Throw Traps with Dip Net Extraction

Please Note:  Information in this section is based on Darby et al. (1999) and discussions with Phil Darby (Univ. of West Florida).

What they are:

• The 1–m² aluminum throw trap is a 60 cm high, 1 m x 1 m box that does not contain a top or a bottom.

  

• Constructed of a welded aluminum pipe frame enclosed with 0.08-inch-thick aluminum sheeting; weighs ~ 18 kg. 

• Snails are sampled, or extracted from the trap, using a dip net (shown in picture above).  The dip net is made of welded aluminum pipe, and consists of a 1.5 m handle on a 0.30 m high x 0.66 m long frame.  The frame is covered with 1.3 cm mesh netting.  Dip nets proved most effective across a range of habitats in a comparative study (Darby et al. 1999).   

What the method is used for:

• For obtaining density estimates, or the number of snails per square meter.  Only snails 13 mm or larger are captured given the mesh size of the nets.

• Very effective in low stem density prairie and slough with water depths < 60 cm.  Darby's crews routinely complete a site (50-70 traps) in 2-3 days.

• 1–m² throw traps have been used to sample other wetland animals, such as fish and crayfish.  

How they are used:

• The trap is deployed by throwing it, then immediately pushing it into the substrate to prevent the escape of animals under the trap.  Vegetation within the trap is then uprooted, rinsed well, and examined for snails.

  

•  When sampling in water depths between 60 and 100 cm, a removable 40 cm extension can be placed on the top of the trap.  However, this adds considerably to sampling time. 

• 2 sweeps of the dip net cover the trap.  The net is passed through the trap until 20 consecutive sweeps contain no snails (see picture below).

  

• A large number of throw traps per site may be required to obtain reasonable precision of the estimate (coefficient of variation (CV) of 20 to 30%).  

• To balance effort and CV, Darby (persoanl communication) established criteria for determining sample size (Table 1).

  



• Because the proportion of snails being counted varies between habitats, this proportion (known as capture probability, or recovery probability) must be determined.  Marked snails can be used to obtain this estimate (as described in Darby et al. 1999).  The absence of such a recovery probability could lead to misinterpretation of the data.

Their limitations:

• This method alters the habitat (via uprooting vegetation); an alternative protocol is to snip the vegetation at substrate level rather than uprooting it, but it is more time consuming.  If one samples about 50 traps in a minimum 2500 m² plot, approximately 2% of the area is altered.

• Labor intensive due to the vegetation removal, awkward, heavy equipment, and large number of samples needed for reliable estimates.

• Not practical in densely vegetated areas, such as sawgrass marsh or cattail marsh.

Literature Cited:

• Darby, P.C., R.E. Bennetts, J.D. Croop, P.L. Valentine-Darby, and W.M. Kitchens.  1999.  A comparison of sampling techniques for quantifying abundance of the Florida apple snail (Pomacea paludosa Say).  Journal of Molluscan Studies 65: 195-208.

Related Publications:

• Bennetts, R.E., P.C. Darby, and L.B. Karunaratne.  2006.  Foraging patch selection by snail kites in response to vegetation structure and prey abundance and availability.  Waterbirds 29(1): 88-94.

• Darby, P.C., P.L. Valentine-Darby, H. Franklin Percival, and W.M. Kitchens.  2004.  Florida apple snail (Pomacea paludosa SAY) responses to lake habitat restoration activity.  Arch. Hydrobiol. 161(4): 561-575.

• Darby, P.C., R.E. Bennetts, and L.B. Karunaratne.  2006.  Apple snail densities in habitats used by foraging snail kites.  Florida Field Naturalist 34(2):37-47.

• Karunaratne, L.B., P.C. Darby, and R.E. Bennetts.  2006.  The effects of wetland habitat structure on Florida apple snail density.  Wetlands 26(4): 1143-1150.