First-feeding by European eel larvae: A step towards closing the life cycle in captivity

Abstract

Breeding European eel in captivity is a complex task. However, recent advances in assisted reproduction and culturing techniques now allow mass production of high-quality gametes, embryos, and first-feeding larvae. Here, three studies were conducted to determine whether dietary regime, chemoattractants, and light intensity have an impact on the incidence of larval first-feeding, gut fullness, and behavior. Firstly, larvae at 12, 16, and 20days post-hatch (DPH) were allowed to forage on one of five diets, rotifer paste (RP), RP+cod roe, RP+octopus juice, RP+live rotifers, or Sargasso Sea plankton for 2h and afterwards evaluated for presence or absence of ingested diet. Secondly, light effects on feeding at 15 and 16DPH were tested, using the following intensities: High light at 21.5±3.9μmolm−2s−1; intermediate at 6.8±1.4μmolm−2s−1; low at 0.6±0.2μmolm−2s−1; darkness; and a control where no feed was added in darkness. Lastly, behavioral observations were recorded on larvae at first-feeding. Results showed no evidence of first-feeding at 12DPH. At 16DPH, 23–50% of larvae had ingested feed when offered either RP, RP+live rotifers, or RP+cod roe. At 20DPH, the highest incidence of feeding was detected when larvae were fed RP and chemoattractants; 8–30% of larvae ingested feed. Light had a positive impact on feeding at 15 and 16DPH, such that incidence of first-feeding and gut fullness increased at the intermediate to high light intensities, where up to 50% of larvae ingested the RP diet and had 9% gut fullness. Swimming of larvae was characterized by bouts produced by undulations of the caudal region, followed by pauses where larvae remained motionless. Duration of swimming increased from 41 to 218s at 13–17DPH, respectively. Larvae exposed to RP-based diets swam more than the control (no feed). Larval attacks were observed in benthic and pelagic zones of the tank, however were more frequently observed in the benthos. Here, 8–10%, 60–73% and 69–86% of larvae were observed grazing on RP-based diets at 13, 15 and 17DPH, respectively. Typically, attacks were associated with S-bend posture and upon initiation of S-bends the larvae remained stationary before an explosive lunge. At 13–17DPH, larvae fed the RP-based diets exhibited a higher incidence of attacks than the control. Together, this work provides benchmark diets and conditions for future feeding and growth trials. Statement of relevanceFirst-feeding by European eel larvae in captivity.