Diving activity of migrating silver eel with and withoutAnguillicola crassusinfection

Abstract

Summary Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite‐free eels in the Baltic using data recovered from data storage tags attached to migrating silver eels. In both areas, eel activity was characterized by frequent diving behaviour throughout the water column during the night, with reduced activity during the day. Despite substantial damage of the swim bladder, the behaviour of the infested eel from the North Sea was within the same range of migrating and diving activity parameters as the three parasite‐free eels from the Baltic Sea. All eels had a similar frequency distribution of descent or ascent speeds and a similar average horizontal migration speed. The diving speeds and dive ranges exclude the possibility that the eels were in continuous hydrostatic equilibrium during their migrations and suggests therefore that the role of the swim bladder in vertical migration is likely to be more complex than currently thought. Our results suggest that eels infested by Anguillicola crassus are capable of diving in a similar manner to uninfested eels during the first stretch of their spawning migration.

Important questions to progress science and sustainable management of anguillid eels

“…Laboratory research indicates that infected eels show a more pronounced stress response when exposed to hypoxic conditions (Gollock et al, 2005), that infected silver eels may not be able to reach the spawning area due to the metabolic impacts of the parasite (Palstra et al, 2007), and physiological modelling suggests that the trans‐oceanic migration would be significantly affected (Barry et al, 2014). Field evidence from silver eels migrating along the Swedish Baltic coast confirmed adverse effects on the swimming abilities and survival prospects of A. crassus infected eels (Sjöberg et al, 2009), although other studies have not come to the same conclusion (Simon et al, 2018). It seems certain that a chronic infection with this parasite alone, or associated with other impacts, will affect the ability of eels to migrate and reproduce effectively, but more work is needed on the impacts of A. crassus and how eels are adapting to infestation as it transitions from an acute to a chronic threat.…”

Mapping silver eel migration routes in the North Sea

“…Five of these included only a limited number (≤ 14) of tracked eels and did not describe any eel exiting the basin 18 – 22 . Although these studies are limited, they have shown that at least a proportion of the European eels migrate south-westwards towards the English Channel to reach the Atlantic Ocean 18 , 20 , while others seem to migrate north 21 . The two remaining studies, based on a much larger number of individuals, reported a ‘Nordic’ route for eels leaving the Baltic along the Norwegian Trench over the British Isles 12 , 16 ; this route is probably joined by the northward migrating individuals reported in the other studies.…”

Towards an in situ non-lethal rapid test to accurately detect the presence of the nematode parasite, Anguillicoloides crassus, in European eel, Anguilla anguilla

“…The presence of the parasite may impact on the eel's migration speed in rivers (Newbold et al ., 2015 ) and in the ocean as the energy demand increases (Pelster, 2015 ), due to the reduction of the swim bladder elasticity. The presence of the parasite appears not to affect the speed and migratory behaviour during the first phase of the migration in shallow water (Simon et al ., 2018 ). However, where deep diving is required in the ocean, damage to the integrity of the swim bladder is believed to seriously impact on an infected eel's chances of survival (Lefebvre et al ., 2012 ; Righton et al ., 2016 ).…”