” (mmm, yum). She found that in highly turbid water, either visual or olfactory cues could be used detect the bloodworm, but in highly turbid water, adding the masking cue INCREASED stickleback foraging success. If they needed olfactory cues to locate prey, then surely adding extra cue everywhere when they couldn’t see should make it more difficult? Maybe, we thought, the smell of food everywhere makes them more motivated to find food (although they didn’t spend more time swimming around)?
Freya then went on to repeat the experiment to look at the effect of prey aggregation (open access) on predation risk, in the lab and in the field . In the lab, aggregated prey in turbid water survived the best initially, but were quickly eaten once the group was discovered (presumably because she was using dead bloodworm, so they couldn’t escape once they were found). So dispersed prey had better long-term survival, particularly in turbid water. In the field, she placed the prey in ‘feeding stations’ made from plastic pots covered in tights, so the predators could smell, but not see the prey. In this situation, aggregated and semi-dispersed prey survived better than dispersed prey. This suggested that aggregation can benefit prey when predators hunt using olfaction.
What about the sensory refuges? Freya’s latest paper  has just (finally!) been published (also open access). Here, she looked at the abilitty of sticklebacks to find prey in flowing water, using a Y-maze design (less fancy than it sounds). She then disrupted the flow of water by placing some obstacles between the prey and the predator, messing up the flow of olfactory cues from the prey (although she used different concentrations of bloodworm extract rather than groups of actual prey, which might be important). She found that the strong and medium extract of bloodworm meant fish were easily able to locate the prey source in smoothly flowing water, but they could only find the strongest source in the disturbed flow. This suggests that disturbing the water flow can create a sensory refuge that might allow at least some groups of animals to hide.
 Weissburg, MJ, Ferner, MC, Pisut, DP & Smee, DL (2002) Ecological consequences of chemically mediated prey perception. Journal of Chemical Ecology 28: 1953-1970
 Smee, DL & Weissburg, MJ (2006) Clamming up: Environmental forces diminish the perceptive ability of bivalve prey. Ecology 87: 1587–1598
 Johannesen, A, Dunn, A & Morrell, LJ. (2012) Olfactory cue use by three-spined sticklebacks foraging in turbid water: prey detection or prey location? Animal Behaviour 84: 151-158 [link] [OA version]
 Johannesen, A, Dunn, AM & Morrell, LJ. (2014) Prey aggregation is an effective olfactory predator avoidance strategy. PeerJ 2:e408 [open access]
 Johannesen, A, Dunn, AM & Morrell, LJ. (2017) Disturbed flow may create a sensory refuge for aggregated prey. PeerJ 5:e3121 [open access]