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A Contribution to the Life History of Tarpon (Megalops atlanticus) in the Northern Gulf of Mexico
William Stein III, Jonathan Shenker, and Martin T. O’Connell

Southeastern Naturalist, Volume 15, Issue 3 (2016): 496–512

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Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 496 2016 SOUTHEASTERN NATURALIST 15(3):496–512 A Contribution to the Life History of Tarpon (Megalops atlanticus) in the Northern Gulf of Mexico William Stein III1,*, Jonathan Shenker2, and Martin T. O’Connell1 Abstract - Megalops atlanticus (Tarpon) is a highly migratory species that supports an economically important recreational fishery in the Gulf of Mexico. Despite this, little is known about Tarpon life history in the northern Gulf of Mexico. Collections of Tarpon leptocephali, young-of-the-year, juveniles, sub-adults, and adults from waters of southeastern Louisiana and the adjacent northern Gulf of Mexico suggest that the northern region may provide valuable habitats for the species at the present time, and potentially be of greater importance as climate change progresses. Introduction Megalops atlanticus Valenciennes (Tarpon) is a culturally and economically important marine fish that supports artisanal, commercial, and recreational fisheries across the tropical and temperate western Atlantic Ocean, Caribbean Sea, and Gulf of Mexico (GOM). Tarpon are fished commercially for consumption in Venezuela, Colombia, parts of Central America, and Mexico (Adams et al. 2014, Cruz-Ayala 2002, Silgado 2002), and sport fishing for Tarpon is an important source of revenue on the coasts of the GOM, southeast United States, and Caribbea n Sea. It is generally accepted that Tarpon stocks began to decline during the 1960s– 1970s (Adams et al. 2014, Dailey et al. 2008). However, how much of the observed decrease is a result of overfishing, loss of nursery habitat, climate change, anthropogenic impacts, or other unrecognized factors has yet to be determined (Adams et al. 2014). Despite their importance as a gamefish, we lack many specifics on Tarpon ontogeny, and have only a general idea of their life history. Successful conservation strategies depend on determining which life stages occur in which locations, understanding the vulnerability of the critical habitats, and assessing the temporal, spatial, and ontogenetic connectivity between habitats. Tarpon have a complex life cycle that incorporates many habitats (Cocheret de La Morinière et al. 2002, Coleman et al. 2000, Scharf 2000, Winemiller and Rose 1992). Previous studies have demonstrated that Tarpon spawn in the southern GOM and Caribbean Sea (Ault et al. 2008; Crabtree 1995; Crabtree et al. 1992, 1995). Spawning may occur at depths off the edge of the continental shelf. Tarpon larvae (leptocephali) have been collected in open-ocean plankton samples off the shelf edge in the Bay of Campeche, the Yucatan Channel, and off the southwest coast of Florida (Adams et al. 2014, Ault 2008, Crabtree et al. 1992, Eldred 1968, Smith 1980). Leptocephali grow in offshore waters and move inshore on currents, entering coastal marshes within 30 1Pontchartrain Institute for Environment Sciences, University of New Orleans, New Orleans, LA 70148. 2Department of Biological Science, Florida Institute of Technology, Melbourne, FL 32901. *Corresponding author - Manuscript Editor: Dave Blewett Southeastern Naturalist 497 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 to 40 days (Shenker 2006, Zerbi et al. 2001). Although the cues for transformation from leptocephalus to juvenile Tarpon remain elusive, the process begins during the inshore migration (Crabtree et al. 1992, Shenker et al. 2002, Zerbi et al. 2001). Young-of-the-year (YOY) Tarpon occur in mangrove marshes in the Caribbean, southern GOM, and east coast of Florida (Jud et al. 2011, Shenker 2006, Zerbi et al. 2001). They are commonly collected along the southwest coast of Florida and from the Yucatan Peninsula (Crabtree 1995, Smith 1980). Young-of-the-year Tarpon also have been reported from Alabama (J. Franks, Gulf Coast Research Laboratory, University of Southern Mississippi, Ocean Springs, MS 2015 