Environmental Factors Affect Daily Increment Deposition and Otolith Growth in Young Colorado Squawfish

Abstract

Otolith microstructure of endangered Colorado squawfish Ptychocheilus lucius was investigated to determine patterns of otolith growth and to validate daily deposition of increments. Sagittae and lapilli formed prior to hatching. After fish hatched, otolith increments were deposited daily whether larvae were reared at a constant 22°C temperature or subjected to fluctuating temperatures (±2.5°/d) centered at 18, 22, or 26°C. Otolith increments were clearer and counts of increments were more accurate for fish reared at fluctuating than at constant temperatures. Otolith growth was lower at 18°C than at 22 or 26°C, but evidence of a direct effect of temperature on otolith growth was inconclusive. Lapillus diameters of slow-growing Colorado squawfish were larger than those of similar-sized but fast-growing fish, indicating that fish and otolith growth rates were not proportional. When larvae were starved, growth in body length generally ceased immediately but otolith growth continued for up to 15 d. Otolith growth was reduced for up to 5 d after starved fish began to feed. Timing of starvation and reduced growth may not be accurately recorded by reduced otolith increment spacing. Low-contrast otolith increments in wild fish may indicate periods of low food abundance and starvation. Increased otolith growth early in life could reflect the start of exogenous feeding by Colorado squawfish larvae, a habitat shift to warmer water, or both. Otolith analysis will be useful for elucidating age, growth, and recruitment patterns of young Colorado squawfish.