Glass Eel Fishery Research Articles

Are the fisheries in the Tagus estuary sustainable?

The comparative sustainability of seven Tagus estuary fisheries – beam trawl, boat dredge, nets for glass eel, gill nets, eel basket, squid jig and octopus traps – is analysed using RAPFISH. This technique relies upon ordination of scored attributes, grouped in fields that cover ecological, economic, social, technological and ethical sustainability, performed using multi-dimensional scaling. Fisheries were analysed within each discipline and using all the attributes for an interdisciplinary approach. Leverage analysis is used to determine how much each attribute influences the ordination. Concerning the cross-discipline ordination, the studied fisheries lay around the mid-range of sustainability. The most sustainable fishery is octopus traps (57%) followed by squid jig (56%) that has the same results except for technological dimension. Eel basket and gill nets show the next best sustainability scores (55% and 53%, respectively). Glass eel fishery presents a poor sustainability (46%), lying close to the boat dredge and beam trawl, the fisheries with lowest sustainability (44% and 43%, respectively). The low leverage values show that ordinations are truly multivariate and that the results are not dominated by any one attribute. More studies should be conducted on the Tagus estuary fisheries in order to get more information about resources, fishers and their activity, which will contribute to improve sustainability.

EFFECT OF RE-OPENING OF A MIGRATORY PATHWAY FOR EEL (ANGUILLA ANGUILLA, L.) AT A WATERSHED SCALE

The construction of the Arzal dam eel ladder in 1996 allowed enhancing fluvial recruitment from a negligible level, limited to yellow eels crossing sluice and dam overflow, to a level ranging from 0.2 to 2.4 million glass eels per year. The effect of such recruitment on the eel population (Anguilla anguilla) of the Vilaine watershed was analysed at 19 electrofishing sampling stations. From 1998 to 2003, average densities of eel varied from 0.34 to 0.72 eels.m-2, with a maximum reached in 1999 of 0.82 eels.m-2. Fluvial recruitment failure, together with density dependent mortalities, explains the drop in yellow eel densities observed from 1998 in the downstream area (< 50 rkm) and from 2000 in the middle stream area (50100 rkm). The largest densities consisted of a large proportion eels classified as age 1 and the increased density in middle stream sectors was interpreted as the consequence of density-dependent migration at the periphery of a saturated area. The construction of 13 eel ladders on the Vilaine waterway in 1999 and 2000 was followed by increased densities in the upstream area (> 100 rkm) in 2001. The comparison to an electrofishing survey performed at 17 stations in 1981, ten years after dam construction, confirmed that ladder installation increased densities by a factor 6 and modified the population structure, with densities of eels age 0 and 1 multiplied 29 fold. This change corresponded to reduced escapement rates to the glass eel fishery (1% to 5%). Considering this result, a preliminary escapement target of 240 glass eel per km2 of watershed area or 1 500 glass eel per ha of water surface, is proposed and discussed for glass eel fisheries.

Estuarine and fluvial recruitment of the European glass eel, Anguilla anguilla, in an exploited Atlantic estuary

AbstractA glass eel fishery exists downstream of the Arzal estuarine dam in the Vilaine (Brittany, France). Catch statistics were collected between 1996 and 2000, and processed using a subsampling technique which allowed data from a reliable subsample to be extrapolated to the whole fishery. During the same period, glass eel migration into fresh water was monitored using a glass eel trap located on the dam – the upstream limit of the fishery. The analysis of glass eel biology and exploitation shows that the glass eel fishery is very intensive and that there was more or less no escapement during the fishing seasons studied. The proportion of the stock successfully migrating towards fresh water, as compared with the total catch is estimated to range between 0.3 and 3.9%. In such a fishery, the fishing effort affects the abundance. Consequently, the total catch has to be used instead of catch‐per‐unit‐effort to estimate abundance.

