Why Alligator Gar Need Floodwaters, by Don Orth

Can we save one of the largest fish in North America with floodwaters?  The Alligator Gar Atractosteus spatula is the largest of seven species of Gar found in Central America, Cuba, and North America.  The IGFA world record is 279 pounds, but larger ones have been reported indicating that that can grow up to ten feet and and 350 pounds. An 8 ft. Alligator Gar weighed 254 pounds with a girth of 44 inches was snagged in Lake Texoma and is the largest fish ever caught in Oklahoma waters. Alligator Gar are imperiled due to reduced abundance and diminished range.  In the past, little attention was paid to management or conservation of Alligator Gar. However, Alligator Gar are vulnerable to overfishing and rivers in its range are highly altered due to dams, dikes, dredging, and other forms of habitat and flow alteration.  A recent investigation reported by Robertson et al. (2018), confirmed suspicions that the Alligator Gar are dependent on seasonal flooding in large floodplain rivers.

The gar family (Lepisosteidae) have been around since the Cretaceous Period (~100 million years  ago.  Gars and bowfin are the sister group to other teleost fishes and, therefore, of interest to evolutionary biologists. The largest gars are in the genus Atractosteus, the three extant species are Alligator Gar (or Catan in Mexico), the Cuban Gar A. tristoechus or Manjuari from western Cuba, and the Tropical Gar A. tropicus (or Pejelagarto) from southern Mexico and Central America. Among these three, the Alligator Gar is most imperiled. Gar are fascinating and misunderstood creatures, and unfortunately, the influence of habitat restoration for gars has not yet been fully explored.  Efforts are now underway to restore these magnificent creatures via supplemental stocking.  It will take many years, up to 50 years, for stocked Alligator Gar to reach the potential maximum sizes.  Supplemental stocking is an uncertain and expensive short-term strategy.  Until natural spawning and rearing habitats can be restored, supplemental stocking is necessary.

Alligator Gar that weighed 108 pounds was sampled May 27, 2015 by Florida Fish and Wildlife Research Institute. Creative Commons by NC-ND-2.0. Source.

Ten-foot long Alligator Gar photographed in 1920 from Mhoon Landing, Mississippi River. Public Domain

Managers need to understand what drives populations of Alligator Gar if the species has any chance to be restored throughout its range (O'Connell et al. 2007; Buckmeier et al. 2017).  Although the effects of hydrologic modification of rivers is well document, the prevailing questions related to re-establishing ecologically sustainable flows, such as "How much?" and "How often?" remain unanswered (McManamay et al. 2013).  Fully mature Alligator Gar may produce 157,000 large eggs (2-4 mm in diameter).  These BOFFFF (= big old fat fecund female fish) need do be protected and we also need to provide habitat so that they will spawn naturally.  What is suitable habitat?  The life history of Alligator Gar is tuned to life in floodplain rivers where spawning is synchronized with the high flow pulse events (Buckmeier et al. 2017).  The Robertson et al. (2018) study examined the extent of potential spawning habitat in the Trinity River, downstream from Dallas, Texas. Trinity River is supports guided fishing for trophy size Alligator Gar and is becoming a model for Alligator Gar management elsewhere. 

Map of Spawning Habitat in Floodplains of Trinity River, Texas.  Robertson et al. (2018)

This research used hydraulic models to predict water surface elevations and digital elevation models from LIDar (Light detection and ranging) data. These fine scale models quantified the extent to which floodwaters inundated large expanses of vegetated habitats in low lying floodwaters.  Alligator Gar spawning habitat was mapped as floodwaters between 0.2 and 2 meters deep over woody vegetation and open canopy vegetation types. The plot below shows a dramatic increase in total spawning habitat available as the river flow increases enough to spill onto the floodplains. 

Plot of area of Alligator Gar spawning habitat versus river flow.  Robertson et al. (2018).

