The Dynamics of Fish Spooking

Updated: Jul 26



Regardless of your target species, you’re going to spook a fish at some point. This isn't the worst thing in the trout world, but if you’re hunting for carp or bonefish, spooking a fish ruins a valuable opportunity. Let's examine some fish behavior to figure out why a fish might spook.


Bonefish are notorious spooky

Let’s start with conventional wisdom. In the behavioral ecology world, there is a theory known as the 'many eyes' hypothesis that suggests as group size increases, the group should have better predator detection. This effect should entice animals to group together. This is also known as the group-size effect. Bigger groups have greater vigilance against predation; the more eyes watching for the hungry beast, the better. A warning from one individual could save others within the group. In the fishing world, there are situations where this is definitely noticeable. For example, a misplaced fly can spook a group of cruising bonefish. But there are also situations where you would think fish should spook and they don't…


So why wouldn’t a fish spook? You’ve probably seen it when hooking a trout out of a pod of fish. Instead of losing the opportunity to catch other fish from the pod, the rest of the fish continue to feed, unalarmed by the bucking trout on the end of your line. The reason the other fish aren’t spooking? Competition!


Several studies have provided experimental evidence that suggest as group size increases (where fish might congregate for protection) the competition increases, and as a result, individuals within the group increase their rate of foraging. More interestingly, as they intensify their feeding, they also seem to make riskier decisions and forage closer to potential predators or forage outside of their cover. The riskiest decisions are most intense when resources are scarce. Meaning, competition is fierce and food is limited (Grand & Dill, 1999; Hintz & Lonzarich, 2018; Johnsson, 2003). Think: a hatch is going off, and each fish is trying to make the most of it.


Let's think through the relationship between group size and foraging success. Initially, as group size increases, the foraging success will likely increase. Fish are starting to compete more but the group isn't large enough to make resources scarce. Eventually, with more individuals within a group, competition will lead to some winners and some losers and individual foraging success will vary. Not only does this suggest there is an optimal density of group size for each individual to have the most success, but also this also suggests that big groups aren't always better... Like most things, healthy competition is a good thing in moderation. However, optimal group sizes are unlikely to stabilize as individuals tend to join groups rather than go solo, or leave a group due to such intense competition (Hart & Reynolds, 2002).



If you consider these dynamics from a predators perspective, i.e. pike, musky, big trout, other predatory gamefish, these group dynamics help explain why streamer fishing can be so effective. Ex: Sometimes fishing streamers during a hatch can produce the largest fish, as they hunt for prey that are preoccupied by the hatching insects.

Though the 'many eyes' hypothesis can prove true, recent work has reformed the idea of the group-size effect to account for the dynamics associated with increased competition within the group. In fact, when group size increases, distractions from increased competition, as well as decreased individual responsibility in detecting predators suggests that vigilance actually decreases as groups get bigger (Roberts, 1996). In a group of 10, each individual is only (at most) 1/10th responsible for detecting danger. So, a challenging balance exists between fighting amongst the group and trying to hide within the group.


Fortunately, this tricky social balance in groups of fish is what allows predators, and anglers, to capitalize.


What does this mean for fishing?

It obviously depends on the target fish and the situation, but these ideas provide hypotheses as to why fish group together (1), when a fish might spook (2), or why fish don’t spook (3), especially after a fish in the group may be hooked (4). These different dynamics aren't necessarily mutually exclusive either.


  1. Group vigilance is (theoretically) better than individual vigilance. Baitfish and smaller fish have a proclivity to hide in greater numbers

  2. Many eyes can detect predators better than one individual’s eyes

  3. Increases in group size push competition and sway an individual's decisions to become riskier

  4. Individual vigilance (may) relax in larger groups as the risk of predation on the individual decreases

  5. Groups may even be too dense for one individual to maintain sufficient vigilance



On The Water Applications

  • Be aware that fish on their own are likely hyper-vigilant. Approach with extra stealth. Catching fish in these situations are true trophies.

  • Dense groups of rising fish are likely preoccupied with competition and less vigilant in prey detection. Some fish might make riskier choices or forage further from cover as a result. These conditions are much less spooky and potentially very rewarding for anglers.

  • Streamers could yield reactions in dense pods jockeying for space, or on the outskirts of pods where big predators are looking to capitalize on risky group activity. Swinging wet flies into the pod, or stripping larger streamers around the pod should get reactions.



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Sources:

Grand & Hill. 1999. The Effect of Group Size on the Foraging Behavior of Juvenile Coho Salmon: Reduction of Predation Risk or Increased Competition? Animal Behavior, 58-2. https://www.sciencedirect.com/science/article/abs/pii/S0003347299911740


Hintz & Lonzarich. 2018. Maximizing Foraging Success: The Role of Group Size, Predation Risk, Competition & Ontogeny. Ecosphere. https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.2456


Johnsson. 2003. Group Size Influences Foraging Effort Independent of Predation Risk: An Experimental Study on Rainbow Trout. Fish Biology, 63-4. https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1095-8649.2003.00187.x


Hart & Reynolds. 2002. Handbook of Fish Biology & Fisheries. Blackwell Science Ltd. https://onlinelibrary.wiley.com/doi/book/10.1002/9780470693919


Cresswell & Quinn, 2011. Predicting the Optimal Prey Group Size from Predator Hunting Behavior. Journal of Animal Ecology. 80-2. https://pubmed.ncbi.nlm.nih.gov/21244418/


Roberts. 1996. Why Individual Vigilance Declines As Group Size Increases. Animal Behavior, 51-5. https://www.sciencedirect.com/science/article/abs/pii/S0003347296901098