Trophic ecology of bullet tuna Auxis rochei larvae and ontogeny of feeding-related organs

Abstract

The bullet tuna Auxis rochei, Risso 1810, is a small tuna widely distributed in tropical and temperate Seas. The present study reports on the first attempt to jointly monitor diet and food selection in larvae of this species and to assess the influence of the ontogenetic development of feeding-related organs on the diet. A. rochei larvae from 2 to 7 mm long are diurnal feeders and highly active predators, with high values of feeding incidence, gut fullness and number of ingested prey items. The rate of change in prey item size in relation to larval size was higher than in other species. Only small, non-motile prey items are eaten at the onset of feeding. Rapid mouth development and the early appearance of teeth allow larvae from 3 to 5 mm long to ingest a wide range of prey. However, niche breadth decreases at 5 mm, when larvae avoid small prey items in favour of larger ones with a higher carbon content. Chesson's selectivity index indicated that small larvae (from 2 to 3 mm long) selected a variety of small prey items, mainly copepod nauplii. Larvae measuring 3 to 5 mm selectively ate cladocerans and appendicularians, and larvae with lengths ≥5 mm preferred appendicularians and fish larvae. Precocious body development (mouth, teeth, and onset of stomach and caudal fin development) and improved visual acuity (larger lens diameter and angular cone density) are factors that contribute to the feeding success of larvae of this species.

Cannibalism among size classes of larvae may be a substantial mortality component in tuna

“…The visual range required to find enough prey to sustain maximum potential growth rate (d maxgr instead of d, also in m) can then be expressed as: (2) where β zoo is the minimum clearance rate (m 3 s -1 ) required to sustain the maximum potential growth rate at a given temperature T. We assumed that all encounters lead to ingestion, and that there is no prey handling time, therefore β zoo can be estimated as: (3) where δ is the temperature-dependent ingestion required to sustain potential growth rates (in μg dry mass, see 'Bioenergetic model'). t f is the time that larvae spend for feeding during the day, which we assumed to be 14 h d -1 based on findings from surveys of the daily cycle showing tuna larvae to have gut fullness of > 50% over 14 h (Morote et al 2008). Because there is no data on the mesozooplankton prey size spectra or prey species composition from the field sampling, we assumed that at stations with sampled larvae, all zooplankton are potential prey (B zoo in μg DW m -3 ).…”

“…We have made an assessment of zooplanktivory based on field mesozooplankton biomasses sampled for the water column integrating from 0-70 m, thus providing a maximum mesozooplankton biomass and giving a conservative estimate of zooplanktivory. Tuna larvae show high prey selectivity and the taxonomic composition of the diet changes with ontogeny when feeding on plankton (Catalán et al 2007, Morote et al 2008, Llopiz et al 2010. These increase the likelihood of food limitation with a pure plankton diet, especially in larger larvae and in species occupying poorer waters (BFT and ALB).…”

“…Most larval assemblages for tuna have been described in terms of densities , Boehlert & Mundy 1993. The few existing data on larval size distributions provide support for the spatial coexistence of planktivorous preflexion and piscivorous postflexion larval stages of tuna species (Catalán et al 2007, Morote et al 2008, Llopiz et al 2010. Based on the literature cited above, this study assumed that scombrids represent a major proportion in the composition of the larval assemblage.…”

“…Interestingly, the highest densities of tuna larvae are often found in the least productive waters (Alemany et al 2006(Alemany et al , 2010. The actual depth of tuna larval occurrence is usually confined to the upper mixed layer above 20-25 m depth , Morote et al 2008, Llopiz et al 2010, Satoh 2010. We have made an assessment of zooplanktivory based on field mesozooplankton biomasses sampled for the water column integrating from 0-70 m, thus providing a maximum mesozooplankton biomass and giving a conservative estimate of zooplanktivory.…”

“…They generally spawn in oligotrophic areas where the abundance of plankton prey is often low. Their diet shifts from a more zooplankton-oriented to a piscivorous and even cannibalistic diet already during the early larval stages (Uotani et al 1990, Miyashita et al 2001, Catalán et al 2007, Morote et al 2008). To become a piscivore, the larvae need to have large mouth gapes (Ottera & Folkvord 1993, Folkvord et al 1994, Nishimura & Hoshino 1999, and to develop their digestive (Kaji et al 1996) and visual systems rapidly (Margulies 1997).…”