Yellowfin tuna (Thunnus albacares) and spotted dolphins (Stenella attenuata) often swim together in the eastern tropical Pacific (ETP). This fact has intrigued scientists for decades. A simple synopsis of current thinking about this issue is detailed below (with figures).
There have been two leading hypotheses about why it happens:
- finding lunch (the feeding hypothesis), and
- avoiding becoming lunch (the predation hypothesis)
And there have also been many questions:
- Why is it mostly yellowfin tuna and spotted dolphins who swim together?
- What are benefits for each species?
- Is the association necessary for either species?
- Is the association permanent for either species?
- Why does it usually happen in the ETP and not other oceans?
A new scientific paper spreads new light on dolphin-tuna associations (see abstract at bottom). Michael Scott, one of the SDRP co-founders and a senior scientist at the Inter-American Tropical Tuna Commission, along with his co-authors have examined the most prominent hypotheses by tying together three studies:
- a simultaneous tracking study of spotted dolphins and yellowfin tuna
- a study comparing their prey and daily foraging patterns, and
- a study of the oceanographic features correlated with the tuna–dolphin associations.
The tuna follow the dolphins, but the association is neither permanent nor obligatory.
The benefits of the association are not based on feeding advantages. The dolphins and tuna tend to feed at different depths, at different times, and often on different prey.
The predation hypothesis was supported. One or both species likely gain protection from predators (such as large sharks) by forming large, mixed-species groups.
The yellowfin tuna also associate with spinner dolphins, although to a lesser degree than spotted dolphins. The difference is influenced by the different swimming depths of the two dolphin species and the oceanography.
These associations occur under quite specific oceanographic conditions.
How Oceanography Influences the Tuna-Dolphin Association
In most tropical oceans, there are warm, well-oxygenated waters at the surface called the mixed layer (Figure 1). Underneath lies a thick layer of colder, oxygen-poor water called the Oxygen Minimum Zone. Separating these two is the thermocline, a thin layer where the temperature drops rapidly.
What makes the ETP so unusual is that the warm mixed layer and the thermocline are much shallower than in most oceans (10 to 50 meters deep) and the Oxygen Minimum Zone is much thicker and more hypoxic.
As one travels westward in the ETP, the mixed layer and the thermocline deepen (Figure 2). When these depths reach about 50 meters, the association between the yellowfin tuna and spotted dolphin weakens).
These conditions are not unique to the ETP but also occur in a few places in other oceans – the nearshore waters of West Africa and the northern Indian Ocean, for example – and yellowfin tuna have been reported to swim with dolphins in these same waters. The association is most prevalent and consistent in the ETP however where these oceanic conditions are more widespread than in any other ocean.
How Swimming Depths Influence the Dolphin –Tuna Associations
The tuna-dolphin association appears to primarily form during the daytime. The tracking and food habits studies showed that spotted dolphins typically dive deep to feed on fishes and squids at night and travel 15-20 meters below the surface during the day (Figure 3). The yellowfin tuna swim near the thermocline during the day.
The spinner dolphin (Stenella longirostris) is another dolphin species found in the ETP associating with tuna. This is most likely because spinner dolphins, like the yellowfin tuna, join spotted dolphin herds during the day to reduce the risk of predation (by gaining safety in numbers). Interestingly, it was the spinner dolphins that helped explain where the tuna-dolphin association can occur.
Spinner dolphins swim near the surface, and there is usually a substantial vertical difference compared with the tuna swimming near the thermocline. In some areas where the thermocline is very shallow, the yellowfin tuna associate with spinner dolphins without any spotted dolphins.
This suggests that the differences in the swimming depths of the three species, along with the depth of the thermocline influence where the tuna dolphin association forms. As the thermocline deepens, the vertical distance between the yellowfin tuna and the dolphins becomes too great to maintain the association.
As shown in Figure 4, when the thermocline is shallow (less than 25 meters), the yellowfin tuna may associate with both dolphin species.
When the thermocline deepens, the vertical difference in swimming depth between the yellowfin tuna (near the thermocline) and spinner dolphins (near the surface) is too great to maintain the association directly. At this depth, however, spinner and spotted dolphins still may associate during surfacing to breath, but only the deeper-swimming spotted dolphins associate with yellowfin tuna (Figure 5).
When the thermocline sinks even deeper , the vertical difference in swimming depth between the yellowfin tuna (near the thermocline) and both spotted and spinner dolphins is too great to maintain an association (Figure 6).
This Open Access article may be obtained from the journal:
Scott, M.D, Chivers, S.J., Olson, R.J., Fiedler, P.C., Holland, K. 2012. Pelagic predator associations: tuna and dolphins in the eastern tropical Pacific Ocean Marine Ecology Progress Series 458:283–302, or by downloading a pdf.
Questions about the study can be addressed to Michael Scott at email@example.com
The association of yellowfin tuna and pantropical spotted dolphins in the eastern tropical Pacific Ocean (ETP) has been exploited by tuna fishermen and has intrigued scientists for decades, yet we still have questions about what the benefits of the association are—whether the association is obligatory or facultative, why the tuna are most often found with spotted dolphins, and why the species associate most strongly in the ETP. We review the hypotheses that have been proposed to explain the bond and present results from 3 studies conducted to address these hypotheses: a simultaneous tracking study of spotted dolphins and yellowfin tuna, a trophic inter- actions study comparing their prey and daily foraging patterns, and a spatial study of oceanographic features correlated with the tuna–dolphin association. These studies demonstrate that the association is neither permanent nor obligatory and that the benefits of the association are not based on feeding advantages. These studies do support the hypothesis that one or both species reduce the risk of predation by forming large, mixed-species groups. The association is most prevalent where the habitat of the tuna is compressed to the warm, shallow, surface waters of the mixed layer by the oxygen minimum zone, a thick layer of oxygen-poor waters underlying the mixed layer. The association has been observed in other oceans with similar oceanographic conditions, but it is most prevalent and consistent in the ETP, where the oxygen minimum zone is the most hypoxic and extensive in the world.