Planktonic larval duration, early growth, and the influence of dietary input on the otolith microstructure of Scolopsis bilineatus (Nemipteridae)

Abstract

The pelagic larval phase represents a critical period in the early life history of fishes, since larval growth and development can contribute substantially to patterns of survival, dispersal and connectivity. The microstructure of otoliths was examined to investigate events during the early life history of the bridled monocle bream Scolopsis bilineatus (Nemipteridae) on the Great Barrier Reef. We validated a distinct settlement mark, characterized by a rapid transition from wide to narrow increments. The time fish spent in the plankton (Pelagic Larval Duration; PLD) ranged from 17 to 27 days (mean = 21.3 days), which is similar to many other fish families. Three distinct early life history stages occurred: pre-settlers (larvae), newly settled fish, and settled juveniles. Increment width (a proxy for growth) was sequentially narrower with each life history stage; pre-settlers>newly settled>settled juveniles, and growth was reduced as fish approached and underwent settlement to reef habitats. Evidence is provided for a “search phase”, whereby growth was reduced in the three to four days preceding settlement; this pattern may be indicative of changes in the behaviour of larvae immediately prior to settling. Two manipulative experiments investigated the effects of diet ration on otolith increment widths. Reduced food intake resulted in significantly narrower increment widths, with a possible lag effect of at least six days. Experiment results indicated that increment widths can be a reliable proxy of somatic growth, and results from this study overall highlight the utility of otoliths as a tool for investigating the early life history of fishes.