Distinctions in the diets and distributions of larval tunas and the important role of appendicularians

Llopiz, Joel K., David E. Richardson, Akihiro Shiroza, Sharon L. Smith, and Robert K. Cowen

Limnol. Oceanogr., 55(3), 2010, 983-996 | DOI: 10.4319/lo.2010.55.3.0983

ABSTRACT: Monthly plankton sampling across the Straits of Florida (SOF) allowed for a thorough investigation of the feeding ecologies of four taxa of larval tunas (family Scombridae, tribe Thunnini) and the horizontal and vertical distributions of tuna larvae and their dominant prey. Before piscivory, Thunnus spp. larvae had a mixed diet of crustaceans and appendicularians, whereas skipjack tuna (Katsuwonus pelamis), little tunny (Euthynnus alletteratus), and Auxis spp. displayed highly selective and nearly exclusive feeding on appendicularians. The availability of both appendicularians and larval fish prey declined from west to east across the SOF, and appendicularians were notably patchy. In the western SOF where prey was more abundant, all taxa of tuna larvae co-occurred, indicating the sharing of resources by the larvae, in addition to the adults of these taxa using similar spawning habitat upstream in the Florida Current. In the central and eastern SOF, where prey was less abundant, only Thunnus spp. and skipjack tuna co-occurred, and these two taxa exhibited significantly different vertical distributions. Prey removal rates (estimated from gut evacuation rates and daily rations) occurring in the western SOF where tuna taxa co-occurred are likely to be sustainable by appendicularian levels within this region but would potentially not be by levels in the east. The spatial and trophic characteristics of these four abundant larval taxa highlight the potential influence of feeding-related processes on larval and adult behavior, while also illustrating a critical trophic link to the microbial food web provided by appendicularians in this oligotrophic environment.

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