unpubl. data) as well as Mississippi, Texas, and Louisiana (Franks et al. 2008, Howells and Garrett 1992). Louisiana marshes and coastal environments are at or near the northern limit of Tarpon in the Gulf of Mexico, and cold winter temperatures may impact the survival or movement of Tarpon. It remains unknown if YOY Tarpon survive and over-winter in interior marsh nursery areas along the northern GOM coast where water temperatures can fall below 9–10 °C, the presumed lethal threshold for YOY Tarpon (Howells 1985, Howells and Garrett 1992, Rickards 1968, R obins 1977). Young-of-the-year Tarpon typically inhabit small fresh to brackish pools of clear to dark-colored water in natural coastal marshes and managed impoundments that are periodically open to tidal flow (Harrington 1966, Poulakis et al. 2002, Rickards 1968, Wade 1962). The habitats are often stagnant, but juvenile Tarpon are able to withstand low oxygen saturation as they possess a physostomous vascularized airbladder and gulp air at the surface (Lewis and Gilmore 2007, Rickards 1968, Wade 1962). After their initial year in marsh habitats, typically at sizes ranging from 150 to 250 mm fork length (FL) (Cyr 1991, Rickards 1968), they may emigrate into larger embayments and nearshore coastal waters. Female Tarpon reach sexual maturity on the west coast of Florida by age 10 years and 1285 mm FL; Florida males reach sexual maturity by 1175 mm FL (Crabtree et al. 1997). In the western Atlantic, adult Tarpon are found throughout the GOM and Caribbean Sea and along the southeast coast of the United States and the northeast coast of South America. Adults occur in northern GOM nearshore waters between Texas and northwest Florida during summer and fall, and undergo regional, long-distance seasonal migrations (Ault et al. 2008, 2009; Luo et al. 2008). The recreational Tarpon fishery in the temperate regions of the northern GOM and southeastern US coast are supported by adults that seasonally migrate northward from the tropics and subtropics. Temperate regions host multiple life stages of Tarpon. This study used multiple methods to determine the extent to which multiple life stages of Tarpon are present in the northern GOM and coastal Louisiana. These results have important implications with regards to future management efforts and provide a better understanding of Tarpon ecology. Methods We used a variety of methods to assess the occurrence of all life stages of Tarpon: leptocephalus larvae, young-of-the-year (YOY 40–300 mm FL), juveniles/ sub-adult (300–1300 mm FL), and adults (>1300 mm FL). Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 498 Leptocephali (larvae) The National Marine Fisheries Service Southeast Area Monitoring and Assessment Program (SEAMAP) conducts icthyoplankton sampling cruises throughout the northern Gulf of Mexico. From 1980 until 2012, 3 cruises (spring, summer, fall) have been typically conducted each year, with sampling effort and station locations varying among cruises dedicated to different research objectives. Bongo and neuston net collections were made at each station, using methods outlined in Muhling et al. (2010). Leptocephali were taken in subsurface samples by bongo nets with 333-μm mesh. Specimens initially identified as Elopomorph larvae were provided to us by SEAMAP and further identified to species using the morphological and pigmentation characteristics described by Smith (1989). We characterized Tarpon leptocephali as yolk-sac, pre-flexion, and post-flexion developmental stages, and measured notochord length for the first 2 stages and standard length (SL) for postflexion larvae. Sampling areas for leptocephali collected by SEAMAP are identified as site N in Figure 1. We examined collections of the Tulane University Royal D. Suttkus Museum and archived collection data from the Nekton Research Laboratory (NRL) of the University of New Orleans and the Louisiana Department of Wildlife and Fisheries (LDWF) for records of Tarpon leptocephali. Figure 1. Map of locations of samples collected in southeastern coastal Louisiana. A: Lake Pontchartrain Land Bridge, B: Chef Menteur Pass, C: Bayou Bienvenue Hot Water Canal, D: Biloxi Marsh, E: Hopedale, F: Myrtle Grove and Bayou