Passive Larval Traps

Passive larval traps rely on the advection and subsequent retention of larvae that may later be counted to provide a measure of supply to the shore. From deployment to retrieval, an ideal trap will continually and reliably catch larvae over the full range of larval concentrations, thereby providing an exact measure of supply that integrates the spatial and temporal variability in the abundance of planktonic larvae. This is obviously preferential to intermittent sampling procedures using water pumps and plankton nets.

Early designs were based upon modified sediment traps (Yundet al., 1991), or light and emergence traps used primarily to sample larval fishes (Doherty, 1987). Unfortunately these designs were often only used in subtidal areas and wave-protected environments such as bays, inlets and coral reefs (Hannan, 1984). Trap capture rates were also very low compared to adjacent levels of settlement, indicating a potential inefficiency of the trap designs.

A new generation of robust traps was soon developed for deployment in the intertidal of rocky shores. These traps mostly utilised plankton mesh and varied in size from large cylindrical drums (figure 3.1; Moksnes & Wennhage, 2001) to filter-cup traps (Castilla & Varas, 1998; Jeffery & Underwood, 2000; Castillaet al., 2001; Yanet al., 2004). These traps exhibited much improved capture rates for a variety of planktonic

However, designs such as those in figure 3.1 have a fixed orientation and so may struggle to cope with the multidirectional currents present on rocky shores. In particular, if the trap inlet faces down the shore, the trap may be less efficient at catching larvae during the ebb tide. The filter-cup traps do not suffer from such a drawback because the trap inlet is located on top of the trap and is equally exposed to any direction of current.

Figure 3.1. Intertidal plankton trap with cut-away showing PVC pipe and rubber flapper valve.

Components include (A) removable plastic lid with nitex mesh netting, (B) opposing 5.0-g magnets, (C) brass spring clips and (D) stainless steel eye-bolts set in intertidal substratum (taken from Setran, 1992).

Such filter traps have not provided a universal solution to assessing larval supply. Jeffery & Underwood (2000) could only deploy their traps in crevices due to their bulky design. Setran (1992) expressed doubts as to whether his trap would survive in heavy wave crash. Ideally, traps should be robust and small enough to be deployed anywhere and in any wave conditions without capture efficiency being compromised. Trap design may also have to be specific to the species or group under consideration. For example, Valleset al. (2006) used traps containing coral reef rubble to assess coral reef fish larval recruitment.

The reliance of many traps on formaldehyde-impregnated mesh may also provide health concerns for traps deployed in locations easily accessible to the public. Castillaet al. (2001) also reported that after several days the mesh clogged with a resultant decrease in filtering ability. Traps which rely on plankton mesh may be unsuitable in waters with much suspended material, such as sites RW and TM used in this study, or with an abundance of suspended algal fragments, such as site SB. Trapped larvae or even adult barnacle moults also have the potential to clog the mesh, resulting in biased comparisons between high and low larval density sites.

To avoid the use of plankton mesh as a method of catching larvae, Todd (2003) used a modified sediment trap design that was able to obtain representative samples ofS.

balanoidescyprids for comparison with settlement levels at adjacent sites (figure 3.2). The trap utilised a urea killing solution to avoid the use of formaldehyde, thus allowing deployment whilst minimising public health concerns. Internal baffles helped stabilise the internal volume and retain cyprids whilst reducing urea washout. Trap manufacture was cheap and easy and its size allowed easy deployment on most vertical surfaces. Trap servicing could be easily accomplishedin situand trap capture efficiency was found to be similar for both tidal and daily deployment.

Figure 3.2.Diagram (not to scale) of intertidal larval trap. Traps were manufactured from small plastic specimen tubes. Traps were emptied and refilledin situby means of the bottom screw cap (taken from Todd, 2003).

From this original trap employed by Todd (2003), significant design

improvements were made by Toddet al. (2006) and Phelan (2006). Developments were primarily concerned with reducing potential cyprid capture inefficiencies, improving urea retention and increasing durability. Figure 3.3 shows these significant stages of

development. The addition of a more complex baffle system and conical aperture were found to improve urea retention and cyprid capture. The epoxy resin sleeve provided a robust leak-proof coat for the trap. These new features resulted in a durable and reliable trap that captured cyprids in biologically realistic numbers, and that could be deployed in conditions of heavy wave crash.

2.8 cm 10 mm cone baffles 6 mm cone baffle Screw cap PVC sleeves 29 cm quarter cone spiral baffles cast resin sleeve with ring slots for cable ties to strip acrylic mounting strip

Figure 3.3. Diagrams (not to scale) of the significant improvements to Todd’s (2003) original (left) larval trap. The spiral (centre) design features an improved baffle system, whilst the coned (right) trap has an epoxy resin sleeve (taken from Toddet al., 2006).

This chapter focuses on the assessment of differing conical tops (i.e. differing aperture openings) for traps that already featured an improved internal baffle system and epoxy resin sleeve (figure 3.3, right design). 1 cm2and 2 cm2aperture sizes were chosen for investigation, because sizes bigger than this were considered to be too close to the original non-coned trap design. Once a standard design of coned trap had been decided upon, focus then shifted to simpler trap designs that may work as well as the potentially

environmental factors of tidal amplitude, wind and wave action, and intertidal cyprid concentration.

3.2.Methods