Can Bayesian Reasoning Predict Your Chance of Spotting Sea Turtles in Phuket?

Bayesian Method

The author wonders how likely it is to see sea turtles during a Phuket trip, noting the guide’s experience that roughly 5 out of 10 outings result in a sighting. Bayesian reasoning is introduced as a way to update this prior probability with new evidence.

Bayesian reasoning combines prior information with fresh evidence to revise the probability of an event. In this case, the prior is the guide’s empirical rate, and the new evidence includes seasonal and weather factors.

Additional information relevant to turtle sightings includes the fact that turtles are more active in warm months (April to October) and that calm, sunny sea conditions increase their visibility. The author’s trip occurs in April, which should raise the chance slightly.

Weather also matters: observations show higher turtle appearance rates on clear days compared with cloudy or windy conditions, making good weather another important piece of evidence.

Bayesian Formula

The calculation uses the standard Bayes theorem:

P(A|B) – posterior probability of event A given evidence B.

P(B|A) – likelihood of observing evidence B if event A occurs.

P(A) – prior probability of event A before seeing the new evidence.

P(B) – marginal probability of evidence B.

In simple terms, weather conditions significantly affect turtle visibility. Even in the active season, strong winds or high waves may keep turtles underwater.

Assuming an initial (prior) probability of seeing a turtle of 50% (0.5), the new information is incorporated to update this estimate.

Define event A as “seeing a turtle” and event B as “it is April and the weather is clear.” The known quantities are:

Prior probability P(A) ≈ 0.5.

Likelihood P(B|A) – probability of April and clear weather when a turtle is seen.

Marginal P(B) – overall probability of April and clear weather based on seasonal and forecast data.

Applying Bayes’ theorem yields a posterior probability P(A|B) that, under the conditions of April and clear weather, the chance of spotting a turtle is approximately (value omitted in source).

The author feels more confident about seeing turtles after the Bayesian update.

During the trip, the group reached Similan Island, snorkeled, and observed fish and turtles. The guide claimed to have seen turtles, but the author did not.

Despite the missed sighting, the author was impressed by the island’s fine white sand and beautiful scenery.