Bluegills: Biology and Behavior

7: Development, Growth, and Mortality


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