You may find a nice approach in Doledec et al. (2000) - so called OMI (Outlying Mean Index) analysis. Soininen & Heino (2007) used it to study the diatoms niche parameters. There is also R package ADE-4 (Thioulouse et al., 1997) which you can use. Hope it can help you (or at least give some other perspective)!

The simplest answer would be to look at the feeding ecology of the organism. Does it specialize on feeding on just a few other organisms, i.e., it evolved to a point where that is all it eats, perhaps because that is all there is? Does it quickly become a nuisance in the ecosystem, especially after some catastrophic event (massive flood, dumping of waste water into what was previously known to be a clean body of water (here in the US, we just had a massive human-error induced breach of a dam holding back mine water that ended up dumping over 3 million gallons of toxic waste water into previously clean water that will affect many states as the plume moves downstream, but the damage is done. It is a good lesson in what happens when the government causes a major problem that they were supposed to watch over. How do citizens obtain redress, as to sue the government is merely to sue yourself. It will cascade downstream as suspended sediments, and as those drop out in slower water areas, become embedded in the sediments, only to be stirred up at some time in the future by a large flood event, doubling or tripling the original event...it's just a matter of time. But back to the issue at hand). I'm going to start at the producer and primary consumers first (and then move up to fish, particularly salmonids, considering your interest in Salmo cettii, as in the Ecology and Classification of North American Freshwater Invertebrates (Thorpe and Covich, 1991, may have been updated since then), Limnology, Lake and River Systems (Wetzel, 2001), Biodegradation and Bioremediation (Alexander, 1994), and finally, just for a list of places to start, The Ecology of Running Waters (Hynes, reprinted in 2001 by special order...it may be hard to get this one, but it shows how much Hynes knew way back in 1970). You may have also heard of the River Continuum Concept. Google that, and you'll get a graphic of the theory. These deal mainly with invertebrates and algae. For fish, since you seem to be focusing on Salmo, I'd look at the "Feeding Ecology of Fish" (Gerking, 1994), The Behavior and Ecology of Pacific Salmon and Trout (Quinn, 2005), and Influences of Forest and Rangeland Management on Salmonid Fishes and Their Habitats (Meehan, ed., 1991). One last book comes to mind as I was perusing some other reference books on my shelves...If you  are trying to create a model, the Handbook of Environmental and Ecological Modeling (1996, Jorgensen, Halling-Sorensen, and Nielsen) may prove worthwhile. I think you will find that feeding ecology, obviously, of your organism of choice will be the ticket. Methodologies can be found in documents easily Googled  from the Protocols of the USEPA and USFWS,and the USGS. I hope this is of at least some assistance...Cheers...Bob

The simplest answer would be to look at the feeding ecology of the organism. Does it specialize on feeding on just a few other organisms, i.e., it evolved to a point where that is all it eats, perhaps because that is all there is? Does it quickly become a nuisance in the ecosystem, especially after some catastrophic event (massive flood, dumping of waste water into what was previously known to be a clean body of water (here in the US, we just had a massive human-error induced breach of a dam holding back mine water that ended up dumping over 3 million gallons of toxic waste water into previously clean water that will affect many states as the plume moves downstream, but the damage is done. It is a good lesson in what happens when the government causes a major problem that they were supposed to watch over. How do citizens obtain redress, as to sue the government is merely to sue yourself. It will cascade downstream as suspended sediments, and as those drop out in slower water areas, become embedded in the sediments, only to be stirred up at some time in the future by a large flood event, doubling or tripling the original event...it's just a matter of time. But back to the issue at hand). I'm going to start at the producer and primary consumers first (and then move up to fish, particularly salmonids, considering your interest in Salmo cettii, as in the Ecology and Classification of North American Freshwater Invertebrates (Thorpe and Covich, 1991, may have been updated since then), Limnology, Lake and River Systems (Wetzel, 2001), Biodegradation and Bioremediation (Alexander, 1994), and finally, just for a list of places to start, The Ecology of Running Waters (Hynes, reprinted in 2001 by special order...it may be hard to get this one, but it shows how much Hynes knew way back in 1970). You may have also heard of the River Continuum Concept. Google that, and you'll get a graphic of the theory. These deal mainly with invertebrates and algae. For fish, since you seem to be focusing on Salmo, I'd look at the "Feeding Ecology of Fish" (Gerking, 1994), The Behavior and Ecology of Pacific Salmon and Trout (Quinn, 2005), and Influences of Forest and Rangeland Management on Salmonid Fishes and Their Habitats (Meehan, ed., 1991). One last book comes to mind as I was perusing some other reference books on my shelves...If you  are trying to create a model, the Handbook of Environmental and Ecological Modeling (1996, Jorgensen, Halling-Sorensen, and Nielsen) may prove worthwhile. I think you will find that feeding ecology, obviously, of your organism of choice will be the ticket. Methodologies can be found in documents easily Googled  from the Protocols of the USEPA and USFWS,and the USGS. I hope this is of at least some assistance...Cheers...Bob

You may use the Levin's  standardized index,modified Amundsen plot and also inferential statistics such as principal component analysis to investigate if a species is a generalist or specialist.

You may also try calculating a standardized dietary niche breadth value outlined by Lewis et al. (2006).  I used this method to determine degree of dietary specialization in two species of woodpeckers feeding nestlings in a paper I recently published.  Of course, this method works for instances where the frequency of prey items consumed is observed. Citations are as follows:

Lewis, S. B., K. Titus, and M. R. Fuller. 2006. Northern Goshawk diet during the nesting season in southeast Alaska.  Journal of Wildlife Management 70:1151-1160.

Article Nestling provisioning by Hairy and White-headed woodpeckers ...