The foraging specialisms, movement and migratory behaviour of the European eel

Barry, James (2015) The foraging specialisms, movement and migratory behaviour of the European eel. PhD thesis, University of Glasgow.

Abstract

The European eel is a mysterious animal and has a life cycle which has fascinated biologists for centuries. However many basic aspects of its life cycle and migrations remain unknown. European eels play a pivotal role in a balanced ecosystem both as a predator and prey species thus, understanding important ecological aspects of eel behaviour while resident in continental waters is vital in safeguarding and enhancing existing stocks.
In recent years we have witnessed declines in juvenile eel recruitment across Europe. Results from this thesis indicate a drastic decline in yellow eel abundance in a transitional water body in Northern Ireland between 1967 and 2013, with current levels at 3.38% of historical levels in the Foyle estuary. Many populations across Europe are thought to be reduced to approximately 10% of their size in the yellow eel growth phase of their life-cycle and juvenile recruitment to this phase is as low as 5% compared with 30 years ago. However, the continental phase in which eels spend up to 30 years before undertaking their spawning migration allows managers to direct effective conservation strategies.
The existence of two morphotypes “broad-headed” and “narrow-headed” in the European eel has been historically documented and this discrete head shape variation has interested biologists across Europe for a considerable amount of time. This phenotypic variation is widespread across the panmictic eel population. The findings presented in this thesis have highlighted the importance of understanding the ecology of alternative phenotypes which can exist in European eels co-occurring within the same habitat, and results suggest there may be potential consequences on life history as a result of foraging strategy undertaken in a growth habitat, with varying lipid stores and growth rates found between individuals. These alternative foraging strategies’ which manifest themselves in head shape variability corresponded to significant variation in space use and activity patterns in lacustrine growth habitat. This provides the first empirical evidence that observed morphological variation leads to significant differences in movement behaviour.
Feeding specialisations during the eel growth phase can have important consequences for population dynamics. Feeding strategies may incur greater risks from, for example, parasites. Intensity levels of the invasive nematode parasite Anguillicola crassus were associated with differences in ontogeny and trophic ecology. Infestation levels of parasites in affected fish revealed a significant negative relationship between fish length and parasite intensity, with smaller individuals having higher parasite intensity than larger individuals. This study indicates that food intake and infection risk are linked in the host-parasite system.
The growth phase for eels in continental waters ends with a transition called “the silvering process” following which individuals begin migrating downstream towards marine waters to undertake their spawning migration to the Sargasso Sea. Understanding migration behaviour, life stage specific mortality and migration success at this important life stage, is critical to effective conservation management. The unimpeded downstream movement patterns and migration success of small female and male silver eels investigated during this study revealed a low success rate to open ocean. Only 26% of eels which initiated downstream migration were detected at the outermost end of an acoustic array located at the mouth of a sea lough. Telemetry equipment functioned efficiently at all locations, therefore this suggests high levels of mortality during sea lough migration, or less likely, long-term sea lough residence by silver eel emigrants.
The overall research approach employed in this study i.e the combination of morphometric, stable isotope analysis and telemetry has allowed vital information to be gathered. Managers can utilise this information to employ appropriate conservation strategies for Anguilla anguilla as well as guiding future research directions.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Eel; Anguilla anguilla ; Foraging; Movement; Migration; Telemetry
• Subjects: Q Science > QL Zoology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
• Supervisor's Name: Adams, Professor Colin
• Date of Award: 2015
• Depositing User: Mr James Barry
• Unique ID: glathesis:2015-6742
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 08 Oct 2015 09:27
• Last Modified: 13 Oct 2015 07:39
• URI: https://theses.gla.ac.uk/id/eprint/6742