Responses of vertebral numbers in rainbow trout to temperature changes during development

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Abstract
Eggs from one pair of an inbred hatchery strain of Salmo gairdneri were divided into 16 lots, each of which was subjected either to a sustained temperature of 4 or 12 °C, or to a temperature break (one-way transfer between these two temperatures in either direction), or to a temperature pulse (two-way transfer) at various developmental stages. Breaks in either direction produced overcompensation (vertebral counts beyond that produced by sustained rearing at the temperature to which the embryos were transferred) if applied early, or paradoxical reaction (in the unexpected direction) if applied late. Results from the temperature breaks and from the one successful pulse were satisfactorily fitted by one set of parameters computed for a previously described "atroposic" model. This is the first test for any species which combines results both of breaks in two directions and a pulse, all using the same two temperatures and offspring from a single cross; the model therefore gains credence. Contrary to a previous report, rainbow trout do not differ qualitatively in vertebral response from other teleosts; difficulties in fitting of previously published data on the species probably arose from genetic diversity in the experimental material. Evidence is also presented that fin-ray counts are unreliable in fish preserved at fork lengths of under 29 mm, which may account for failure to fit the atroposic model to our or to most other published responses of salmonid fin rays to temperature changes.