A syndrome affecting cultured chinook salmon, Oncorhynchus tshawytscha (Walbaum), characterized by distended abdomens, gastric dilation, air sacculitis (GDAS), increased feed conversion rates and increased mortality has been recognized in New Zealand. Affected fish were most obvious in sea cages but were also present in fresh water. Mortality rates associated with this condition were highest in late summer and approached 6% per month. A dilated and flaccid stomach, without visible rugal folds containing copious oil, watery fluid or undigested feed was typical. Gastric mucosal ulceration or inflammation were not present. The air sacculitis consisted of a thickened, dilated bladder with a mixed mucosal inflammatory infiltrate and a luminal exudate associated with large numbers of morphologically diverse bacteria. Gastric dilation or air sacculitis occurred alone or together in the same fish. In a group of 20 subclinically affected fish with or without gastric dilation, there were no significant differences in weight, length, serum osmolality, sodium, total protein or packed cell volume. Twenty‐three severely affected fish had significantly (P < 0.05) higher serum osmolality but similar sodium and total protein to that of clinically normal fish.

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.

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.

Grossly, NGD can be mistaken for BGD; the distinction is important because chloramine-T, routinely used in the treatment of BGD, is ineffective for the treatment of NGD. Static formalin baths of 170 ppm (37% formaldehyde) for 1 hour, supplemented with oxygen, may reduce amoebic infections but should be used cautiously as treatment can precipitate mortality in fish with gill disease. Cues for the onset of NGD are not completely understood, although poor water quality and elevations in temperature are likely to exacerbate infections. Similarly, the prevalence of this disease is not accurately known, but it is possible that it may be more common among salmonids in New Zealand than is presently recognised.

Chinook salmon smolt in fresh water fed a commercial diet known to produce minimal gastric dilation and air sacculitis (GDAS) were randomly assigned to four experimental tanks with flow-through sea water. All four groups were acclimatized to sea water for 3 weeks and fed a diet of minced fresh seafood. After 3 weeks the groups were fed either; seafood as before, a different commercial pelleted diet associated with the development of GDAS on farms, or either diet supplemented with 500 mg L(-1) putrescine, 300 mg L(-1) cadaverine and 250 mg L(-1) tyramine. Gastric dilation was produced in fish fed the commercial diet for 1 month but not by feeding a diet of minced seafood. The addition of putrescine, cadaverine and tyramine to either diet had no significant effect on the development of gastric dilation. Fish fed the commercial diet had significantly (P < 0.0001) wider weight-adjusted stomach widths, less prominent longitudinal stomach folds (P < 0.0001) and lower (P < 0.0001) stomach-width ratios than fish fed the fresh seafood diet. There was no significant difference in serum osmolality or sodium concentration between fish from groups with or without gastric dilation or fed biogenic amines.