We now have the making of a 'Field of Dreams' hypothesis.  If you build it, they will come.   If floodwaters are held back in reservoirs for release at another time, Alligator Gar may not receive the cue to initiate the courtship and spawning behavior.  However, if we create large expanses of spawning habitat, will the breeding Alligator Gar come and spawn?   Amount, duration, and timing of spawning habitat appeared to correlate with years of exceptional Alligator Gar recruitment in the Trinity River (Robertson et al. 2018).  Larval Alligator Gar are only about 8 mm long upon hatching.  These fish that may grow to ten feet, yet start off as tiny fragile larvae.  Larvae must attach to substrates with an adhesive organ on the snout.  If the flood pulse is artificially shut off after spawning, recruitment will be reduced.  The longer duration of the flood pulse enhances nursery habitats for young Alligator Gar.

In 2014, Kimmel et al. (2014) witnessed spawning of Alligator Gar in floodplain habitat in the Mississippi river floodplains at St. Catherine Creek National Wildlife Refuge.  A large aggregation of Alligator Gar was observed in a flooded ditch, lined with buttonbush, shrubs, and herbaceous vegetation four miles from the main river channel.   These spawning observations help to validate the habitat suitability criteria used by Robertson et al. (2018). 

Spawning behavior displayed by Alligator Gar observed by Kimmel et al.  (2014) in floodplains of St. Catherine Creek, near Natchez, Mississippi. 

Eggs of Alligator Gar deposited in woody debris and vegetation.  Kimmel et al.(2014).

The lessons from the Trinity River study give us optimism for population restoration here and elsewhere.  The demand for water from the Trinity River is growing from population centers of Dallas-Fort Worth and Houston and flood-pulse management may provide for periodic strong Alligator Gar recruitment.  Gar production in hatcheries may help, but they provide an uncertain number of offspring (Schmidt 2015).  While many are experimenting with spawning Alligator Gar (Mendoza et al. 2002), for example the USFWS does hatchery spawning of Alligator Gar, the restoration of natural habitat when and where it is needed has the best likelihood for long-term sustainable populations. 

Larva of the Spotted Gar Lepisosteus oculatus.  Photo by Konrad P. Schmidt.

References

Buckmeier, D.L., N.G. Smith, D.J. Daugherty, and D.L. Bennett. 2017. Reproductive ecology of Alligator Gar: Identification of environmental drivers of recruitment success.  Journal of the Southeastern Association of Fish and Wildlife Agencies 4:8-17.

Kimmel, K., Y. Allen, and G. Constant. 2014. Seeing is believing: alligator gar spawning event confirms model predictions.  Website  https://lccnetwork.org/blog_entry/seeing-believing-alligator-gar-spawning-event-confirms-habitat-suitability-index  Accessed June 20, 2018.

McManamay, R. A., D.J. Orth, J. Kauffman, and M.M. Davis. 2013.  A database and meta-analysis of ecological responses to stream flow in the south Atlantic region.  Southeastern Naturalist 12(Monograph):1-36. 

Mendoza, R., C. Aguilera, G. Rodríguez, M. Gonz.lez, and R. Castro. 2002. Morphophysiological studies on alligator gar (Atractosteus spatula) larval development as a basis for their culture and repopulation of their natural habitats. Reviews in Fish Biology and Fisheries 12:133–142.

O’Connell, M. T., T. D. Shepherd, A. M. U. O’Connell, and R. A. Myers. 2007. Long-term declines in two apex predators, bull sharks (Carcharhinus leucas) and alligator gar (Atractosteus spatula), in Lake Pontchartrain, an oligohaline estuary in southeastern Louisiana. Estuaries and Coasts 30:567–574.

