Novel insights on European eel inferred from acoustic telemetry
On Friday December 7th, Pieterjan Verhelst (Ghent University) publicly defended his doctoral thesis entitled: "European eel (Anguilla anguilla L.) movement behaviour in relation to habitat fragmentation - Novel insights inferred from acoustic telemetry".
Promoters of this PhD are Prof. Dr. Ir. Peter Goethals, Prof. Dr. Tom Moens, Dr. Ir. Ans Mouton and Dr. Jan Reubens, all from Ghent University. This thesis was conducted in close collaboration with the VLIZ data center division and supported by LifeWatch Belgium.
Summary of the thesis:
An ever increasing human population has led to global change, resulting in substantial pressures on ecosystems and diversity loss by (in general) acting on five fronts: (a) climate change, (b) habitat modifications resulting in habitat loss and fragmentation, (c) biogeochemical and hydrological cycles and pollution, (d) biological invasions, and (e) overexploitation of natural resources. Consequently, maintaining species diversity and ecosystem functioning will increasingly require human intervention by adequate management. Aquatic environments are among the most anthropogenically altered systems. Impact often results in changed hydrological conditions due to land reclamation, agricultural drainage, shipping accompanied with shipping locks, construction of pumping stations, dams and weirs, waste water treatment and hydropower. Obviously this affects aquatic life.
A flagship fish species affected by global change, is the European eel (Anguilla anguilla L.). Its spawning grounds are assumed to be in the Sargasso Sea, after which the leptocephalus larvae drift with the Gulf Stream to continental Europe and metamorphose into glass eels. It is this stage which settles in coastal, estuarine and freshwater habitats to grow as yellow eels. Once they reached a certain size and accumulated enough fat, they migrate back to the spawning site as silver eels. Yet, over the last four decades, their glass eel recruitment numbers have plummeted by 90–99%,leading to the ’critically endangered’ IUCN Red List status. This worrisome decline resulted in the adoption of the EU Eel Regulation in 2007 which states that European countries need to take management measures to ensure 40% escapement of the spawning stock biomass, defined as the best estimate of the theoretical escapement rate if the stock were completely free of anthropogenic influences. However, as all five components of global change affect the European eel population, taking effective measures is not straightforward. Currently, management mainly focuses on fisheries regulation and redistribution of glass eels from high abundance to low abundance areas. Yet, effectiveness of these actions is questionable as long as climate change, habitat quality and connectivity, pollution and biological invasions are not addressed. To include those aspects in eelmanagement, a better understanding is required. Consequently, to partly cover this knowledge gap, habitat quality and connectivity were the central focus of this dissertation. Connectivity between freshwater habitats and marine areas is heavily obstructed by anthropogenic structures (e.g. weirs, pumping stations, shipping locks, sluices...), leading to a high pressure on the European eel population. A better understanding of fish migration behaviour in relation to these barriers is needed to take proper mitigation actions. To understand eels’ habitat use, we applied acoustic telemetry to reveal their movement behaviour in various aquatic systems and habitat types. Acoustic telemetry involves acoustically tagged animals which can be detected by a network of acoustic listening stations.
In chapter three, we investigated the movement behaviour of large female yellow eels in a polder area, characterized by interconnected canals, polder ditches and ponds. The study illustrated that yellow eels have a high site fidelity and that their movement is not strongly affected by environmental factors (temperature, precipitation, day length, atmospheric pressure, pumping discharge). This may be attributed to their opportunistic behaviour and productivity of the polder system, which makes such systems highly suitable as eel growth habitats. In view of their high site fidelity, large female yellow eels may not encounter many human-induced connectivity problems in polder systems. In addition, our study indicated that large eels prefer deeper habitats. If measures concerning eel habitat restoration in polders are considered, areas of sufficient depth need to be maintained for large eels. A higher yellow eel survival could in turn result in a higher silver eel escapement, and as such could help meet the 40% escapement obligation imposed by the European Eel Regulation.
In accordance with eels’ opportunistic feeding behaviour, chapter four handles head width distribution of eels in the Schelde Estuary. As eels are opportunistic feeders, it is doubtful that head width follows a bimodal distribution with the dichotomous characterization of narrow and broad headed specimens. Indeed, our study showed a continuum of narrow to broad with a unimodal distribution. This pattern could illustrate there is no resource selectivity and consequently no disruptive selection for the eels in the Schelde Estuary.
