Opportunistic predation in tuna: a size-based approach

Abstract

To test whether predation is an opportunistic size-based process within a tuna community, analyses were carried out on the size composition of stomach contents of bigeye tuna Thunnus obesus (Lowe, 1839) and yellowfin tuna T. albacares (Bonnaterre, 1788) caught in 1995 to 1997 during longline scientific surveys in the French Polynesian Exclusive Economic Zone (EEZ). Prey size distributions were compared with the size distribution of organisms collected by pelagic trawls carried out during the same programme. Relationships between prey size and predator size were studied using quantile regressions, and were related to tuna mouth-gape measurements. The results showed that mean and maximum sizes of prey increased with increasing predator size, and that maximum prey sizes (versus tuna size) were below those predicted by tuna mouth-gape size. Minimum prey size varied little with tuna size, and the size distributions of prey in tuna stomachs were very asymmetrical (lognormal type), confirming that during growth, tunas continue to feed on small prey. Comparison with previous studies on other piscivorous species from different ecosystems underlined that tunas feed on very small prey in relation to their own size. However, comparison of size distributions of prey in stomach contents and prey in pelagic trawls revealed that bigeye tuna select larger prey than yellowfin tuna when such prey are available.

Local Ecological Knowledge (LEK) on Fish Behavior Around Anchored FADs: the Case of Tuna Purse Seine and Ringnet Fishers from Southern Philippines

“…In relation to this, the association of juvenile tunas to an anchored FAD seems to indicate that they feed primarily on prey species found under the FAD because of their rapid growth, 3.8 mm per day (Mitsunaga et al, 2012). The opportunistic feeding behavior of tunas and its predisposition to social interaction (Robert et al, 2013) may have implications on its movement from one anchored FAD to another (Ménard et al, 2006). The attraction distance of 10 km which fishers mentioned about tunas is reasonable given that the usual maximum inter-FAD distances between anchored FADs in the Philippines, is of the same distance (Libre et al, 2015;Macusi et al, 2015).…”

Mesoscale eddies influence distribution and aggregation patterns of micronekton in the Mozambique Channel

“…Our findings provide first evidence that mesoscale eddies can shape the aggregation patterns of the prey of top pelagic predators in this region. Indeed, large fish predators such as tuna feed on various micronektonic organisms ranging from 2 to a few 10s of cm, including numerous fish, squid and crustacean species (Young et al 2001, Ménard et al 2006, Potier et al 2007). Piscivorous predators hunt individual prey but commonly seek out micronekton aggregations, such as shoals and clusters of shoals, in the water column (LebourgesDhaussy et al 2000, Bertrand et al 2002.…”

Context‐dependent planktivory: interacting effects of turbidity and predation risk on adaptive foraging

“…To test whether a hump-shaped pattern existed in stomach content with respect to turbidity, we used quantile regression, an approach that can capture changes in the boundaries of data distributions (Scharf et al 1998) and has previously been used to interpret diet data (Scharf et al 1998, Pinnegar et al 2003, Menard et al 2006 and difficult to analyze, in part due to factors unrelated to the interaction between a consumer and its prey, including post-collection digestion and collection-induced regurgitation (Bowen 1996), which can depress values and are difficult to account for. We dealt with these putative hidden biases by focusing on how turbidity constrained the upper limit of stomach content.…”