Teleost vertebral centra are often similar in size and shape, but vertebral-associated elements, i.e. neural arches, haemal arches and ribs, show regional differences. Here we examine how the presence, absence and specific anatomical and histological characters of vertebral centra-associated elements can be used to define vertebral column regions in juvenile Chinook salmon (Oncorhynchus tshawytscha). To investigate if the presence of regions within the vertebral column is independent of temperature, animals raised at 8 and 12 °C were studied at 1400 and 1530 degreedays, in the freshwater phase of the life cycle. Anatomy and composition of the skeletal tissues of the vertebral column were analysed using Alizarin red S whole-mount staining and histological sections. Six regions, termed I-VI, are recognised in the vertebral column of specimens of both temperature groups. Postcranial vertebrae (region I) carry neural arches and parapophyses but lack ribs. Abdominal vertebrae (region II) carry neural arches and ribs that articulate with parapophyses. Elastic- and fibrohyaline cartilage and Sharpey's fibres connect the bone of the parapophyses to the bone of the ribs. In the transitional region (III) vertebrae carry neural arches and parapophyses change stepwise into haemal arches. Ribs decrease in size, anterior to posterior. Vestigial ribs remain attached to the haemal arches with Sharpey's fibres. Caudal vertebrae (region IV) carry neural and haemal arches and spines. Basidorsals and basiventrals are small and surrounded by cancellous bone. Preural vertebrae (region V) carry neural and haemal arches with modified neural and haemal spines to support the caudal fin. Ural vertebrae (region VI) carry hypurals and epurals that represent modified haemal and neural arches and spines, respectively. The postcranial and transitional vertebrae and their respective characters are usually recognised, but should be considered as regions within the vertebral column of teleosts because of their distinctive morphological characters. While the number of vertebrae within each region can vary, each of the six regions is recognised in specimens of both temperature groups. This refined identification of regionalisation in the vertebral column of Chinook salmon can help to address evolutionary developmental and functional questions, and to support applied research into this farmed species.

No abstract

Summary The prevalence and onset of radiological skeletal deformities in tagged Chinook salmon (Oncorhynchus tshawytscha) were investigated after 129 days in seawater at grading for removal of fish that failed to thrive (mean weight ± SEM, 432 ± 6.5 g) and again at harvest after 395 days (mean weight ± SEM, 3,721 ± 43 g). A scoring system identifying four categories of the radiographic deformity phenotype was established based on literature and evaluation of X‐rays of harvest‐size Chinook salmon. Deformity categories were: spinal curvature or Lordosis, Kyphosis, Scoliosis (LKS); Fusion; Compression and/or reduced inter‐vertebral (IV) space; Vertical shift. Of the 432 fish surviving to harvest and for which there were weights and diagnostic radiographs, 38.4% were affected by at least one deformity. Late onset LKS, detected at harvest, was the most prevalent deformity in 29.4% of all harvested fish. LKS was present alone with no other potentially confounding deformities in 10% of harvested fish. Cranial lordosis, a common LKS variant, in the postcranial region comprised half of LKS‐deformed fish. LKS commonly co‐existed with compression, a combination prevalent in harvest fish at 13.4%. Compression and/or reduced IV space detected in 22.0% of harvested fish was commonly associated with other deformity phenotypes (84/95 of fish with compression). Fusion and vertical shift were present in 7.6% and 4.6% of harvested fish, respectively. More than 77% of fish with any type of deformity developed the deformity within 9 months of harvest. Fusions that were visible in radiographs at grading persisted in fish that survived until harvest. In contrast, LKS, the most visible harvest deformity, was difficult to detect in radiographs from the earlier time point, suggesting that this economically important deformity develops at a relatively late stage of seawater growth. Deformed fish at harvest were smaller (mean ± SEM 3,479 ± 76 g) than normal (mean ± SEM 3,875 ± 51 g). Fish with no deformity grew at a significantly (p < .05) faster rate than fish that developed a deformity during this period or were already deformed at grading. The scoring system performed with a sensitivity of 92.4% and a specificity of 97.6% for Chinook salmon >500 g, and thus has a potential utility for other farmed salmonids.

A total of 777 fish from three growing regions of New Zealand Chinook salmon farms comprising of five sites were tested. Quantitative PCR was used to determine the distribution of New Zealand rickettsia‐like organism and Tenacibaculum maritimum. Genetic information from these bacteria were then compared with strains reported worldwide. Using this information, suggested associations of pathogens with clinically affected fish were made. NZ‐RLO was detected in two of the three regions, and T. maritimum was detected in all regions. Three strains of NZ‐RLO were identified during this study. Based on analysis of the ITS rRNA gene, NZ‐RLO1 appears to be part of an Australasian grouping sharing high similarity with the Tasmanian RLO, NZ‐RLO2 was shown to be the same as an Irish strain, and NZ‐RLO3 was shown be closely related to two strains from Chile. Based on multi‐locus sequence typing, the New Zealand T. maritimum was the same as Australian strains. NZ‐RLOs were detected more frequently in fish with skin ulcers than fish without skin ulcers. While additional research is required to investigate the pathogenicity of these organisms, this is the first time that NZ‐RLOs have been associated with the development of clinical infections in farmed Chinook salmon.

No abstract