Scottish Marine and Freshwater Science Volume 6 Number 3: Effects of Codend Mesh Size and Twine Number on Nephrops Selectivity: Report of Fishing Industry Science Alliance (FISA) Project 03/13

Appendix: Tables and Figures

Note: for the relative catch rate figures below a solid line denotes where the catch rate is significantly different from 100 %. A broken line shows where the rate is not significantly different.

Figure 1. Comparison of raised nephrops length frequency for the control net (white bars) and the three test nets (black bars). Results are amalgamations of all hauls for the particular test case.

Figure 2. The relative catch rate of nephrops by codend from smoothed data.

Figure 3. Haddock - top row: combined number caught v length by the control (white bars) and test nets (black bars). Bottom row: focus on the numbers from the test net.

Figure 4. The relative catch rate of haddock by codend from smoothed data. Note that in the case of the 120 mm codend the analysis shows that the trend with length continues and that for haddock >30 cm the relative catch rate is not significantly different from the control net, however, this is not illustrated here.

Figure 5. Whiting - top row: combined number caught v length by the control (white bars) and test nets (black bars). Bottom row: focus on the numbers from the test net.

Figure 6. The relative catch rate of whiting by codend from smoothed data

Figure 7. Plaice combined numbers caught at length by the control nets (white bars) in comparison to the test nets (black bars).

Figure 8. The relative catch rate of plaice for 100 mm codend from smoothed data.

Figure 9. Witch: combined numbers caught at length by the control nets (white bars) in comparison to the test nets (black bars).

Figure 10. The relative catch rate of witch for 100 mm codend from smoothed data.

Figure 11. Lemon sole: combined numbers caught at length by the control nets (white bars) in comparison to the test nets (black bars).

Figure 12. Hake: combined numbers caught at length by the control nets (white bars) in comparison to the test nets (black bars).

Figure 13. Bottom: approximate percentage 'fate at length' of commercially caught nephrops from the North and South Minch (source: MSS observer data 2013). Fates are illustrated as a percentage of three available to each mm carapace length grouping - light grey area: discarded, dark grey area: landed as tails, cross-hatched area: landed as whole. Top: relative catch rates of the 3 test gears as Figure 2 which can be applied to each length/fate. The length range illustrated is the same as that analysed during these trials.

Percentage fates as illustrated represent an amalgamation of observer data covering both the North and South Minch during all 4 quarters of 2013 and incorporate information from both 80 mm and 100 mm codends. The figure, then, should not be taken as definitive but as a guide. As in other areas market forces, seasonal forces, working logistics and individual vessel practice all heavily influence the patterns of tailing and landing of whole nephrops in the Scottish west coast fishery.

Figure 14. Approximate midpoints (open circles) of hauls undertaken during the trials.

Table 1. Weight (kg) of Nephrops, haddock, whiting, other commercial species (others 1), non-commercial species and invertebrates (others 2) by haul number and test configuration (mesh size and whether fished port or starboard on the twin-rig). Missing haul numbers refer to foul hauls.