Bell, G: Port Sulphur Roadside Ditch, H: Grand Isle and Port Fouchon, I: Fourleague Bay, J: Bayou Black: Gibson, K: Bayou Rambio, L: Main Pass Lease Blocks, M: South Pass of the Mississippi River and West Delta Lease Blocks, N: Offshore Locations of SEAMAP leptocephali collections, O: Grand Isle Lease Blocks, P: South Timbalier Lease Blocks, Q: Lake Quitman and Bayou Carlin. Southeastern Naturalist 499 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 Fishery surveys for YOY, juvenile/sub-adult, and adult Tarpon From May 2010 through December 2012, we conducted several intensive fishery surveys that provided data on distribution and abundance of YOY, juvenile/subadult Tarpon in the inland marshes and coastal waters of southeastern Louisiana. The New Orleans Land Bridge Study (NOLBS) study was conducted on the New Orleans Land Bridge east of the Gulf Intracoastal Waterway, from Chef Menteur Pass to the Louisiana–Mississippi State Line east of Pearl River Island (sites A and B in Fig. 1). During 29 consecutive months (April 2010–July 2012), we took 2360 samples with a 1.8-m–radius cast net (6-mm monofilament net) and collected over 66,000 fishes and decapods in this survey (Stein 2013). The Port Sulphur Study (PSS), near the coast of Barataria Bay, targeted 2 sites (sites F and G in Fig. 1) of Louisiana marshland, again using a 1.8-m cast net, in October through December 2010 and once in November 2011. We also sampled 1 of these sites once in October 2011 by electrofishing (Fig. 1, site G). In addition to our own sampling, databases from 3 fisheries-independent studies were examined for records of larval, YOY, and juvenile/sub-adult Tarpon (1970 through 2012): (1) the LDWF Finfish and Shrimp monitoring program from 1990 through 2010 (methods outlined in LDWF 2002); (2) University of New Orleans NRL gillnet, trawl, and seine net sample records from 2002 through 2012 (O’Connell et al. 2004, 2014); and( 3) samples from the Tulane Museum of Natural History. We also collected fisheries-dependent data that relied on the contributions of citizen scientists (Henderson 2012). Unlike surrounding states, Louisiana does not have any restrictions on the capture of Tarpon by anglers or divers. As part of our Angler Program, we contacted anglers across southeastern Louisiana at marinas, boat launches, and through the Louisiana Tarpon Club. Several educational talks were given to anglers explaining the purpose of the research and how it was to be conducted. We encouraged participating anglers to measure, photograph, release, and report their Tarpon catches. However, some anglers, fishing tournaments, and divers kept Tarpon after capture, enabling us to examine the size, age, and reproductive status of some fish. Sporting goods stores, boat launches, bait suppliers, boat storage facilities, fishing clubs, and individuals were contacted across southeast Louisiana from October 2009 through July 2012 and encouraged to report specimens of YOY and juvenile/ sub-adult Tarpon. We distributed 250 four-color, 8.5 x 11 inch flyers that included a photograph of a YOY Tarpon and a request to call the researchers when one was caught. We asked for photographic evidence of capture, size, and GPS coordinates from the anglers. Larger Tarpon are generally fished from July until October. We asked anglers to report catches of juvenile/sub-adult (300 mm and 1300 mm FL) and adult Tarpon (>1300 mm FL) and to photograph and weigh each fish, measure its FL, record the location and date of capture, note the presence of any witnesses, and release their catch alive. During this study, we attended 17 fishing rodeos and tournaments to obtain carcasses for dissection to assess gonadal condition. Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 500 For the Diver Program, we contacted commercial and recreational (SCUBA and free) divers from southeast Louisiana through the Louisiana Council of Underwater Dive Clubs (LCUDC) and enlisted their members to assist with observations and collections of juvenile/sub-adult and adult Tarpon. We gave a series of educational talks to explain our research goals and requirements at LCUDC and individual member club meetings. We asked members of the LCUDC to document all sightings of Tarpon, and several divers provided specimens collected by spear from March 2010 through June 2012. Visual surveys were performed by LCUDC divers at petroleum platforms in the northern GOM off Louisiana and Mississippi (sites L, M, N, O, P in Fig. 1) during this same time period, weather permitting. Tarpon collected by divers were weighed (kg) and measured (FL, mm). We recorded the Bureau of Energy Management (BOEM) Gulf of Mexico lease block numbers of collection location, name of diver, and depth (m), temperature (°C), date, and time of collection for each fish where provided. We asked divers to obtain video whenever possible to document the presence of juvenile/sub-adult and adult Tarpon from March 2010 through June 2012, and to review their earlier video records to provide data on seasonality and location of fishes observed prior to the present study Finally, to develop a historical perspective on the Tarpon fishery in Louisiana, we reviewed hook-and-line records from 1984 through 2011, documented and kept by the Louisiana Outdoor Writers Association, and the Louisiana Official Spearfishing Records from 1995 through 2010, maintained by the LCUDC (LCUDC Records), to determine when and where Tarpon were collected and the sizes of the fish. Environmental data We obtained surface-water temperature data from 2 sites in the Barataria Basin from USGS water-quality monitoring buoys. USGS Station 292800090060000 is located in the upper basin in Little Lake near Bay Dosgris east of Galliano, LA, and Station 291929089562600 is located in the lower basin just north of Grand Terre Island (sites 1 and 2 in Fig. 2; NWIS 2015a, b). Near-surface sensors measure temperature continuously at both sites. Results Our collections during 2010 through 2012 confirmed that 4 life stages of Tarpon (leptocephali, YOY, juveniles/sub-adults, and adults) were present in southeastern Louisiana estuarine and coastal waters. Leptocephali An analysis of the leptocephali from the SEAMAP plankton trawls confirmed 23 yolk-sac Tarpon leptocephali less than 5 mm notochord length were collected on the Louisiana coast in August 2009 and September 2011 and 2012 (Fig. 2, Table 1). These leptocephali were collected at 4 separate SEAMAP sites located along the edge of the continental shelf south of the Mississippi River delta. These samples represent the first confirmed Tarpon leptocephali collected from Louisiana off-shore waters. Southeastern Naturalist 501 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 A review of the Tulane Museum records revealed that only a single 27-mm (SL) Tarpon leptocephalus is currently accessed into the museum collections (TU164723). Records indicate this specimen was collected with a seine in the Freeport Canal (15 km south of G in Fig. 1) in fall 1973. A review of the data collected by the NRL yielded no records of Tarpon leptocephali in the Pontchartrain Estuary or in the LDWF fishery-independent database. No leptocephali were collected during the NOLBS sampling. Young-of-the-year Nearly 300 YOY juveniles were collected by the various research sampling programs and by volunteer anglers (Table 1, Fig. 3). The YOY were generally captured from marsh sites within 60 km of the coastline. Habitats ranged from a deep, narrow, freshwater bayou 60 km from the coast bordered by overhanging cypress and oak trees to open, oligohaline, shallow ponds adjacent to the Gulf of Mexico. The majority of the YOY Tarpon were collected from shallow Spartina marshes and narrow channels connected to the Gulf of Mexico. Young-of-the year Tarpon were collected every month of the year except January. The northernmost YOY Tarpon was a single specimen among the more than 66,000 fishes and decapods collected during NOLBS sampling (at site A in Fig. 1). Closer to the coast of Barataria Bay, Figure 2. Map showing collection locations for a total of 23 Tarpon leptocephali taken off the Louisiana coast during 2001, 2009, and 2011 NOAA/SEAMAP plankton sampling (solid triangles) and locations of adult Tarpon (>1300 mm fork length) collected in Louisiana during 2011–2012 (solid dots), along with location of photographs and video winter adults in 1999 and 2005 (WA99, WA05, respectively): spawning-capable female (SCF), spent female (SF), and spawning-capable males (SCM). Sites of water-temperature plots: (1) Bay Dosgris, (2) Barataria Bay at Grand Terre Island. Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 502 from PSS sampling, 79 YOY Tarpon were caught in 5 separate collections with cast nets from October through December 2010, and a single YOY Tarpon was collected at the same location in November 2011 (site G in Fig. 1). In October 2012, another YOY was collected there by electrofishing in the low-salinity (4 pp t) water. Monthly LDWF seine, gill net, and otter trawl collections from 1990 to 2010 contained records of 14 YOY Tarpon (Table 1, Fig. 3). All of these were collected from April through December in bays and bayous of southeast Louisiana. Eight of these were collected at the same location in Bayou Rambio during 5 different years (site K in Fig. 1). Two were collected on different dates during the same year at Bayou Bell (10 km west of K in Fig. 1). Two YOY Tarpon were collected in the upper Barataria Basin (15 km north of H in Fig. 1) and 2 more in the marsh east of the Mississippi River (10 km east of F in Fig. 1). Water temperatures ranged from 10 °C to 26 °C and salinity from 0.8 ppt to 12.6 ppt. There were no samples of YOY Tarpon in the NRL or Tulane Museum collections. Table 1. Collections of Tarpon by life stage. Data source abbreviations: Tulane University Royal D. Suttkus Museum (TU), Southeast Area Management and Assessment Program (SEAMAP), Louisiana Department of Wildlife and Fisheries Finfish and Shrimp Monitoring Program (LDWF), Port Sulphur Study (PSS), Louisiana Council of Underwater Dive Clubs (LCUDC), New Orleans Land Bridge Study (NOLBS), University of New Orleans Nekton Research Laboratory (NRL). Gear type abbreviations: PN (plankton net), H (hook and line), CN (cast net), T (otter trawl), EF (electrofishing), SG (spear gun), V (video), and NA (not available). Life Stage/data source Number Collection dates Size (mm) Gear Leptocephalus Fishery-Independent TU 1 1975 Stage ? PN SEAMAP 23 Aug. 2009–Sept. 2011 Stage 1 PN Young-of-the-Year Fishery-Independent LDWF 14 Nov. 1990–Nov. 2010 less than 300 T PSS 79 Oct. 2010–Nov. 2011 55–285 CN PSS 1 Oct. 2012 193 EF NOLBS 1 Oct. 2011 177 CN Fishery-Dependent Angler Program 186+ Aug. 2010–June 2012 56–280 H, CN NRL 7 Sept. 2012 less than 350 H Juvenile/sub-Adult Fishery-Dependent Angler Program 26 May 2010–Dec. 2012 less than 1200 H Dive Program 30 July 2011–July 2012 760–1180 SG LCUDC Video 14 2005 NA V Adult Fishery-Dependent Angler Program 10 July 2011–July 2012 1676–1892 H Dive Program 3 July 2011–July 2012 1575–1867 SG Shrimp Net 1 Dec. 2012 NA LCUDC Records 11 1999–2005 >1550 SG LCUDC Video 100s 1999, 2005 NA V Southeastern Naturalist 503 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 Fisheries data provided by volunteer anglers identified 19 additional sites from which 186 YOY Tarpon were collected. More than 100 YOY Tarpon were collected in the months of July and August in 2009 and 2010 by cast net in Hopedale (site E in Fig. 1). A cast-netter in the marsh north of Fourleague Bay (site I in Fig. 1) collected more than 30 YOY Tarpon on 30 October 2012 (Table 1, Fig. 3). On one day in fall 2012, two members of the NRL staff hooked and clearly identified, but did not land, 7 YOY Tarpon in the Biloxi Marsh (site D in Fig. 1). Most of the YOY Tarpon collected by anglers were taken during the months of June through December. However, members of the Louisianna Tarpon Club reported also catching them in February and March in a “hot-water” discharge canal for a petroleum processing plant on Bayou Black near Gibson, LA, 60 km from the Gulf of Mexico (site J in Fig. 1). During June through September 2010, ten YOY Tarpon were caught in this location. Temperature in the canal stays between 30 °C and 36 °C year round, and salinity is less than 1.5. Digital photographs and 5 specimens between 180 mm and 400 mm FL were submitted by members of the Louisianna Tarpon Club during 2011 from this bayou. Club records indicate YOY Tarpon were collected there yearly from 2007 to 2011. Juvenile/sub-adult Volunteer anglers and divers provided data describing the observation or capture of 70 juvenile/sub-adult Tarpon (Table 1, Fig. 4). We examined a total of 26 juvenile/sub-adult Tarpon that were collected by the Angler Program from March 2010 through June 2012. The initial fisheries-dependent collection of Tarpon was at the Southwest Pass Jetty of the Mississippi River in May 2010 (site M in Fig. 1). Figure 3. Map of the locations (solid dots) of young-of-the-year (YOY) Tarpon (~40 mm to 300 mm fork length) collected in southeast Louisiana during 199 0–2012. Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 504 Juvenile/sub-adult Tarpon were frequently reported released by anglers during July and August 2010 in the Barataria Bight in bays and around jetties of the Mississippi River, and around shallow oil-production platforms (sites M, N, O, and P in Fig. 1; Fig. 4). A single 850-mm Tarpon was caught in the Pontchartrain Estuary in Lake Borgne in August 2011. Juvenile/sub-adult Tarpon were also found in specialized habitats in southern Louisiana. On Christmas Day 2011, seven juvenile Tarpon were captured and released in a “hot-water” canal from a power-generation plant at Bayou Bienvenue in eastern New Orleans (confirmed via photographic evidence; site C in Fig. 1). The angler caught and released 9 more Tarpon at the same location during the first week of December 2012. Photographs from the summers of 2010 and 2011 confirmed 10 juvenile/sub-adult, and reports were received of at least 35 more landed by anglers from Bayou Black during June, July, and August 2010 (site J in Fig. 1). Fisheries-independent data from the Tulane Museum and LDWF did not include any records of juvenile/sub-adult Tarpon. Collections by NRL in Lake Pontchartrain and Lake Borgne as well as collections in bays and bayous of southeastern Louisiana did not include any juvenile or sub-adult Tarpon. Results from the Diver Program included over 40 records (collections and sightings) of juvenile/sub-adult Tarpon (Table 1, Fig. 4). Thirty specimens were collected and submitted by spear fishers from coastal Gulf of Mexico waters less than 20 m deep within 15 km of land. Two of these Tarpon were collected on 10 December 2012 by divers at different oil-production platforms in the same area. Divers also recorded video of 14 juvenile/sub-adult Tarpon schooling in Barataria Bight during October 2012. Figure 4. Map of locations (solid dots) of juvenile and sub-adult Tarpon (~300 mm to 1300 mm fork length) collected by divers and fishers in Louisiana dur ing 2011–2012. Southeastern Naturalist 505 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 Adult All of the adult Tarpon landed or filmed as part of the Angler and Dive programs were from the open waters of the GOM within 50 km of the Louisiana coast (Table 1, Fig. 2). During 2011 and 2012, fourteen adult Tarpon were collected and examined from anglers; all adult Tarpon reported to us by anglers for this study prior to June 2011 were released alive and not landed. The first adult Tarpon landed by an angler and reported to us was on 2 July 2011 at the Port Fouchon Tarpon Rodeo. Subsequently, 4 adult Tarpon, 2 males and 2 females, were dissected and found to be capable of spawning (Stein et al. 2012). Three adult Tarpon were landed as part of the Dive Program; all were collected immediately adjacent to oil-production platforms. The only documented adult Tarpon in this study, not collected offshore, was caught in Chef Menteur Pass (Pontchartrain Basin) on 25 November 2012, in the wing net of a boat shrimping between the Highway 90 Bridge and the railroad bridge to the south (Table 1; site B in Fig. 1). This Tarpon was photographed and released alive, but no measurements were obtained. Video and still photographs supplied by members of LCUDC as part of the Dive Program included 3 videos of Tarpon congregating in large numbers at oil-production platforms in Main Pass Block 299 on 14 February and 21 February 1999, and at Main Pass Block 296 on 19 March 2005 (site L in Fig. 1, WA99 and WA05 in Fig. 2). We interviewed divers present on each of these dives and authenticated the videos. Records of the largest adult Tarpon collected by divers from 1995 through 2010, obtained from the official LCUDC record book, confirm that 9 of