Spotted gar (Lepisosteus oculatus) COSEWIC assessment and status report: chapter 6
Biology
General
The known maximum age is 18 years and maximum length and weight are 1120 mm and 2700 g (Coker et al. 2001). Spotted gar caught in Point Pelee National Park in 2002 and 2003 (n=19) ranged in length between 133 mm and 629 mm and had a maximum weight of 1087 g. Studies on the growth rate of young spotted gar from Oklahoma suggested a growth of 1.7 mm (1 g) per day during July and August (Carlander 1969). Young spotted gars reach a length of 250 mm after the first year of life (Pflieger 1975).
An adhesive organ on their snout, oval-shaped pigmented eyes, and an ovoid and elongated yolk sac characterize recently hatched gars (Simon and Wallus 1989). Spotted gar larvae are darkly pigmented subdermally on the dorsum (Simon and Wallus 1989). Although capable of swimming, they often remain hanging vertically, relatively inactive, attached to underwater structures by their adhesive snout. The yolk sacs are absorbed at lengths greater than 17.6 mm and the spotted gars then become more dispersed and begin to feed (Simon and Wallus 1989).
Reproduction
Spotted gar are 4 years old at onset of maturity and approximately 522 mm in length (Scott and Crossman 1998, Coker et al. 2001). Love (2002) described sexual dimorphism in spotted gar from southern Louisiana. Females had longer bodies and long snouts. He attributed this difference in length between the sexes as likely due to reproductive investment. Females have larger gonads than males per unit of body mass as they are extremely fecund. The large snout of females may enable greater foraging success, possibly indicating that nutritional requirements are greater for females (Love 2002).
Increasing photoperiod and water temperatures at 15°C initiated spring spawning in Louisiana (Snedden et al. 1999), with the most intense spawning occurring mid-May in Oklahoma (Tyler and Granger 1984). Cudmore-Vokey and Minns (2002) reported spawning temperature to range from 21°C to 26°C.
Tyler and Granger (1984) described the spawning behaviour of spotted gar in Oklahoma. One large female, closely accompanied by three to five much smaller males, swam slowly through densely vegetated areas. The female deposited her eggs as she jerked and thrashed in the shallows. The demersal, adhesive, oval (approximately 2.5 mm in diameter) eggs (Simon and Wallus 1989) are formed in masses held together by a clear gelatinous substance and are attached to aquatic vegetation (Scott and Crossman 1998, Coker et al. 2001). The eggs hatch within a week (Cudmore-Vokey and Minns 2002).
Survival
The known maximum age is 18 years (Coker et al. 2001). Survival rates are unknown.
Physiology
The preferred temperature of spotted gar has been reported as 16°C (Coker et al. 2001). The spotted gar possesses a physostomous gas bladder and can breathe air (Scott and Crossman 1998).
Movements/dispersal
The spotted gar exhibit significantly greater movement rates in Louisiana during the spring (flood pulses) as they move to spawning areas (Snedden et al. 1999). This follows a consistently low activity period from December to February, and then in June and July activity declines to pre-flood levels. However, it will move large distances to regions of more suitable habitat, but has distinct home ranges during the spring (Snedden et al. 1999). In Louisiana, spotted gar are more active at night than during the day except during the spring (Snedden et al. 1999).
Nutrition and interspecific interactions
The spotted gar is primarily a piscivorous ambush predator, but does consume crayfishes and aquatic insects (Carlander 1969, Scott and Crossman 1998, Tyler and Granger 1984, Coker et al. 2001, Snedden et al. 1999). Fish species consumed vary with studies and seem to indicate that spotted gar feed on the most vulnerable or most available prey items (Dugas et al.1976). Dugas et al. (1976) indicated that spotted gar in Louisiana primarily consumed small, non-game species and did not pose as much of a threat to game fishes as previously thought. Feeding intensity varies throughout the day, with most feeding activity occurring in the early morning and, secondarily, at night (Carlander 1969, Snedden et al. 1999). Feeding took place around complex structures where prey items would be found. Relatively little food was consumed during the day (Snedden et al. 1999).
Spotted gar use asymmetrical movements of muscles on either side of the head to manipulate fish after capture so that the prey can be swallowed head first (Lauder and Norton 1980). This allows prey to be swallowed more successfully despite the relatively small opening of the buccal cavity and the direction of the scale rows on the prey.
It has been previously thought that the eggs of spotted gars were toxic to higher invertebrates and possibly vertebrates (Scott and Crossman 1998); however, Ostrand et al. (1996) found that green sunfish (Lepomis cyanellus) and channel catfish (Ictalurus punctatus) that fed on the eggs of spotted gar showed no evidence of ill effects. Therefore, the ichthyotoxin of gar eggs may not act as a protective mechanism from fish predators (Ostrand et al. 1996). However, fish fed on the eggs of spotted gar showed the least amount of weight gain compared to those fed eggs of other gar species.
The spotted gar is present in Point Pelee National Park where the longnose gar is absent. Although spotted gar are present in Long Point and Rondeau bays where longnose gar are present, they are absent from the many suitable habitats in southwestern Ontario where longnose gar are abundant (N.E. Mandrak, unpubl. data). Further study is required to determine if this observation is the result of interspecific interactions or other factors.
Behaviour/adaptability
Gars are among the most abundant piscivores in structurally complex shallow water habitats in the southern United States. This high relative abundance and predatory potential suggest that they are key components of the food web (Snedden et al. 1999). As a result of their ability to breathe air, spotted gar are physiologically well adapted to heavily vegetated ecosystems and can exploit seasonally hypoxic (dissolved oxygen concentrations of less than 2 mg/L) habitats that typically exclude other piscivores (Snedden et al. 1999).
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