Evaluating Tuna Management in the Eastern Pacific Ocean

We answer four questions about tuna management in the eastern Pacific Ocean: What are the management objectives? Are they reasonable? Can we determine whether they have been achieved? Have they been achieved? There are explicit, overarching management objectives including keeping populations at levels that will permit maximum sustainable yields, consideration of the precautionary approach, and consideration of ecosystem consequences, but although reasonable, these objectives are vague from a stock-assessment perspective. Objectives of individual participants are unlikely to be the same and collectively make achieving the overarching objectives more difficult. For data-rich stocks, we can determine whether the yieldbased objectives have been achieved, but even these results have alternative interpretations. Unfortunately, when data are limited, the management objectives cannot be evaluated. In the eastern Pacific, species differ in achievement of objectives. Yield-based objectives are probably being achieved for yellowfin tuna—yields may increase if effort is reallocated among fishing methods—whereas for bigeye tuna, biomass levels are falling below that necessary for maximum sustainable yield. For skipjack, although the population level is healthy, restrictions designed to protect bigeye and yellowfin tuna prevent fishermen from achieving maximum sustainable yield. By-catch of other species in the tuna fisheries is a management concern. The status and management of large pelagic fish populations worldwide is causing concern (Pauly et al., 1998; Jackson et al., 2001; Baum et al., 2003; Myers and Worm, 2003). On the basis of a meta-analysis, Myers and Worm (2003) estimated that biomass of large predatory fish today is only about 10% of preindustrial levels, but their work is highly controversial, and they have been charged with selective use of data and use of inappropriate assumptions (Walters, 2003; Hampton et al., 2005; Polacheck, in press). Similar criticism has been directed at the other studies (e.g., by Burgess et al., 2005). Despite the controversy, evaluation of the management of large pelagics is important; it will identify areas of improvements and allow a continued sustainable and beneficial use of the resource. Here, we discuss an approach to evaluating management of tuna stocks and apply it to the stocks in the eastern Pacific Ocean (EPO). To evaluate management of tunas, we address four questions: What are the management objectives? Are they reasonable? Can we determine whether they have been achieved? Have they been achieved? The final question is discussed only in the specific case of the management of tunas in the EPO. We answer these questions from our perspective as stock assessment scientists, so economic and social issues, although important, are given only cursory treatment. The Tuna Fisheries in the EPO Tuna are fished in the EPO by vessels of many nations and by various fishing methods (Table 1). The majority of catch is taken by longlines—used by Japan (Okamoto and Bayliff, 2003), the Republic of Korea, Chinese Taipei, and the Peoples Republic BULLETIN OF MARINE SCIENCE, VOL. 78, NO. 3, 2006 594 Ta bl e 1. C ha ra ct er is ti cs o f m aj or fi sh in g m et ho ds f or tu na s in th e ea st er n Pa ci fic O ce an . Fi sh in g m et ho d T un a ca tc h B yca tc h C ou nt ri es C ur re nt m an ag em en t m ea su re s L on gl in e L ar ge b ig ey e an d ye ll ow fi n tu na s, al ba co re tu na B ill fis h, s ha rk s, tu rt le s, s ea bi rd s Ja pa n, R ep ub lic o f K or ea , C hi ne se Ta ip ei , P eo pl es R ep ub lic o f C hi na C at ch li m its f or b ig ey e tu na Pu rs e se in e, fl oa tin g ob je ct s Sk ip ja ck tu na , s m al l b ig ey e, a nd ye llo w fin tu na s V ar io us s pe ci es C en tr al a nd S ou th A m er ic an n at io ns , E ur op ea n U ni on Te m po ra l c lo su re Pu rs e se in e, f re esw im m in g sc ho ol s Sk ip ja ck , s m al l t o m ed iu m ye llo w fin tu na V ar io us s pe ci es C en tr al a nd S ou th A m er ic an n at io ns ; E ur op ea n U ni on Te m po ra l c lo su re Pu rs e se in e, d ol ph in -a ss oc ia te d L ar ge y el lo w fin tu na D ol ph in s M ex ic o an d V en ez ue la Te m po ra l c lo su re MAUNDER AND HARLEY: STATUS AND MANAGEMENT OF TUNA 595 of China—and purse seines (used by other nations). These vessels catch yellowfin, Thunnus albacares (Bonnaterre, 1788), bigeye, Thunnus obesus (Lowe, 1839), and skipjack, Katsuwonus pelamis (Linnaeus, 1758), tunas and several other species of lesser commercial importance (IATTC, 2000). The purse-seine method can be divided into three set types, that on tunas associated with floating objects, that on tunas associated with dolphins, and that on free-swimming schools of tunas. Importantly, more than one species are often caught in the same set. Fisheries for tunas capture a variety of by-catch (Hall, 1998; see Table 2). The dolphin-associated sets encircle dolphins but usually release all of them. The Inter-American Tropical Tuna Commission (IATTC) is responsible for management of tropical tunas and tuna-like species in the EPO. It also has significant responsibilities for the implementation of the International Dolphin Conservation Program and provides the secretariat for that program. Stock assessments for the main tropical tuna (yellowfin, bigeye, and skipjack) are based on an age-structured statistical catch-at-length analysis (A-SCALA) that combines multiple sources of information (Maunder and Watters, 2003) and are published by the IATTC (e.g., Harley and Maunder, 2005; Maunder and Harley, 2005a,b); stock status appears in the annual “Fishery Status Reports.” These reports and information about other IATTC activities are available on the IATTC website (www.iattc.org). The IATTC, in collaboration with member countries, runs a comprehensive observer program that Table 2. Average annual estimated by-catch (1993–2003) by Class-6 (≥ 364 mt) purse-seine vessels in the eastern Pacific Ocean by set type. Average number of sets by classes 1–5 are given for comparison. Data compiled from IATTC (2004a, b, c). See Olson and Watters (2003) for a finer breakdown of species and limited by-catch data for the longline fisheries. Units for tuna catches are in metric tons; those for other species are numbers of individuals. Floatingobject Freeswimming Dolphinassociated Average number of sets, class 6 4,459 4,882 9,367 Average number of sets, classes 1–5 850 5,698 9 Retained yellowfin tuna 32,615 83,509 174,668 Retained skipjack 110,749 4,4502 2,586 Retained bigeye tuna 37,604 2,588 28 Discarded yellowfin tuna 3,995 943 1,170 Discarded skipjack tuna 17,570 2,332 635 Discarded bigeye tuna 2,973 23 0 Dorado, Coryphaena hippurus Linnaeus, 1758 523,537 10,349 328 Wahoo, Acanthocybium solandri (Cuvier, 1832) 259,240 1,067 378 Rainbow runner, Elagatis bipinnulata (Quoy & Gaimard, 1825), and yellowtail, Seriola lalandi Valenciennes, 1833 101,921 18,298 1,206 Sharks 37,011 6,957 3,930