Spawning site distribution of a bluefin tuna reduces jellyfish predation on early life stages

Abstract

Bluefin tunas across the world migrate long distances to spawn in particularly warm and oligotrophic areas constrained by oceanographic fronts. The low abundance of predators in these areas increases survival chances of their early life stages, but its importance for choice of spawning habitat is unknown. Here, we use estimated clearance rates and data on spatial distributions of Atlantic bluefin tuna larvae Thunnus thynnus and metaephyrae of the jellyfish Pelagia noctiluca to quantify predation at a major spawning ground in the Mediterranean Sea. We found that high densities of P. noctiluca can rapidly deplete tuna eggs and preflexion larvae, but their patchy distribution and low spatial and temporal overlap results in overall low predation. The specific distribution of the spawning sites suggests that bluefin tunas may use local oceanography as cues to spawn outside areas with high predator densities.

Singularities of surface mixing activity in the Western Mediterranean influence bluefin tuna larval habitats

“…The fitness of bluefin tuna during the early life stages could therefore be benefited by hydrodynamic environments associated with low to moderate FSLE values, that is in the absence of intense LCS which occurs in the Balearic Sea in comparison to other areas in the Western Mediterranean. This agrees with identified strategies of bluefin tuna larvae, that occupy areas spatially distributed to avoid predation from species such Pelagia noctiluca ephyrae (Ottmann et al 2021) and for which cannibalistic strategies seem to play a key role for survival strategies (Reglero et al 2011, Uriarte et al 2019. At a regional scale, our results reveal that during the time of spawning, the predominant mean values of the FSLEos in the Balearic Sea are lower (close to 0.1 d −1 ) than in surrounding areas of the Western Mediterranean Sea (Figs.…”

“…Firstly, the data set of larval abundances is strongly skewed to low values, making the binomial distribution adequate to explore relations with environmental variables. This is the same approach applied in the framework of the International Commission for the Conservation of Atlantic Tunas (ICCAT), where larval abundances are assessed through delta-log normal models, in which presence probabilities play a main role (Ingram et al 2017, Alvarez-Berastegui et al 2021. Secondly, it is the most relevant statistical parameter used today to identify habitats of tuna larvae in the study area, not only for bluefin tuna but also for other tuna species (Alvarez-Berastegui et al 2016, 2018 3.…”

“…Firstly, it identifies cues related to ecological processes behind the selection of these reproductive areas, and secondly, it allows the design of ocean indicators used to properly monitor interannual changes potentially affecting the species of interest. Previous studies within the Balearic Sea (Western Mediterranean), one of the most relevant spawning grounds of bluefin tuna (Rooker et al 2007, Alemany et al 2010, have shown that during the spawning season these areas present temperature ranges favoring egg survival and larval growth (Reglero et al 2018), low levels of primary production (Alvarez-Berastegui et al 2016, Druon et al 2016, association with the presence of oceanic density fronts (Alvarez-Berastegui et al 2014) and a spatial−temporal segregation with main predators such Pelagia noctiluca ephyrae (Ottmann et al 2021). Some of these characteristics are similar in other important spawning grounds (Muhling et al 2017).…”

Small fish eat smaller fish: A model of interaction strength in early life stages of two tuna species

“…Nonetheless, the increase in BFT piscivorous pressure is just one of several contributing processes driving the ALB decay, as the larval abundances of both species are not always negatively correlated. In 2017, for instance, larval abundances of both species increased, suggesting that factors like warm water temperature, food availability, or low abundance of invertebrate predators (Fiksen and Reglero 2021;Ottmann et al 2021) could favor both species.…”

“…During the tuna spawning season in the Mediterranean Sea, invertebrate predators are few and patchy (Ottmann et al 2021), but piscivorous fish larvae may be important predators of smaller larvae. The fact that BFT and ALB can produce large batches of larvae that are restricted to the warm water above the shallow ($ 20 m depth) thermocline (Torres et al 2011;Reglero et al 2018a) increase encounter rates and strengthen interactions.…”