Standardising fish stomach content analysis: The importance of prey condition

Comparisons of fish trophic data are limited by the range of methods used to quantify dietary composition, with scientists yet to agree on a standard approach to stomach content analysis. This study examined how prey type and condition of stomach contents influenced identification of prey and the ability to estimate dietary importance by methodologies based on volume, weight, number and frequency of occurrence. A total of 154 stomachs were examined from six trophically diverse, temperate fish species. The condition of prey i.e. entirety, digestion state, and presence of mucus were recorded for each stomach, and the taxonomic level to which prey could be identified to assessed. The influence of prey condition on the application of each metric was then assessed. Descriptions based on prey volume or weight were significantly affected by differences in prey condition. In contrast, the simple presence/absence or frequency of occurrence approach (%F) provided a rapid, unambiguous and reliable account of diet composition and was not affected by the condition of prey. It was the only approach able to quantify the full spectrum of prey types in a consistent manner, making it the most practical metric. Variable prey condition also highlighted uncertainties in prey identification. We recommend routine reporting of how prey condition influences identification, the specific approaches used, and any assumptions made in identifying prey. In addition, %F data should be reported as a nested hierarchy of taxonomic levels which allows these data to be readily standardised across studies and used in meta-analyses.