Robertson, C.R., K. Aziz, D.L. Buckmeier, N.G. Smith, and N. Raphelt.  2018.  Development of flow-specific floodplain inundation model to assess Alligator Gar recruitment success.  Transactions of the American Fisheries Society DOI: 10.1002/tafs.10045

 

Schmitt, K.  2015. Gar farming.  American Currents  40(4):3-9

Sargassum: Essential Habitat or Beach Nuisance, by Don Orth

A vacation at a beach resort near white sandy beaches and aqua blue waters was the plan.  However, the white sandy beaches and surf were covered with a thick mats of Sargassum, a brown macroalgae or 'seaweed.'  As I waded out beyond the floating seaweed, I could not see my feet in the sand, the water was that murky. The resort was along the Caribbean coastline known as the Riviera Maya, Mexico. This coastal zone is famous for its beaches and reef dependent recreation.  Tourism contributes 8.6% to the national GDP of Mexico and 45% of tourists choose the coastal zone as their destination (UNWTO 2016). Pristine beaches and associated recreational amenities are key attractions to coastal tourism worldwide (Onofrio and Nunes 2013). Hotel room prices are positively correlated to the beauty of the beach and recreation (Mendoza-González et al. 2018).  However,  'seaweed' is not part of the marketing plan for resorts.

 

Sargassum on the beach.

I know only a little about Sargassum, and that is about its role as offshore habitat for fish and invertebrates.  There are many species of Sargassum, bu  two species, Sargassum fluitans and Sargassum natans, dominate the floating mats. When trolling for pelagic fish the boat captain will seek out rafts of Sargassum, which attract numerous species (Dooley 1972). Some fishes will complete their entire lives in Sargassum, others use it for breeding, and others use it only for larval and juvenile habitats.  The Sargassumfish Histrio histrio is a frogfish (Antennariidae) that lives its entire life amidst the Sargassum (Pietsch and Grobecker 1987).  Sargassumfish are camouflaged with colors that change and weedy projections so that it blends into the floating seaweed. It dangles its esca as a fishing lure to attract small fish, shrimp and other invertebrates close enough to capture them.  

Floating raft of Sargassum photographed June 3, 2018 near Playa del Carmen, Mexico.

Sampling fishes in and around the floating Sargassum mats is a complicated process.  Therefore, I was not surprised by how few studies investigated the association of Sargassum and fishes. The studies I reviewed showed that small individuals from four or five fish families (Antenariidae, Carangidae, Monacanthidae, Syngnatidae, Tetraodontidae) typically made up over 90% of the sampled catch.  Depending the study duration and lcoation, between 36 to 110 fish species were identified (Dooley 1972; Bortone et al. 1977; Moser et al. 1998; Wells and Rooker 2004; Cassaza and Ross 2008; Moritz 2015).   Larger fishes, such as dolphinfishes, jacks, wahoo, and billfishes aggregated below the weedlines using the Sargassum for feeding habitat.  In addition to the fishes, Sargassum habitat provides feeding habitat for marine mammals, sea birds, and sea turtles (Cassaza and Ross 2008).

Fishes along Sargassum weed line near Cape Hatteras, North Carolina.  A. small planehead filefish; B. larger jacks below weed line; C. large predators, dolphinfish; D. schooling unicorn filefish; E. planehead filefish feeding on ctenophore;  and F. edge of Sagarssum weed line.  (Cassaza and Ross 2008).  

Realizing the importance of Sargassum to recruitment of many marine animals, NOAA approved a Sargassum Management Plan in 2003.  See the final rule here.  Development of this management plan was controversial over the interpretation of Sargassum as an essential habitat versus a harvestable product.  Larval and juvenile fishes of many species will be taken by Sargassum harvesters and effects of harvesting are unstudied. However, the current estimated harvest is miniscule compared with the estimated biomass of Sargassum.

Sargassum underwater near beach at Playa del Carmen, Mexico, photographed June 5, 2018. 

The Saragassum is widely distributed but the occurrence of Sargassum strandings on Caribbean beaches has become more common since 2011 (Franks et al. 2016).  Warming climate and changes in oceanic currents influence the spatial distribution and some researchers believe increases in nutrient loading may be stimulating an increase in Sargassum.  A forecast system based on satellite measurements shows promise for predicting the distribution and timing of Saragassum blooms, thereby providing localities with advance warnings of beaching events (Wang and Hu 2017).   

