Characterization of Compensatory Growth in Hybrid Striped Bass (Morone chrysops X Morone saxatilis): Hormones and Mechanisms

Abstract

Compensatory growth (CG) is a period of growth acceleration that exceeds normal rates after animals are alleviated of certain growth-stunting conditions. While the endocrine control of growth in fishes and other vertebrates is regulated primarily through the growth hormone (GH) ⁄ insulin-like growth factor (IGF) axis, the hormonal dynamics regulating accelerated (compensatory) phases of growth are poorly understood. We assessed whether CG responses could be induced in hybrid striped bass (HSB; Morone chrysops X Morone saxatilis) through feeding and/or temperature manipulation, and then examined whether components of the GH/IGF endocrine growth axis could be contributing to the response.

We found that CG responses can be elicited in HSB following periods of 1) partial feed restriction, 2) complete feed restriction and 3) a combination of low water temperatures and complete feed restriction, with full catch-up growth being achieved in the latter. The most dramatic periods of CG were preceded by a catabolic state marked by body weight loss, hepatic steatosis, depressed hepatosomatic index values and an endocrine state of GH resistance. Upon alleviation of the growth-stunting condition, HSB displayed significant elevations in growth rate that exceeded controls by as much as 4 times. This CG response was accompanied by increases in feed intake (hyperphagia), improved feed conversion and a reversal of steatotsis and GH resistance.

The onset of CG in feed manipulation studies was marked by significant elevations in total hepatic IGF-I mRNA and plasma IGF-I, indicating that elevated synthesis and secretion of the growth factor may mediate rapid growth responses. Strong correlations between systemic IGF-I and SGR suggest that circulating levels of IGF-I may serve as a biomarker of growth in HSB and perhaps other fishes. The 40kDa IGF binding protein (IGFBP) may also contribute to CG, since plasma levels increased in concordance with IGF-I and growth. Although little is known about the mitogenic effects of IGF-II in adult vertebrates, changes in hepatic IGF-II mRNA paralleled changes in body weight prior to and during CG, strengthening the hypothesis that it is an important regulator of variable growth rates in fishes. Associated with declines in growth and IGF-I, we found that feed deprivation caused reductions in GH receptor (GHR) gene expression which were subsequently restored during realimentation. Hence, the GHR may be a critical mediator of the changes in IGF-I and growth rates observed prior to and during CG.

Aside from endocrine (circulating) contributions, insulin-like growth factors may also act locally to regulate tissue growth. A ten-fold increase in skeletal muscle IGF-I mRNA from previously depressed levels was seen during realimentation, suggesting the growth factor may act in a paracrine⁄autocrine fashion to stimulate cell proliferation and facilitate rapid growth characteristic of CG.

Taken together, these data show that HSB undergo considerable phases of accelerated growth when preceded by sufficient catabolism, and that an up-regulation of endocrine (IGF-I, -II, 40kDa IGFBP) and paracrine⁄autocrine (IGF-I) components of the GH⁄IGF axis likely facilitate the response. Compensatory growth protocols in HSB provide a good model system to elucidate the underlying metabolic and endocrine mechanisms of poor, normal and accelerated (compensatory) growth in fishes. Our results with cold-banking and feed deprivation show that CG protocols can improve overall feed conversion by as much as 30% with no loss in biomass, providing a practical method for reducing production costs of HSB, an important aquacultured species.