Dawn Robinson

Can physical variation affect fitness in European Eels?

Wednesday 16th July 2014

European eels are a critically endangered species, therefore study and research is needed to monitor any population changes in order to predict and help maintain the survival of the species in the future. A study sample area was taken from a side channel of the River Frome, Dorset, in order to see if there were any positive correlations between morphological differences already observed and the eel’s foraging strategies and especially their level of fitness. The main morphological differences observed of the eels in this area were a range of head and gape sizes, with several at the extremes of broadest and narrow. This morphological change within one species in a small spatial area can be explained by individual specialism, which leads to exploitation of other resources. However, previous studies have not monitored if this specialism and resulting morphological differences affect the individual fitness of the eel. In this study, the body condition was used as evidence for fitness levels and it was found that eels that did not have broad or narrow head morphology, were in general at a lower fitness level than eels with extreme head differences. This was taken into consideration with different ecological niches within the sample area as well as different trophic levels resulting from diet and foraging differences between the eels.

During the study, electric fishing was used for capture-recapture techniques and tagging, stable isotope analyses and telemetry were carried out during the capture process. The area of the River Frome in which the sampling was taken was also studied for micro-habitats and different ecological niches within a small spatial area in order to investigate if eels of a certain morphological type primarily lived in certain environmental conditions. This was undertaken by recording ecological characteristics, such as channel width and vegetation cover. The results indicate that narrower headed eels occupy a habitat niche nearer the river bank than those with broader heads but more research is needed as to the extent this is linked with their foraging habits.

The stable isotopic analysis was undertaken on both the captured eels and a large range of potential prey fish from that area in order to determine to what extent different prey fish contributed to the eel diet. The study uncovered that the mean contributions of prey fish in the eel ranged from 26.7 to 82% but that was a positive correlation between broader headed eels eating a significantly higher amount of prey fish to those with narrower heads. The study suggests that the high fitness level of the broader head eels may be linked with the larger amounts of energy needed to consume large amounts of prey fish.

Read the full paper, Fitness consequences of individual specialisation in resource use and trophic morphology in European eels, here.

Image via Flickr user Wild Journeys.

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