Table S3. Equations used in calculations of habitat and diet metrics
The total fullness of stomachs was visually determined on a percentage scale ranging from empty (0
%) to full (100 %). The prey-specific abundance (Pi) to the total stomach contents was subsequently
estimated according to Amundsen, Gabler & Staldvik (1996):
  Si
Pi  
  St
i


  100


where Si is the stomach content comprised of prey i, and Sti the total stomach content in only those
predators with prey i in their stomach.
Schoener’s (1970) similarity index (α) was used to study the habitat and dietary overlap between all
charr and trout, and the dietary overlap between the pelagic- and littoral-caught charr. The index is
calculated as:


n
  1  0.5  Pxi  Pyi 
 i 1

where Pxi is the proportion of habitat/ prey group i used by species/subpopulation x, Pyi is the
proportion of habitat or prey group i used by species/subpopulation y, and n is the number of habitat/
prey categories. The overlap is considered high when the index value exceeds 60 % (Wallace 1981).
Levin’s (1968) index (B) was calculated to estimate the dietary niche width for all charr and trout, and
separately for pelagic- and littoral-caught charr:
B
1
 p 2j
where pi is the proportion of each prey type i in the diet expressed as fraction rather than percentage.
A proportional similarity (PSi) index (Bolnick et al., 2002) was calculated to measure individual
dietary specialization within all charr and trout, and within the pelagic- and littoral-caught charr. The
index compares each individual’s diet to the overall diet of the population as:
PSi  1  0.5 pij  q j   min  pij , q j 
j
j
where pij is the proportion of prey group j in the diet of individual i, and qj is the proportion of prey
group j in diet of the population as a whole. PSi approaches 1 when the individual has a similar diet to
that of the population.
The overall prevalence of individual specialization within all charr and trout, and within the pelagicand littoral-caught charr was further calculated as the average individual PSi values:
IS 
1
 PSi
N
The inverse 1 – IS was calculated to make high estimates to correspond high prevalence of individual
specialization in the population and vice versa.
References
Amundsen, P.-A., Gabler, H.-M. & Staldvik, F.J. (1996) A new approach to graphical analysis of
feeding strategy from stomach contents data –modification of the Costello (1990) method. Journal
of Fish Biology, 48, 607–614.
Bolnick, D.I., Yang, L.H., Fordyce, J.A., Davis, J.M., Svanbäck, R. (2002) Measuring individuallevel resource specialization. Ecology, 83, 2936–2941.
Levins, R. (1968) Evolution in Changing Environments: Some Theoretical Explorations. Princeton
University Press, New Jersey.
Schoener, T.W. (1970) Nonsynchronous spatial overlap of lizards in patchy habitats. Ecology, 51,
408–418.
Wallace, R.K. (1981) An assessment of diet-overlap indexes. Transactions of the American Fisheries
Society, 110, 72–76.