Early life history of the American conger eel ... - ResearchGate

482Fig. 5 Conger oceanicus. Profiles of the mean increment widththroughout the otolith’s increment countable zone. Specimens weregrouped by the time when the width of daily rings abruptlyincreased (see arrows)Fig. 4A, B Conger oceanicus. SEM micrographs showing theotolith microstructure of a late metamorphosing conger eelleptocephalus (total length: 88.0 mm, preanal length/total length:0.53); A whole view of the otolith with accessory growth centres(arrows); B marginal region of the otolith (detail of the regionindicated by a box in panel A) showing the disturbed arrangementof the peripheral rings (AGC accessory growth centres; ICZincrement countable zone; DZ diffuse zone; arrow in panel B clearmark)averaged 0.25±0.08% in the core and increased outwardsto a maximum level of about 0.39±0.06%, atages 70–120 days. Subsequently Sr content decreasedrapidly to about 0.28±0.03% at the outermost regions.Calcium (Ca) content was almost constant (26.2±1.8%)throughout the otolith, except for the core region, whichrecorded a low value (25.9±1.8%). Sr:Ca ratios changedin a similar manner to Sr content (Fig. 6) andaveraged 8.5·10 3 (1.0·10 3 SD) in the core and rapidlyincreased soon after hatching. The ratios reached amaximum level of about 15.4·10 3 (1.8·10 3 SD), some70–120 days after hatching, i.e. at the end of DLGZformation. Subsequently, the ratios sharply decreasedand maintained a stable minimum value of 11.9·10 3(0.9·10 3 SD) outwards to the otolith edge (Fig. 7). Thesudden increase in otolith increment width (WIZ), atages 70–120 days, coincided with the rapid drop inotolith Sr content and Sr:Ca ratios, and marked theonset of metamorphosis (see ‘‘Discussion’’) (Fig. 8).Age and hatching timeThe ages at recruitment of the leptocephali to the estuary,while still in an early developmental stage (PAL/TL‡0.63), and thus without a DZ in the otolith, rangedfrom 149 to 183 days (Table 1). Hatching dates, backcalculatedfrom these daily estimates, indicated that thespawning season lasted about 3 months, from lateOctober to mid-December.Age at metamorphosisThe duration of the developing leptocephalus stage (i.e.the number of increments in the DLGZ) ranged between70 and 120 days, with a mean value of 92±16 days. Theage at metamorphosis was negatively correlated with themean increment widths of the DLGZ (r 2 =0.62, n=20,P