Mapping silver eel migration routes in the North Sea

Recent developments in tracking technology resulted in the mapping of various marine spawning migration routes of the European eel (Anguilla anguilla). However, migration routes in the North Sea have rarely been studied, despite many large European rivers and hence potential eel growing habitat discharge into the North Sea. In this study, we present the most comprehensive map to date with migration routes by silver European eels in the North Sea and document for the first time successful eel migration through the English Channel. Migration tracks were reconstructed for 42 eels tagged in Belgium and 12 in Germany. Additionally, some eels moved up north to exit the North Sea over the British Isles, confirming the existence of two different routes, even for eels exiting from a single river catchment. Furthermore, we observed a wide range in migration speeds (6.8–45.2 km day−1). We hypothesize that these are likely attributed to water currents, with eels migrating through the English Channel being significantly faster than eels migrating northward.

Development of the silver eel (Anguilla marmorata) gonad during downstream migration in the Lake Poso system, Central Sulawesi

The Lake Poso system is located in Central Sulawesi and is connected to the Tomini Bay by the Poso River. It is known that five out of nine Indonesian eel species were found at the Poso River. Anguilla marmorata is the most caught species. However, uncontrolled catching during downstream migration and the construction of a hydropower plant threaten the silver eel in the Lake Poso system. Research on silver eel (A. marmorata) in Lake Poso was conducted to determine and compare the condition of gonad development as part of the reproductive process. This represents essential information in eel fisheries management in Lake Poso. The eels were collected through bamboo traps (waya masapi). The present study calculates the GSI and HSI values, and histological analyses characterize the gonad. The GSI of eel ranges from 1.95 to 5.69%, and the HSI value ranges from 0.83 to 1.16%. Histological observation showed that eels from Tentena (Lake Poso outlet) and from the estuary of Poso River have ovaries in the early vitellogenic stage (III) and the vitellogenic stage (IV).

Population structure of tropical eel (Anguilla bicolor bicolor) in Cikaso River, West Java

A. bicolor bicolor is a tropical eel subspecies that dominate catches in the coastal areas of West Java, and its conservation status is near threatened. Cikaso River, West Java, becomes eel migration path, while unfortunately, information concerning the stock of eel in this river is still inadequate. The objective of this study was to determine the population structure (size distribution, length-weight relationship, and condition factor) of A. bicolor bicolor in the Cikaso River. The research was carried out in the main streams of the Cikaso River and the coastal marsh of Ciroyom from January to December 2020. Eel samples were collected using lift-net, fishhook, and traps. The number of eels caught is counted, then the total length and weight are measured. We collected nine elvers and 89 yellow eels, but no silver eel was caught during the study. The elver's total length (TL) ranged from 10.2 to 20 cm, and the TL of yellow eel ranged from 20.1 to 42.2 cm. The catch of eel is dominated by the size of 25-30 cm. The growth pattern of eel in the Cikaso River is positive allometric, with the condition factor of eel ranging from 0.450 to 1.917 (1.160±0.3096).

Studi Perbandingan Kandungan Vitamin B Ikan Sidat (Anguilla marmorata (Q.) Gaimard) Fase Yellow Eel dan Silver Eel Asal Sungai Palu Sulawesi Tengah

Ikan sidat (Anguilla marmorata (Q.) Gaimard) memiliki keunggulan gizi atau nutrisi yang tinggi seperti protein, asam lemak, dan vitamin. Penelitian ini bertujuan untuk membandingkan kadar vitamin B (B1, B2, B3, B5, B6, B7, B9, dan B12) pada ikan sidat fase yellow eel dan silver eel asal Sungai Palu Sulawesi Tengah. Metode yang digunakan yaitu purposive sampling berdasarkan pada tempat, ukuran, dan fase ikan. Hasil pengujian vitamin B (B1, B2, B3, B5, B6, dan B9) menggunakan Kromatografi Cair Kinerja Ultra menunjukkan bahwa vitamin B2 fase yellow eel sebesar 0,07 mg/100 g; silver eel sebesar 0,05 mg/100 g; vitamin B3 fase yellow eel sebesar 0,3 mg/100 g; silver eel sebesar 1,16 mg/100 g; dan untuk vitamin B1, B5, B6, dan B9 tidak terdeteksi kadarnya. Sedangkan pengujian vitamin B7 dan B12 menggunakan Kromatografi Cair-Spektrometri Massa menunjukkan bahwa vitamin B7 fase yellow eel sebesar 58,35 µg/100 g; silver eel sebesar 10,61 µg/100 g; vitamin B12 fase yellow eel sebesar 0,52 µg/100 g; dan silver eel sebesar 0,27 µg/100 g. Kesimpulan yaitu vitamin B7 dengan menggunakan Kromatografi Cair-Spektrometri Massa memiliki kadar vitamin tertinggi sebesar 58,35 µg/100 g pada fase yellow eel dan untuk vitamin B3 (Nicotinamide) menggunakan Kromatografi Cair Kinerja Ultra memiliki kadar vitamin terendah sebesar 0,3 mg/100 g pada fase yellow eel.

Conservation restocking of the imperilled European eel does not necessarily equal conservation

To stop the decline of the European eel population, one of the measures taken is translocating eels for restocking, despite its conservational value being largely unknown. We aimed to contribute to this knowledge gap by (i) investigating the origin of eels caught in coastal waters of Estonia and Finland using otolith microchemistry and (ii) directly estimating restocked eel escapement from Narva River Basin District (NRBD), which is part of the primary Eel Management Unit in Estonia. In Estonia, 74% of the sampled eels (n = 140) were natural recruits and 26% were restocked. In Finland, 27% of the sampled eels (n = 235) were natural recruits and 73% were restocked. Only 1% of all the coastally collected eels were originally restocked to NRBD. These new data together with the reported commercial landings from the escapement route conflict with the current silver eel escapement estimation for NRBD and question the accuracy and value of such indirect calculations compiled for most Eel Management Units throughout the European Union. It is concluded that restocking eels to freshwaters may be futile as a conservation measure in some situations, and better escapement is likely achieved in restocking eels to coastal waters or undammed freshwater systems with a direct connection to the sea.

Route selection, migration speed, and mortality of silver eel passing through two small hydroelectric facilities

The European eel is a highly threatened species according the European Inland Fisheries Advisory Commission (EIFAC) and the Food and Agriculture Organization of the United Nations (FAO). The recruitment of this species has collapsed over the last fifty years primarily because of the destruction of free migration routes and overfishing. One of the most important factors linked to population decline is mortality during catadromous migration caused by hydroelectric facilities. The aim of the present study was to assess the mortality rate of silvers eel passing through two small hydroelectric facilities. Total mortality at the site was 5%, but it was 15% for fish passing through the two hydroelectric facilities. However, the cumulative mortality in the river basin studied, which has many hydroelectric facilities, indicated that silver eel escapement from the Słupia drainage basin was very low.