Short-Term Growth Bioindicators : Do Laboratory Results Coincide with Field Measurements?

In North Carolina and elsewhere there is concern that excessive nutrient loading and resulting hypoxic conditions in coastal ecosystems is adversely affecting native fauna, but quantifying the effects on fish can be difficult.  Previous research indicates that indirect effects (e.g., density-dependent reductions in growth and survival as fish crowd into oxygenated refuges) may actually be more detrimental than direct physiological effects of hypoxia because most fish can detect and avoid dissolved-oxygen levels that cause mortality or reduce growth.  Given the fine spatial and temporal scale of oxygen dynamics in the Neuse River Estuary (NRE), evaluating the impacts of hypoxia on fish growth requires novel, short-term growth indicators that integrate the effects of rapidly changing environmental conditions.  Specifically, we are interested in assessing changes in short-term (e.g., one to several days) growth of individual fish, which can be a used as a reliable  indicator of  the effects of changing environmental conditions such as episodic hypoxia.  We experimentally determined the sensitivity and response time of three bioindicators of recent growth, RNA:DNA ratios in muscle tissue, liver expression of insulin-like growth factor-I (IGF-I), and Hepatosomatic Index (HSI), to changes in fish growth in a laboratory setting.  Spot (Leiostomus xanthurus), a common estuarine fish, were collected from the NRE, acclimated to laboratory conditions, and divided into two temperature treatments (temperatures bracketed the low, 24˚C, and high, 28˚C, range spot experience over the summer).  To provide a thorough test of each bioindicator’s ability to measure changes in short-term growth, we manipulated growth via five food ration treatments ranging from ad libitum to starvation.  Spot were sampled after one, two, and three weeks of treatment; weight, length, a tissue sample and liver were taken at the beginning of the experiment and at time of sacrifice.  Spot were also collected from the NRE weekly throughout the summer along with water quality monitoring and measurements of food availability.    In the laboratory, HSI was significantly responsive (p<0.001) to changes in food ration, but appeared to be a less reliable indicator of changes in fish conditions in the field.  However, preliminary results indicate that RNA:DNA ratios and IGF-I expression (measured by qPCR) could be more sensitive to fish growth and condition in the field.  If ground-truthed by laboratory experiments, using these bioindicators alone or together could prove useful in quantifying how fish are affected by fluctuating oxygen conditions and resultant changes in movement, food availability, and density-dependent processes.