1. Growth of the body length in vertebrates is well known to be unidirectional, with organisms progressively increasing in body size as they become older. However, there is evidence that body length shrinkage is a survival strategy for some vertebrates under unfavourable environmental conditions. Here we report both experimental and field evidence that the body length of young stream-dwelling salmonids can decrease in winter.
2. In examining how juvenile salmonid fish responded to harsh environmental conditions, we were faced with unexpected and previously undocumented observations in terms of growth performance, indicating that fish do shrink in harsh winter conditions. Young salmonids showed significant shrinking of individual body length, up to 10% of the body length, over the course of winter. The dynamics of the growth in length of these fish can be explained by a combination of anorectic stress and environmental conditions. Under stable, sheltered underwater conditions fish were best able to maintain positive growth in length.
3. We propose that growth in body length of a vertebrate animal can be temporally negative, individuals suffering from nutritional deficits shrinking in their length in addition to losing their body mass. There is circumstantial evidence that subsequent compensatory growth can have unexpected and dramatic longer-term costs. Experimental approaches, both field- and laboratory based, are sorely needed to reveal how common a phenomenon negative structural growth is among animals, and what the consequences are for individual performance, and, furthermore, for population dynamics.