When Fish Are Friends Not Food

In the 2003 hit film Finding Nemo, there’s a scene where a shiver of sharks are attending a meeting not unlike Alcoholics Anonymous in which the sharks repeat the pledge that “fish are friends not food”.

Recent research undertaken by the Royal Society suggests this isn’t altogether just a fantasy from an animated film. Not the actual meeting as such but it would seem (on occasion), predators aren’t in the mood to be wanton killing machines. In fact, after a good feed and their belly is full, they probably just cruise around quite comfortably.

Now while this doesn’t seem hugely surprising, as I tend not to walk into a supermarket wanting to eat everything after a meal, what is surprising is the prey animals seem to know when they’re not in danger.

In the study titled Fine-scale behavioural adjustments of prey on a continuum of risk which also involved researchers from the University of Sydney, it was found prey animals change their behaviour depending on how active their predators are.

The study investigated the behaviour of mosquitofish and their predator, the jade perch, and found that mosquitofish swim slower and closer to the jade perch when the predator appears to be uninterested. The mosquitofish will actively swim faster and away from the mouth of the jade perch when it becomes active and potentially ready for a meal. The researchers suggest that prey are able to constantly assess the risk of a predator and adjust their own behaviour in response.

In the wild, prey species often live in the vicinity of predators, rendering the ability to assess risk on a moment-to-moment basis crucial to survival.

Visual cues are important as they allow prey to assess predator species, size, proximity and behaviour.

However, few studies have explicitly examined prey's ability to assess risk based on predator behaviour and orientation. Using mosquitofish, Gambusia holbrooki, and their predator, jade perch, Scortum barcoo, under controlled conditions, the researchers found the mosquitofish constantly seemed to monitor predator risk by calculating the distance and angle from the predator’s mouth and their overall behaviour.

Put simply: when the predators were inactive after a good solid meal, they posed less of an immediate threat, so the prey slowed down. Conversely, when predators became active, prey swam faster in a tighter formation.

Most importantly, this study provides evidence that prey do not adopt a uniform response to the presence of a predator. One hopes the next stop is lions.

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