Habitat analysis of the commercial tuna of the Eastern Tropical Pacific Ocean

Abstract

We have extracted information on the habitats of bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) in the Eastern Tropical Pacific Ocean by matching the spatial‐temporal distribution of catch and effort of purse seine and longline fleets collected by the Inter‐American Tropical Tuna Commission with oceanographic conditions and subjecting the matched data to Quotient Analysis and General Additive Models (GAMs). These analyses yielded the following results. The habitats defined by the GAM analysis of young fish differ significantly between two periods, one before and one after the introduction of fish aggregation devices (FADs). This was not true for the older fish caught by longline. We speculate that these changes were caused by the extensive use of FADs. Younger bigeye and yellowfin caught by the purse seine fleet have a different preference of environmental variables compared to older fish caught by longline. This is to be expected since tuna of different age groups have different sizes, metabolic capabilities and swimming skills. Moreover, as revealed by GAMs, the habitats of young fish differ between species to a much larger degree than those of older fish. Our results indicate the fundamental differences between fishing methods, targeted species, and operating region of the two fisheries. Specifically, young bigeye occupy equatorial waters farther from the coast and where the hypoxic layer is deeper, young skipjack occupy more productive waters associated with equatorial and coastal upwelling, and young yellowfin occupy broad areas where waters are underlain by a shallow hypoxic layer.

ENSO drives lateral separation of FAD-associated skipjack and bigeye tuna in the Western Tropical Pacific

“…These findings are in contrast to those of Hu et al (2018) [19], who found that hypoxic layer depths, rather than temperature-related conditions and SSHAs, were most useful for laterally demarcating preferred skipjack and bigeye habitats in the ETP. There are several potential reasons for these differences.…”

“…Quantification of habitat preferences was done using quotient analysis, which determines whether a given environmental condition is preferred, tolerated, or avoided by a certain species based on how frequently that species appears within that condition (e.g., [19,38,52]). The steps involved in the quotient analysis performed here are as follows:…”

“…Here we expand upon this previous work by analyzing lateral FAD-associated, purse seine skipjack-bigeye catch separability in the Western Tropical Pacific (WTP), where the majority of skipjack tuna are caught [2]. Oceanographic conditions are quite different in the WTP compared to the ETP regions examined by Harley & Suter (2007) [18], Hu et al (2018) [19], Lennert-Cody et al (2008) [20], and Schaefer & Fuller (2013) [23] (e.g., [24,25]). Furthermore, new growth rate measurements suggest that Tropical Pacific bigeye potentially consist of two separate stocks (though with mixing in between), divided between the WTP and ETP at approximately 150°W [9,13].…”

“…Leroy et al (2009) [21] also concluded that the potential for targeting specific species through fishing depth selection is limited, based on an analysis of the vertical behaviors of the same three species associated with anchored FADs within Papua New Guinea’s exclusive economic zone (EEZ). Given the difficulties associated with separating bigeye and skipjack vertically and/or via time-area closures, Hu et al (2018) [19] sought to separate the two species laterally by investigating differences in their habitat preferences using 1°-latitude by 1°-longitude monthly purse seine catch and effort data in the Eastern Tropical Pacific (ETP). They found that within the ETP, purse seine-caught bigeye occupy waters further from the coast where the hypoxic layer is deeper, while purse seine-caught skipjack occupy more productive waters associated with equatorial and coastal upwelling.…”

“…They found that within the ETP, purse seine-caught bigeye occupy waters further from the coast where the hypoxic layer is deeper, while purse seine-caught skipjack occupy more productive waters associated with equatorial and coastal upwelling. Thus, to reduce incidental bigeye catch rates, Hu et al (2018) [19] suggested targeting skipjack where hypoxic layers are shallowest (i.e., where there is coastal upwelling).…”