Why are migrating male shortfinned eels (Anguilla australis) in Lake Ellesmere, New Zealand, getting smaller but not younger?

Lake Ellesmere is a large coastal lake in the South Island of New Zealand which supports important commercial fisheries for both feeding and migratory shortfinned eels Anguilla australis. Over the past 50 years, the sex distribution of migratory eels in the lake has changed from a dominance by females to a dominance by males. Age and size data of males have been collected intermittently since 1974. During this time, the mean age has remained relatively constant while the average length has declined by almost 20%. Despite this reduction, length distributions are less variable than weight, condition, or growth rate distributions, and length is considered to be the primary factor associated with the onset of migration. The reduction in average size does not appear to be a result of selective harvest of larger individuals, but must reflect changing environmental conditions in the lake itself - such conditions could include loss of macrophytes, and a reduction in the proportion of larger eels of both species with resultant proliferation of the small benthic bully Gobiomorphus cotidianus. The suggested management strategy is to maximise harvest of migratory male eels and thus reduce the capture of females which are large enough to prey on bullies

A stage-structured model to predict the effect of temperature and salinity on glass eel Anguilla anguilla pigmentation development

The pigmentation development process of glass eels Anguilla anguilla from stage V-B to VIA3 was modelled by gamma cumulative functions. These functions varied with respect to the factors temperature and salinity whose effects were adjusted by beta functions. Temperature was shown to accelerate pigmentation, while salinity acted as a secondary factor slowing down the pigmentation. The model fits the development of 15 samples kept at various temperatures and salinities in the Vilaine River, as well as samples monitored at other dates and places in Europe. It allows the prediction of the duration of estuarine residency for glass eels, in winter and spring, in the Atlantic estuaries

Swimming physiology of European silver eels (Anguilla anguilla L.): energetic costs and effects on sexual maturation and reproduction

The European eel migrates 5,000–6,000 km to the Sargasso Sea to reproduce. Because they venture into the ocean in a pre-pubertal state and reproduce after swimming for months, a strong interaction between swimming and sexual maturation is expected. Many swimming trials have been performed in 22 swim tunnels to elucidate their performance and the impact on maturation. European eels are able to swim long distances at a cost of 10–12 mg fat/km which is 4–6 times more efficient than salmonids. The total energy costs of reproduction correspond to 67% of the fat stores. During long distance swimming, the body composition stays the same showing that energy consumption calculations cannot be based on fat alone but need to be compensated for protein oxidation. The optimal swimming speed is 0.61–0.67 m s−1, which is ~60% higher than the generally assumed cruise speed of 0.4 m s−1 and implies that female eels may reach the Sargasso Sea within 3.5 months instead of the assumed 6 months. Swimming trials showed lipid deposition and oocyte growth, which are the first steps of sexual maturation. To investigate effects of oceanic migration on maturation, we simulated group-wise migration in a large swim-gutter with seawater. These trials showed suppressed gonadotropin expression and vitellogenesis in females, while in contrast continued sexual maturation was observed in silver males. The induction of lipid deposition in the oocytes and the inhibition of vitellogenesis by swimming in females suggest a natural sequence of events quite different from artificial maturation protocols

Fish under exercise

Improved knowledge on the swimming physiology of fish and its application to fisheries science and aquaculture (i.e., farming a fitter fish) is currently needed in the face of global environmental changes, high fishing pressures, increased aquaculture production as well as increased concern on fish well-being. Here, we review existing data on teleost fish that indicate that sustained exercise at optimal speeds enhances muscle growth and has consequences for flesh quality. Potential added benefits of sustained exercise may be delay of ovarian development and stimulation of immune status. Exercise could represent a natural, noninvasive, and economical approach to improve growth, flesh quality as well as welfare of aquacultured fish: a FitFish for a healthy consumer. All these issues are important for setting directions for policy decisions and future studies in this area. For this purpose, the FitFish workshop on the Swimming Physiology of Fish (http://www.ub.edu/fitfish2010) was organized to bring together a multidisciplinary group of scientists using exercise models, industrial partners, and policy makers. Sixteen international experts from Europe, North America, and Japan were invited to present their work and view on migration of fishes in their natural environment, beneficial effects of exercise, and applications for sustainable aquaculture. Eighty-eight participants from 19 different countries contributed through a poster session and round table discussion. Eight papers from invited speakers at the workshop have been contributed to this special issue on The Swimming Physiology of Fish

Comparative ecology of the European eel, Anguilla anguilla (L.1758), in a large Iberian river

A total of 1,816 eels were sampled in 1988, from seven sampling areas. Four areas were located in brackish water and the remaining three were located in freshwater reaches of the Tagus river basin. Eels were more abundant in the middle estuary and decreased both in the upstream and in the downstream directions, with a predominance of males in higher density areas. Smaller individuals preferred more peripheral areas, such as margins and upper reaches in the brackish water zone, and the tributaries of the freshwater habitats. It was assumed that this distribution pattern resulted from three main factors: (i) the dominance of larger specimens; (ii) the need to avoid predators and; (iii) the search for better trophic conditions. The condition of the individuals generally decreased toward the upper reaches, apparently due to a corresponding decrease in feeding intensity. The presence of the Belver dam in the main river, 158 km upstream from the sea, seemed to impose major alterations to the described patterns. The concentration of specimens below this impassable obstacle yielded a reduction in the proportion of females and a decrease in the condition and survival of the eels, contributing to a reduction in the spawning success of this population. Suggestions to diminish the effects of the dam, and to preserve the fishery are also presente

Why are migrating male shortfinned eels (Anguilla australis) in Lake Ellesmere, New Zealand, getting smaller but not younger?

Lake Ellesmere is a large coastal lake in the South Island of New Zealand which supports important commercial fisheries for both feeding and migratory shortfinned eels Anguilla australis. Over the past 50 years, the sex distribution of migratory eels in the lake has changed from a dominance by females to a dominance by males. Age and size data of males have been collected intermittently since 1974. During this time, the mean age has remained relatively constant while the average length has declined by almost 20%. Despite this reduction, length distributions are less variable than weight, condition, or growth rate distributions, and length is considered to be the primary factor associated with the onset of migration. The reduction in average size does not appear to be a result of selective harvest of larger individuals, but must reflect changing environmental conditions in the lake itself - such conditions could include loss of macrophytes, and a reduction in the proportion of larger eels of both species with resultant proliferation of the small benthic bully Gobiomorphus cotidianus. The suggested management strategy is to maximise harvest of migratory male eels and thus reduce the capture of females which are large enough to prey on bullies