Fishing tuna around Fish Aggregating Devices (FADs) vs free swimming schools: Skipper decision and other determining factors

Abstract

Fish Aggregating Devices (FADs) are increasingly used by tuna purse-seine fleets all around the world, modifying the species catch composition. We analyse the determinants of FAD vs free school (FS) fishing, including the skipper effect, and environmental and economic factors. A multivariate and econometric analysis of a panel dataset for the French purse-seine fleet in the Indian Ocean (1980–2007), complemented by a survey of fishers, demonstrates the influential role of climate and prey on FAD fishing and also emphasises individual skipper preference. However, we found that the major determinant of FAD fishing remains the growing fishing capacity (use of bigger vessels, satellite buoys, echo-sounders, supply vessels), thus modifying the species catch composition towards smaller tuna. Other things being equal, raising the proportion of FAD sets by 1% would increase the catches of skipjack by 1.3% and decrease those of large yellowfin tuna by 1.7%. This result shows that the control of effort and investment could be adapted through management measures to encourage one fishing method.

Introduction

Since the early 1990s, Fish Aggregating Devices (FADs; i.e. man-made floating objects) have been increasingly used around the world by the purse-seine fleets targeting the three main species of tropical tuna: skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) (Fonteneau et al., 2000, Miyake et al., 2010). FADs, like any floating object, attract tropical tuna and other tropical species (Taquet et al., 2007, Moreno et al., 2007a). The main advantage of FADs for fishers is that they increase the catchability of tuna, as compared to sets on free-swimming schools (FS). Firstly, they help fishers locate the fish (reducing search time). This is particularly true at present with satellite tracking buoys attached to FADs (Moreno et al., 2007a, Moreno et al., 2007b), which allow the long term tracking of FADs. Secondly, fishing around FADs provides high successful set rates compared to those for free swimming schools (Miyake et al., 2010, Suzuki et al., 2003). Squires and Kirkley (1999) report that skippers usually prefer a high probability of positive sets with small catches (typical with FADs) to a low probability of positive sets with higher catches (typical with FS).

Another factor may have an effect on FAD fishing. The species catch composition and value is different depending on whether vessels are targeting tunas associated with FADs or in FS. The proportion of high-valued large yellowfin is greater with FS than with FADs (Campbell and Nicholl, 1994, Fonteneau et al., 2000), while FAD catches are mainly characterized by lower-value skipjack and small yellowfin and bigeye tuna. As a result, fishers can choose between a lower probability (i.e. a greater number of null sets) of catching a large proportion of high-priced large yellowfin tuna (on FS) or a greater probability (i.e. a lower number of null sets) of a large proportion of low-priced skipjack¹ and small yellowfin and bigeye tuna (<3.4 kg), skipjack representing on average 87.6% of this small tuna category (average proportion between 1991 and 2009, source Observatoire thonier tropical – IRD). The central issue of this research is whether individual fishers have a preference for one strategy or the other, or is the strategy entirely dependent on exogenous conditions?

While choosing one strategy over the other could provide different economic outputs, they could also have different ecological impact. The Regional Fisheries Management Organisations (RFMOs) in charge of tropical tuna are concerned by the high percentage of small yellowfin and bigeye tuna captured around FADs, as well as the by-catch of the technique (Hallier and Gaertner, 2008). However, high catches of adult yellowfin could also affect the sustainability of the fishery (see the exceptional catch records of large yellowfin tuna in the Indian Ocean between 2003 and 2005; IOTC, 2006, p. 66).

Very little research has been conducted to describe and analyse the motivations of fishers choosing one strategy over another (FADs or FS) in the long run. Is there any skipper preference behind such a choice? Are there “FAD fishers” and “FS fishers”? Do fishers alternate between the two strategies, according to the relative abundance of species/size-classes? Do they respond to economic circumstances (vessel size, costs of effort, relative price of species)?

This study examines the influence of three sets of factors: skipper skill and experience, environmental conditions and economic incentives on the choice of FAD or FS fishing. The objectives of the present work are (1) to determine if there are different categories of fishers who vary in their preference for FAD vs FS fishing; (2) to assess the role of ecological phenomena (such as climate oscillation and prey episodes) on this preference, and (3) to evaluate the impact of economic incentives (relative species price, fishing costs, investment in buoys, vessel size) on FAD fishing. We rank these factors in decreasing order of importance.