Haul no.	Test Config	Control Net						Test Net						% bulk Wt. reduction
		Nephrops	Haddock	Whiting	others 1	others 2	Totals	Nephrops	Haddock	Whiting	others 1	others 2	Totals
1	80 Stbd	39.8	14.7	7.5	0.2	50	112	35.1	13.5	3.9	1.9	26	80	28
2	80 Stbd	127.2	62.8	11.0	4.7	124	330	92.7	21.6	5.4	10.4	93	223	32
3	80 Stbd	59.9	12.4	1.0	3.5	105	182	56.2	8.1	1.4	6.5	65	137	25
5	120 Stbd	136.5	53.6	8.6	3.7	44	246	61.3	2.7	0.5	4.7	13	82	67
6	120 Stbd	115.6	30.4	10.6	6.3	61	224	44.5	1.7	0.9	2.8	32	82	63
7	120 Stbd	126.1	24.8	6.4	6.0	72	235	39.6	1.9	0.2	0.8	34	76	68
8	100 Stbd	96.3	94.3	29.8	6.7	58	285	78.0	15.3	2.2	5.7	14	115	60
9	100 Stbd	151.1	83.4	33.0	10.2	107	385	122.8	8.1	2.4	5.4	25	164	57
10	100 Stbd	123.7	14.2	5.1	4.9	74	222	67.1	6.4	0.1	6.4	18	98	56
11	100 Stbd	55.6	14.4	4.5	8.1	205	288	26.4	10.2	2.5	5.9	126	171	41
12	100 Port	59.4	78.9	1.1	3.2	47	190	31.2	51.1	0.4	2.1	29	114	40
13	100 Port	26.4	80.1	0.7	2.1	66	175	11.8	33.9	0.7	3.0	25	74	58
14	100 Port	178.6	38.7	8.3	10.3	74	310	92.3	5.5	1.4	3.8	15	118	62
15	80 Port	89.2	35.6	7.2	3.7	98	234	63.1	6.4	5.8	4.2	15	94	60
16	80 Port	53.1	27.4	5.7	1.8	79	167	38.4	3.0	2.5	4.8	21	70	58
17	80 Port	54.0	44.3	6.8	2.7	103	211	33.5	13.6	2.3	4.1	17	71	67
18	80 Port	136.8	85.7	2.0	3.7	81	309	102.0	2.8	1.1	2.6	16	124	60
19	80 Port	203.0	30.2	5.9	9.4	79	327	133.5	1.8	1.3	8.8	31	176	46
20	80 Port	73.6	30.6	3.4	6.6	89	203	58.1	2.4	1.4	6.2	17	85	58
21	80 Port	100.0	28.3	5.0	7.4	52	193	75.2	1.4	0.2	1.1	33	111	42
23	100 Port	73.0	27.5	5.8	3.5	43	153	55.1	1.3	0.1	3.2	5	65	58
24	100 Port	136.0	33.0	4.9	3.3	72	249	70.0	1.8	0.3	1.5	8	82	67
25	100 Port	88.2	37.0	6.8	7.0	86	225	50.7	2.1	0.8	3.0	11	68	70
26	120 Port	44.0	9.7	0.3	5.2	101	160	21.4	9.7	0.0	2.5	24	58	64
27	120 Port	58.0	27.5	1.0	2.9	67	156	23.5	16.0	0.9	1.4	19	61	61
28	120 Port	58.3	11.9	4.9	2.8	74	152	25.9	7.4	0.2	1.6	22	57	62
29	120 Port	91.2	6.8	17.4	30.8	60	206	39.8	0.6	1.0	6.0	10	57	72
30	120 Port	60.3	1.6	5.3	9.1	38	114	28.3	0.0	0.9	11.3	11	52	55
31	120 Port	79.6	10.8	14.2	68.7	41	214	37.9	1.6	2.4	62.9	11	116	46
32	120 Port	88.6	9.4	10.7	16.3	40	165	42.3	4.1	3.4	34.9	25	110	33
33	100 Stbd	78.2	13.5	35.8	8.5	62	198	69.3	5.6	3.5	6.6	32	117	41
34	100 Stbd	63.9	9.3	12.8	14.5	91	191	48.9	5.0	3.2	8.0	44	109	43
35	100 Stbd	110.8	10.1	5.6	26.5	99	252	86.7	2.7	1.8	7.6	49	148	41
36	100 Stbd	85.6	6.7	10.0	45.6	149	297	64.8	3.0	0.7	13.6	30	112	62
37	120 Stbd	157.8	51.4	11.0	7.6	120	348	81.7	0.8	0.5	3.2	5	91	74
38	120 Stbd	67.4	50.5	50.4	10.4	112	291	36.1	1.8	0.5	2.0	52	92	68
39	120 Stbd	94.1	47.4	21.6	4.9	329	497	68.1	19.9	3.7	3.1	72	167	66
40	120 Stbd	154.8	40.4	23.2	4.6	343	566	73.8	16.1	7.3	3.9	52	153	73
41	80 Stbd	132.7	23.1	7.2	6.1	430	599	137.6	4.4	1.8	9.4	53	206	66
42	80 Stbd	85.9	52.5	24.7	4.7	340	508	73.9	1.8	2.5	3.3	27	108	79
43	80 Stbd	44.6	54.6	17.5	1.4	318	436	46.6	4.6	1.5	1.2	26	80	82

Table 2. Retention (% by weight) for the three main commercial species caught. It is important to note that these retention values will apply only to the length frequencies encountered (as illustrated in Figures 1, 3 and 5).

