Differential larval growth strategies and trophodynamics of larval Atlantic bluefin tuna (Thunnus thynnus) from two discrete spawning grounds

Abstract

Atlantic bluefin tuna (ABT), Thunnus thynnus, is a globally important fishery resource. This oceanic top predator undertakes long-distance migrations from its nutrient-rich feeding grounds in the north Atlantic to oligotrophic spawning grounds. Larvae are born mainly in the corresponding ecosystems of its two well-established spawning areas: the Gulf of Mexico (GoM) and the Mediterranean Sea (MED). Stable isotopes analyses (SIA) of δ15N and δ13C have proven useful for understanding marine food webs and have evaluated larval ABT signatures in their respective spawning areas. This study is among the first to use larvae collected during the same spawning season (2014) from the GoM and one of the main spawning areas within the MED (the Balearic Sea) to standardize methodologies, compare larval growth, and relate SIA with larval postflexion growth using GAMs for the two spawning areas. The main findings identify significant population differences, with comparatively faster growth in GoM larvae. The GoM population had larger otoliths with wider increments, as well as higher δ15N values and higher trophic position for postflexion stages. Collectively, these findings highlight the importance of not only developing well-calibrated growth curves, but also integrating the distinct early life history dynamics for each spawning area to properly support ABT’s ongoing management efforts.

Maternal Effects and Trophodynamics Drive Interannual Larval Growth Variability of Atlantic Bluefin Tuna (Thunnus thynnus) from the Gulf of Mexico

“…Similar observations have been reported for larval Shortbelly Rockfish where larvae with a lower TP were heavier and grew faster [101]. The TP reflects how the energy is transferred from the base of the food web up to the larva [99,102], and its estimation can be influenced by food chain efficiency [103,104], which causes a wide range of TP estimates in the GOM [64].…”

“…δ 15 N levels are enriched with each trophic transfer, providing information about the TPs of consumers [50,99]. In previous studies, better larval growth was found to be associated with higher TPs [56,64,100], which was interpreted as reflecting greater trophic specialization. According to our results, however, larvae with better growth from GOM18 had lower TPs than GOM17 (Table A1).…”

“…Each of the Bongo 90 and Bongo 20 nets were equipped with a General Oceanics flowmeter to measure the volume of water filtered during each tow (m 3 ). Temperature ( • C) and salinity (psu) profiles for the upper 25 m were determined from CTD casts conducted concurrently at each station (see [64] for hydrographic sampling details).…”

“…Otolith radius (RADIUS, µm), daily increments (AGE, days) and mean increment widths (MIW, µm) were also measured by Leica image analysis software. Reading criteria for ABFT larval age estimations were previously applied [34,65,66] and detailed by Malca et al [64].…”

“…We applied a basal trophic position (TP BASAL ) of 2, assuming micro-zooplankton are primary consumers [68]. For the nitrogen isotopic discrimination factor (∆ 15 N), we used the muscle tissue value for juveniles ABFT (1.46‰) proposed by [69] and previously applied to ABFT larvae by [64].…”