Session C5: Downstream Migration of the European Eel (A. anguilla): Movement Patterns and the Potential Impact of Environmental Factors

Abstract: Recruitment of European eels (Anguilla anguilla) has declined to the extent that they have been added to the IUCN Red List of Threatened Species. Therefore, it is critical to ensure that eels complete their outward river migration in order to contribute to the available spawning stock. We conducted a four-year (2007-2011) telemetry study to understand the migratory behaviour and potential impact of environmental factors on the eel during this critical life stage. Out of 399 female eels tagged with acoustic transmitters, only 28 % demonstrated clear downstream migratory behaviour. Fifty-five percent were detected exhibiting no downstream migration behaviour and 17 % were not detected at any monitoring station. Movement patterns of downstream-migrating (silver) eels were characterised by nocturnal activity and seasonal migration, with distinct peaks in autumn and spring. Migration was often discontinuous and exhibited phases of active locomotion and expanded stopovers. The most important determinants of movement activity were water temperature, cumulative precipitation, and moonlight, although the significance varied by season and location in the river basin. Our results evidence a discontinuous, stepwise migration over an extended period. Furthermore, our findings indicate that migration success depends on holding duration prior to tagging, and environmental predictors with varying importance depending on the season, as well as the locations of capture, tagging and release

Effects of sex, stock, and environment on the shape of known-age Atlantic cod ( Gadus morhua ) otoliths

The effects of sex, stock, and environment on the shape of known-age Atlantic cod (Gadus morhua) otoliths from the Faroe Islands were investigated. Moreover, the feasibility of otolith shape analysis for stock identification was evaluated. The shape was described by using several normalized Fourier descriptors and morphometric variables. There were no consistent differences between the left and right otoliths and between sexes within different age classes, stocks, and environments. With our experimental design, we could evaluate the relative importance of genetic and environmental conditions (water temperature and diet regime) on otolith shape and morphometrics. Using otolith shape, cod individuals were significantly separated into Bank and Plateau stocks. Total classification success was between 79% and 85% between stocks and between 85% and 96% between environments for the different age classes. The significant differences in otolith shape between Faroe Bank and Faroe Plateau cod stocks provided a phenotypic basis for stock separation. Stock and environmental influences were substantial in determining the shape of cod otoliths. [References: 38

Evaluating the performance of otolith morphometrics in deriving age compositions and mortality rates for assessment of data-poor tropical fisheries

© International Council for the Exploration of the Sea 2015. All rights reserved.Deepwater snappers (Family Lutjanidae) are important to artisanal and subsistence fisheries in Pacific Island countries. Most species of deepwater snapper are considered vulnerable to exploitation due to their extended longevity and low natural mortality rates. However, the sustainability of deepwater snapper fisheries in Pacific Island countries remains uncertain because there are limited resources available to collect the required data for comprehensive stock assessments. Reliable estimates of the age composition for exploited deepwater snapper populations are limited primarily because of the lack of skills and resources required for routine age estimation from sectioned otoliths. The development of alternative low-cost approaches to derive estimates of age for deepwater snappers is required. We evaluated the performance of using otolith morphometrics (weight, length, width, and thickness) to obtain estimates of age for the most important target species in these fisheries: Etelis carbunculus, E. marshi, E. coruscans, and Pristipomoides filamentosus. We compared age compositions and fishing mortality rates (F) derived from otolith morphometrics with those derived from counts of annual increments in otoliths. We then used the ratio of F to natural mortality (M) as a biological indicator to evaluate the potential effects on management responses by comparing estimates of F/M derived from otolith morphometrics with those derived from annual increment counts. Age compositions and estimates of F and F/M did not differ significantly between those derived from otolith morphometrics and those derived from annual increment counts for all species. These results demonstrate that management responses would likely be similar whether based on age estimates derived from sectioned otoliths, or predicted from otolith morphometrics. In the absence of sufficient resources to section otoliths for age estimation, we recommend that otolith morphometrics be used as a proxy for age in assessments of deepwater snapper fisheries in Pacific Island countries, and potentially for other similar data-limited fisheries