A Procrustean assessment of the European eel stock

No assessment of the state of the European eel stock is available due to the absence of adequate data for many areas of the continent. In contrast to past efforts which turned to complete the traditional catch composition data, and which met with little success, we will try to develop a simplified cohort-model (based on life stage, rather than age or size), simple enough for the available data. Under the (incorrect) assumption of stable recruitment and exploitation, the catch-at-life-stage analysis yields a preliminary assessment of the entire European stock, and for the glass eel importing and exporting countries. Recruitment is estimated at about 2000 million eels annually, most of whom enter countries around the Bay of Biscay, supporting intensive glass eel fisheries. Elsewhere, the natural recruitment is outnumbered by imported and transported glass eel. The fishing mortality rate accumulated over the total life span is estimated at 5.21 (=99%) for glass eel exporting countries and 3.25 (=96%) for glass eel importing countries. This Procrustean assessment provides a limited view on continent-wide stock. Substantially improved assessments are unlikely at the time-scale at which management action is required. However, the development of a co-ordinated system of inter and intra-national management (i.e., the only effective levels) will benefit the assessment of the European eel stock.

Intraseasonal characterization of glass eel migration in the River Tiber: space and time dynamics

Glass eel (Anguilla anguilla) upstream migration was studied in the River Tiber estuary to obtain a better understanding of spatial and temporal migration dynamics within the season of ascent. Using data from glass eel fisheries, time series analysis of daily catches per unit of effort revealed a fortnightly cycle that can be related to invasion waves possibly corresponding to tidal currents. The amplitude of these waves appeared to correspond to the tidal area of the estuary. Furthermore, glass eels apparently had a delay in this area before resuming upstream migration.

Intraseasonal characterization of glass eel migration in the River Tiber: space and time dynamics

Glass eel (Anguilla anguilla) upstream migration was studied in the River Tiber estuary to obtain a better understanding of spatial and temporal migration dynamics within the season of ascent. Using data from glass eel fisheries, time series analysis of daily catches per unit of effort revealed a fortnightly cycle that can be related to invasion waves possibly corresponding to tidal currents. The amplitude of these waves appeared to correspond to the tidal area of the estuary. Furthermore, glass eels apparently had a delay in this area before resuming upstream migration.

Seasonal dynamics of estuarine migration in glass eels(Anguilla anguilla)

An analysis of statistical data available in eight glass eel fisheries distnbuted over the French Atlantic coastline is used to define the upstream migration dynamics of Anguilla anguilla in estuaries. The CPUE (Catch Per Unit of Effort) data reveal three different types of seasonal trends that reflect different migration kinetics. The recruitment abundance is studied compared to the salinity of the estuary. The first two types of trends correspond to years of high recruitment andor of reduced fishing effort. In brackish estuarine zones, the abundances then follow a symmetrical curve, with a peak in JanuaryFebruary. This time course is due to the passage of a wave of migrants. Further upstream, in tidal freshwater zones, the abundances follow an asymmetrical curve which may describe the progressive accumulation of eels between November and March/April. When recruitment is low andlor the fishing effort is high, CPUEs vary during the season following a plateau curve, with values staying more or less constant between November and March. This latter trend represents a balance status between immigration and capture. In this case, abundance indices can only be established with total catches. The curves are explained by the migration behaviour of glass eel which exhibit a resting phase in brackish zones and an accumulation in tidal oligohaline zones. These results have direct consequences particularly for the determination of recruitment abundance indices.

The Seasonal Occurrence of Glass Eels (Anguilla Anguilla L.) in the Rio Minho between 1991 and 1993 (North of Portugal)

AbstractGlass eels were sampled monthly at two stations in the Rio Minho. The experimental fishery was carried out in the estuary between November 1991 and July 1992 and at an upstream site between December 1991 and March 1993. The yield differed at each station, especially in summer and between the official seasons of the experimental glass eel fishery. Early pigmentation stages were more frequent in the estuary and the maximum lengths and weights were observed in autumn and winter. The official statistics of the professional glass eel fishery are briefly mentioned.

By-catch of juvenile flounder,Platichthys flesusL. 1758, in the glass-eel fishery of the Loire Estuary, France

By-catch of 0-group flounder, Platichthys flesus L. 1758, in the Loire glass-eel fishery was estimated for 1987 and 1988. Samples were taken throughout the estuary but the by-catch mainly occurred in the Central area. This pattern was observed in both surveys despite different hydrological conditions. The overall by-catch was 41 million animals for 1987 and 7 million for 1988. This difference was not only a consequence of a lowered fishing effort, it also suggests a difference in year-class strength. Recause of the great variability of postlarval distribution the accuracy of the estimates was very poor and only orders of magnitude can be discussed.