Environ Biol Fish (2016) 99:593–602 DOI 10.1007/s10641-016-0501-1 Otolith chemistry chimes in: migratory environmental histories of Atlantic tarpon (Megalops atlanticus)caught from offshore waters of French Guiana Mehis Rohtla & Markus Vetemaa Received: 26 November 2015 /Accepted: 20 July 2016 /Published online: 30 July 2016 # European Union 2016 Abstract The migratory life histories of 37 Atlantic Introduction tarpon (Megalops atlanticus) caught from offshore wa- ters of French Guiana were quantified using otolith Sr:Ca Tarpons are euryhaline migratory fish of the and Ba:Ca ratios. In general Sr:Ca ratio was more infor- Megalopidae with Atlantic tarpon Megalops atlanticus mative marker for inferring movements along salinity and Pacific tarpon Megalops cyprinoides being the only gradients, but Ba:Ca ratio was used to infer movements two species of the family. They are the most ancestral to estuarine mixing zones with high Ba availability. All amongteleostfishandalsoshareacommonoceanic sampled individuals had made use of marine and brack- offshore spawning trait and leptocephalus larval stage ish water but only 8 % had experienced fresh water and (Forey et al. 1996; Adams et al. 2014). Atlantic tarpon 24 % hypersaline water. The newly-hatched post-meta- are large predatory fish that mainly inhabit the tropical morphic larvae recruited to brackish (49 %), sea (43 %), and subtropical regions of the Atlantic Ocean and can fresh (5 %) and hypersaline (3 %) water nursery areas. tolerate wide range of environmental conditions includ- Following the settlement to nursery areas Atlantic tarpon ing oxygen-poor and hypersaline water (Adams et al. likely displayed facultative habitat selection and complex 2014). In these regions Atlantic tarpon can be found set of inter-habitat shifting among waters of various anywhere between offshore marine areas and sea- salinity. The combined use of both markers allowed us connected freshwater lakes, however, they usually inhab- to conclude that brackish (estuarine) areas are likely the it coastal areas and estuaries (Crabtree et al. 1995;Brown main habitat for Atlantic tarpon in French Guiana, and and Severin 2008;Adamsetal.2014). Atlantic tarpon fresh water use is surprisingly rare. As no general migra- havealonglifespanofupto50oreven80years,and tory life-history emerged, and because Atlantic tarpon relatively late maturity (i.e. 7–12 years depending on sex seemed rather facultative in its habitat use, it should be and region) (Crabtree et al. 1995; Andrews et al. 2001; regarded as ‘euryhaline marine migrant . Adams et al. 2012). Although world-renowned as a sport fish, the economic importance of Atlantic tarpon varies Keywords Atlantic tarpon . Otolith microchemistry. among regions with catch-and-release angling being French Guiana . Migration more widespread in Gulf of Mexico and Caribbean Sea, and commercial-artisanal fishing in equatorial South America (de Menezes and Paiva 1966; Adams et al. 2012; Mateos-Molina et al. 2013). Atlantic tarpon is currently listed as BVulnerable^ in the IUCN Red List * : M. Rohtla ( ) M. Vetemaa as the population abundances are still declining since the Estonian Marine Institute, University of Tartu, Vanemuise 46a, 51008 Tartu, Estonia 1950 s mainly due to consumptive fisheries, habitat deg- e-mail: [email protected] radation, and bycatch mortality (Adams et al. 2012). 594 Environ Biol Fish (2016) 99:593–602 Considerable ambiguity exists in the literature Sr:Ca and salinity exists for Pacific tarpon (Chen et al. concerning the life-history of Atlantic tarpon. While 2008). The aims of the present study were to: (i) inves- researchers seem to agree that Atlantic tarpon spawn tigate the variability of individual migration patterns of in unknown offshore areas and larvae recruit to various Atlantic tarpon using otolith Sr:Ca and Ba:Ca profiles, coastal and inland waters ranging from 0 to 45 PSU and (ii) if possible look into detailed movements pat- (Crabtree et al. 1992; Adams et al. 2014), there is still terns between habitats with different salinity. considerable uncertainty around the migratory ecology. According to Myers (1949) and Riede (2004)Atlantic tarpon is amphidromous. Brown and Severin (2009) Materials and methods state that Atlantic tarpon is catadromous, but then go on and note that it is not obligated to migrate into fresh Atlantic tarpon samples (n = 37) were purchased from a water. Finally, McDowall (1988) suggests that Atlantic local fish market at Cayenne (French Guiana) in tarpon are not strictly diadromous at all. Although it has October of 2013. These fish were caught with drift been suggested that individuals inhabiting Lake gillnets (BGuyana seine^) by commercial fishers from Nicaragua may complete their life cycles in fresh water various offshore sites near Cayenne. Although Atlantic (see reference in Jones et al. 1977), this conclusion was tarpon are not specifically targeted by fishers, they are most likely incorrect because Brown and Severin (2008) landed selectively and marketed if they weigh <15 kg. recently demonstrated that the individuals sampled from Otoliths were extracted from the heads, cleaned and Lake Nicaragua actually originated from the ocean. In stored dry in micro-tubes. Biological parameters of the fact all Atlantic tarpon (n = 12) studied by Brown and fish were also recorded when feasible (Table 1). In most Severin (2008) clearly made use of freshwater habitats of the individuals an additional length measurement (some large specimens for extended time periods), (termed JOL in Table 1) was taken from the tip of the therefore rendering the possibility of freshwater or at closed lower jaw to the tip of the opercule to help to least estuarine spawning. Hence, further studies on in- estimate the size of the individuals in which only heads dividual variability in life-time migration patterns and were available. No Atlantic tarpon with a total weight the importance of freshwater habitats are warranted to (TW) of >20 kg was sampled due to the reasons men- elucidate the migratory ecology of Atlantic tarpon. tioned above. Sex could not be determined in most of The chemical composition of otoliths can be used to the sampled fish as only gutted individuals were avail- retrospectively determine the environmental life-history able at the fish market. Two individuals were purchased of Teleostei. Otoliths offer a continuous and permanent unprocessed (including the largest one in the sample), growth history of fish, but additionally incorporate mi- but only small juvenile gonads were found. In Brazil, nor and trace elements from the water (Campana 1999). the smallest mature male and female Atlantic tarpon These chemical markers, if incorporated in proportion were 95 and 125 cm (FL) respectively (de Menezes with ambient concentrations, can be then used to infer and Paiva 1966), and therefore it can be assumed that fish migration patterns among habitats with different the majority of Atlantic tarpon sampled in the present water chemistries (Elsdon et al. 2008; Walther and study were immature. Limburg 2012). Strontium-to-calcium ratio (Sr:Ca) is In the laboratory, otoliths were embedded into epoxy one of such markers that generally displays a positive resin and transverse thin-sections were subsequently relationship with salinity (Secor and Rooker 2000; obtained with a low-speed saw. These thin-sections Walther and Limburg 2012), and has been successfully were ground and then polished with abrasive paper (grit used in various environmental settings (e.g., Rohtla size P1200 to P4000) until the core area was visible. et al. 2014; Payne Wynne et al. 2015). Barium-to- Finally they were glued onto standard glass slides and calcium ratio (Ba:Ca), however, generally displays a stored in clean plastic bags for later analysis. Otolith negative relationship with salinity (Walther and 43Ca, 86Sr and 137Ba were quantified with laser ablation Limburg 2012), but the usefulness of this marker seems inductively coupled plasma mass spectrometry (LA- to vary among locations (e.g., Crook et al. 2006;Rohtla ICPMS) in WM Keck Collaboratory of Plasma et al. 2015). Although the relationships between otolith Spectrometry (Oregon, USA). A VG PQ ExCell Sr:Ca or Ba:Ca and salinity have not been validated for ICPMS with a New Wave DUV193 excimer laser was Atlantic tarpon, a validated relationship between otolith used. Before analysis, all otoliths were ultrasonically Environ Biol Fish (2016) 99:593–602 595 Table 1 Recorded biological parameters of sampled Atlantic cleaned for 15 min in NANOPure water and dried in a tarpon from the offshore waters of French Guiana. Asterisk de- laminar flow hood. The laser was set at 7 Hz with a notes the individuals in which only the heads were obtained. All μ μ −1 measured lengths are in cm and masses in kg. JOL denotes the 40 m ablation spot size and a scan speed of 10 ms . additional length measurement that was taken from the tip of the A continuous line scan was traced from the core area to closed lower jaw to the tip of the opercule to help to estimate the the ventral edge (Fig. 1), although not all transects size of the individuals in which only the heads were available actually reached the edge of the otolith because some ID TL FL SL JOL SW TW edges were still embedded into epoxy resin. Helium was used as a carrier gas. A glass reference material 1 130 112 108 - 14.3 - (NIST612) was analysed before and after every 6–10 2 111 94 92 - 8.9 - otoliths. Data reduction was achieved by following the 3 918176-4.9- methods of Miller (2007) as described in Rohtla et al. 4 988581216.5- (2014). A nine-point running mean followed by nine- 5 102 96 86 22 7.6 - point running median was used to reduce the noise and 6 121 108 105 - 12.1 - smooth the Sr:Ca and Ba:Ca data. 7 120 106 102 23 11.5 - In order to infer detailed fish migration patterns using 8 847673-4.5- otolith Sr:Ca and Ba:Ca profiles threshold values of the 9*---17-- relevant biomes or habitats must be established.