Journal of Experimental Marine Biology and Ecology 448 (2013) 228–239 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Impact of Costa Rican longline fishery on its bycatch of sharks, stingrays, bony fish and olive ridley turtles (Lepidochelys olivacea) Derek Dapp a, Randall Arauz b, James R. Spotila a,⁎, Michael P. O'Connor a a Department of Biodiversity, Earth and Environmental Science, Drexel University, Philadelphia, PA 19104, USA b Programa Restauración de Tortugas Marinas (Pretoma), 1203-1100, Tibás, San José, Costa Rica article info abstract Article history: We used data collected by an observer program to assess the impact of the Costa Rican longline fishery on Received 4 January 2013 numbers, capture locations, seasonality and body sizes of silky sharks (Carcharhinus falciformis), pelagic thresher Received in revised form 22 July 2013 sharks (Alopias pelagicus), olive ridley turtles (Lepidochelys olivacea) and other bycatch species in the Central Accepted 24 July 2013 American Pacific. The longline fishery caught a large number of mahi-mahi (Coryphaena sp.) and silky sharks, Available online xxxx but also caught a large number of olive ridley turtles and pelagic stingrays (Pteroplatytrygon violacea). We esti- mated that longline fisheries caught 699,600 olive ridleys, including 92,300 adult females, from 1999 to 2010. Keywords: Geospatial analysis These captures were associated with a decline of nesting populations at nearby arribada beaches. There were sta- Longline fishery tistically significant size decreases from 1999 to 2010 in mature olive ridley turtles and from 2003 to 2010 in silky Marine conservation sharks. Average fork length of silky sharks in 2010 was 97.3 cm, which was far below observed fork length at Marine protected areas maturity, 144 cm. Pelagic thresher sharks were small and fluctuated in size over the study period. Capture of Observer program large numbers of juvenile blacktip sharks (Carcharhinus limbatus) indicated a nursery area near the Osa Peninsula. Populations Geospatial analysis indicated shifts in mahi-mahi abundance on a temporal scale but fishing efforts did not shift with the shift in mahi-mahi abundance. Yellowfintuna(Thunnus albacares), Indo-Pacific blue marlin (Makaira mazara) and Indo-Pacificsailfish (Istiophorus platypterus) catches varied seasonally and were most abundant out to sea and south of Panama. Marine protected areas and/or time area closures are needed to reduce the impact of the Costa Rican longline fishery on sea turtles and sharks. © 2013 Elsevier B.V. All rights reserved. 1. Introduction boats reported over 55,000 turtles captured by longlines around the world. Therefore, bycatch may be a major problem for survival of sea Within the past few decades, there has been increasing interest turtle populations (Wallace et al., 2010). For olive ridley (Lepidochelys in the effects of longline fisheries on marine species (Lewison and olivacea), loggerhead (Caretta caretta) and leatherback (Dermochelys Crowder, 2007; Myers et al., 2007). In particular, species of sea turtles coriacea) turtles, mortality on fishing lines is highly variable, but is typ- and sharks display an intrinsic sensitivity to mortality from longline ically low (Koch et al., 2006; Lewison and Crowder, 2007). However, fisheries because they exhibit life history traits such as high adult survi- poor handling practices, retention of turtles for consumption and injury vorship, low fecundity and late age of maturity (Cope, 2006; Gilman caused after capture may contribute to increased mortality (Koch et al., et al., 2008; Lewison and Crowder, 2007) that limit their response to 2006; Watson et al., 2005). Overall, post-hooking mortality of sea turtles high rates of adult mortality. ranges from 4% to 27% in longline fisheries (Camiñas, 2004; Lewison and Fisheries bycatch is a primary driver of population declines in elas- Crowder, 2007). In the olive ridley turtle, mortality caused by longlining mobranchs and sea turtles (Wallace et al., 2010). Shortfinmakosharks is typically low in Costa Rican longline fisheries and olive ridleys are (Isurus oxyrinchus) suffer high fishing mortality throughout their usually released alive, with minimal short-term post-capture mortality range (Dulvy et al., 2008)andfishing routinely depletes marine fish (Swimmer et al., 2006; Whoriskey et al., 2011). populations by 50 to 70% (Baum et al., 2003), with losses exceeding However, populations of olive ridleys at the two main nesting 90% becoming more common (Myers and Worm, 2005). From 1990 to beaches in Costa Rica have declined since the 1980s. Olive ridleys nest 2008 bycatch of sea turtles per unit effort was particularly high for long- in mass synchronous nesting aggregations, or arribadas, and two of lines in the Northwest Atlantic, Mediterranean, Southwest Atlantic and the six major arribada beaches in the world are in Costa Rica. The pop- Eastern Pacific and for gillnets in the Eastern Indian Ocean and trawls in ulation at Nancite beach