Pesca continental

FAO European Inland Fisheries Advisory Commission; International Council for the Ex‐ ploration of the Sea. Report of the 2007 session of the Joint EIFAC/ICES Working Group on Eels. Bordeaux, France, 3–7 September 2007. EIFAC Occasional Paper. No. 39. 

Managing inland fisheries

This report summarizes the presentations, discussions, and recommendations of the 2007 session of the Joint EIFAC/ICES Working Group, which took place in CEMAGREF, Bor‐ deaux, France, from 3 to 7 September 2007. Information on recruitment, stock, and fisheries reviewed by the WG continues to support and reinforce the advice that the global European eel stock has declined in most of the distribution area and is outside safe biological limits. Recruitment of glass eel to the con‐ tinental stock remains low, with no obvious sign of recovery. Most recent data indicate a continued downward trend in the stock–to–recruit relationship, heightening concern over possible depensation and concerns about the stock’s ability to recover, even in the long term. The WG concluded that, before the practice of stocking was introduced, the pres‐ ence of eel in the river systems connected to the Black Sea and the Danube was, at best, sporadic, and concluded that the area around the Black Sea is at the extreme limits of the natural geographic range for eel. The WG welcomed the EU regulation that establishes measures encouraging the recovery of the stock and emphasized that this be implemented urgently. The urgent need to re‐ duce mortality as soon as possible is clear. The objective of the regulation is the protection and sustainable use of the natural eel stock. To achieve this, Member States will develop eel management plans for their river basins, aiming at a reduction in anthropogenic mor‐ talities so as to permit the escapement to the sea of at least 40% of the biomass of silver eel, relative to the best estimate of escapement that would have existed in the absence of anthropogenic influences on the stock. There are strong indications that recruitment might be impaired by the low spawning stock. Prioritization of restrictions on anthropo‐ genic impacts will be consistent with the risk‐averse strategy of the precautionary ap‐ proach, while making best use of positive actions that potentially boost the stock. In the 1970s, recruitment of the glass eel was still at historically high levels. This indicates that SSB was not limiting the production of recruits at that time. Quantification of the 1970s spawner escapement therefore is the simplest derivation of the reference level. Note that in this case, the full escapement of the silver eels in the 1970s (given the anthropo‐ genic mortality of that time) corresponds to the escapement level advised by ICES. That is, one should set the reference point either at 100% of the 1970s silver eel escapement, or at a percentage of the notional pristine state. It is of utmost importance that existing recruitment monitoring be continued and im‐ proved, easing the dependence on commercial fisheries, and extended where inadequate. A radical improvement in the assessment of the current state of the stock, including quan‐ tification of the impact of anthropogenic mortalities, is urgently needed and required in the Eel Management Plans (EMPs). Although comprehensive datasets exist in some river basins, this assessment will not be achievable in most river basins from currently limited data. Therefore, it is proposed that concerted research be initiated urgently to compile datasets, develop regression models of the relation between impacts and stock, and in‐ ter/extrapolate to the data‐poor situations. Though density‐dependence has been demonstrated in several life stages by means of various mechanisms, it is not well understood, and knowledge is certainly inadequate to predict where it will occur or on what processes it will act (i.e. growth, colonization pat‐ terns, sexual differentiation and sex ratios, mortality). This affects both the calculation of restoration targets, and the assessment of the impact of fisheries and other anthropogenic factors. Sampling methods available to provide the data for these population status and manage‐ ment action assessments were considered. Experience gained from the EU INDICANG programme, previous WG reports, published scientific literature, and the knowledge of iv | EIFAC/ICES WGEEL Report 2007 the WG participants was used to consider a pragmatic approach, given the short time available to develop the EMPs. These and other methods will also be used in further im‐ proving EMPs and in the post‐evaluation of the effectiveness of the management meas‐ ures. Stocking and transfers of juvenile eel have been discussed at length by the Working Group (most recently ICES, 2006), mainly in conceptual and theoretical frameworks ow‐ ing to a lack of hard data. Given that stocking is listed in the forthcoming regulation as one management option among others to aid stock recovery, it was agreed that the major need is robust evidence of the extent to which stocking and transfer on local, national, and international scales can contribute to improved spawner escapement. Glass eel to silver eel production and growth rates may be non‐linear over the ranges of input stock densi‐ ties in local studies (100–1600 glass eels ha⁻ 2 ) reported to date, suggesting that optimum output expressed per glass eel equivalent is to be gained at the lower stocking densities. Little information is available on the effect of stocking on‐grown eels, although studies suggest that growth, sex ratio, and return rates (numbers of silver eels per glass eel) prior to spawner emigration are in the same order of magnitude as in natural eel populations. Stocking programmes should be aware of the need to simulate natural processes as far as possible, because this is most likely to lead to high success rates, measured in biomass or numbers of silver eel escaping to the sea. Considerable advances have been made in the collection of data on contaminants, para‐ sites, and fat levels in eel, with many Member States commencing the monitoring of eel quality. WGEEL has established a European Eel Quality Database (EEQD). The eel quality data are highly variable, and identification of “black spots” is possible for low‐quality eels. The parasite Anguillicola crassus appears to be widespread across Europe. Relating the database parameters to assess their impact on the stock or to management targets was not possible during WGEEL 2007, but eel quality parameters should be included in eel stock assessments. The working group reviewed the anthropogenic impacts of impediments to upstream migration, downstream migration barriers, and turbine mortality. Assessing anthropo‐ genic range restriction is therefore an important consideration in the assessment of com‐ pliance with the target of 40% of natural spawner‐escapement levels. A system for categorizing the severity of individual and cumulative effects of obstacles to upstream eel migration into a single score, based on obstacle height, gradient, surface roughness, and bank characteristics, has been developed. It is important to maintain upstream continuity, especially where downstream access to the sea for spawners is assured. Hydropower has been recognized as one of several factors contributing to the dramatic decline in the eel population, and eels tend to have considerably greater mortality rates from downstream passage at hydropower stations than other fish species. There is a need to quantify these impacts further, as well as those possibly occurring at cooling water intakes, pumping stations, and tidal power plants. A variety of behavioural and mechanical mechanisms for reducing or avoiding mortality in turbines are discussed. Natural mortality of eels is a major but relatively unknown factor in the population dy‐ namics of the species. Mortality caused by predation is but one of the factors contributing to natural mortality (and may be compounded by other issues such as disease and parasit‐ ism). The EU regulation lists reducing predation as a possible management option that could be employed when attempting to reach escapement targets. Methods for assessing the levels of eel in the diet of predators were discussed, in particular for cormorants and otter, although determining the level of impact on the stock is difficult. Crude calculations indicated cormorant consumption of eel on the order of 30–50% of the 1993/94 commercial European catch. Measures for reducing the level of predation are presented and discussed in relation to the various protective regulations and directives.