Spiny softshell turtle (Apalone spinifera) COSEWIC assessment and status report: chapter 6

Biology

General

No new information.

Reproduction

From the recent (1994-2000) work conducted in Ontario, clutch size ranges from 6-36 eggs, and it is possible that during long warm summers that double clutching occurs (Fletcher 1999). Softshells are solitary breeders but, with the lack of available nest sites, many individuals will lay their clutch in the same nesting area. The requirements for these areas are that they are open, have a sandy or gravelly substrate, and are in close proximity to water. Along the river, these sites tend to be sand deposits at riverbends or on islands. On the lake, preferred sites appear to be those where a narrow strip of sand separates the lake from a pond or other quiet water such as a bay. Although there was a certain amount of fidelity to these sites during the drought conditions of the late 1990s, softshells were observed to start nesting on areas that in other years would have been underwater (Fletcher 1998). A few nests were destroyed in 1996 (Fletcher 1996) and 1997 (Fletcher 1997) and close to 100% of nests failed during the flood year of 2000 (Fletcher, pers.obs.). There is a 1:1 sex ratio of hatchlings. Sources and rates of egg mortality vary from site to site, but major causes of mortality are nest depredation by raccoon or fox, nest infestation by sarcophagid fly maggots, egg infertility, and egg collection by humans. Predation of eggs could become a factor in survival of local subpopulations as 100% of softshell nests that were not protected by the research team were preyed upon, as were more than 650 nests of other turtle species (Fletcher 1999).

Spiny softshells seem to require unvegetated areas of fine gravel or coarse sand on which to nest. They may be well adapted to nesting on this substrate because unlike other species of turtles in Canada, A. spinifera has a hard-shelled egg that is fairly impermeable to water (e.g. Packard et al. 1979). This may allow them to nest successfully in dry sandy areas that are unsuitable to other turtles. These sandy beaches are also prime habitat for recreation.

Survival

There are few factors that currently affect adult survival other than incidental mortality due to boat collisions and capture through commercial and sport fishing. Softshell turtles should closely follow the expected age dependant survival rates outlined by Congdon et al. (1993) for Blanding’s turtle (Emydoidea blandingii) that show that survival rates during the early stages of life are very low, whereas during the late juvenile and adult stages they are very high. The potential problem that the Ontario softshell population may face is that recruitment may not be sufficient to compensate for natural adult mortality. Survey work along the Sydenham River in particular only found adult softshells and most of these were large and therefore likely older adults (Fletcher and Gillingwater 1994). In general, 6 years of intensive work in Ontario have provided very few observations of softshells under the age of 5 and even fewer sightings of young adults (Fletcher 1999). Success of nest predators has been shown to be quite high in some areas and, although this is to be expected, when our high observed level of depredation is combined with the fact that in some areas we may have a population consisting of older adults with few or no juveniles the future of these populations may be in doubt. Such a skewed age distribution has been associated with population collapse in models of long-lived species. A collapse is more likely if adults are being subjected to increased rates of mortality by humans (boat collisions, incidental trapping, etc.).

Physiology

No new information.

Movements

Radio-telemetry studies throughout the Canadian range indicate that over the course of the year softshell turtles can travel more than 30 km between nesting and hibernation sites (Fletcher 1996, Galois 1997). To date there has not been a large amount of time spent studying the characteristics of the sites chosen for overwintering by the Ontario population, but the work on Lake Champlain has shown softshells leave the lake to overwinter in some of the tributary rivers of the lake (Galois 1997). This could also be true of the Ontario lake populations as the author was informed by a trapper that softshell turtles were being caught in traps set for snapping turtles in some of the tributary streams around Rondeau Bay during October.

On the Thames River, there are two dams in the City of London preventing migration of individuals past these points. There are turtles both upstream and downstream of these dams (part of this population is between the two dams) and more extensive migration and interaction within the population would most likely occur without this blockage of access. Other factors that might interrupt migration movements are large scale construction projects along the river, such as bridge and pipeline crossings, which may not completely block access through these areas, but may deter softshells from passing through the area because of increased noise and activity.

Nutrition and interspecific interactions

There is no new information on nutrition. However, it is important to consider that spiny softshells feed heavily on crayfish and molluscs, two taxa that are themselves declining in southern Canada (Campbell et al. 1985; McCoy 1982). Thus, decline in food supply may be a problem for spiny softshells.

Behaviour/adaptability

No new information.

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