In chapter five, the spatio-temporal silver eel migration behaviour was studied in a barrier-free system, i.e. the Schelde Estuary. The results of this study demonstrated that silver European eels use selective tidal stream transport and underlines the importance of tides in eel migration. Hence, eels can make a distinction between ebbing and flooding tide, indicating that cues other than currents play a role in orientation. Apparently, tides help eels to migrate in a bioenergetically efficient way through estuaries, just like for other diadromous fish species such as salmonids and flounder. Therefore, restoration of estuaries, for example via tidal barrier management, may not only aid recovery of the European eel population, but of diadromous fish species in general.
The next two chapters relate to spatio-temporal silver eel migration behaviour in anthropogenically regulated systems. Regulated waterways come in various forms depending on anthropogenic needs (agricultural drainage, transport, waste water...) and accompanied migration barriers. Chapter six handles about silver eel migration behaviour in a polder system. A polder system has a lower water level than the sea and consequently needs to be drained by a pumping station to prevent it from flooding. Accompanied with a pumping station is a weir and both act as migration barriers. In chapter seven we studied a shipping canal, i.e. the Albert Canal, which is characterized by shipping locks, turbine stations and tidal sluices. Both studies showed that migration was highly hampered by the various types of migration barriers, leading to significant delays and exploratory behaviour near the barriers. In addition, the substantial low migration speeds in the canal sections of the Albert canal are likely a consequence of the highly regulated water flow, preventing a unidirectional cue for the eels to orient towards the sea. Delays can have a serious impact on eels since their energy resources are limited for a successful trans-Atlantic migration. In addition, delays and exploratory behaviour can also increase predation and disease risk. The results indicated that adequate management measures need to be taken to make migration barriers passable and stimulate as ubstantial unidirectional water flow as eels are likely depending on this for orientation. As this can be economically challenging, management could act during specific migration windows when it is likely the majority of the eels are migrating. In general silver eel migration occurs from August till December at night during moments of increased precipitation and consequently increased flow. Although these studies were conducted on a small geographical scale relative to the wide distribution of the European eel, management at different geographical locations may have substantial various outcomes on the population. In chapter eight we describe a new migration route which at least a part of the eels take to leave the North Sea. Different migration routes may have different bio-energetic implications as some routes may be energetically more demanding, leaving less energy for spawning. Consequently, local management may have an important impact on the population. However, we do not have enough information yet to determine whether something significant is happening during marine migration and whether the inland management actions taken are effective.
In this dissertation, we illustrated that migration barriers such as weirs, pumping stations, shipping locks and tidal sluices substantially hamper silver eel migration via delays, disorientation and exploratory behaviour. Therefore, adequate management measures such as fish friendly screws or gravitational flow stimulation during specific migration windows need to be taken to aid recovery of the European eel population. Yet, such management demands a constructive cooperation between ecologists and engineers. Not only the European eel, but other diadromous and potamodromous fish species will benefit from this as well.
Thesis available here.
Summary of the thesis:
An ever increasing human population has led to global change, resulting in substantial pressures on ecosystems and diversity loss by (in general) acting on five fronts: (a) climate change, (b) habitat modifications resulting in habitat loss and fragmentation, (c) biogeochemical and hydrological cycles and pollution, (d) biological invasions, and (e) overexploitation of natural resources. Consequently, maintaining species diversity and ecosystem functioning will increasingly require human intervention by adequate management. Aquatic environments are among the most anthropogenically altered systems. Impact often results in changed hydrological conditions due to land reclamation, agricultural drainage, shipping accompanied with shipping locks, construction of pumping stations, dams and weirs, waste water treatment and hydropower. Obviously this affects aquatic life.
A flagship fish species affected by global change, is the European eel (Anguilla anguilla L.). Its spawning grounds are assumed to be in the Sargasso Sea, after which the leptocephalus larvae drift with the Gulf Stream to continental Europe and metamorphose into glass eels. It is this stage which settles in coastal, estuarine and freshwater habitats to grow as yellow eels. Once they reached a certain size and accumulated enough fat, they migrate back to the spawning site as silver eels. Yet, over the last four decades, their glass eel recruitment numbers have plummeted by 90–99%,leading to the ’critically endangered’ IUCN Red List status. This worrisome decline resulted in the adoption of the EU Eel Regulation in 2007 which states that European countries need to take management measures to ensure 40% escapement of the spawning stock biomass, defined as the best estimate of the theoretical escapement rate if the stock were completely free of anthropogenic influences. However, as all five components of global change affect the European eel population, taking effective measures is not straightforward. Currently, management mainly focuses on fisheries regulation and redistribution of glass eels from high abundance to low abundance areas. Yet, effectiveness of these actions is questionable as long as climate change, habitat quality and connectivity, pollution and biological invasions are not addressed. To include those aspects in eelmanagement, a better understanding is required. Consequently, to partly cover this knowledge gap, habitat quality and connectivity were the central focus of this dissertation. Connectivity between freshwater habitats and marine areas is heavily obstructed by anthropogenic structures (e.g. weirs, pumping stations, shipping locks, sluices...), leading to a high pressure on the European eel population. A better understanding of fish migration behaviour in relation to these barriers is needed to take proper mitigation actions. To understand eels’ habitat use, we applied acoustic telemetry to reveal their movement behaviour in various aquatic systems and habitat types. Acoustic telemetry involves acoustically tagged animals which can be detected by a network of acoustic listening stations.
