Quantitative methods to reconstruct length and weight of Great Lakes prey fishes from dietary observations of partially digested prey

Dietary studies of fish inform many aspects of fisheries science and ecosystem management, providing critical information to signify fluctuations in ecosystem functions. Bony structures are persistent, remaining in the stomach longer than soft tissue. We evaluated the utility of bony structures (cleithra, standard length, full vertebral column, and partial vertebral column) for reconstituting length and weight of thirteen prey fish species commonly found in the diets of piscivores in the Great Lakes. Standard length and full vertebral measurements were the most accurate for estimation of total length. Cleithra provided the third most accurate length estimates with exceptions for deepwater and slimy sculpin where extraction was more difficult. Partial vertebral measurements resulted in less variable results for species with low total vertebrae counts. Sculpin, a species with lower numbers of total vertebrae, produced more reliable estimates from partial vertebral measures than rainbow smelt or bloater which have more vertebrae. The accuracy of partial vertebrae measurements is improved when larger numbers or a greater proportion of the vertebral column is measured. Access to more broadly available digital X-ray technologies allowed for more rapid processing by eliminating the need for tissue removal to measure vertebrae and facilitated estimation of numeric relationships for prey length reconstitution. Using the data and mathematical relationships provided herein, investigators can easily reconstitute many of the diet items observed in Laurentian Great Lakes piscivores and can tailor methodologies and structures utilized to minimize estimation error and maximize efficiencies in evaluating stomach contents.