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Cannibalism among size classes may reduce starvation and improve survival of larval tuna in oligotrophic ocean areas, but it may also be a substantial mortality component depending on the availability of alternative prey. Here, we combine laboratory and field data on tuna larvae with a model of larval foraging and bioenergetics to explore the role of cannibalism in cohort development at different temperatures, durations of hatching period, hatching larval densities and natural mortality rates. Prey fields (zooplankton densities and co-occurrence of different larval stages of 3 species of tuna) were established from cruises in a main tuna spawning area around the Balearic Islands (Mediterranean Sea). Results suggest that a pure zooplankton diet is frequently insufficient to sustain larval growth. Piscivory can be a major source of larval mortality among tuna species and larvae hatched early can feed on abundant larvae of smaller size and have fewer predators themselves. We show how the intensity of cannibalism depends on the temperature dependent growth rate and the resulting relative size distribution when eggs are released continuously over a period of a few weeks. The predator-prey size distribution and the relative densities of these voracious larvae may produce overcompensation in recruitment under some environmental conditions.

To analyze the potential trophic interactions among tuna larvae and other fish species, we studied the larval fish assemblages inhabiting the surface mixed layer around the Balearic Islands using data from two surveys carried out in summer 2006 and 2008. Despite the high diversity, Thunnus alalunga, Auxis rochei, Cyclothone pygmaea and Ceratoscopelus maderensis clearly dominated the catch in both years. The spatial distribution of several larval fish assemblages, in which tuna larvae were among the dominant species, was related to the presence of fronts and mixed Atlantic waters (AW). Different developmental stages of tuna were found in similar hydrographical scenarios. Smaller tuna larvae and plausible piscivorous tuna were found in the areas with mixed AW. Later in the season, potential piscivorous tuna were found together with small larvae from other non-tuna species that had been spawned later and transported by the fresh AW flow to offshore stations. That the different life stages of tuna co-occur with mesopelagic species in the mixed layer reveals the interesting possibility of ecological interactions between the different larvae.

a b s t r a c tIn marine ecosystem models, the underwater light intensity is commonly characterized by the shading of phytoplankton in addition to a background light attenuation coefficient. Colour dissolved organic matter (CDOM) is an important component of the background light attenuation, and we investigate how variation in CDOM attenuation affects euphotic zone properties in a general marine ecosystem model. Our results suggest that euphotic zone properties are highly sensitive to CDOM variations occurring in nature. While the nutrient input to the euphotic zone scales the magnitude of the primary production, the vertical structure of nutrients and phytoplankton is largely determined by the variation in CDOM attenuation in our simulations. This suggests that knowledge of CDOM variation is useful to constrain uncertainties in predictions of water column structure in marine ecosystem modelling, but also in analyses utilizing the oceanic nutricline depth as proxy for primary production. Finally, according to our sensitivity analysis, many coastal areas experiencing high loads of terrestrial CDOM are expected to show eutrophication symptoms induced by altered optics.

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