Factors driving spatial and temporal variation in production and production/biomass ratio of stream-resident brown trout (Salmo trutta) in Cantabrian streams

1. The objective was to identify the factors driving spatial and temporal variation in annual production (P-A) and turnover (production/biomass) ratio (P/B-A) of resident brown trout Salmo trutta in tributaries of the Rio Esva (Cantabrian Mountains, Asturias, northwestern Spain). We examined annual production (total production of all age-classes over a year) (P-A) and turnover (P/B-A) ratios, in relation to year-class production (production over the entire life time of a year-class) (P-T) and turnover (P/B-T) ratio, over 14 years at a total of 12 sites along the length of four contrasting tributaries. In addition, we explored whether the importance of recruitment and site depth for spatial and temporal variations in year-class production (P-T), elucidated in previous studies, extends to annual production. 2. Large spatial (among sites) and temporal (among years) variation in annual production (range 1.9-40.3 g m(-2) per year) and P/B-A ratio (range 0.76-2.4 per year) typified these populations, values reported here including all the variation reported globally for salmonids streams inhabited by one or several species. 3. Despite substantial differences among streams and sites in all production attributes, when all data were pooled, annual (P-A) and year-class production (P-T) and annual (P/B-A) and year-class P/B-T ratios were tightly linked. Annual (P-A) and year-class production (P-T) were similar but not identical, i.e. P-T = 0.94 P-A, whereas the P/B-T ratios were 4 + P/B-A ratios. 4. Recruitment (Rc) and mean annual density (N-A) were major density-dependent drivers of production and their relationships were described by simple mathematical models. While year-class production (P-T) was determined (R-2 = 70.1%) by recruitment (Rc), annual production (P-A) was determined (R-2 = 60.3%) by mean annual density (N-A). In turn, variation in recruitment explained R-2 = 55.2% of variation in year-class P/B-T ratios, the latter attaining an asymptote at P/B-T = 6 at progressively higher levels of recruitment. Similarly, variations in mean annual density (N-A) explained R-2 = 52.1% of variation in annual P/B-A, the latter reaching an asymptote at P/B-A = 2.1. This explained why P/B-T is equal to P/B-A plus the number of year-classes at high but not at low densities. 5. Site depth was a major determinant of spatial (among sites) variation in production attributes. All these attributes described two-phase trajectories with site depth, reaching a maximum at sites of intermediate depth and declining at shallower and deeper sites. As a consequence, at sites where recruitment and mean annual density reached minimum or maximum values, annual (P-A) and year-class production (P-T) and annual (P/B-A) and year-class P/B-T ratios also reached minimum and maximum values.