SJR Dolphin SCA: Degradation Scores, OL Length and Identifications of Otoliths Collected from Bottlenose Dolphin Stomachs

Otoliths were collected from stomach contents of stranded bottlenose dolphins (Tursiops erebennus) in the St. Johns River in Jacksonville, Florida. Otoliths were analyzed by a panel of 3 reviewers to determine the level of otolith degradation that occurred during digestive processes. Otoliths with scores ≤ 3 were measured. Otolith length measurements were used to estimate the size of most species by applying standard regression equations developed from fish species collected from a nearby water system, the Indian River Lagoon, and for one species, equations developed from violet gobies collected from the St. Johns River. These equations enabled estimation of the mass of each prey species in each dolphin’s stomach, then the calculation of their relative proportions of reconstructed mass across all stomachs. For otolith identification purposes, a panel of 4 reviewers assigned each otolith with a family-level and species-level identification. Each identification was given a confidence code ranging from 1 (no confidence) to 4 (certainty). When the average code for all reviewers was < 3, the otolith was considered unidentified. If two of the reviewers agreed with the “weight” reviewer and all gave scores ≥ 3, the score of the outlying reviewer was discarded. Identification was assigned when the average confidence code was ≥ 3.
The minimum number of species per dolphin stomach was determined by counting the left and right otoliths for each species separately, using the higher count as the minimum prey number. Unidentified species were counted, and half of their sum was considered the minimum prey number. The frequency of occurrence (%FO, or proportion of stomachs in which a species was detected) and numerical proportion (%N, or proportion of a given species pooled across all stomach samples) of each prey species were then calculated.