Data in each file were collected in the field in 2013. The files include the main data used for this paper (Fish_data.csv and Scan_data.csv) and auxiliary data analysed and discussed in the paper. Column headings (including measurement units) are defined below. Please contact Stefan Oli Steingrimsson (stefan@holar.is) with any questions. Fish_data.csv Round - Experimental round Enclosure - Enclosure number one through eight (1-4 in Round 1, 5-8 in Round 2) Treatment - Low- or high-density treatment Tagged - Denotes whether a fish was tagged ("Tag) or untagged ("UT") Code - Assigned to each fish. Tagged fish are individually designated with the two letters indicating the colors of their tag (Green, Yellow, Orange, Red) and the enclosure number (1-4) followed by the round number (1-2). Untagged fish are designated as "UT" followed by the enclosure and round number. Growth - Standardized mass-specific growth rate, expressed as the daily change in mass as a proportion of body size Activity - Proportion of scans during which the particular fish was observed active Mean time of activity - Mean time of activity, in radians Dispersion of activity - Standard deviation of the mean time of activity, in radians Scan_data.csv Round - Experimental round Enclosure - Enclosure number one through eight (1-4 in Round 1, 5-8 in Round 2) Treatment - Low- or high-density treatment Julian date - Day of scan, counted from beginning of experiment Time - Time of scan Water temp. - Water temperature in degrees Celsius Light intensity - Light intensity in kilolux Water level - Depth of water at a fixed point, in centimeters Activity - Proportion of fish observed active during a particular scan Water_depth_data.csv Enclosure - Enclosure number one through four Xcoor - Centimeters from front (upstream side) of enclosure Ycoor - Centimeters from side of enclosure Depth 1-Jul, etc. - Water depth, in centimeters, recorded on a particular date Water_current_data.csv Enclosure - Enclosure number one through four Xcoor - Centimeters from front of enclosure Y coor - Centimeters from side of enclosure Current 1-Jul, etc. - Water current velocity, in meters per second, recorded on a particular date Substrate_data.csv Enclosure - Enclosure number one through four Square - Designates 1 m2 within enclosure Quarter - Designates 25 cm2 within square Substrate R_1 - Substrate classification in Round 1, according to modified Wentworth scale (see Table 1 of article) Substrate R_2 - Substrate classification in Round 2, according to modified Wentworth scale (see Table 1 of article) Drift_data.csv Round - Experimental round Enclosure - Enclosure number one through eight (1-4 in Round 1, 5-8 in Round 2) Treatment - Low- or high-density treatment Date - Date sample taken Time - Time sample taken Drift density - Density of invertebrate drift expressed as items per cubic meter