Locations of pelagic Sargassum and current vectors.  Franks et al. 2016. 

While, we were beach combing, my wife, Valerie, noted a small dead fish in the surf.  We began searching the recently beached Sargassum and found many others, all about 2-3 cm long.  She thought it was a pufferfish and my initial identification was incorrect.   After posting on All Fish Species Identification Group on Facebook, Pete Liptrot and Roy Hemdel identified it as Sharpose Puffer Canthigaster rostrata. 

Sharpnose Puffer Canthigaster rostrata and one puffer Sphoeroides sp. (bottom left) from beached Sargassum.

Sharpnose Puffer Canthigaster rostrata

Tourists in the region quickly learn that Sargassum is a common occurrence, but the quantity that reaches Caribbean beaches has increased.   The Caribbean Alliance for Sustainable Tourism and the Caribbean Hotel and Tourism Association created Sargassum: A Resource Guide for the Caribbean.   This guide examines possible impacts on beach tourism, uses of Sargassum, and management practices for beach resorts. Sargassum is an excellent medium for plant and crop growth and may become a new source of revenue.   Beach cleaning machines are available, but many resorts use manual removal in order to save costs and avoid issues with sea turtles. 

Gabriel raking Sargassum on beach.  Burying the Sargassum provides some relief to the problem.

Sargassum is a living renewable marine resource, a primary producer, and an essential habitat for many marine organisms, including highly migratory species.  For that reason, Sargassum should be managed and protected as essential habitat.  However, tourists expecting the picturesque sandy beaches will be dismayed to walk amidst the decaying Sargassum on the beaches. In the future, Caribbean beach resorts will need to adapt to changing climate conditions and the delivery of excessive amounts of Sargassum to pristine beaches. 

References

Casazzarro, T.L., and S.W. Ross. 2008. Fishes associated with pelagic Sargassum and open water lacking Sargassum in the Gulf Stream off North Carolina.  Fisheries Bulletin 106:348-363.

Dooley, J. K. 1972. Fishes associated with the pelagic Sargassum complex, with a discussion of the Sargassum community. Contributions in Marine Science 16:1–32.

Franks, J.S., D.R. Johnson, and D.S. Ko. 2016. Pelagic Sargassum in the tropical North Atlantic.   Gulf and Caribbean Research 27: SC6-11.

Mendoza-González, G., M.L. Martínez, R. Guevara, O. Pérez-Maqueo, M.C. Garza-Lagler, abd A. Howard, 2018. Towards a sustainable sun, sea, and sand tourism: The value of ocean view and proximity to the coast. Sustainability 10:1012-1026.

Moritz, T. 2015. Fishes of a stranded Sargassum meadow at Punta Cana, Dominican Republic. Bulletin of Fish Biology 15:141-146.

Moser, M.L., P.J. Auster, P.J., and J.B. Bichy. 1998. Effects of mat morphology on large Sargassum-associated fishes: observations from a remotely operated vehicle (ROV) and free-floating video cameras.    Environmental Biology of Fishes 51: 391-398.  

Onofri, L., and P.A.L.D Nunes. 2013. Beach ‘lovers’ and ‘greens’: A worldwide empirical analysis of coastal tourism. Ecological Economics 88:49–56.

Pietsch, T.W., and D.B. Grobecker.  1987. Frogfishes of the world. Stanford University Press, Stanford, CA, p 420

United Nations World Tourism Organization (UNWTO). Tourism Highlights. 2016. Available online: http://www.e-unwto.org/doi/pdf/10.18111/9789284418145 (accessed on 11 June 2018).

Wang, M. and  C. Hu. 2017. Predicting Sargassum blooms in the Caribbean Sea from MODIS observations. Geophysical Research Letters 44:3265-3273.

Wells, R.J.D., and J.R. Rooker. 2004. Spatial and temporal patterns of habitat use by fishes associated with Sargassum mats in the northwestern Gulf of Mexico.  Bulletin of Marine Sciences 74:81-99.