Are Life History Traits of Mesopelagic Fish Stable? An Examination of Reproductive Traits of a Lanternfish in the Central Pacific Ocean

Mesopelagic fishes likely dominate the total fish biomass of the world’s oceans and constitute important prey items for commercially valuable species such as tuna and swordfish. The lanternfish Diaphus schmidti is among the most numerous mesopelagic fishes from Hawaiian waters. Despite this, little is known about the stability of life history traits for this species through time. From specimens obtained from midwater trawls during 2005-2008 in Hawaiian waters, we undertook a histology-based study of the reproductive traits of D. schmidti in order to describe aspects of gonad anatomy and development, compare the effectiveness of morphological vs. histological gonad classification schema, identify the mode of sexuality, determine the apparent and functional sex ratio and size at reproductive maturity, describe aspects of oocyte growth and development, and quantify the relationship between fecundity and female size. To examine the stability of these reproductive traits, we also compared our results to those published from the late 1970s for D. schmidti populations in the Central Pacific Ocean. We found no evidence of hermaphroditism, suggesting that the species is gonochorous, similar to prior work. Our results also showed differences in length-at-maturity between sexes, with males maturing at larger sizes, and size-specific sex ratios heavily skewed toward females at smaller lengths and males at the largest sizes. These findings diverge from prior work three decades earlier that suggested no differences in maturation size between sexes and balanced sex ratios for D. schmidti. These differences suggest instabilities in reproductive traits for a mesopelagic fish over several decades which could reflect changes in ecological interactions or environmental conditions.