Bluegills: Biology and Behavior

7: Development, Growth, and Mortality

doi: https://doi.org/10.47886/9781888569933.ch7

Development of the eggs and larvae of bluegills (Figure 62) has been described several times.⁹²¹ Identifying larvae of the genus Lepomis to species is difficult. Two reports offer advice based on observational comparisons,⁹²² and another describes an allozyme technique.⁹²³ Bluegills develop vertical bars that become visible on the lateral surfaces at ~20 mm TL and increasingly distinct by ~24 mm.⁹²⁴ The lateral bars of pumpkinseed are similar, becoming visible at ~21.5 mm TL, but by 26 mm spots have appeared between them. Bluegill eggs are adhesive and demersal,⁹²⁵ ranging in diameter from 1.16 to 1.37 mm.⁹²⁶ Egg diameter in fishes limits the size of the hatched larva according to in which TL = total length (mm) at hatching and D = egg diameter (mm).⁹²⁷

Bluegill eggs ordinarily hatch ~40 h postfertilization at 24.3–25.4°C,⁹²⁸ but higher and lower temperatures prolong it: 28.5°C (27–29 h) and 18.5°C (75–85 h).⁹²⁹ Eggs can hatch at temperatures of 18–36°C, and the range of thermal tolerance is eggs < fry < juveniles.⁹³⁰ Maximum normal hatch (45%) reportedly occurs at 28°C, although percentages at temperatures ranging from 22–32°C are not significantly lower.⁹³¹ The hatch was 24.5% at 34°C, 2.5% at 20°C and 36°C, and zero at temperatures not bracketed by this range. Bluegill eggs in Alabama hatch at ~27°C.⁹³² Annual variation in seasonal temperature, especially in the upper Midwest and Canada, can accelerate or delay hatching by 2 weeks or more.⁹³³

Newly hatched larvae are 3.75–4.05 mm TL with a yolk sac, 29–30 myotomes, and numerous melanophores distributed evenly over the body.⁹³⁴ Another description puts the average at 3.23 mm TL with a yolk sac of 1.13 0.9 mm and no pigmentation on the body.⁹³⁵ Newly hatched bluegills reflect a bright golden color, which fades as the yolk sac is gradually absorbed.⁹³⁶ Within 5 d the fry are swimming.⁹³⁷ Others have reported the yolk sac being absorbed by 10 d and 6.05–6.35 mm TL.⁹³⁸ Yolk sac larvae are sedentary, occupying interstices in the substratum.⁹³⁹ The mortality of larvae is greater over muddy and silty bottoms that lack a system of such cracks and crevices, and in unguarded nests the death rate correlates negatively with increasing proportions of coarse particles.⁹⁴⁰