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Aquaculture CRSP 21st Annual Technical Report
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69
IGF as a Growth Rate Indicator in Oreochromis niloticus

Tenth Work Plan, Reproduction Control Research 3 (10RCR3)
Final Report

Christopher L. Brown
Marine Biology
Florida International University
North Miami, Florida, USA

Remedios B. Bolivar and Emmanuel M. VeraCruz
Freshwater Aquaculture Center
Central Luzon State University
Science City of Mu–oz, Nueva Ecija, Philippines

Abstract

Insulin-like growth factor-I (IGF-I) is considered to be the primary proximal cue for the peripheral signalling of growth reponses in tilapia as in other vertebrates. It is therefore possible that the detection of IGF-1 may be a precise and practical means of determining the growth status of a fish. Our research thus far lays the groundwork for establishment of a method of detecting IGF-1 in the Nile tilapia as a means of assessing growth status, that offers the potential of more precision than other indicators (growth hormone levels, RNA/DNA ratios, etc) and which is potentially far more efficient and cost-effective than measuring fish following an extended period of growout.

We undertook a series of studies to clone the cDNA encoding a portion of the IGF-I gene for Nile tilapia (Oreochromis niloticus) and to examine whether liver IGF-I gene (mRNA) expression differs in fish grown at normal and slow rates. We also examined tissues from tilapia on intensive growth and maintenance diets. This work is the first part of a doctoral program being carried out by Mr. Emmanual Vera Cruz from Central Luzon State University, who is approaching the end of his first year of study. Plans are to complete this work under the coming work plan or work plans.
Introduction

The proposed program of doctoral research consisted of a focused collaborative effort to clone the Insulin-like Growth Factor (IGF-I) gene for Nile tilapia (Oreochromis niloticus), to use this gene to generate one or more probes that can be used in the detection of the gene product, and to conduct a series of preliminary examinations in order to determine the efficacy of this approach as a means of determining instantaneous growth rate in the tilapia.

Studies carried out under the support of the Aquaculture CRSP, including our own studies under Work Plan 10 (10PDVR2), involve the growout of tens of thousands of fish under controlled (on-station) or semi-controlled (farm) conditions. Typically, this approach is used to optimize production parameters such as feeds, fertilization, and other controllable variables. To date, we have carried out a series of on-station and on-farm trials that have led us to conclusions about the cost-effectiveness of feeding strategies (for
example, see Bolivar et al., 2001, 2003; Brown et al., 2000). Surprisingly, we have found in the course of these studies that reducing rations by as much as 50% do not impact or barely impact growth and yield. The cost of such studies is considerable. A total of nine farms each grew fish over a period of more than four months, and throughout these studies, tens of thousands of fish must be weighed in order to ascertain the effect of a particular feeding strategy or method on growth. These are exceedingly simple experiments, in which hundreds of man-hours are required to carry out a statistically valid comparison of a single pair of feeding techniques, and we have carried out four such studies over the past two biennia, or an average of one growout study per year.

It is theoretically possible to ascertain growth rate without going through a large-scale, long duration growout process. The proposed approach is to develop a molecular method for detection and quantification of IGF-I, which is activated by the presence of growth hormone and which in