Microstructure of otoliths in larval and juvenile Shishamo smelt Spirinchus lanceolatus, a cold-water fish species

Abstract

We examined the daily periodicity of otolith increment formation in reared Shishamo smelt Spirinchus lanceolatus larvae, and investigated the growth history of wild-caught larvae. The mean body length of newly hatched larvae was 8.4 mm, and the mean otolith radius was 12.7 µm (sagitta) and 12.4 µm (lapillus), respectively. Both otoliths displayed a check mark at hatching (1st check), and an additional check mark was observed later (2nd check). The relationship between the number of increments formed outside the 1st check on both otoliths (I_s, I_Ɩ) and age (A_c, days) was described by regression lines (I_s = 0.86A_c (n = 38, r² = 0.992, p < 0.01), I_Ɩ = 0.49A_c (n = 24, r² = 0.932, p < 0.01)), where the slopes significantly differed from 1 (t-test, both p < 0.01). We also examined the relationship between the number of increments outside the 2nd check (Iʹ_s, Iʹ_Ɩ) and age (A_c, days) for both otoliths. As a result, the regression lines (Iʹ_s = 0.99A_c–19.5 (n = 26, r² = 0.996, p < 0.01), Iʹ_Ɩ = 0.61A_c–16.3 (n = 15, r² = 0.928, p < 0.01)) showed that the slope for the sagitta was not significantly different from 1 (t-test, p = 0.38), while the lapillus was significantly different from 1 (t-test, p < 0.01). These results suggest that daily countable increments only form after the 2nd check mark in the sagitta. One possible reason why daily periodicity was not confirmed between the 1st and 2nd checks on both otoliths is that the increment width during this period was so narrow that it could not be distinguished under an optical microscope. This phenomenon has also been reported in cold-water fish species such as Atlantic herring and European sardine. Therefore, for cold-water fish species, which have increments after hatching that are generally difficult to distinguish, it is reasonable in age estimation to count increments only after they are clearly visible, and to compensate for earlier periods using check mark positions and growth indices. Based on the results of the rearing experiments, we examined the number and width of otolith increments in Shishamo smelt larvae and juveniles collected by sledge net along the Mukawa and Sarugawa coasts during June–July in 2023 and 2024. The ranges of hatching dates in both 2023 and 2024, calculated from age and sampling date, were respectively March 29–April 27 and March 23–April 27. A significant allometric relationship (r² = 0.959, p < 0.001) was found between otolith radius (OR, µm) and body length (BL, mm). The mean back-calculated body lengths of wild-caught individuals in 2023 and 2024 estimated using the Biological Intercept Method were respectively 11.4 mm and 11.3 mm at 20 days old, 17.0 mm and 17.6 mm at 40 days old, 21.8 mm and 23.3 mm at 60 days old, and 27.6 mm and 31.2 mm at 80 days old.