Appendix S2: Electivity Index with Standardized Lengths

Size- and condition-dependent predation: a seabird disproportionately targets substandard individual juvenile salmon 1 1 Appendix S2: Electivity index with standardized lengths

2 To account for any temporal mismatches between survey and colony sampling and the potential for

3 growth, we recalculated the Electivity Index (ɛ; Chesson 1983) by standardizing all fish lengths to July

-1 4 1 by assuming an average growth rate of 1mm•d (Groot and Margolis 1991). For example, if a 120

5 mm fish was captured July 5, its standardized length would be 116 mm. We assumed condition index

6 (+/-) was the same as it is not expected to vary that quickly. In fact, controlled starvation experiments

7 demonstrate that salmon can be food deprived for 3 weeks without seeing any effects on the sign of

8 length-weight residuals; only after 4 - 6 weeks of starvation is a decline noted (Hertz et al. submitted-

9 currently in revision).

11 Fish (trawl and predated) were classified into 4 categories based on fork length and condition. Fish

12 were either small or large relative to the pooled mean fork length of 122 mm and in good or poor

13 condition depending on a positive or negative weight residual respectively. The proportion of fish in

14 each category for each species and year was calculated for trawl-caught and predated fish separately.

15 Note that the trawl proportions were adjusted by the total catches (all fish were counted) given the

16 selective sampling of these species (with respect to the maximum number of individuals measured

17 aboard the research vessel). For each prey category, Electivity Index ɛ contrasts the diet proportions to

18 the relative abundance as indexed by the trawl survey:

20 where m = number of potential dietary types (in this case 12: 3 species x 2 size classes (small or large)

21 x 2 condition levels (good or poor)) and αi = Manly’s Selection Index for prey type i:

22 Size- and condition-dependent predation: a seabird disproportionately targets substandard individual juvenile salmon 2 23 where ri,rj = proportions of prey types i and j in the diet (i and j= 1, 2, 3,..., m); ni, nj = proportions of

24 prey types i and j available in coastal waters. Values of ɛ vary from -1 (strong avoidance) to +1 (strong

25 preference).

26 Although specific values of ɛ changed slightly from the original, trends were similar as there were no

27 substantial switches in selectivity/avoidance (Figure S1). In both years, the electivity index was higher

28 for small salmon of either good or poor condition, with the exception of large pink salmon in poor

29 condition in 2013. Within a size category, fish in poor condition consistently had the higher electivity

30 index. Small sockeye salmon in poor condition were strongly selected in both years relative to all other

31 prey classes and large positive sockeye salmon and chum salmon were very strongly avoided in both

32 years, while large positive pink salmon were strongly avoided in 2012 and moderately avoided in 2013.

33 In 2012, small positive sockeye salmon were moderately selected along with small negative pink

34 salmon. In 2013, large and small negative pink salmon were also selected. In contrast to the original

35 results (main text Figure 4), small-poor condition chum in 2012 went from weakly selected to weakly

36 avoided and in 2013 there was increased selection for small-poor condition pink.

38 References

39 Chesson, J. 1983. The estimation and analysis of preference and its relationship to foraging models.

40 Ecology 64:1297-1304.

41 Groot, C. and Margolis, L. (eds). 1991. Pacific salmon life histories. University of British Columbia

42 Press, Vancouver.

43 Hertz, E., M. Trudel, M.K. Cox, and A. Mazumder. Reconsidering the effects of starvation in food web

44 studies. Ecology and Evolution in revision.

45 Size- and condition-dependent predation: a seabird disproportionately targets substandard individual juvenile salmon 3 46 Figures

47 Figure S1: Electivity index (ɛ) for different salmon prey classes using standardized lengths where

48 SE=sockeye salmon, PK=pink salmon, CM=chum salmon, large >122mm, small<122, + (white bars) is

49 a positive weight residual and - (black bars) is a negative weight residual. Fish lengths were

-1 50 standardized to July 1 by assuming an average growth rate of 1mm•d (Groot and Margolis 1991). Size- and condition-dependent predation: a seabird disproportionately targets substandard individual juvenile salmon 4

SE large + 2012 SE large - SE small + SE small - PK large + PK large - PK small + PK small - CM large + CM large - CM small + CM small -

SE large + 2013 SE large - SE small + SE small - PK large + PK large - PK small + PK small - CM large + CM large - CM small + CM small -

-1.00 -0.50 0.00 0.50 1.00

53 Figure S1