declined precipitously in the 1980s and has the Eastern Pacific and Mediterranean. Observers on board b1%–5% of not recovered (Fonseca et al., 2009; Honarvar et al., 2008, 2011). The population at Ostional is very large (about 500,000) but also has ⁎ Corresponding author. Tel.: +1 6094405158; fax: +1 2158951273. declined in recent years (Valverde et al., 2012). Several arribada popula- E-mail address: [email protected] (J.R. Spotila). tions in Mexico have essentially disappeared (Koch et al., 2006; 0022-0981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jembe.2013.07.014 D. Dapp et al. / Journal of Experimental Marine Biology and Ecology 448 (2013) 228–239 229 Marquez et al., 1996). Although Plotkin et al. (2012) report that popula- sharks (Carcharhinus limbatus), a species considered to be globally tions are recovering in Mexico and that similar trends are occurring in near threatened. Costa Rica, the most recent and complete data (Valverde et al., 2012) demonstrate that the olive ridley population at Ostional has actually de- 2. Methods creased and the population at Nancite is still very low (Fonseca et al., 2009). Despite the seemingly low rates of mortality caused by longlines 2.1. Observer program in the Costa Rican fishery, it is essential to monitor the potential impacts fi of the shery on olive ridleys because even slight decreases in adult The program began in 1999 and longline monitoring was continuous survivorship have substantial impacts on sea turtle populations (Koch until 2010, with the exception of 2001, when no observations were fi et al., 2006; Wallace et al., 2010)and sheries in areas that have large conducted. Efforts of the program varied between years, with a mini- numbers of mature olive ridleys could be extremely detrimental to mum of 15 longlines observed in 2000 and maximum of 88 longlines turtle populations. observed in 2008. Observer effort also varied by month. Most observa- While considerable effort has been made to reduce sea turtle tions took place in May, which had 14.3% of the total observed lines. bycatch on longlines, comparatively few efforts have been made to Observations were conducted onboard six medium scale (10–15 ton eliminate shark bycatch on longlines (Gilman et al., 2008; Swimmer capacity with iced holds) vessels out of a Costa Rican fleet of 350 vessels. et al., 2011). Shark and sea turtle bycatch varies greatly depending Boats were owned by Papagayo Seafood S.A. vessels and sailed from upon spatial location and methods utilized (Gilman et al., 2006, 2008). Playa del Coco. fi Mahi-mahi (Coryphaena hippurus and Coryphaena equiselis) sheries Observers were trained to identify and record species, sex, reproduc- fi in the Central American Eastern Paci c may be particularly prone to tive state and dimensions of all animals captured onboard longline fish- catching sea turtles and sharks due to their shallow set depth, which ing trips. Prior to 2003 silky sharks and blacktip sharks were recorded as typically has a higher catch rate of sharks and sea turtles (Lewison and Carcharhinus spp. so we could not analyze individual species, and sizes of Crowder, 2007; Whiting et al., 2007a; Whoriskey et al., 2011). Unlike fish were not recorded. Observers also recorded information pertaining sea turtles, shark bycatch on longlines in Costa Rica typically results in to individual longlines, such as location, set and haul back times, hook fi mortality, because shermen retain sharks, removing and selling their type, hook number, bait used, target species and total number of hooks. fi ns for export and selling their meat for domestic consumption All hooks used were circle hooks (#14, 15, 16) with and without 10° (Arauz, pers. obs.). Capture stress can cause high rates of immediate or offset (Swimmer et al., 2011). Set specific data on bait used were incom- delayed mortality in sharks (Frick et al., 2010). Shark populations are plete. Observers remained on fishing vessels throughout a voyage and fi very sensitive to over shing as indicated by substantial decreases in recorded all data from each line set. Trips typically lasted about two fi many shark populations worldwide following heavy shing pressure weeks and vessels set approximately 18-mile longlines with wire leaders (Baum et al., 2003; Clarke et al., 2006; Heithaus et al., 2008; Myers and 650 hooks per set. et al., 2007; Robbins et al., 2006). In Costa Rica, silky shark, Carcharhinus falciformis, and pelagic thresher shark, Alopias pelagicus, populations are fi declining (Whoriskey et al., 2011). 2.2. Examining shing effort and catch rates Sharks regulate marine communities and provide stability to each ecosystem they inhabit, both through direct predation (Bascompte Using the total number of hooks on the longline, and number of et al., 2005; Myers et al., 2007) and by altering the habits of various spe- individuals caught of each species we calculated the rate of catch per cies in their presence (Heithaus and Dill, 2006; Heithaus et al., 2007, 1000 hooks set.