In chapter three, we investigated the movement behaviour of large female yellow eels in a polder area, characterized by interconnected canals, polder ditches and ponds. The study illustrated that yellow eels have a high site fidelity and that their movement is not strongly affected by environmental factors (temperature, precipitation, day length, atmospheric pressure, pumping discharge). This may be attributed to their opportunistic behaviour and productivity of the polder system, which makes such systems highly suitable as eel growth habitats. In view of their high site fidelity, large female yellow eels may not encounter many human-induced connectivity problems in polder systems. In addition, our study indicated that large eels prefer deeper habitats. If measures concerning eel habitat restoration in polders are considered, areas of sufficient depth need to be maintained for large eels. A higher yellow eel survival could in turn result in a higher silver eel escapement, and as such could help meet the 40% escapement obligation imposed by the European Eel Regulation.
In accordance with eels’ opportunistic feeding behaviour, chapter four handles head width distribution of eels in the Schelde Estuary. As eels are opportunistic feeders, it is doubtful that head width follows a bimodal distribution with the dichotomous characterization of narrow and broad headed specimens. Indeed, our study showed a continuum of narrow to broad with a unimodal distribution. This pattern could illustrate there is no resource selectivity and consequently no disruptive selection for the eels in the Schelde Estuary.
In chapter five, the spatio-temporal silver eel migration behaviour was studied in a barrier-free system, i.e. the Schelde Estuary. The results of this study demonstrated that silver European eels use selective tidal stream transport and underlines the importance of tides in eel migration. Hence, eels can make a distinction between ebbing and flooding tide, indicating that cues other than currents play a role in orientation. Apparently, tides help eels to migrate in a bioenergetically efficient way through estuaries, just like for other diadromous fish species such as salmonids and flounder. Therefore, restoration of estuaries, for example via tidal barrier management, may not only aid recovery of the European eel population, but of diadromous fish species in general.
The next two chapters relate to spatio-temporal silver eel migration behaviour in anthropogenically regulated systems. Regulated waterways come in various forms depending on anthropogenic needs (agricultural drainage, transport, waste water...) and accompanied migration barriers. Chapter six handles about silver eel migration behaviour in a polder system. A polder system has a lower water level than the sea and consequently needs to be drained by a pumping station to prevent it from flooding. Accompanied with a pumping station is a weir and both act as migration barriers. In chapter seven we studied a shipping canal, i.e. the Albert Canal, which is characterized by shipping locks, turbine stations and tidal sluices. Both studies showed that migration was highly hampered by the various types of migration barriers, leading to significant delays and exploratory behaviour near the barriers. In addition, the substantial low migration speeds in the canal sections of the Albert canal are likely a consequence of the highly regulated water flow, preventing a unidirectional cue for the eels to orient towards the sea. Delays can have a serious impact on eels since their energy resources are limited for a successful trans-Atlantic migration. In addition, delays and exploratory behaviour can also increase predation and disease risk. The results indicated that adequate management measures need to be taken to make migration barriers passable and stimulate as ubstantial unidirectional water flow as eels are likely depending on this for orientation. As this can be economically challenging, management could act during specific migration windows when it is likely the majority of the eels are migrating. In general silver eel migration occurs from August till December at night during moments of increased precipitation and consequently increased flow. Although these studies were conducted on a small geographical scale relative to the wide distribution of the European eel, management at different geographical locations may have substantial various outcomes on the population. In chapter eight we describe a new migration route which at least a part of the eels take to leave the North Sea. Different migration routes may have different bio-energetic implications as some routes may be energetically more demanding, leaving less energy for spawning. Consequently, local management may have an important impact on the population. However, we do not have enough information yet to determine whether something significant is happening during marine migration and whether the inland management actions taken are effective.
In this dissertation, we illustrated that migration barriers such as weirs, pumping stations, shipping locks and tidal sluices substantially hamper silver eel migration via delays, disorientation and exploratory behaviour. Therefore, adequate management measures such as fish friendly screws or gravitational flow stimulation during specific migration windows need to be taken to aid recovery of the European eel population. Yet, such management demands a constructive cooperation between ecologists and engineers. Not only the European eel, but other diadromous and potamodromous fish species will benefit from this as well.
Thesis available here.