Haul	80mm			Haul	100mm			Haul	120mm
	Nephrops	Haddock	Whiting		Nephrops	Haddock	Whiting		Nephrops	Haddock	Whiting
1	88	92	52	8	81	16	7	5	45	5	6
2	73	34	49	9	81	10	7	6	38	5	8
3	94	65	146	10	54	45	3	7	31	8	4
15	71	18	80	11	47	71	55	26	49	100	0
16	72	11	44	12	53	65	37	27	41	58	89
17	62	31	34	13	45	42	89	28	44	62	4
18	75	3	54	14	52	14	16	29	44	9	6
19	66	6	22	23	75	5	1	30	47	1	16
20	79	8	41	24	51	5	6	31	48	15	17
21	75	5	5	25	57	6	12	32	48	44	32
41	104	19	26	33	89	42	10	37	52	2	5
42	86	3	10	34	77	54	25	38	54	3	1
43	104	8	8	35	78	27	31	39	72	42	17
				36	76	45	6	40	48	40	31
Overall	79	17	30	Overall	67	22	12	Overall	47	20	12

Table 3. Mean wingspreads (m) by haul. Missing values refer to periods of no data. Missing haul numbers refer to foul hauls.

Haul no.	wing spreads		Haul	wing spreads
	Port	Stbd	no.	Port	Stbd
1	-	-	24	-	-
2	-	-	25	8.4	8.6
3	-	-	26	-	9.4
5	8.4	8.7	27	8.1	8.6
6	8.1	8.1	28	-	8.2
7	-	8.6	29	8.5	8.6
8	8.2	9.9	30	8.9	8.7
9	8.0	8.0	31	8.6	8.5
10	-	8.5	32	8.8	8.4
11	-	7.6	33	8.7	-
12	8.6	-	34	8.5	9.1
13	8.4	8.4	35	8.3	8.6
14	8.2	8.4	36	9.2	8.8
15	8.1	8.5	37	8.3	8.4
16	8.6	8.5	38	8.2	8.4
17	8.8	8.6	39	-	7.6
18	8.5	9.4	40	8.5	7.9
19	8.6	8.4	41	-	-
20	8.0	8.2	42	8.8	8.0
21	8.0	8.2	43	8.7	8.2
23	-	-

mean overall wingspreads: Port - 8.4 Stbd - 8.5

Table 4. P-values: pairwise comparisons between test gears and control. Shaded areas indicate significant difference (P<0.05).

Species	Window	Half-Range	Test Gear	80mm	100mm	120mm	control
Nephrops	21	19.5 - 54.5	80mm	-	0.022	0.000	0.000
			100mm	-	-	0.000	0.000
			120mm	-	-	-	0.000
Haddock	9	8.5 - 25.5	80mm	-	0.569	0.012	0.000
			100mm	-	-	0.022	0.000
			120mm	-	-	-	0.000
Whiting	13	9.5 - 32.5	80mm	-	0.024	0.000	0.000
			100mm	-	-	0.004	0.000
			120mm	-	-	-	0.000
Plaice	9	16.5 - 30.5	100mm	-	-	-	0.000
Witch	15	13.5 - 30.5	100mm	-	-	-	0.000

Table 5. Test codend makeup. All codends and straight sections were constructed from Brezline ( PE).

Test mesh (nominal)	Approx. mesh size pre-Trial	% > nominal size	Codend twine number and thickness	Codend no. meshes deep	Codend approx. length	Straight section twine number and thickness	Overall Length	No. open meshes across
80	83	3.8 %	4mm single 80mm	-	-	4mm single 80mm	12m	51
100	104	4.0 %	5mm double 100mm	50	5m	5mm single 100mm	12m	44
120	124	3.3 %	5mm double 120mm	50	6m	5mm single 120mmm	12m	44

Table 6. Mean codend mesh measurements (30 measurements on each of top and bottom sheet over the area of selectivity during the trials). All measurements were taken with a standard Omega gauge on wet meshes.

mesh	pre-trials mean size	post-trials mean size
80mm	82.7	81.8
100mm	103.8	102.4
120mm	123.9	122.0