ALBERT

Description: Knowledge on life history traits of mesopelagic fishes is still sparse. This dataset contains reproductive data of the myctophid Electrona risso (Cocco, 1829) and the melamphaids Melamphaes polylepis Ebeling, 1962 and Scopelogadus mizolepis (Günther, 1878). The specimens were collected in March and April 2015 during cruise 383 of the fishery research vessel "FFS Walther Herwig III" in the Eastern Central Atlantic. A pelagic midwater trawl ("Aalnet", Engel Netze, Bremerhaven Germany) with multiple opening-closing device with three net bags of 20 mm mesh size and 1.8 mm mesh size in the cod-end container was used for catching fish at three depth strata per station between 50 and 700 m depth. Fish was stored in phosphate buffered 4% formalin solution and later on land in laboratories, specimens were measured, weighed, and dissected to examine gonads and oocytes under a stereo microscope. Reproductive phases were assigned based on macroscopic inspection following the standardised terminology for reproductive development in fish by Brown-Peterson et al. (2011, doi:10.1080/19425120.2011.555724). Histological cross sections of selected individuals were used for validation. The Gonadosomatic Index was calculated as GSI = 100 x gonad mass / (total mass – gonad mass). Batch fecundities were determined with the gravimetric method, counting the most advanced eggs in weighed subsamples.

Description: This study was the first to investigate the key reproductive traits of the electric lantern fish Electrona risso (Myctophidae, n = 918) and the bigscale fishes (Melamphaidae) Melamphaes polylepis (n = 260) and Scopelogadus mizolepis (n = 649). Specimens of these mesopelagic species were collected in March and April 2015 in the eastern Central Atlantic (0–24° N, 20–26° W). Sex ratio was not significantly different from 1:1 in E. risso and M. polylepis but significantly skewed toward female dominance in S. mizolepis. Reproductive phases were determined macroscopically and by histological analyses on selected individuals. Female length at 50% maturity (L50) was 55.1 mm standard length (LS) in E. risso, with an observed female maximum length (Lmax) of 81.2 mm LS. M. polylepis females had an L50 of 40.2 mm LS and an Lmax of 86.7 mm LS. S. mizolepis had an L50 of 46 mm LS and an Lmax of 97.9 mm LS. The three species show histological features of iteroparity, but the E. risso population appears to occur in two year-classes and experience only one spawning season per lifetime in the study region. All three species are batch-spawners. A batch fecundity of 2668 eggs was estimated from one E. risso individual, with a relative batch fecundity of 369 eggs g−1 gonad-free body mass. M. polylepis had a batch fecundity of 1027 eggs and a relative batch fecundity of 149 eggs g−1 (n = 3). S. polylepis had a batch fecundity of 1545 eggs and a relative batch fecundity of 215 eggs g−1 (n = 21). The median gonado-somatic index during the actively spawning phase of E. risso was 4.5, significantly lower than that of M. polylepis (7.5) and S. mizolepis (7.1). No regressing or regenerating phases were observed in this study. Batch-spawning in all three species is suggested to be advantageous to cope with intra-annual variability in food supply and other risks for offspring survival. With what appears to be in effect a (facultative) semelparous strategy in combination with a short life span in E. risso, interannual differences would have a great effect on population dynamics of this species. Knowledge is still lacking on temporal aspects of reproduction such as the duration of the spawning season and the frequency of spawning, as well as age and growth.