the 11 largest Tarpon (>1500 mm) were landed during February and March. Water temperature Temperature records from November 2007 through November 2014 were obtained from 2 USGS water-monitoring buoys in the upper and lower Barataria Basin. Water temperature remained above 9° C in 3 of 7 years in the lower basin and in 2 of 7 years in the upper basin (Fig. 5). During the winter of 2008, water temperature dropped below 9° C on 2 successive days in the upper basin and during 1 day in the lower basin. During winters 2009–2010, 2010–2011, and 2013–2014, surface-water temperatures fell to approximately 5° C on multip le days. Discussion This study provides evidence of spawning and occurrence of 4 life stages of Tarpon in the northern GOM. The lone leptocephalus collected in 1973 provided the first tenuous suggestion of spawning activity on the Louisiana coast. The recent SEAMAP sample data showing the presence of less than 5-mm yolk-sac leptocephali infer the occurrence of spawning activity as late as August or September along the edge of the continental shelf south of the Mississippi River delta (at site N in Fig. 1). The SEAMAP samples were limited in time, location, and sampling effort, but the collection of newly hatched Tarpon leptocephali shows this region is a spawning area for Tarpon. Our report of the spawning-capable Tarpon, based on histology, is Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 506 further evidence that spawning occurred (Stein et al. 2012). The absence of leptocephali in the other collections may be related to the sampling gear because seines, otter trawls, and gill nets with mesh sizes greater than 9 mm were utilized in these studies (O’Connell et al. 2004, 2014). During the spawning season (June–July), persistent southwest and south winds create onshore transport (Walker et al. 2005), forcing water from the spawning grounds (Fig. 2) towards the northeast into Barataria Bight, at which point the flow nears the coastal current proximal to the Barataria Bay inlets. Wind reversals during the summer coupled with summertime flushing processes between Barataria Bay and the proximal coastal ocean (Li et al. 2011) create opportunities for larvae to enter Barataria Bay, or continue westward, following the downcoast currents towards Atchafalaya Bay (Li et al. 2011, Walker et al. 2005). Since larval duration is about 20–25 days, these larvae could be advected into the coastal marshes during tidal exchange. Likewise, leptocephali spawned east of the Mississippi River Delta could be entrained northward along the Delta into the Chandeleur and Mississippi sounds and then eastward along the coast of Mississippi, where they have been documented in Ocean Springs, MS (J. Franks, Gulf Coast Research Laboratory, University of Southern Mississippi, Hattiesburg, MS, unpubl. data). Sampling in coastal marshes of southeastern Louisiana regularly produced YOY Tarpon that presumably were derived from spawning in the northern GOM. Young- Figure 5. Temperature plots for Barataria Basin for 7 years, 2008–2014, at Grand Terre Island (USGS Station 291929089562600, top) and Bay Dosgris (USGS Station 292800090060000, lower). Shaded line = lower tolerable temperature limit for Tarpon. Southeastern Naturalist 507 W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 of-the-Year Tarpon were collected or observed from a wide range of habitats such as mesohaline ditches, open bays (e.g., Barataria Bay), and tidal sloughs. These are similar types of habitats that support YOY Tarpon in Florida (Gilmore et al. 1982, Poulakis et al. 2002, Zale and Merrifield 1989). Although YOY were found across Louisiana coastal marshes, they disappeared from these shallow habitats by early winter (except those in heated-effluent channels). The lethal minimum temperature for YOY Tarpon is thought to be about 9° C–10° C (Howells 1985, Robins 1977, Wade 1962, Zale and Merrifield 1989), although this may be a function of the rate of temperature decline. In some years, near-surface water temperatures did not fall below 9° C in the Barataria Basin, while near-surface water temperature fell below 9° C for several hours to a day or longer during other winters. The occurrence of juvenile/sub-adult