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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 - wstein1@uno.edu.
Manuscript Editor: Dave Blewett
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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).
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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.
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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
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2016 Vol. 15, No. 3
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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. A special acknowledgment is given to the hard
work and dedication of the numerous citizen scientists who assisted with the collection of
specimens including members of the Louisiana Council of Underwater Dive Clubs, the Hell
Divers, Sea Tigers, Aqua Aces, and members of the Louisiana Tarpon Club. The Louisiana
Department of Wildlife and Fisheries, the Tulane University Royal D. Suttkus Museum,
and the Aquarium of the Americas made available valuable samples and data. We thank the
SEAMAP program for allowing us access to their ichthyoplankton samples and database,
and for all their efforts in collecting and processing the samples.
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