Fig 1.
The ‘standard’ Newhaven dredge (a) and modified ‘skid’ dredge (b). The skid frame is attached to the back end of bottom of the collector bag (i.e. furthest away from the mouth of the dredge).
Table 1.
Scallop dredge vessel and scallop dredge characteristics used in the experiment, and regionally relevant legislation.
Fig 2.
Location of the fishing grounds where fishing trials compared the standard and modified ‘skid’ dredge design.
Fishing trials took place in Scottish waters in two areas within the Moray Firth (Moray Firth A: MF_A, Moray Firth B: MF_B) and in two areas in Welsh waters (Northeast Anglesey: W_NEA, Cardigan Bay: W_CB). Sixteen (fifteen in Moray Firth A) replicate hauls were carried out within each area (inset). Basemap created in ArcGIS using freely available data from Natural Earth [23].
Table 2.
Scallop dredge tow characteristics and environmental variables (mean ± SE) at each of the four study areas.
Fig 3.
Principle component analysis of the environmental variables experienced during the survey in Scotland (A) and Wales (B) showing the clear differentiation in environmental conditions at each of the two survey sites. The angle between the arrows reflects the correlation between the variables they represent; the smaller the angle the higher the correlation. The first axis (PC1) accounted for 41.0% of the explained variance in the Wales sites (W_CB and W_NEA) and 33.9% in the Scotland survey sites (MF_A and MF_B). The second axis (PC2) accounted for 27.1% in Wales and 27.4% in Scotland. Thus, the first two axes explained 68.1% of the total variance in Wales and 61.3% in the Scotland sites.
Fig 4.
The influence of environment and operation characteristics on the relative catch (response ratio (lnRR) of WPUA, kg ha-1 ±95% confidence intervals (CI)) of scallops caught in the Scottish survey areas by skid and standard dredges.
The dashed horizontal line (0) represents equal catches between skid and standard dredges. Positive lnRR values indicates higher WPUA in skid dredges compared to standard dredges, negative lnRR values indicates lower WPUA in skid dredges. Error bars in (A) and (B) indicate the 95% CI around the mean lnRR. The number of tows included in the analysis is given below the error bar in the plot (A). Dotted blue lines and grey interval in (C) indicate the fitted relationship and the 95% CI interval for variables that the GLM model analysis found to have a significant influence on the catch ratio between skid and standard dredge (lnRR).
Fig 5.
The influence of environment and operation characteristics on the relative catch (response ratio (lnRR) of WPUA, kg ha -1 ±95% confidence intervals (CI)) of scallops caught in the Welsh survey areas by skid and standard dredges.
Table 3.
The estimated parameters, standard error, T and p values for the best fit generalised linear models describing the relationship between the environmental parameters and the relative catch (lnRR of WPUA, kg/ha) of scallops.
Fig 6.
(A,C,E,G) The modelled ln response ratio, lnRR (± 95% confidence intervals (CI)) curve showing difference between the skid dredge and standard dredge across scallop size classes from the four survey areas. Bubbles represent number of scallops of each size class. The dotted horizontal line (0) represents equal catches, by abundance, between skid and standard dredges. Significant differences between the catches of the two dredges occur when the confidence interval (grey shading) does not overlap the horizontal dotted line (lnRR = 0). (B,D,F,H) Size frequency of catch distributions of scallops caught in the skid and standard dredge plotted for each survey area. The dashed vertical line in all plots represents the minimum landing size in Scotland (105mm) and Wales (110mm).
Fig 7.
The relative catch (ln response ratio (lnRR) of WPUA, kg ha -1 ±95% confidence intervals (CI)) in skid and standard dredges of all bycatch species combined (A, B) and of different taxonomic groups caught at Welsh (C, D) and Scottish (E, F) sites. ACT = Actinopterygii; AST = Asteroidea; BIV = Bivalvia; CEP = Cephalopoda; CHO = Chondrichthyes; ECH = Echinoidea; GAS = Gastropoda; MAL = Malacostraca; OPH = Ophiuroidea.
Fig 8.
The relative catch (ln response ratio (lnRR) of WPUA, kg ha -1 ±95% confidence intervals (CI)) of debris and stones landed in the skid and standard dredges in each area.