Early weaning of southern flounder, Paralichthys lethostigma, larvae and ontogeny of selected digestive enzymes

There is considerable interest in rearing Southern flounder, Paralichthys lethostigma, for commercial production and for stock enhancement. Both goals depend upon excellent larval nutrition for the production of robust juveniles. The current use of live prey for larviculture is an expensive and time consuming process that can be alleviated by weaning larvae onto dry feed. A study was conducted to assess the potential for early weaning of southern flounder larvae onto a microdiet (MD). In addition, the activity of selected digestive enzymes was measured during ontogeny to evaluate the digestive capabilities of the larvae over time. Pancreatic enzyme activities (U larva(-1)) were very low or undetectable at hatching and a marked increase in activity was not observed until the larvae reached 4 mm (similar to 11 dph) in standard length for chymotrypsin (24-44,000) and 6 mm (similar to 25 dph) for amylase (< 1-24), trypsin (1-18) and bile salt-dependent lipase (0-443). Acid protease activity (similar to 10) was detected once the larvae were 8.5-9.0 min (37-39 dph) in length although a sizeable increase in activity (> 10.0) was not observed until after complete metamorphosis (> 110 mm: 40-45 dph). Feeding regimes employed for the weaning study consisted of a live feed control (C) and a combination of live feed and MD in which the addition of the MD was initiated on 11 dph and live feed terminated on 17 (T17), 23 (T23) or 29 (T29) dph. At the end of the study (35 dph), mean standard length and the percent of settled fish were significantly greater for fish in the control treatment (8.3 mm. 21.1%) than for fish fed any combination of live prey and MD (6.4 mm: 2.0%). Average survival was 27.7% and no significant differences were noted among treatments However. the number of fish exhibiting spinal deformities, lordosis. was significantly lower in the control and T29 treatments(1.7%) than the T17 and T23 treatments (25%) The results of this study indicate that southern flounder larvae readily wean onto dry feed prior to the onset of metamorphosis. However, decreased growth and a high incidence of lordosis emphasize the need for the development of a more appropriate MD for this species when digestive enzyme activities are relatively low and gastric digestion is absent. (C) 2009 Elsevier B V. All rights reserved.