A ten year gap between sighings of three photo-identified Risso's dolphins provides insights into their behaviour, occurrence, and distribution.

With over ten years of active field work collected by MLS researchers, we are beginning to discover interesting factors in the ecology of the marine mammals of Monterey Bay, California.

Our current study on the population ecology of Risso's dolphins uses the combined tools of photo identification, randomized line transects, focal follows and behavioural budgets, in order to understand the life history and daily behaviour of these charismatic dolphins.

While analyzing data in the lab today, we made an exciting discovery involving three individual Risso's dolphins. By looking at the natural markings on each of the dolphin's dorsal fins, we were able to identify the three animals from two separate encounters. One encounter was in 2006, and the another in 2017. This turned out to be a ten-year distributional gap, the longest we have recorded for this species in Monterey Bay (based on our data).

This ten-year gap between sightings maybe our longest, but we have additional sightings throughout the study period showing smaller gaps in time between sightings of other catalogued dolphins - a pattern we believe may signify some sort of site fidelity within Monterey Bay.

On both occasions all three dolphins swam with different herds of up to 40 dolphins. Members of the two herds were composed of subadults, females with calves, and juveniles. Often, we noted the cluster of three to be swimming at the periphery of the herd. We now believe these three dolphins are all older males that may travel between female herds. It has been found that as Risso's dolphin age, they become increasingly white in colour. This is due to scars, gouges, and nicks that are obtained from interactions with conspecifics; as well as their main prey, cephalopods. When first identified in 2007, these three males were ghost white from years of scarring, suggesting they were mature in age.

 

Figure 1. Three Risso's dolphins showing characteristics of scars and nicks from interactions with conspecifics and cephalopods. It is unknown the longevity of the species, but it is believed that Risso's dolphins can live up to 30 years of age or more. Photograph taken under NOAA NMFS Permit 15621.

What is even more interesting is that all three males had changed very little in their markings over the ten-year period, when last encountered in 2017. The reason for this is not known, and we hope with continued research we will be able to answer this question.

 

Figure 2. Photograph of male Risso's dolphin Monterey Jack (MGg_0152) taken in 2007. Left side of dorsal fin of the dorsal fin showing distinguishable feature. Photograph by Marinelifestudies.org taken under NOAA NMFS Permit 15621.

 

Figure 3. Photograph of male Risso's dolphin Monterey Jack (MGg_0152) taken in 2017. Photograph showing the right side of the dorsal fin and how very little has changed in marking since 2007. Photograph by Marinelifestudies.org, taken under NOAA NMFS Permit 20519.

This is one of many interesting observations and discoveries our team has made while studying Risso's dolphins. Understanding the population size will unlock additional questions into the distribution, occurrence, and foraging of this cryptic and interesting species.

With your support our team at Marine Life Studies will be able to continue our long-term study, and provide many updates the coming years. What an exciting time to be a citizen scientist!

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