Skipjack Tuna Availability for Purse Seine Fisheries Is Driven by Suitable Feeding Habitat Dynamics in the Atlantic and Indian Oceans

Jean-Noël Druon,Emmanuel Chassot,Jon Lopez,Hilario Murua

doi:10.3389/fmars.2017.00315

Jean-Noël Druon, Emmanuel Chassot + Show 2 more

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https://doi.org/10.3389/fmars.2017.00315

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Abstract

An Ecological Niche model was developed for skipjack tuna (Katsuwonus pelamis, SKJ) in the Eastern Central Atlantic Ocean (AO) and Western Indian Ocean (IO) using an extensive set of presence data collected by the European purse seine fleet (1998 – 2014). Chlorophyll-a fronts were used as proxy for food availability while mixed layer depth, sea surface temperature, dissolved oxygen, salinity, current intensity and height anomaly variables were selected to describe SKJ’s abiotic environmental preferences. The resultant ecological niche included both mesoscale eddy-type productive features that displayed latitudinal range in the IO to large scale upwelling systems that shrink and swell seasonally in the AO. Overall, 83% of all free swimming school sets (FSC) and 75% of drifting fish aggregating device sets (dFAD) that contained SKJ occurred within 25 km of favorable feeding habitat. In the AO, 34% of dFAD sets were made more than 100 km away from this habitat, mostly in the surface chlorophyll-a poor environment of the Guinea Current. These distant sets represent 10% of dFAD sets in the IO and 8% of all FSC sets. Our results suggest that the Mozambique Channel in the IO, with its simultaneously favorable feeding and spawning conditions, may seasonally offer a better SKJ nursery habitat than the Guinea Current which shows a substantially poorer feeding capacity. With the exception of this latter area, our results also suggest that fishing accessibility will be higher in months where the size of the favorable feeding habitats are reduced, likely because this reduction drives a geographical contraction in SKJ populations. The observed relationship between the annual size of favorable feeding habitat and both annual catch rates and total catches in the IO is consistent with the near-full exploitation of this stock that has occurred since the 2000s. Moreover, it suggests that annual habitat size could be used as an indicator of growth capacity for this highly productive stock. Habitat monitoring, as part of a dynamic fisheries management approach, should contribute to the sustainable exploitation of SKJ by providing information on the climate-dependent aspects of stock variability and the effects of dFAD deployment in food-rich habitats.

Highlights

Global annual catches of tropical and temperate tunas have been steadily increasing over recent decades, reaching 4.5 million tons (t) in 2015 (ISSF, 2017)
Our Ecological Niche Model approach (ENM) was composed of a five-step methodology (Figure 1) that aimed to: (1) identify the main behaviors and ecological traits of SKJ based on current literature and expert knowledge; (2) collect and process gridded data for defined study areas; (3) perform a cluster analysis to identify a set of variable thresholds that characterized SKJ feeding ecology; (4) derive the habitat model equation that classified the degree to which each portion of the study area was either suitable or unsuitable habitat on a daily basis; and (5) perform, by geographical area and fishing mode, a seasonal and inter-annual analysis using the distance of presence data to the closest favorable feeding habitat as a metric
Except during brief migrations periods, SKJ presence can be used to identify the existence of nearby favorable feeding habitat

Summary

Introduction

Global annual catches of tropical and temperate tunas have been steadily increasing over recent decades, reaching 4.5 million tons (t) in 2015 (ISSF, 2017). When purse seine vessels arrived in the early to mid-1980s, catches rapidly increased on free swimming schools (FSC), peaking at around 615,000 t in 2006 (IOTC, 2016). During this time, drifting fish aggregating devices (dFADs) were increasingly deployed. These devices are known to attract a range of pelagic species including SKJ (e.g., Lopez et al, 2017). In the Atlantic Ocean (AO), SKJ catches increased from the early 1950s and in recent years, have fluctuated around 230,000 t (ICCAT, 2016)

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