The otter is a semi-aquatic carnivore that spends a lot of it is active time in the water (Chanin 1993). Thus it is specialized in an aquatic diet with a few terrestrial species included (Heggberget and Moseid 1994). The otter’s diet remains of fish, crayfish, frogs, birds (Erlinge 1971) and mammals (Conroy et al. 2005).
The interest in the otter’s diet has been considerable during a long time and several studies have been conducted. Traditionally three methods have been used to determine what otters eat. One is direct observation of individuals feeding, another is analysis of stomach contents and a third is analysis of spraints (Conroy et al. 2005). A fourth method that has been increasingly used for other species is analysis of stable carbon (C) and nitrogen (N) isotopes. This method gives time-integrated information about feeding relationships and energy flow through food webs (Vander Zanden and Rasmussen 1999). However, I haven’t found any study of stable isotopes that has been applied on otters.
In animal tissue there is a natural ratio of stable isotopes of for example N and C (Dalerum and Angerbjörn 2005). These stable isotopes can reflect the food intake of a consumer over time because the isotopes derive from assimilated food (Das et al. 2003). Animals seem to have more of the heavier isotopes N and C than their food and there is a stepwise enrichment that occurs on each trophic level in the food chain (Olive et al. 2003). The stable nitrogen isotope ratio can be used to assess the trophic position of the animal (Post 2002) and the stable carbon isotope can be used as an indicator of for example an inshore versus offshore diet source (Das et al. 2003). For carbon the enrichment in trophic level is roughly 1‰ (Angerbjörn et al. 2006, Newsome et al. 2006) and for nitrogen the enrichment is 3-4‰ (Kelly 2000). Often the trophic level enrichment in heavy isotopes is used to characterize terrestrial and marine food webs and also to determine the major source(s) of prey for both carnivores and herbivores in various ecosystems (Newsome et al. 2006).
Different tissues have different turnover rates so every tissue reflects a diet over a specific time window. The muscle tissue has a relative high metabolic rate and thus a high molecular turnover and therefore this tissue will only reflect the consumer’s diet for the season the muscle was collected (Dalerum and Angerbjörn 2005).
The strain of pollutants, especially PCB, in otters is shown to be higher in southern than in the northern Sweden (Roos et al. 2001). PCB also shows a great variation in concentration according to different fishes in the northern and southern Sweden. The reasons for the higher concentrations of PCB in the south of Sweden seem to be due to local contaminant sources, the high numbers of urbanized areas and industries and to airborne fall out (Roos et al. 2001). Observed dietary patterns of otters in Europe has been associated with two climate regions, temperate climate in central and partly northern Europe and Mediterranean climate in southern Europe. Thus the otters in the different climate regions could have different food and foraging behavior (Clavero et al. 2003). In spite the fact that the otters main prey are the same everywhere in Europe their consumption of secondary preys shows a variation in different regions (Clavero et al. 2003) and therefore they could eat on different trophic levels in different regions.
The differences in contaminant concentration that occurs in otters from different regions could be connected to a different habitat use of the otters and thereby a different diet. Based on other studies I examined if there are any differences in prey choice between sexes, seasons and regions. I also hope to se a trophic level difference in the otters since a regional difference in prey choice can give a change in the trophic level of an animal. To test these hypothesis and questions I used two methods of diet analysis on a set of data spanning over 30 years.
Contents
Abstract
Introduction
Material and methods
- Age determination
Stomach & intestine content
Stable isotope
Niche breadth
Study region
Statistics
Results
- Stomach and intestine content
Niche breadth and Availability
Stable isotope
Nitrogen
Carbon
Starvation
The otter and its prey
Discussion
Acknowledgements
References
APPENDIX 1
APPENDIX 2
APPENDIX 3
- Figure 1
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