Frédéric Ménard

Bioenergetics, Trophic Ecology, and Niche Separation of Tunas

Tunas are highly specialized predators that have evolved numerous adaptations for a lifestyle that requires large amounts of energy consumption. Here we review our understanding of the bioenergetics and feeding dynamics of tunas on a global scale, with an emphasis on yellowfin, bigeye, skipjack, albacore, and Atlantic bluefin tunas. Food consumption balances bioenergetics expenditures for respiration, growth (including gonad production), specific dynamic action, egestion, and excretion. Tunas feed across the micronekton and some large zooplankton. Some tunas appear to time their life history to take advantage of ephemeral aggregations of crustacean, fish, and molluscan prey. Ontogenetic and spatial diet differences are substantial, and significant interdecadal changes in prey composition have been observed. Diet shifts from larger to smaller prey taxa highlight ecosystem-wide changes in prey availability and diversity and provide implications for changing bioenergetics requirements into the future. Where tunas overlap, we show evidence of niche separation between them; resources are divided largely by differences in diet percentages and size ranges of prey taxa. The lack of long-term data limits the ability to predict impacts of climate change on tuna feeding behaviour. We note the need for systematic collection of feeding data as part of routine monitoring of these species, and we highlight the advantages of using biochemical techniques for broad-scale analyses of trophic relations. We support the continued development of ecosystem models, which all too often lack the regional-specific trophic data needed to adequately investigate climate and fishing impacts.

Length and weight estimates from diagnostic hard part structures of fish, crustacea and cephalopods forage species in the western Indian Ocean

To estimate the original prey size of well-digested prey (fish, cephalopod and crustacean) of large pelagic fish predators representing 17 species in eight families (Scombridae, Xiphiidae, Istiophoridae, Carangidae, Coryphaenidae, Alepisauridae, Sphyraenidae and Carcharhinidae), we presented regression equations relating the length and weight of the prey to lengths of diagnostic hard part structures recovered from stomach contents. Stomach samples were collected in the western Indian Ocean between 2000 and 2008 from predators caught by three fishing gears: longline, purse seine and troll lines. In addition, fresh specimens were collected from trawls nets carried out during scientific cruises at depths ranging from the surface to 500xa0m. Parameters of the least-square regression equations were estimated between different diagnostic hard parts and the length and the weight of the prey. These relationships are useful for estimating the reconstructed weight of the diet of top predators and for estimating the predator size-prey size ratios. This work is the first reference on such relationships for the forage fauna of the western Indian Ocean.

Variability in conspecific predation among longnose lancetfish Alepisaurus ferox in the western Indian Ocean

Conspecific predation among longnose lancetfish Alepisaurus ferox was investigated in four spatio-temporal strate of the western Indian Ocean. The cannibalism level varied from 0 to 45.5% by frequency of occurrence and was negatively related with abundance of non-evasive prey (such as crustaceans Charybdis smithii and Natosquilla investigatoris) and foraging success. Predation by lancetfish is often described as a non-selective process, constrained by local prey availability and by its feeding speed during an attack of prey. Our results show that lancetfish may adapt its opportunistic foraging behavior, feeding on non-conspecific abundant prey such as crustaceans when available, and switching to a high level of conspecific predation in poor waters.