Behaviour, predator-prey and fisheries interactions of the Ocean sunfish (Mola mola) in the north-east Atlantic

Abstract

Over recent years, the availability of satellite telemetry has offered unparalleled opportunities to better understand the behavioural ecology of marine predators. One such predator for which little is known despite the high levels of bycatch in various fishing activities, is the ocean sunfish Mola mola. The work presented here tracked sunfish with three different types of satellite transmitter,
revealing unknown spatial dynamics and selected habitats for this species in the north-east Atlantic. Tracked fish displayed seasonal movements that were primarily driven by water temperature, while exhibiting pronounced site fidelity to productive frontal regions. Moreover, there was an apparent size-related variation in dispersal, with larger fish moving farther and positively rheotacting in relation to major oceanographic currents. Furthermore, diving behaviour varied both within and between tracked fish, and both reverse and normal DVM were detected.
However, as these different diving patterns did not correlate with geographic region or water column stratification, the observed variability is likely driven by prey distribution oscillations. To investigate this further, DNA barcoding of sunfish stomach contents revealed that while the diet of larger fish included cnidarian species, smaller individuals had a more generalist diet. Thus, although no adult sunfish were sampled, these data support the previously reported ontogenetic shift in diet. Additionally, sunfish foraging success was estimated in relation to simulated planktonic prey-fields (e.g. teleost and invertebrate larvae, gelatinous zooplankton). Tracked sunfish performed better than random null tracks in simulated prey-poor areas, whereas they performed equally in more productive regions. Fine-scale GPS tracking of sunfish behaviour also revealed that area restricted search, a proxy for foraging activity, was associated with areas of likely higher productivity. Lastly, coupling the sunfish behavioural and movement patterns, we explored for the first time the extent of sunfish and pelagic longliners co-occurrence. This revealed that 56% of the observed sunfish habitat was concomitantly used by longliners, representing an average of 2 days-at-risk per month – data that can be further used to inform potential management options.
Overall, it is presented here the results of an exceptional opportunity to characterise the behavioural ecology, including interactions with prey and fisheries, of the world’s heaviest teleost, the vulnerable sunfish.

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