Development of habitat prediction models to reduce by-catch of sailfish (Istiophorus platypterus) within the purse-seine fishery in the eastern Pacific Ocean Raul O. Martinez-Rincon A,E , Sofia Ortega-Garcia A , Juan G. Vaca-Rodriguez B,D and Shane P. Griffiths C A Instituto Polite ´cnico Nacional – Centro Interdisciplinario de Ciencias Marinas (CICIMAR), Departamento de pesquerı ´as, Avenida IPN s/n, La Paz, B.C.S. 23096, Me ´xico. B Programa Nacional de Aprovechamiento del Atu ´ n y de Proteccio ´ n de Delfines (PNAAPD), Km 107 Carretera Tijuana-Ensenada, campus CICESE, Ensenada, Baja California, Me ´xico. C Commonwealth Scientific and Industrial Research Organisation (CSIRO), Oceans and Atmosphere Flagship, GPO Box 2583, Brisbane, Qld 4001, Australia. D Present address: Facultad de Ciencias Marinas, Universidad Auto ´ noma de Baja California (UABC), Km 103 Carretera Tijuana-Ensenada, campus UABC, Ensenada, Baja California, Me ´xico. E Corresponding author. Present address: Centro de Investigaciones Biolo ´ gicas del Noroeste (CIBNOR), Avenida IPN s/n, La Paz, B.C.S. 23096, Mexico. Email: raul.martinez.rincon@gmail.com Abstract. Sailfish (Istiophorus platypterus) is an important apex predator in neritic and oceanic pelagic ecosystems. The species is also a primary target of important catch-and-release sport fisheries that the support local economies of developing countries. However, commercial purse-seine fisheries that target tuna in the eastern Pacific Ocean (EPO) incidentally catch and discard large numbers of sailfish. Sailfish by-catch data recorded by scientific observers in the Mexican tuna purse-seine fleet in the EPO from 1998 to 2007 was used in generalised additive models (GAMs) to predict environmental and spatial preferences of sailfish. GAM predicted the highest sailfish catches to occur in coastal waters during El Nin ˜o events during late autumn and winter, with sea surface temperatures .268C, with negative values of deviation in sea surface height (,À10 cm), and low chlorophyll-a (,0.25 mg m À3 ). GAM predicted that the catch probability for sailfish increased 1.8-fold during El Nin ˜ o events in coastal waters and 1.5-fold under La Nin ˜a. However, the spatial distribution of sailfish remained largely unchanged during El Nin ˜ o and La Nin ˜ a events. Our models may be an additional fisheries management tool that may be used to support temporary spatial-temporal throughout the fishing season to reduce sailfish by-catch in the EPO. Additional keywords: environmental predictors, generalised additive model, spatial predictors. Received 5 March 2014, accepted 8 September 2014, published online 19 February 2015 Introduction There is increasing evidence that depletion of the ocean’s top predators by large-scale fisheries are compromising the integrity and functioning of large marine ecosystems (Scheffer et al. 2005; Heithaus et al. 2008). In a subtropical Pacific ecosystem, Polovina and Woodworth-Jefcoats (2013) showed that the commercial pelagic longline fishery was likely responsible for a significant change in the structure of the ecosystem. This change was caused by a significant decline in the abundance and size of target tuna species, which allowed proliferation of unmarketable species, including lancetfish and snake mackerel. In the Gulf of Thailand, fisheries sequentially moved to target species occupying lower trophic levels, as larger and more desirable species were fished to such low abundances that they were no longer economically viable to target, leaving the ecosystem in a highly degraded state (Pauly and Chuenpagdee 2003). Examples of ‘fishing down the food web’ (Pauly et al. 1998) demonstrate the need for fishery managers to take a holistic approach to management that ensures the long-term sustainability of target and by-catch species that play an important role in maintaining the integrity of pelagic ecosystems. The eastern Pacific Ocean (EPO) supports one of the world’s largest and most valuable pelagic fisheries for high trophic level predators, including bigeye, yellowfin, skipjack and albacore CSIRO PUBLISHING Marine and Freshwater Research http://dx.doi.org/10.1071/MF14062 Journal compilation Ó CSIRO 2015 www.publish.csiro.au/journals/mfr