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MacNamara, R., and McCarthy, T. K. 2012. Size-related variation in fecundity of European eel (Anguilla anguilla). – ICES Journal of Marine Science, 69: . Declining European eel (Anguilla anguilla) recruitment has focused attention on conservation of potential spawners leaving continental waters. Fecundity of wild, seaward-migrating silver-phase eels was shown to be size-related and higher than previously reported from artificial maturation experiments. Reliable information on fecundity is essential for stock modelling and future development of eel management policies.

The European eel (Anguilla anguilla) is in decline throughout its distribution and accurate regional information, particularly on the silver eel life-stage, is an essential component of stock recovery plans. Therefore, silver eel population dynamics and production were investigated on the Irish River Shannon. Size frequency analysis during the annual seaward migration showed that between 2008 and 2011, seasonal trends in both sex ratio and female size occurred. Catch analysis and mark-recapture experiments were undertaken during the same period to determine silver eel production. Due to extreme discharge in 2009, no catch data was available for 17 nights, and a novel protocol was developed using a Dual Frequency Identification Sonar (DIDSON) to quantify silver eel migration. In 2008, silver eel production was estimated at 57.3 t (1.35 kgÁha À1 ), but an additional 18.3 t of potential migrants were estimated to have been removed by the summer yellow eel fishery. In 2009, following closure of the yellow eel fishery, silver eel production increased to 68.6 t (1.62 kgÁha À1 ), before declining to 62.7 t (1.47 kgÁha À1 ) in 2010 and 61.6 t (1.45 kgÁha À1 ) in 2011. Modelling, based on long-term stocking data and retrospectively estimated silver eel production (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) suggests that despite commercial fishery closure on the river, the decline in River Shannon silver eel production will continue for at least the coming decade.

Invasive bigheaded carps, genus Hypophthalmichthys, are spreading throughout the Mississippi River basin. To explore the efficacy of a consumer-based market (i.e., invasivorism) to manage them, we developed a conceptual model and evaluated three harvest approaches—direct contracted removal, volume-based incentives (“fisher-side” control), and set-quota harvest (“market-side” control). We quantified the efficacy of these approaches and potential population impact in the Illinois River. Contracted removal was effective for suppressing small populations at the edge of the range but cannot support a market. “Fisher-side” removals totaled 225,372 kg in one year. However, participation was low, perhaps due to reporting requirements for fishers. The “market-side”, set-quota approach removed >1.3 million kg of bigheaded carp in less than 6 months. Larger, older fish were disproportionately harvested, which may hinder the ability to suppress population growth. Total density declined in one river reach, and harvest may reduce upstream movement toward the invasion fronts. With sufficient market demand, harvest may control bigheaded carp. However, lack of processing infrastructure and supply chain bottlenecks could constrain harvest, particularly at low commodity prices. Given the geographical scale of this invasion and complicated harvest logistics, concerns about economic dependence on invasivorism that encourage stock enhancement are likely unmerited.

Knowledge of silver eel production and escapement is essential for conservation of the European eel (Anguilla anguilla) stock. Silver eel trap and transport, being undertaken (2009)(2010)(2011)(2012) to mitigate for negative impacts of hydropower on eels in some Irish rivers, was therefore monitored in the River Erne (catchment area 4375 km 2 ). Eel population analyses at an experimental fishing weir involved mark-recapture experiments and sonar (DIDSON) surveys. Silver eel mortality rates at two hydropower stations were assessed by acoustic telemetry. The combined results of these studies enabled estimation of silver eel production and escapement for two migration seasons. For the 2010 season, production and escapement estimates were 42.5 t (1.62 kgÁha À1 ) and 38.8 t (1.48 kgÁha À1 ), respectively. For the 2011 season, production and escapement estimates were 44.6 t (1.70 kgÁha À1 ) and 41.8 t (1.59 kgÁha À1 ), respectively. Closure of all commercial eel fisheries and initiation of silver eel conservation fishing in 2009 resulted in 9.36, 19.33 and 25.25 t of silver eels being transported downstream of the hydropower dams in 2009, 2010 and 2011, respectively. The protocols developed in this study can be used to test models being used for eel population management in Ireland.

The European eel (Anguilla anguilla), once abundant throughout much of Europe and North Africa, has recently been classified as critically endangered. Information on its biology from the eastern Mediterranean is lacking, especially in relation to spawner quality. Therefore, silver eels were sampled during their seaward spawning migration from Vistonis Lake in Greece. Characteristics linked to reproductive output and success (i.e. body size and condition, sex ratio, silvering, Anguillicola crassus infection, fecundity and oocyte diameter) were examined. The lake produced large (687-1138 mm), exclusively female silver eels, 61.7% of which were infected by A. crassus. Silver eel fecundity, the first estimates from the southern part of the species range, was positively related to body length (R 2 = 0.693; P < 0.001) and body weight (R 2 = 0.731; P < 0.001). Fecundity did not differ between A. crassus infected and uninfected silver eels, but Greek silver eels were significantly more fecund than those in northwest Europe. The reproductive potential of Vistonis Lake silver eels and their contribution to the A. anguilla spawning stock is discussed.

This paper reports the first study of breeding in the boreo-arctic barnacle Semibalanus balanoides in which latitudinal variation in timing of egg mass hardening has been examined simultaneously over the geographical scale involved, thereby excluding temporal confounding of the data. The timing of autumn egg mass hardening on the middle shore was established in 2002 and 2003 at ten stations ranging latitudinally from Trondheim (63°24′N) to Plymouth (50°18′N). To assess variation at local scale (<10 km), breeding was studied on three shores at each of two Irish locations (Cork and Galway). At Oban (Scotland) and Cork, the effect of shore height on timing of breeding was investigated. A strong influence of latitude and day length on timing of breeding was found in both 2002 and 2003. In both years, barnacles bred much earlier (when day length was longer) at high rather than low latitudes. No significant effect of environmental temperature or insolation on timing of breeding was detected. Shores no more than 10 km apart showed minimal difference in middle shore breeding date (<4 days). However, upper shore barnacles bred significantly earlier (by 7–13 days) than middle shore animals. The data indicate that breeding is controlled by period of daily darkness, with high shore animals encountering longer effective ‘nights’ because of the opercular closure response to emersion (which will reduce light penetration to tissues). Predictions concerning the effects of global changes in climate and cloud cover on breeding and population distribution are made. It is suggested that increased cloud cover in the northern hemisphere is likely to induce earlier breeding, and possibly shift the present southern limit of Semibalanus southwards.