A nonlethal, rapid method for assessing the somatic energy content of migrating adult Pacific salmon

Traditional methods for determining the energy content of fish involve either chemical assays of lipid and protein levels (proximate analyses) or tissue combustion (bomb calorimetry). In studies of migrating Pacific salmon Oncorhynchus spp., entire fish need to be homogenized prior to analysis, as energy reserves are stored along a head-to-tail gradient and change along this gradient depending on the stage of upriver migration. The logistics and costs associated with transporting carcasses to the laboratory can be prohibitive. Also, many populations of Pacific salmon are at risk of extinction, so lethal sampling is often not an option. Few reliable and practical methods exist that enable rapid and nonlethal energy determinations of large fish in the field. We evaluated a handheld microwave energy meter as a means of estimating whole-body energy concentrations. In 2002 and 2003, we collected sockeye salmon O. nerka from several stocks during their coastal and upriver migration through the Fraser River watershed (British Columbia). For each stock, we sampled fish from various locales ranging from ocean to spawning areas. Fish somatic tissues were interrogated at four body positions; however, the two most anterior positions produced the most accurate energy information. It took less than 30 s per fish to collect these data. We found strong regression relationships between somatic lipid percentage (R² = 0.93; P < 0.001) and gross somatic energy density (R² = 0.94; P < 0.001) measured by whole-carcass proximate analyses and log_e transformed energy meter readings. The slopes and intercepts of these relationships did not differ among stocks or years.