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