Tarpon in coastal marshes during the summer months suggests that at least some YOY may survive some winters. Habitats for juvenile Tarpon in Florida are generally larger and deeper than YOY habitats, and often include vegetation-lined waterways. These juveniles have a wide salinity tolerance, occurring in fresh, mesohaline, and full seawater (Gilmore et al. 1982, Jud et al. 2011, Rickards 1968, Shenker 2006, Wade 1962, Zerbi et al. 2001). Analogous habitats in southeastern Louisiana are dominated by Juncus roemerianus Scheele (Black Needle Rush), Spartina alterniflora Loisel (Smooth Cordgrass), and Spartina patens (Aiton) Muhl (Saltmeadow Cordgrass) rather than mangroves as in Florida. Although juvenile/sub-adult Tarpon were observed in marsh habitats only during summer months (except at warm-water discharge sites), larger juveniles/ sub-adults exceeding 800 mm were observed or collected in nearshore waters of the GOM, including many collections in the Barataria Bight during both warm and cold months. Our data suggest that YOY and juvenile/sub-adult Tarpon utilize shallow marsh habitats for a period of growth during warmer months, and begin to move out of these habitats when temperatures drop, moving towards open bays and coastal waters where waters are deeper and temperatures are likely to be more regulated. Videos obtained during the winters of 1999 and 2005 showing adult Tarpon at offshore oil-production platforms in the northern GOM demonstrate that not all Tarpon migrate out of the northern GOM in winter as has been suggested (Luo et al. 2008). Collectively, our observations suggest that successful recruitment of juvenile Tarpon follows a “boom-or-bust” cycle. The boom years are a result of successful spawning, transport of leptocephali into marsh nursery areas, or a result of warm winters and successful over-wintering by YOY, or all of these. Tarpon are highly mobile and have been demonstrated to migrate from Mexico and Florida to Louisiana coastal waters near Marsh Island and the mouth of the Mississippi River in summer (Ault et al. 2008, 2009; Luo et al. 2008). Adult Tarpon occur yearly in Lake Pontchartrain and in large bays and tidal inlets of the GOM, and are pursued by local anglers. It is unknown whether or not there are multiple Tarpon stocks in the GOM (Garcia De Leon et al. 2002), but gene flow appears to be extensive and suggests one large meta-population with little divergence except Southeastern Naturalist W. Stein III, J. Shenker, and M.T. O’Connell 2016 Vol. 15, No. 3 508 perhaps in South Florida and Costa Rica (Blandon et al. 2003, Garcia De Leon et al. 2002, McMillen-Jackson et al. 2005, Seyoum et al. 2008, Ward et al. 2008). All life stages of Tarpon utilize the northern GOM: they spawn offshore, YOY and early juveniles use marshes, and sub-adults and adults use offshore habitats where some are resident year-round. It is unknown whether or not early life stages inhabiting the marshes and coastal waters survive and enter the adult population. Our observational and temperature data suggest that the northern GOM marshes are a marginal habitat that may permit survival in some years but not in others when temperatures decrease below lethal minima. The northern GOM may thus make some contribution to the overall GOM population, but that contribution may vary widely among years. As the GOM region undergoes climate change and increasing temperatures, there is potential, under certain conditions, for this contribution to become more important (Church and White 2006, Parmesan and Yohe 2003). Although increasing temperatures may increase survival and growth of early life stages, it is unknown how climate change will alter available habitats, food and prey resources, and predator levels (Doney et al. 2012, Portner and Peck 2010, Rijnsdorp et al. 2009, Scavia et al. 2002). Acknowledgments We express our sincere appreciation to the anonymous reviewers whose insightful comments were helpful in the preparation of this manuscript and to the invaluable assistance of the editor who helped get our manuscript into final form. Dr. Ioannis Georgiou’s assistance with the figures was gratefully appreciated. 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