Trap design comparisons

Multiple trap arrays were constructed and deployed at three sites in two years so that comparisons of inlet designs could be made in the field. Data for cyprid capture of two species of barnacleSemibalanus balanoidesandBalanus crenatus, proportional trap urea retention and volume of sand capture were routinely recorded. Traps were re-filled after two tides, for as long as day light conditions permitted. When retrieval was restricted by daylight the deployment was extended for a third tide and servicing resumed the following morning. These three-tide deployments were approximately fortnightly and are not included in the analysis here as no study was done to examine the effectiveness of the traps beyond two tides.

In 2004 multiple trap design arrays were deployed at Fife Ness and Tentsmuir. At Tentsmuir a 9-trap array was deployed with 0.5cm2and 1cm2coned trap designs and a cylindrical 5cm2unconed spiral design. Triplicate traps for each of the three designs were deployed. At Fife Ness in 2004 a 12-trap array was deployed. This array consisted of 2 sets of 6 slot positions either side of an artificial settlement substrate array because it was part of the mesoscale study described in Chapter 5. 0.5cm2, 1cm2 and 2cm2coned designs and 5cm2unconed design was deployed in triplicate.

In 2005, the 9-trap array was moved to Reres Wood. Coned and spiral trap designs with 0.5cm2and 1cm2inlets and a 5cm2unconed spiral design were deployed in triplicate once again. Part way through the season the 5cm2unconed spiral design was replaced by a 0.25cm2coned and spiral design because it was apparent that the 0.5cm2design was still able to capture large numbers of cyprids despite its small aperture size. In 2005, the 12-trap array was re-deployed at Fife Ness in the same location as in 2004. On this occasion, 10 slots were used, 5 either side of the artificial substrate array. 0.5cm2coned and spiral trap designs and a 5cm2unconed spiral design were deployed in triplicate once again. 1cm2traps were deployed in

quadruplicate due to their involvement in the 2005 mesoscale study (Chapter 5). Part way through the season the 5cm2unconed spiral design was replaced by a 0.25cm2 coned and spiral design as at Reres Wood.

Analysis of the data was undertaken using ANOVA. Todd (2003)

demonstrated the original trap design resulted in detectable slot positional effects, possibly due to consistent small scale hydrodynamics across a trap array. Due to the possibility of consistent positional effects, trap positions were randomised prior to each daily deployment in the field. Following Mauchly’s test for sphericity, the existence of positional effects for each array, in each year, was determined by

balanced repeated measures design ANOVA on data forSemibalanus balanoidesand Balanus crenatuscyprid capture, urea retention and sand capture. The amount of sand andBalanus crenatuslarvae was insufficient at Fife Ness to perform the analysis for this site on these dependent variables.

Data for cyprid capture of both species and urea retention was transformed where appropriate to normalise the data. Sand was measured only if >1ml was recorded. If data for the dependent variable, cyprid capture rate, was less than 1 for

any slot position on any day that day was removed from the analysis because it would provide no relative information on the performance of that slot position. For the urea analysis any leakage of a trap meant the exclusion of these data for that day from the analysis.

Differences in trap designs and between days were examined with balanced factorial ANOVA because this is the most robust method when data fail the

assumptions of ANOVA. Bonferroni multiple pairwise comparisons were performed on design types when a significant difference was found from ANOVA. Transformed data forSemibalanus balanoidescyprid capture, urea retention and sand capture were used when appropriate.

At Tentsmuir and Reres Wood it was necessary to adjust urea retention for sand capture. Due to the density of concentrated urea there is a steep seawater/urea gradient resulting in a recognisable interface between the killing solution and the washout zone in a trap whilst it was deployed in a field environment. Therefore urea concentration was not judged to be distributed in a uniform manner throughout the trap and the crucial factor linking urea concentration to cyprid capture was judged to be contact with a sufficiently concentrated urea solution rather than total trap

concentration. Sand capture displaces urea at the bottom of the tube forcing the more concentrated urea solution towards the inlet. As a result it was necessary to determine the volume of washed out urea rather than retained urea. The volume of sand was subtracted from the total volume of the trap. This was multiplied by the proportion of urea washed out. The volume of washed out urea was then divided by the total volume of the trap and this was expressed as proportion of urea retained. Data for percentage urea retained was then arcsine transformed into degrees.